Air Gap + and -

[brad.arcova]

Is an air gap manifold a good choice for a street driven cruiser?

Read through A LOT of the threads and links on this topic. Generally, all the publications love the air gap manifolds. On one of the other sites though, there's a lot of bad mouthing of air gap main for street use. Not sure what to believe. None of the magazines want to offend their advertisers so I don't completely trust them. On the other hand, I suspect some the loud voices on the forums are actually no smarter than I am so I don't trust them completely either.

This seems like a nice quiet place with smart people, so, is an air gap manifold a good choice for a street driven cruiser? Naturally I'm concerned about my own vehicle, a 3500 lb car with a 383, an air gap, a 234/244 cam, a 670 Holley street avenger, a manual trans and a 3.36 rear end. It's a little undercarbed for maximum performance but I really don't care about peak horsepower; I just love that torque from 2000 -4000 rpm. Struggling to get it set up, it seems to be running lean, based on plugs, even after going up 4 jet sizes, but I have a couple of things to check on before considering that the air gap manifold is a problem.

Thanks,
Brad

 

[DorianL]

I have have had several Edelbrock intakes. They are good kit: good design and construction.

The Air-Gap comes in several configurations... Performer and Performer RPM. Power ranges from idle-5000, 1000-5500 and 1500-6500. There's nothing outright wild here -all will work- and probably not a huge difference between them. Which one do you have?

Quite frankly, I think the runner design is properly tuned. My sense is the Air-Gap is mostly for visual appeal: seeing those runners snake into your heads. I am not saying that the "air gap" doesn't give you a "cooler, denser charge." It will. I am saying that the effect is less significant than runner cross section, length and overall design.

Which heads do you have? Exhaust manifold? Based on what you write - my feeling is something is amiss, possibly in the carb tuning. There is some math out there to verify, but I think the carb could be big enough to serve you well.

What is your timing curve like?

(Sorry - I had to edit a couple of times. I missed a couple of your points.)

 

[grumpyvette]

your cars transmission gearing, rear gear ratio, and weight, transmission type, converter stall speed if its an auto trans,and exhaust scavenging efficiency (headers,restrictive exhaust) will effect your choice
for probably 80% or more of the engines run on the street the edelbrock air gap or wieand AIR STRIKE dual plane intakes are an excellent choice!
the intake design is supposed to reduce the plenum exposure to hot oil splashing on the plenum base from the lifter gallery to slightly reduce the plenum temperatures which it does, but once the engines up to operating temps the difference is not huge as aluminum conducts heat rather well and the heads absorb and transfer a good deal of heat to the runners.
you'll occasionally see online discussions about how the plenums not heated and how that makes the car run badly and while that might be a factor in some areas where theres extremely low temperatures, for the first few minutes run time, your average engine compartment tends to heat up fairly quickly and thats rarely in issue after the first 5 minutes of run time, in fact most of the time it works to your advantage in that a cooler denser fuel/air charge tends to make better power.most guys find that the carb runs better if its cooler thats why theres phenolic carb spacers
Ive built engines for freinds in COLORADO and it can get cool there, and they use an edelbrock air gap so its not like the first cold day will render a car equipped with an air gap intake un-drivable

http://store.summitracing.com/parts/edl ... dia/images
extended divider carb spacer
and if for some reason you want to run a heated carb there are heated carb spacers that allow you to route coolant thru the spacer to heat the carb.
let me say that on most street/strip engines I build the air gap intakes my first choice in a dual plane intake

read thru this
viewtopic.php?f=55&t=58

viewtopic.php?f=55&t=4362

viewtopic.php?f=55&t=2857

 

[grumpyvette]

BTW if your thinking of building a hot street/strip 383-406, heres a known effective combo
for a 10.2:1-10.7:1 cpr engine with a manual transmission or 3500 stall converter.

AIR GAP INTAKE http://www.summitracing.com/parts/EDL-7501/

CRANE 119651 roller cam..http://www.cranecams.com/product/cart.p ... il&p=24187

brodix heads..http://www.jegs.com/i/Brodix/158/1021001/10002/-1
OR
AFR heads..http://www.summitracing.com/parts/AFR-1054/

short block..http://ohiocrank.com/chev_sb_shortb.html

rockers,..http://www.summitracing.com/parts/CCA-1601-16/

carb..http://www.summitracing.com/parts/HLY-0-3310C/

add a decent set of headers, a 7-8 quart baffled oil pan and a 3.73-4.11:1 rear gear

 

[brad.arcova]

... but once the engines up to operating temps the difference is not huge as aluminum conducts heat rather well and the heads absorb and transfer a good deal of heat to the runners.
you'll occasionally see online discussions about how the plenums not heated and how that makes the car run badly and while that might be a factor in some areas where theres extremely low temperatures, for the first few minutes run time, your average engine compartment tends to heat up fairly quickly and thats rarely in issue after the first 5 minutes of run time, in fact most of the time it works to your advantage in that a cooler denser fuel/air charge tends to make better power.most guys find that the carb runs better if its cooler thats why theres phenolic carb spacers
Ive built engines for freinds in COLORADO and it can get cool there, and they use an edelbrock air gap so its not like the first cold day will render a car equipped with an air gap intake un-drivable

That's what I thought; thanks for the reply. I couldn't quite buy into those particular online discussions.

Brad

 

[DorianL]

Reading some back threads - the hads are Vortecs. Exhaust is unknown.

Grump, what do you think about that 670 Avenger carb? Too small at WOT?

 

[grumpyvette]

Reading some back threads - the hads are Vortecs. Exhaust is unknown.

Grump, what do you think about that 670 Avenger carb? Too small at WOT?

Ive always preferred a HOLLEY OR DEMON 750 vacuum secondary carb on almost any small block configuration, the 670 Avenger carb is not a bad selection on a daily driver or mild performance application combo.

http://www.summitracing.com/parts/HLY-0-80670/?rtype=10

http://static.summitracing.com/global/i ... 0219-3.pdf


Ive usually selected one of these

http://www.summitracing.com/parts/HLY-0-9015-1/

http://www.summitracing.com/parts/HLY-0-3310S/



IVE had good results with the 750 cfm demon carbs lately, and if I was building the combo thats what ID select
http://www.jegs.com/i/Barry+Grant/132/1 ... E/10002/-1

 

[brad.arcova]

Thanks for all the help!

update: been reading a lot of your threads, doing the work to fix the obvious, and am finally getting this thing tuned and running right. It's quite a bit of fun now.

383 crate engine from Blueprint (JEGS catalog)
http://www.blueprintengines.com/index.php/gm-383-bp3830ct1
no name cast iron vortec heads
* 2.02 swirl polished intake valves
* 1.60 swirl polished exhaust valves
flat tappet cam: .487Int / .508 Exh & 234 Int / 244 Exh duration @ .050 - 112 degree lobe sep.
Hypereutetic pistons
air gap dual plane manifold
HORSEPOWER: 405 @ 5500
TORQUE: 440 @ 4200
COMP RATIO: 9.5 to 1 (requires 91 octane fuel)
IGNITION TIMING: 34 Degrees Total (at 4000 RPM), 15 at 1000 RPM; vacuum advance voids warranty
3900 lb 1964 Impala with fuel and driver
Tremec TKO 500 5sp OD,3.27 1st, .68 od
3.36 rear end
27" diameter tires
1840 rpm 5th gear, 65 mph
Holley street avenger 670cmf carb, kind of small but this engine will never see the drag strip or 4500 rpm
Top dead center identified w/piston stop
1000 rpm idle speed, stalls out at every stop light at lower rpm
12-13" vacuum
160 deg thermostat; takes a long time to warm up, electric choke goes off long before it's warmed up
driven as a cruiser, almost always in 1600-2200 rpm range

How it ran at first:
bad exhaust popping at cruise
poor fuel mileage 8 mpg
plug reading: extreme rich idle, very lean cruise
bad lean surge
bad hang up on shift
engine cuts off when clutch pushed in but easily restarts
very stinky exhaust
almost undrivable in civilized company
very smooth running at ~2000 rpm and above, no vibration at all

1) replaced 2" exhaust with very restrictive bends with new 2.5" ram horn cast iron manifolds and 2.5" pipes with perfect mandrel bends (no cross-over, weird Impala x-frame) and 18" magnaflow mufflers. That got rid of the exhaust popping.
2) rebuilt carb, no change
3) went up 6 jet sizes in three increments. Lean surge almost gone except at very low rpm, plugs gone from white centers to almost good tan
4) cracked open secondary idle to enable closing primary throttle a bit at idle
5) drilled primary and secondary throttle plates. exhaust stink gone, black going away on plug outside rims. Changes adjustability on on idle mixture; takes more turns for same effect.
6) got timing advanced all the way up to dead nuts on engine makers warranty limit of 34 deg at 4000 rpm. Even though I was only off by 1.5 deg, this DRAMATICALLY improved performance; still no pre-detonation at all under any conditions but plugs indicate timing advanced a bit much. always using 93 octane.
7) engine doesn't cut off any more when warmed up and clutch is pushed in, occasionally before warmed up
8) fuel mileage gone from 8.0 to 10.5 mpg in several steps; might just be getting broken in. Got 800 miles on the engine now.
9) excellent response on shift if take it up 3000 rpm before shift, slight catch if shift at lower RPM shift
10) still very smooth running at ~2000 rpm and above, no vibration at all
11) very drivable

In hind sight, this might be a higher performance engine than I should have gotten since I tend to keep the RPMS 1600-2200 but I've never driven a high performance engine before and I wanted to see what it was like. It accelerates like a scalded dog when I take the rpms higher, even at 40, 50 or 60 mph.

next steps
1) enrich idle a bit to keep engine running without blipping the throttle between the time when the electric choke goes off and the engine is fully warmed up.
2) check plugs again for cruise mixture, at 6 jet sizes might almost be out of room to go up more; I've heard that going beyond that usually exceeds adjustabilty and doesn't help
3) test thermostat to make sure it isn't stuck open and see when it does open.

Future Goals: perfect driveability, high fuel mileage, lower idle (maybe 700) and long life. It's already got more power than I'm ever going to use.

maybe
1) replace cam with RV/towing roller cam since I like to drive at low rpm, would that improve fuel mileage?
2) install holley avenger TBI with computer controlled distributor to improve mileage; kind of hate to spend the money though. Even at $4/gal it would probably take a decade or more to get my money back at 2000-3000 miles/year.

Comments?

 

[grumpyvette]

the cam you selected is definitely a bit too large, if your rarely going to spin the engine over 4500rpm, something in the 215-220 intake duration range seems like a better match to the application from what your saying now.
swapping to the smaller cam will boost low rpm torque and increase the engine response, allow the car to run smoother and get a bit better mileage, but cost you a bit on peak hp.

BTW I don,t know very many people that have NOT installed a cam that is a bit too large for the application at some time in this hobby so chalk it up as a learning experience most of us went thru!, having those impressive peak hp numbers is almost as addictive as drugs untill you realize you rarely need or use them, its the low and mid rpm torque curve thats used 90% of the time

a cam like this seems about correct
http://www.cranecams.com/product/cart.p ... il&p=23791

 

[Indycars]

Have you tried adjusting the electric choke. Below are the Holley instructions for your carb.

http://www.holley.com/data/Products/Technical/199R10219-3rev2.pdf


Electric Choke:
IMPORTANT: The STREET AVENGER carburetor has been factory wet-flowed and calibrated. The “out of the box” settings should be very close for all adjustments. The following tuning section is included ONLY to aid you in fine tuning adjustments.

1. You can control the choke operation by rotating the choke cap. If the choke comes off too soon, loosen the three screws and rotate the choke cap counterclockwise one notch at a time, until the choke operation is satisfactory. Rotate the choke cap clockwise, if the choke comes off too late. After making the final adjustments, start the engine and make sure the choke plate opens completely.

.......... A. A choke that comes off too soon could exhibit one or more of the following symptoms: stalling, surging, backfiring, stumbles, or poor vehicle driveability when the vehicle is cold.

..........B. A choke that comes off too late could exhibit one or more of the following symptoms: black smoke from the tail pipe, poor driveability when cold, poor gas mileage, misses, rough idle.

2. From the factory, the choke cap has built-in limiters. If choke operation is unsatisfactory and you have adjusted the choke cap in either direction to the limiters with unsatisfactory results, recheck your positive electrical line connection.

3. If the fast idle RPM is too low or too high for your preferences, TURN THE ENGINE OFF. Advance the throttle to wide-open, exposing the fast idle set screw below the choke housing (See Figure 9).

4. Using a 1/4” open end wrench, turn the screw clockwise to increase the RPM or counterclockwise to decrease the RPM. The factory setting should give you a 1500-1600 RPM fast idle speed.
NOTE: All vacuum ports must be plugged at this time.

5. Return the throttle to the fast idle position, as described in step 2. Restart the engine, and recheck the fast idle RPM. Repeat steps 2 & 3 until the desired fast idle RPM is met.

 

[brad.arcova]

Thanks guys!

I'll keep driving it to see how much fun it really is but I'll probably put that smaller cam in this winter when there's snow on the ground. Yeah, you look at the numbers and you WANT IT! No regrets though, you only live once, it was now or never. But then again, it really sounds bad a__, and it's a lot of fun taking it from 30 - 70 mph with no effort while all the rice burners are running out of breath....

As to the electric choke, I'll work that a bit more. The idle, fast idle, idle mixture and choke setting seem to work as a set. I'll work on getting the mixture a tad richer, the idle speed a tad lower and try enriching the choke just a bit. Might have to enlarge the holes in the throttle plates a bit too make all that happen; right now I can't make everything work with less than 1000 rpm idle. That particular throttle plate drilling trick is pure magic. In the first go-round it went from stinky exhaust and gasping for air idle to no stink and nice lopey cammed up idle.

Of course replacing the cam will screw up the carb tuning but at least I know where I started. New throttle plates are available from Holley and I have all the old jets.

Brad

 

[brad.arcova]

More questions:

1. When reading plugs, do you need to start with a fresh set of plugs? In other words, I had a lot of carbon on the outer electrode then made some changes to try to lean things out. Pulling the plugs again, I still see carbon but am wondering if it's left over from before I made changes or if it's accurate reflection of the current tune. Haven't heard or seen anything in years about cleaning plugs. Thought I heard that because of expensive plating on some plugs, cleaning might actually ruin them.

2. Couldn't find anything about using a v-shaped wire as an idle feed restrictor. Here's where I'm at:

Recall that I'm running a 234/244 duration cam, an air-gap manifold and a Holley 670 cfm street avenger. After doing the following, the car runs much better than it ever has:
a. drilled 1/8" holes in all throttle plates, the secondary throttle stop is at the stock setting (fully closed then open 1/4 turn)
b. smooth idle at ~800 rpm, idle-transfer slot not over-exposed
c. excellent throttle response and no hesitation anywhere in power band
d. lean surge gone
e. changed from 160 deg to 180 deg thermostat, car still takes forever to warm up though, there's no valve in the coolant circuit through the heater core so there's always lots of by-pass.
f. adjusted timing to ~ 11 deg advance, reading plugs shows its just about perfect (IAW builders warrenty, no vacuum advance)
g. about 12" vacuum at idle

The problem is that it's still pretty stinky at idle and the outer rings of the plugs are very black. The mixture screw appears correctly set because if I lean it out more any more at all, the engine shuts off when I let off he throttle and disengage the clutch.

My theory is that the air/fuel emulsion coming through the idle feed is is too rich because of low vacuum. The idle mixture screw only changes the amount of emulsion, not its mixture. Because it's so rich, its not atomizing properly and causing the engine to run rich. Leaning out the idle mixture screw doesn't change the emulsion mixture and doesn't help it atomize any better. If I insert a wire ino the idle feed as a restriction, less fuel will come into the emulsion for a given amount of bleed air. The same effect would be achieved with a larger idle air bleed but mine aren't changable. A leaner emulsion should atomize better and burn fully.

Is this BS or should I try idle feed restrictors? Is this unusual for a cam like this? Am I missing something else? Since the car runs perfect, should I just leave well enough alone?

Brad

 

[grumpyvette]

you have no idea how great it is to see clear pictures of spark plugs, its so exceedingly rare that I get asked questions with clear pictures to work from!

it should be obvious that if plugs have been run over a long period of time under various conditions that the plugs will tend to reflect that in the conditions shown and that you tend to see BOTH the AVERAGE condition and the MORE recently endured conditions

ok, looking at the plugs condition a few things stand out,your obviously running a bit cooler and richer than ideal most of the time at lower rpms indicated by the dark oilly black ash on the plugs being rather consistent on the plug threads, and obviously the rather clean burn ash on the ground strap extending almost down to the threads indicate higher combustion temps under higher rpms, this could be the result of a lean f/a ratio in the upper rpm range OR timing thats a bit to far advanced, but the porcelain insulator ash dispersion pattern tends to favor the lean upper rpm f/a ratio, as the advanced timing would be more consistent in the ash burning off the insulator
Id probably try to jet the primary jets a bit leaner and use the next richer power valve, and check that the float levels in the carb are currently correct as a first step

viewtopic.php?f=55&t=109

viewtopic.php?f=55&t=1639

viewtopic.php?f=55&t=5229

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[brad.arcova]

Got it. One question though, should I also go with a hotter plug?

I've gone up 6 jet sizes to compensate for a lean surge at very light throttle when I think the problem was actually an uncovered transition slot. I drilled the throttle plates and now have them in the correct postion at idle. Hopefully I won't have any lean surge now at light throttle even if go back 4 jet sizes (or more.)

The power valve is the stock 6.5; I'll go for a 7.5.

Brad

 

[grumpyvette]

Id suggest an 8.5 or even a 9.5 power valve and I doubt swapping to a hotter plug will significantly improve things and dropping back on the jet sizes is a good idea

 

[brad.arcova]

Even with only 13" vacuum at idle you'd go for an 8.5 or 9.5 power valve?

 

[grumpyvette]

Even with only 13" vacuum at idle you'd go for an 8.5 or 9.5 power valve?

keep in mind your trying to tune the way the fuel/air ratio changes as the rpms increase and in an ideal world you want to run about a 14.6:1-14.8:1 fuel / air ratio at idle, and maintain that ratio up too about 3000rpm, past that you want too slowly but consistently and predictably transition over the next few thousand rpm o reach about a 12.6:1-13:1 FUEL AIR RATIO at WOT. that will normally require thoughtful changes ,you may want to go to a smaller set of carb jets if the plugs indicate your running to rich, at idle with the higher power valve but the plugs seem to indicate your running rich at idle and low speeds but lean on the upper rpm range, exactly opposite what you are trying to do. jets control much of the fuel flow at lower rpms, the power valve allows additional fuel as the rpms increase and vacuum decreases, its a balance that can be reached if you understand when and why each part of the carb and its transition circuits work

 

[brad.arcova]

Went down 4 jet sized to a #67 from #71 (stock is #65). Went up 2 power valve sizes to 8.5 from the stock 6.5. It appears that the lean surge has not returned now that I've got the throttle plates positioned correctly with respect to the transition slot. I've got a bad stumble though on light acceleration between 1500 & 2000 rpm that wasn't there before. My belief is that the primary jets are ok but that being 4 sizes richer was covering up for an accelerator pump shot that should be a little bigger. The 670 cfm carb is a little small for a 383 so that wouldn't be surprising.

Just got a new box of accelerator pump cams in the mail today and will try one that puts in more pump shot quicker. On the other hand some people seem to mess with squirters rather than cams.

This article http://www.stockcarracing.com/techarticles/scrp_0601_holley_accelerator_pump_adjustment/index.html seems to say go higher on the quirter before going up on the pump cam. The current (stock) pump nozzle size is 0.031.

Any thoughts or am I just over-analyzing this? :geek:

Brad