throttle body size, 52mm, vs, 58mm


Staff member
I get asked frequently if installing a larger than the stock 48mm TPI or LT1-4 throttle body will significantly help on a corvette engine, Ill save you some reading right off if you want,
problem one, is that the STOCK TPI MAF sensor inside diameter,and available cross sectional area, as a flow restriction, and the stock cylinder heads and stock TPI or LT1 intake design , air flow restrictions.
NOT the 48mm throttle body.
the O.E.M. cam is also very restrictive if your looking to maximize the LT1 or TPI engines potential

is significantly less than the total area of a twin 58mm throttle body which is slightly more than 8 square inches, while the stock throttle body twin 48mm is a better match to the MAF sensor size, being almost 6 square inches in area, so unless you increase the maf sensor flow which is also about 6 square inches in cross sectional area,the air flow feeding the 58mm throttle body will remain restricted



BTW the stock HOLLEY FUEL PRESSURE REGULATOR ON THE STEALTH RAM, is know to have frequent failures, heres a link to a higher quality unit, part #6547 ... 36547.html


Keep in mind even the factory 502 cubic inch big block used the stock L98 TPI throttle body size and made 500 horse power

yes a 58mm can potentially increase hp if your engines highly modified and your still using a stock 48mm throttle body, but its hardly a big restriction, and your probably going to gain more with other mods
If you calculate the point where dual 48mm throttle bores become a significant restriction to flow on a 383 SBC
youll find you need to spin it to well over 8,000 rpm ... -body.html

Ive used this type on several engines, swapping to the 52-or-58mm won,t hurt or help much on a mildly modified engine but you might want to go that route if your contemplating future mods.


but the restriction a stock 48mm throttle body represents is not as significant and the gains to be had are not that large, or non-existent in most cases,, if the engine does not have better heads, and a cam that allows significantly higher than stock flow levels and even then a minimum of about a 383 displacements required to see minor gains




your STOCK intake manifold and cam plus your heads are the major restrictions. the stock 48mm throttle body flows about 700cfm and has more than enough flow to support about 500hp-550hp, bored to 52mm it will support about 550hp,-575hp, a 58mm can support about 600hp-750hp, but keep in mind theres lots of factors other than just throttle body size that determine potential hp
youll need to port most intake manifolds to open the plenum entrance to get the full 58 mm flow potential

stock plenum has 48mm openings for Throttle-body (TB) and 39mm openings for stock runners.
if you fail to read the sub links youll miss a great deal of info


you'll need this info in these links if you do install a new throttle body




using or modifying a throttle body to flow more than 1000cfm -1200 cfm like a 58mm twin bore potentially flows is usually a waste of money, heres why
the largest displacement sbc your likely to build is a 434cid, that requires a 4" stroke crank and you'll be limited to about 6500rpm even with top quality parts if you want the engine to last any time at all, now you might not realize it but the time necessary to fill the cylinders gets limited very quickly at higher rpms and the volumetric efficiency falls off quickly after about 5000rpm simply because theres just not en ought time to allow the cylinders to fill completely. at 5000 rpm theres 41.6 intake strokes per second, per cylinder, by 6500 rpm its up to 54 intake strokes per second, per cylinder thats only 0.018 seconds for the intake port to flow into the cylinder per intake stroke. but theres yet another factor!!! thats the time for the full intake stroke!!!, while theres 720 degrees in the repetitive cycle and Ive given you a full 360 degrees worth of time, the truth is that your cam/engine combo, seldom allows anywhere near that duration!!!! even a radical race cam seldom exceeds 250 degrees in effective duration for port flow, so you'll need to knock about 45% off that time, now your down to only about 0.012 seconds or less of effective flow per intake stroke per cylinder, is it any wonder the cylinder can,t fully fill at high rpms??
now the theoretical 434 were working with here even if it could fill its cylinders 100% at 6500rpm which is totally impossible at that rpm would require 434 (the displacement) x 3250(the number of intake strokes)=1410500 (the full 100% in cubic inches /1728(to change to cfm) = only 816cfm.... now even if you could ram tune the exhaust and intake pulse perfectly and get the cylinders to pack the cylinders to 120% (a total impossibility without a supercharger or turbo) thats till only 979 cfm
but in the real world your volumetric efficiency falls off fast after 5000 rpm with a 4" stroke and you'll be exceptionally lucky to pull 80% efficiency at over 5000rpm
drop your displacement to a more common 383 and your rpm limit to the fairly common 6500rpm and you'll drop even the theoretical air flow requirement to below 875cfm

heres the ramjet ZL1 454

yeah you might not believe it but it got a stock 48mm twin bore TPI style chevy throttle body feeding that 454 engine, do you think chevy would do that if it was a significant restriction to hp production??

the restriction is not the throttle body its the port size, valve size and flow the heads have and intake manifold runers restriction,and cam lift and duration... install an intake manifold,cam and heads that can flow 300cfm and the 58mm aftermarket throttle body will still effectively feed the engine, remember that while even 300cfm ports at full flow only pull air about 250 degrees out of 720 degrees in the cycle and are spaced 90 degrees apart, that means that 1000cfm can feed all 8 cylinders a full 833cfm they could in theory flow ,and that 300cfm per port can support roughtly 600hp

why would you buy a 52mm vs a 58MM throttle body????

now PLEASE don,t get me wrong,
the 52MM has a nice improvement in air flow over the stock throttle body, but in most cases that extra air flow is not used, especially on a mild engine or one with stock gearing and displacement, and the 58mm won,t in most cases increase the performance, and yes IM well aware from testing that the MAF sensors and the intake manifolds , and heads are the big restrictions, and that with out inproving those areas the stock throttle body works just fine.
I tried BOTH on my 383 with BOTH the TPI and STEALTH RAM intakes, (I borrowed a friends 52mm to test) and there was no real differance untill I was up in the 6000rpm range, and even then the differance was minor, but again WHY buy the smaller unit, that could potentially be a restriction with future mods done??
its NOT like a carb, you won,t get a big loss of throttle response, in fact I could detect ZERO difference once it was correctly tuned) or for that matter experience any down side to the larger unit provided of course your tuning skills are decent.
and before you guys get crazy, do some research!
I don,t remember ever seeing a dyno on anyone else s engine EITHER that showed a LOSS to the larger unit, just a few that showed no IMPROVEMENT in a particular application, and if your thinking of pointing out a lack of throttle response thats been repeatedly shown to be due to tuning issues like low fuel pressure, too small of injectors,or the TPS voltage was off the mark, the IAC was not set correctly or the ignition timing curve was not ideal, or in a few cases the compression ratio was to low for the cam selected, get the LCA or duration wrong for the application and AIR FLOW REVERSION in the runners CAN cause problems, like a bog when you transition from idle to W.O.T. almost instantly, but in no case IVE seen was the extra potential air flow a problem, especially once you realize that the plenum/ports carry only AIR, and the fuels injected in the last 4 inches of the runners.

for some reason many guys seem to think the throttle body size is far more restrictive than it is while IGNORING the fact that as your engines horse power expectations increase the need to supply fuel to the engine also will increase and they find the engines tendency to loose power at 4500 rpm-5000 rpm , and blame it on a lack of air flow exclusively, when in fact its generally BOTH a lack of fuel and air flow rates that cause the power curve to nose over as rpms increase.
yes youll need a larger flow rated set of fuel injectors AND A BETTER FUEL PUMP, if you plan to exceed 400 plus hp

Inj Flow Rate (@ 40psid) Naturally Aspirated hp (@ 0.50)
19 lb/hr 258 hp @ 85% Duty Cycle
24 lb/hr 326 hp @ 85% Duty Cycle
30 lb/hr 408 hp @ 85% Duty Cycle
32 lb/hr 435 hp @ 85% Duty Cycle
39 lb/hr 530 hp @ 85% Duty Cycle
42 lb/hr 571 hp @ 85% Duty Cycle
47 lb/hr 639 hp @ 85% Duty Cycle
60 lb/hr 816 hp @ 85% Duty Cycle
BTW To TEST an INJECTOR with a multi meter for OHMS resistance, remove the connector from the injector, push carefully in the middle of the locking clip, with your thumb, This will make the sides, and tips move OUT WARD in their connector slots allowing the tips that hold the injector into the connector to slide out of the way from the injector body, and you can then pull the sides away. WATCH the locking clip - IT can and WILL come off and get lost, if your not careful put your V.O.M. meter on OHMS and put the leads across the two connectors ON THE INJECTOR ,it should read about 11-16 OHMS in most cases, a few are designed to read up to 19 ohms, or as low as 9 ohms , BUT GENERALLY READINGS LOWER THAN 11 OHMS tends to point to a problem with the injector , The Ford motorsport injectors in the 24lb-hr flavor test a little lower (14.1-14.3 ohms) than the bosch's @ 22lbs/hr. (15.9-16.3) BUT ALL INJECTORS IN A SET SHOULD READ WITHIN ONE OHM OF EACH OTHER if its out of that range by a wide margin ITS MORE THAN LIKELY DEFECTIVE, checking the OHMS reading as a first step will frequently detect a defective injector, naturally using your trouble codes , a shop manual and a fuel pressure gauge won,t hurt and a shop scope , or engine analyzing software with a read out to your laptop computer helps and can further isolate the problem, at IDLE speeds placing a finger tip on the side of the injector will usually allow you to feel it (CLICKING) adding a couple cans of injector cleaner and about a pint of MARVEL MYSTERY OIL to a full tank of fuel can sometimes free a sticky injector that partly clogged, but don,t do it frequently as its very hard on the CATS and O2 SENSORS if done constantly



partly clogged injectors tend to reduce power and increase emissions

notice, the injector resistance is usually stated in the description

Brand: ACCEL
Product Line: ACCEL Fuel Injectors
Part Type: Fuel Injectors
Part Number: ACC-150826
Injector Advertised Flow Rate (lbs/hr): 26 lbs./hr.
Injector Advertised Flow Rate (cc/min): 269.0cc/min
Injector Impedance: 14.4 ohms
Driver Type: 12 V saturated circuit
Overall Height (in): 2.880 in.
Seat to Seat Height (in): 2.270 in.
Manifold O-Ring Outside Diameter (in): 0.573 in.
Fuel Rail O-Ring Outside Diameter (in): 0.574 in.
Outside Diameter (in): 0.943 in.
Injector Plug Style: Bosch/Amp-style
Wiring Harness Included: No
O-Rings Included: Yes






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Staff member
Re: throttle body size

ID just like to point some thing out
(very useful info here)


the largest common sbc engine size is 434 cid, youll have a hard time spinning that 4" stroke sbc engine to over 6500rpm, so lets look at what that engine requires in the way of air flow
- FIRST TPI Intake.(good choice)



intake comes with a new throttle body

it would require about 850cfm at 100% efficiency (WHICH YOULL NEVER REACH)
a stock 48mm throttle body flows about 700cfm, a 52mm about 870cfm and a 58 mm throttle body flows over 1000cfm
so the only thing you can use a 1300cfm throttle body for is a turbo and even thats debatable!

CHEVY even used the STOCK 48mm throttle body on the 502 bbc engine to increase throttle response, so why a 1300cfm????

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btw Im making close to 500hp/500ft lbs with a 58mm throttle body and theres almost no vacume at full throttle , indicating almost zero restriction and thats mostly the air filterJEGS has the 58mm for $319



For those guys thinking of slapping a 58mm throttle body and 1.6:1 ratio rockers on their engine, or even a hot cam upgrade, Ill point out a few things
look a 383 spinning 6400rpm , let alone a 350 sbc
could in theory pump 697 cubic feet of air thru its cylinder,s but the truth is that youll seldom exceed 90% volumetric efficiency even on a race engine at over 5000rpm with a sbc,ESPECIALLY WITH STOCK HEADS ANDASTOCK EXHAUST, but lets say your at 90%, thats roughtly 630 cfm the engine could be useing with a 383, given the fact your even thinking about the hot cam or any cam in the 225-235 duration range youll never come close to those figures. thatswhy the stock tb. will work just fine and why a 58mm won,t do much but reduce the crispness in the throttle responce

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Staff member
Re: throttle body size

like this dyno test

before you go crazy over worrying about the difference in flow potential between a 52mm and 58mm throttle body, thats mounted on your TPI intake,to increase air flow and potential power, take a deep breath and remember the stock M.A.F. sensor, and both the stock cylinder head ports and stock T.P.I. intake runners and manifold are a larger restriction to air flow and power than the throttle body is more restrictive to air flow


Keep in mind even the factory 502 cubic inch big block used the stock L98 TPI throttle body size and made 500 horse power

yes a 58mm can potentially increase hp if your engines highly modified and your still using a stock 48mm throttle body, but its hardly a big restriction, and your probably going to gain more with other mods
If you calculate the point where dual 48mm throttle bores become a significant restriction to flow on a 383 SBC
youll find you need to spin it to well over 10,000rpm

ITS been my experience that their worth maybe 1-2 hp in RARE cases, (certainly Ive never seen the 10-15 hp they advertise) but ONLY at very high engine rpms (5000rpm and up) and only with low restriction air filters and a good tune. if you want to spend $30-$40 get one but don,t expect much results

read thru this, its about an LT1 but much of the info translates too the L98 ... sults.html

ID suggest doing this mod,

its worth 5 hp and costs very little to do and you can always reverse it back to stock should you choose too

what most guys mis-understand is that those throttle body air foils potentially do work noticably better ON the stock throttle body with its abrupt entrance angles and on a larger displacement engine configs and when the rpms are higher so the flow rates are effected to a noticably greater degree, but a 350 displacement and having an intake design that basically maxes out on flow at about 4500-5000rpm like a L98, and with its heads that only flow 200 cfm , and a cam thats maxed out at about 5500rpm its basically semi optomistic to think they will provide much improvement.
you might also remember that SAME stock L98 throttle bodies used on the 502 BBC engine with EFI RAM JET INTAKE and it feeds even that engines needs so a 350 has hardly any restriction.


bytor said:
Came across this info while doing some research and thought I'd share.
TPI Intakes and runners

The following airflow tests were performed on the University of Northwestern Ohio's SuperFlow SF600 Flow Bench. All CFM values are corrected for airflow at 28 inches of water. Injector flow rates are flowed at 43.5 PSI on an injector flow bench using test fluid with same density as gasoline.


Stock TPI/LT1 48mm Throttle Body w/o airfoil -- 783.0 cfm

Stock TPI/LT1 48mm Throttle Body w/ airfoil -- 821.9 cfm

TPI/LT1 52mm Throttle Body w/o airfoil -- 848.9 cfm

TPI/LT1 52mm Throttle Body w/ airfoil -- 898.8 cfm

Stock 98 Camaro 3800 II Throttle Body -- 554.3 cfm

Stock TPI Bosch MAF sensor w/ screens -- 517.8 cfm

Stock TPI Bosch MAF sensor w/o screens -- 658.4 cfm

Stock 87 GN 3.8L Turbo AC MAF sensor w/ screen -- 584.2 cfm

Stock 86 2.8L AC 5-wire MAF sensor w/ screen -- 576.2 cfm

Stock 96-up AC 3100 V6 MAF sensor w/ screen -- 616.4 cfm

Stock 96-up AC 3100 V6 MAF sensor w/o screen -- 670.7 cfm

Stock 94-up LT1 MAF Sensor w/o screen -- 719.0 cfm

Stock 85-87 Firebird TPI airbox mid piece -- 499.3 cfm

Stock 4.3/5.0/5.7 2bbl TBI complete -- 574.1 cfm (dry)

Stock 4.3/5.0/5.7 2bbl TBI w/o injectors -- 584.7 cfm

Stock 3800 vin L throttle body w/ screen -- 419.1 cfm

Stock 3800 vin L throttle body w/o screen -- 444.8 cfm

4bbl MPFI Holley Throttle Body -- 1287.6 cfm

Another source sent in these flow numbers

Flow and HP ratings for Throttle-bodies:

Flow (cfm) Max. NA HP
Stock 668 300
Stock w/airfoil 710 350
52MM w/airfoil 835 400
54MM (AS&M) 900 450
58MM 1050 500

TPI Intakes and runners flow rates

Stock intake manifold with runner
Stock....................198.72 cfm
ACCEL................213.52 cfm
Extrude/ACCEL....217.11 cfm
Super Ram............220.67 cfm

the stock TPI has a hard time flowing 230cfm even with minor port work, look here
most of this info is right off the accel,holley,edelbrock, and TPIS sites, add a little math and the results become much clearer!!!

Intake....... length ....... port in -- out
Stock GM Base--- 6.375"------ 1.47"- 1.96x1.2
TPiS base------ -6.125"------ 1.75"- 2.09x1.28
Accel base----- -6.125"------ 1.75"- 2.09x1.28
Holley base------- 6” runner 2.3”- 1.9”x 1.23 (2.337 sq inches)
Stock TPI----- -- 7.250"------1.470" round(1.70 sq inchs)
SLP ----------- - 6.625"------1.600" round (2.01 sq inchs)
Accel LTR------- 6.625"------1.615" round (2.05 sq inchs)
TPiS----------- 7.625"------1.660" round (2.168 sq inchs)
Mini ram -----3.5”
LT1 ----------3”

Runners (measured individually)
Stock....................203.17 cfm
ACCEL................242.02 cfm
Extrude/ACCEL...275.83 cfm
Super Ram............289.18 cfm
Intake manifold with 3/8 inch radiused intlet.............................222.45 cfm
Holley stealth ram ………..275cfm

Stock intake manifold with runner
Stock....................198.72 cfm
ACCEL................213.52 cfm
Extrude/ACCEL....217.11 cfm
Super Ram............220.67 cfm
Holley stealth ram …..275cfm

ACCEL Hi-Flow intake manifold with 3/8 inch radiused inlet.........251.51 cfm

ACCEL Hi-Flow intake manifold with runner
Stock....................215.83 cfm
ACCEL................232.53 cfm
Extrude/ACCEL....243.21 cfm
Super Ram............240.24 cfm

Extrude-Honed ACCEL Hi-Flow intake manifold with 3/8 inch radiused inlet ...............275.83 cfm
Extrude-Honed ACCEL Hi-Flow intake manifold with ACCEL runner ..............266.94 cfm
Edelbrock Performer RPM manifold (Stock)..........286.51 cfm
Edelbrock Victor Jr. ............275.24 cfm

the HOLLEY STEALTH RAM FLOWS at 275cfm out of the box, and has the potential when matched to the correct heads and cam to totally out flow most other intakes available,can easily reach 300cfm with minor port work and costs much less
Stock…………………………… 275cfm

Runner lengths
Stock tpi manifold 8” runners 11.25”, cylinder head 6” total 25.25”
Accel super ram manifold 8” runners 7” cylinder head 6” total 21”
Holley stealth ram manifold 6.26” ” cylinder head 6” total 12.26”
Edelbrock performer RPM runners 6” ” cylinder head 6” total 12”
Edelbrock vic jr , runner length 5.5” ” ” cylinder head 6” total 11.5”

Also interesting TPI mods. Not so sure I agree with the one on bumping up the initial timing.

Basic TPI modifications
Friday, May 23, 2014

Basic TPI Mods: by DEBBIES87

There are a few basic mods that should be done to any TPI car, I call these "entry-level" modifications.

1) Cut the air box open without letting air in the engine without going through the air filter & remove the additional plastic piece below the air box to get more air into the engine.

2) Modify the mass air sensor; remove the screens, & and if your brave cut down the fins.

3) Port the plenum (upper intake). There are two small ridges sticking up in the entry-behind the throttle body, remove these. The throttle body openings are already 52mm, so don't go crazy when porting.

4) While the plenum is off for porting, it is now a good time to get a adjustable fuel pressure regulator & a MAT sensor relocater kit. The Adj. FPR will aid in tuning, and the new MAT sensor will improve throttle response as well. The new LT1 style MAT sensor is placed in the air box instead of the plenum, this allows it to read cooler air and advance the timing accordingly.

5) Advance the base timing to 12-13 degrees. Note: the harmonic balancers on these cars tend to drift with age, adjust timing to where the car still starts good and doesn't detonate.

6) If legal in your area, gut the catalytic converter and remove the smog pump belt (85-87), or run a shorter serpentine belt to bypass the pump (88-92); you will need to remove the smog pump on the 88-92's to reroute the belt properly. Check local smog laws first.

7) On automatic cars, adjust T.V. cable (throttle valve cable), so that the car shifts out between 4200-4500 RPM under wide open throttle. TPI motor power drops off drastically after 4500-4700 RPM, they peak at about 4200-4400 RPM. No need to over-rev these motors.

8) Bypass fan relay to make fans come on manually, or install aftermarket relay to turn on fans sooner.

All together these mods should make the car perform like it should have from the factory, gains should be between .6-.9 tenths of a second from these modifications.

CALCULATORS ... old-design

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Staff member
IVE always been amazed at the lack of actual testing people who own vettes do on the plenum vacuum on the L98 TPI cars, anyone whose ever tested knows they pull about 1.4 inches of vacuum at wide open throttle indicating that they can use a bit more flow, (remember ideal should read about .5 inches of vacuum at wide open throttle,but its not just the plenum, runners,throttle body its the MAF sensor and duct work also



If you calculate the point where dual 48mm throttle bores become a significant restriction to flow on a 383 SBC
youll find you need to spin it to well over 10,000rpm

BTW the stock HOLLEY FUEL PRESSURE REGULATOR ON THE STEALTH RAM, is know to have frequent failures, heres a link to a higher quality unit, part #6547 ... 36547.html

read this ... index.html




HERES A FEW OPTIONS and related info

"The FIRST® manifold is now available for the LT1 motor for all of you LT1 guys that want to bring back some of that lost torque!"


IF I was building that engine and my goal was having the best combo for 90% plus street use ID be very prone to buying a FIRST TPI LT4 intake designed for those LT1 heads as it has significantly larger TPI style runners that would allow the engine to spin 6000rpm with that cam you listed and ID get the DYNAMIC compression down to 8:1, theres little doubt that that FIRST intake may cost you a few peak horsepower but its also likely to produce a far higher and more street friendly increase in mid rpm range torque ... -lt4.shtml ... to_03.html ... to_22.html ... ewall.html


READ THIS LINK ... ttle-body/




IF you already spent the cash to buy a twin 58 mm throttle body it only makes sense to make up an adapter to use on the FIRST INTAKE as a throttle body, as it flows enough air and makes installing the stock TPI throttle linkage far easier







be aware the TPS and IAC sensors,and the connecting wiring,for the different year throttle bodies, and THROTTLE LINKAGE for the cars and transmissions varies a great deal between different corvette, years and between the TPI vs LT1 throttle bodies






this type of air foil thats retained by the plastic air tube and worm gear clamp, rather than a bolt in the air foil body is far less likely to cause engine failures


IF you do install one of these air foils retained to the throttle body with a bolt be DAMN SURE you use GREEN LOCTITE on the BOLT THREADS , Ive seen several of those come apart over time, get sucked into a runner and do really nasty things to the valves and piston of the cylinder it fell into


this extra 2 hp looks like its hardly worth the effort but its basically semi-free upper rpm hp, and rarely causes problems if the air foils properly installed
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Staff member
87vette81big posted this info

"I am using the stock GM throttle body on my 87.
There has been many recent reports of problems using BBK's twin 52-58 mm throttle bodies.
I would check your IAC counts with an automotive scanner.
Target IAC counts is typically 15 to 30 numerical as will be shown on the data tables with a scanner.
Maybe you need to reset the Minimum Idle air setting.
Procedure is Documented in GM service manual for your year of Corvette.
One issue I have read about with BBK throttle bodies is that the throttle blades are not located on true center with air blades closed as they should be.
Assembly error from the factory &/ or machining problem(s).
Holding the throttle body up to a shop drop light would confirm this problem with the BBK throttle body air blades fully closed."


Staff member
Hey grumpy, I installed one of those BBK 58mm throttle bodys and now my engine idles a bit higher at idle, WHY?

youll need to reset the IAC and TPS , this link should help

read the links


Ive got a 58mm BBK throttle body on my 383 corvettes engine and have had zero issues, check for vacuum leaks and/or a miss adjusted throttle blades, Ive seen the TV trans cable hold the blades open slightly numerous times on cars with aftermarket throttle bodies, or throttle bodies blamed for
a badly adjusted TPS,IAC or leaking injectors etc.
Ive found that if you don,t assume a damn thing, before you start testing and just test every possible component you'll have more facts, and can make logical conclusions.
keep in mind the engine speed is the result of both air flow and ignition advance curve but its air flow restriction that the throttle and IAC combined controls that dictates about 90% of that engine idle speed, and if it idles significantly higher than desired its usually an indication of a leaking gasket or missing vacuum line or a leak like a cracked brake booster, but also remember some cams with longer than stock duration will require higher idle speeds, just to maintain a steady idle.
stick something like a 250 @ .050 lift duration cam on a 106 LSA in a 350-383 displacement SBC and use a 4000 rpm stall converter and you can forget about idle speeds under 1300rpm.


1700cfm single blade throttle body

1800 cfm single blade throttle body

you might also find reading thru these threads helpful












Throttle Body maintenance 101: READ THESE INSTRUCTIONS ENTIRELY BEFORE STARTING THIS PROJECT! (YOU WILL NEED A DIGITAL MULTIMETER IF YOU TAKE THE TPS OFF! DON’T ATTEMPT TO REMOVE THE TPS IF YOU DON’T HAVE ONE, IT IS MANDATORY!) You should also buy the gasket kit from GM that includes the 4 gaskets and costs about $25. Using your screwdriver remove the air duct in front of the throttle body, now open the throttle by hand (engine off of course) and feel behind the throttle blades. Did your fingers go all black and gungy (yucky isn't it). You need to get that throttle body off and scrub it cleaner than clean can be. Preparation: Go and buy some throttle body cleaner (NOT CARBURETOR CLEANER), and buy yourself a new toothbrush, your old one just got a new life. Get an old bowl or something similar, rags and perhaps some of those lovely rubber surgical gloves.
Getting right on in there remove the 3 pin connector from the throttle position sensor, and remove all the pipe work going to the throttle body, there is a little water spillage, not much.
Also remove the IAC connector (a 4 wire square connector) it's a bit fiddly to get to.
Now the tricky part, the throttle cables need to be released, (manual and auto cars are different), the cables are fixed with wonderful things called Jesus clips, you slide a screwdriver under it and ping it disappears, you mutter 'Jesus where did that go' - what a name for an item. Best to cover the area with a cloth to capture any pinging that goes on.

With the cables released the throttle body should be free to remove. Undo the four long bolts holding it to the plenum, careful when prizing it apart, do not damage the gasket unless you are replacing it.
Now you thought you were clear didn't you, nope, GM put a little booby trap underneath. There is a small vacuum tube with a rubber elbow that needs to be pulled off, be careful with this hose, they tend to get brittle and break with age. You're free! Now comes the really dirty part. Put the throttle body in the bowl and spray with throttle body cleaner and scrub like crazy, especially around the throttle blades. It’s great, really messy but the final spray just cleans it all off and there you have a gleaming throttle body. Now using a 30-32mm spanner remove the IAC valve (that's the large black looking thing on the side), mind the gasket. Clean the spring and pintle and if you can get inside the orifice and clean the seat area. Make sure that the small hole going from the throttle mouth to the IAC is clear. Finally just spray the spring and pintle with WD40 and replace the IAC valve.

Final check - and perhaps another blast with the cleaner to remove any surface muck still there. You may want to soak a rag with throttle body cleaner and clean as far in the plenum as you can get. Allow to dry and replace the throttle body on the plenum, taking great care with the gasket ensure it is positioned correctly. Ensure all the connectors cables and pipes are back on (don't forget the little tube underneath). This may be a good point to check and adjust your throttle position sensor. Get two paper clips, straighten out and slide into the end of the TPS sensor connector, A (earth) and B (tps output), keep the wires separate and turn the ignition on (car off) and measure the voltage at the wires and adjust by loosening the torx bolts, if necessary, to 0.54 volts - this is the normal idle setting.
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The Grumpy Grease Monkey mechanical engineer.
Staff member


How to Adjust your Early C4 TPS and Idle Speed
By Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO

This tech paper will discuss the procedure for correct adjustment of the Minimum Idle Speed and for adjustment of the Throttle Position Switch (TPS) on the early C4 Corvette TPI systems. These steps apply specifically to the 1985 model year, and it general to other years. Later models do not have adjustable TPS's.

Idle speed and off-idle response on the early TPI systems is determined by correct adjustment of the minimum idle speed screw combined with a correct setting of the TPS. I've seen many of these cars that have had their idle speed "corrected" by well-intentioned mechanice and owners by simply screwing the minimum idle speed screw in a few turns. This really messes up the settings, and will not make your car perform properly. Doing a correct setup of the TPS is one of the easiest ways make your car feel and respond better. To maximize the benefit of this procedure, I recommend that you first remove your Throttle Body (TB), disassemble it (it's incredible easy - there are a total of about 5 pieces in it...), clean the TB up really good with some spray carb cleaner, and put it back together. A nice clean TB will really put an edge on the performance improvement you will get by doing this procedure.

The Service Manual has instructions for doing these operations, but the directions are scattered through several sections of the Manual. Here is the complete, step-by-step process for doing this (not including TB rebuild). All specs and steps are taken directly from the Manual (all 3 different sections), and this process is absolutely correct.

Tools and Equipment
You will need the following tools and equipment:
1. A set of Torx wrenches. You can buy a complete set in a nice, genuine plastic pouch at Sears
2. A good digital voltmeter that will read voltages less than 1 volt
3. A paper clip
4. A small screwdriver

There are two electrical components on the TB that you will be working with: The TPS and the Idle Air Control Valve (IAC). Make sure that the connectors for these two components are easily accessible and that you can easily disconnect the IAC. You will also be playing with the diagnostic connector under the dash. Remove the cover (if it’s still in place). Bend your paper clip into a “u” shape. You will be playing with the two top right hand terminals (“A” to “B”) in the connector.

1. First step is to set the minimum idle speed. If nobody has messed with this on your car before, the set screw will be covered by a pressed-in plug. It’s located on the driver’s side of the TB. Remove this plug if it’s there.
2. With the IAC connected and the ignition “OFF,” stick the paper clip into the diagnostic connector from “A” to “B.” This grounds the diagnostic lead.
3. Turn the ignition to the “ON” position without starting the engine. Wait 30 seconds.
4. Now, with the ignition still in the “ON” position, disconnect the IAC connector at the IAC.
5. Remove the paper clip from the diagnostic connector.
6. Start the engine and allow it to reach normal operation temperature. The idle speed will probably be really low, and you may have to coax the engine a bit with the gas pedal to keep it running for a while.
7. If your car is an automatic, set the parking brake and put the transmission in “DRIVE.” If your car is a manual, leave it in neutral.
8. Adjust the idle speed screw to obtain 400 rpm in drive or 450 in neutral.
9. Shut off the engine and reconnect the IAC.

That’s it for idle speed. Now on to the TPS.
There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires. You can either buy a special harness connector that breaks these wires out (from Min America), or gently pierce the insulation of the wires with the pointy prongs on your volt meter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. Whatever is easiest for you.

1. Turn the ignition to the “ON” position without starting the engine.
2. Loosen the TPS Torx adjustment screws.
3. Set your volt meter to the low scale DC volt setting that will accurately read less than 1 volt.
4. Measure the voltage between the two top TPS wires.
5. Adjust the TPS by rotating its position until you get a reading of .54 volts.
6. Tighten the Torx screws and recheck the voltage. Readjust if necessary to make sure voltage is right at .54
7. Turn the ignition “OFF.”

Addition by CorvetteForum Member Charles Warner

As an addendum to “How to adjust your early C4 TPS and idle speed,” I would like to add that checking your WOT TPS voltage is also, potentially, a worthwhile endeavor. Frequently the TV cable that controls shifting pressure to the transmission binds the throttle linkage thereby not allowing for WOT. After measuring and, if necessary, adjustiong the idle voltage (nominally .54 volts) check the WOT voltage and ensure that approx. 4.5 volts is realized. A minimum of 4 volts at WOT are required for the ECM to go into fuel enrichment mode (FEM). If you are not seeing over 4 volts at WOT you are probably not opening the throttle to the stops and are not seeing FEM. A significant performanceimprovement is possible with this adjustment.
Charles Warner