305 and 307 sbc info


Staff member
the 305 and 307 sbc engines are far from the ideal starting points for a performance engine build-up.

but they are far from being a total lost cause, EITHER!
EXAMPLE pay attention to the bore stroke ratios, a 307 with its greater bore and shorter stroke , and slightly ;larger displacement and less valve shrouding should ALWAYS show a slight advantage in performance over a similar smaller bore longer stroke 305

Chevy V8 bore & stroke chart

Post by RebStew on Fri 08 Feb 2008, 3:28 pm

262 = 3.671" x 3.10" (Gen. I, 5.7" rod)
265 = 3.750" x 3.00" ('55-'57 Gen.I, 5.7" rod)
265 = 3.750" x 3.00" ('94-'96 Gen.II, 4.3 liter V-8 "L99", 5.94" rod)
267 = 3.500" x 3.48" (Gen.I, 5.7" rod)
283 = 3.875" x 3.00" (Gen.I, 5.7" rod)
293 = 3.779" x 3.27" ('99-later, Gen.III, "LR4" 4.8 Liter Vortec, 6.278" rod)
302 = 4.000" x 3.00" (Gen.I, 5.7" rod)
305 = 3.736" x 3.48" (Gen.I, 5.7" rod)
307 = 3.875" x 3.25" (Gen.I, 5.7" rod)

325 = 3.779" x 3.622" ('99-later, Gen.III, "LM7", "LS4 front wheel drive V-8" 5.3 Liter Vortec, 6.098" rod)
327 = 4.000" x 3.25" (Gen.I, 5.7" rod)
345 = 3.893" x 3.622" ('97-later, Gen.III, "LS1", 6.098" rod)
350 = 4.000" x 3.48" (Gen.I, 5.7" rod)
350 = 4.000" x 3.48" ('96-'01, Gen. I, Vortec, 5.7" rod)
350 = 3.900" x 3.66" ('89-'95, "LT5", in "ZR1" Corvette 32-valve DOHC, 5.74" rod)
364 = 4.000" x 3.622" ('99-later, Gen.III, "LS2", "LQ4" 6.0 Liter Vortec, 6.098" rod)
376 = 4.065" x 3.622" (2007-later, Gen. IV, "L92", Cadillac Escalade, GMC Yukon)
383 = 4.000" x 3.80" ('00, "HT 383", Gen.I truck crate motor, 5.7" rod)
400 = 4.125" x 3.75" (Gen.I, 5.565" rod)
427 = 4.125" x 4.00" (2006 Gen.IV, LS7 SBC, titanium rods)

Two common, non-factory smallblock combinations:

377 = 4.155" x 3.48" (5.7" or 6.00" rod)
400 block and a 350 crank with "spacer" main bearings
383 = 4.030" x 3.75" (5.565" or 5.7" or 6.0" rod)
350 block and a 400 crank, main bearing crank journals
cut to 350 size

given a choice a 383 or 396 stroker combo built on a 350 block is a FAR BETTER starting point than what most guys refer too as a crummy 305,or 307 sbc with its limitations,
but there's dozens of guys stuck with those 305s and 307 sbc engines due to economic or lack or easily accessed basic engine core,reasons so I figured Id give them some useful info!

the standard VORTEC heads , with some basic port and bowl clean-up and better valve springs are about the best, available and most cost effective OEM sbc heads available for the smaller bore SBC builds
in salvage yards for those low budget engine builds, and a decent vortec intake
matched to those heads goes a long way to allowing the smaller engines to breath, if matched to a decent cam with about a minimum of a 220-225 duration at .050 lift and a .460 lift at the valve, and about a 9.5:1 compression ratio is where youll want to start.
while its not reasonable to start at a disadvantage then try to overcome it, before building an engine , you need to realize that many guys feel they must work with what they currently have,personally Ive always found it advantageous to build a second PERFORMANCE engine in the garage and swap it in,after its complete with the correct parts for the application, rather than constantly doing minor mods to a street engine! the advantages are many! but the main one is that you can drive the car within the weekend should the performance engine fail, by swapping back too or swapping in the standard engine, if the other one has problems so your not stuck more than a weekend without dependable transportation.!
keep in mind a 350 block is NOT generally all that hard to locate or expensive

https://secure.pawengineparts.com/shopp ... catid=1477

maybe this INFO BELOW will be of interest

http://www.chevyhiperformance.com/proje ... index.html




http://www.popularhotrodding.com/engine ... index.html


if your thinking the only difference between the 305-307 and a 350 is the displacement advantages...
the 350 has several other advantages other than the increased displacement
it can use larger valves and clear the bore, it has less valve shrouding even if the same valve size is used,thats easily a 5% increase in breathing potential alone for the 350, vs the 305, if both use the largest valves that will clear the bore wall its has the extra 45 cubic inches of displacement as mention but it gets that from a larger bore so a side benefit is the larger surface area of the piston, that allows more total effective pressure, against the piston even if they are exactly the same peak pressure in the cylinders, resulting in more torque,
youll have a much harder time getting over 400hp with a 305 than a 350, using just the port flow rates alone as a guide
then lets assume you get 1.2hp per cubic inch from engines using similar parts
350=420, add the extra 5% breathing potential and your at 440hp , a significant advantage if the larger valve heads are used with the larger bore
now add a 3.75" stroker crank and a .030 over bore and build a 383 and you can easily add an additional 35-40 hp and ft lbs of torque over the similar 350, now 450-475 hp is well within the engines potential if its correctly thought thru and built.
COMPRESSION ratio increases generally give a 3% average boost in torque for each point gained in compression in the 8:1-11:1 range
if both a 305 and a 350 have true flat top pistons and lets assume a 68cc average combustion chamber size , a .018 head gasket and a .023 deck height the 305 will have about a 9.5:1 cpr and a similar 350 would be at about 10.3:1, or a 2% increase in torque from that factor alone across the usable rpm band., you could used dished pistons and drop the ratio but the difference in cpr will be similar if both engine use similar piston designs

305 = 3.736" x 3.48" (Gen.I, 5.7" rod)

350 = 4.000" x 3.48" (Gen.I, 5.7" rod)

13% more surface area on the bore diam.on the 350.

13% more displacement for the 350

1.84 valves on the 305
1.94 valves on the 350

5% more curtain area for the 350 at any valve lift

slightly less valve shrouding on the cylinder wall to valve tangent area in the combustion chamber for the 350.

if you can fill the combustion chambers with a 5% lower flow restriction,thru the use of slightly larger valves, and you burn approximately 13% more fuel/air mix per power stroke and apply that extra force over a 13% larger piston surface area, you get a noticeable advantage
IM not saying you can,t get decent power from a 305, but anything a 305 can do a 350 can do noticeably better, when the criteria you judge by is the power produced

add to that the fact that you can find 383 stroker kits, and 350 rebuild kits at the same or lower price in some cases to 305 parts, that will add an additional increase in displacement and you've got a significant reason to build with the larger bore combos.

BOTH the 305 and 350 have the SAME stroke and rod length
the larger surface area and larger displacement DO make a difference
a 305 has a piston dome area of about 11 sq inches, a 350 with its larger bore has a piston dome area of about 12.59 sq inches, if both have a peak cylinder pressure of 600psi that 6600 pounds of force exerted, for an instant or two on the 305 and 7550 pounds of force exerted pounds of force exerted, for an instant or two on the 350 piston

typical high performance pressure curves


305 = 3.736" x 3.48" (Gen.I, 5.7" rod)

350 = 4.000" x 3.48" (Gen.I, 5.7" rod)

if your thinking the increased power is all the result of the increase in displacement alone,
again, comparing apples/oranges the displacement went up about 13% due to an increase in bore diam of about 13%, but the stroke remained constant, between the two engines and the torque did go up as did the hp. even thou the stroke remained a constant, torque went up mostly due to the larger bore, and the resulting increase in displacement ,hp went up, due to the higher torque at any given rpm.
now a more reasonable comparison to show the effective result of increasing the bore but keeping the displacement nearly constant would be to compare the 305 with its 3.736" bore to a 302 with its 4" bore
305 = 3.736" x 3.48"stroke (Gen.I, 5.7" rod)
302= 4" bore x 3" stroke(Gen.I, 5.7" rod)
what you find is the larger bore and the shorter stroke , but nearly equal displacement move the power curve up about equal to the change in piston speed

at 3000FPM in piston speed a 305 is turning 5172rpm
at 3000FPM in piston speed a 302 is turning 6000rpm
or about 800 rpm faster, if both make the same 300ft lbs of tq at the same piston speed you get
305 is turning 5172rpm x the same 300ft lbs of tq =295hp
302 is turning 6000rpm x the same 300ft lbs of tq=343hp
which shows the advantage of the same displacement but a larger bore and shorter stroke at similar stress levels

as you can see in this chart most of the stock heads don,t flow as well as the AFR 195cc heads and there ARE better flowing heads

now some guys are required to maintain the stock block, or basic displacement, and in those cases you might also want to think about supercharging the engine, its no big trick to install a decent supercharger on your basic 305-307 and make 400-450 plus hp either
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while were talking about the smaller displacements in the SBC someone always points out the older 302 z28 engine with its 3" stroke and 4" bore,and starts talking about..... back in the day,..... a 283 crank could be swapped into a 327 with its 4" bore , you added 11:1-12:1 cpr pistons, SUNOCO 260 ,110 octane gas was easy to get a service stations in 1967-1968,the crap we have now is camel piss in comparisson, fuelie heads and a decent intake and a radical solid lifter cam like a CRANE #110921 could be installed and you could make much better hp than the standard 300-365hp 327sbc, but that was mostly because the available heads maxed out thier flow with a 300-330 displacement at the time, any larger displacement found the heads were a restriction in the upper rpm ranges, but we have advanced a long way from the mid 1960s head designs.
keep in mind they were almost always matched to a manual 4 speed transmission and 4.11 rear gears in a light car, and never expected to get great mileage, pull the car smoothly at 2000rpm or work with an over drive transmission or an automatic transmission, they had a solid lifter cam and it was expected the guys that bought them knew how to adjust valves and tune carbs,
the killer for any 302 sbc your dreaming up is always the valve train. The valve train components to make a 302 live and make modern power are very expensive. YES Im only too aware your buddies used to spin thiers to 8000rpm all the time, and You're looking at a 7.5K-8.5K rpm redline in a 302 combo, you want to build. yes ,You can make 400-450 peak hp, with the correctly match parts, in a 302 sbc but a correctly built 383 can do the same or probably a good deal higher horsepower in the 475-525hp range, with a much more managable 6.5K redline (and a hell of a lot more torque), if built to similar specs, and with todays aftermarket heads theres no reason to build anything under a 350 displacement. almost any combo you can build or dream up with a 302 displacement can be easily exceeded in power with a similar but larger displacement version in the 350-441 cubic inch sbc range today.
over the last 20 plus years Ive helped a number of guys improve the performance on various 1980-1990 year cars with first generation chevy engines of various types, the one factor that remained fairly consistent was that even after you build a 305-307 engine up so it makes decent power levels the owners almost invulnerably, eventually decide they want to upgrade to a 383-406 sbc.its not that a 283-305-307 can,t make decent power, its just that starting with a 350-383-406 doesn,t cost all that much more by the time your thru but they produce much better power once your done!
most guys will find buying a decent spare performance engine, slowly over time , with exactly the components they want and once its complete spending a weekend swapping engines has huge advantages, one is limited down time and the second is having the option to swap back to the know working dependable stock engine should anything happen to the weekend toys power plant.
now you can swap in some components you might need well before the actual engine upgrade like a bigger radiator, better brakes , etc. but the idea here is not to be in a position where your primary daily transportation's screwed up, and your both broke financially from parts purchases and have to get the car up and running. Its far better to pick a long weekend or a weeks vacation to do the engine swap, then test drive it a few days and not have huge expensive surprises, this route is far better than tearing the engine down, ordering parts or waiting on a machine shop to complete work while your cars not drivable, it also allows you to keep your pride and joy engine and sell the car with its original engine once your tired of the car rather than being forced to take a huge financial hit buy including an engine that the new owner surely won,t pay you full value on when you sell the car, after your done playing with it!

heres a few hours worth of reading links and sub links that will help prevent you wasting time and money



http://www.popularhotrodding.com/engine ... index.html

http://www.chevyhiperformance.com/techa ... index.html

http://www.chevyhiperformance.com/techa ... index.html