Which can you get more power out of? I have a 87 5.0 carb 305 and a 89 5.7 TPI 350. Which responds better to mods and is the 350 significantly faster than the 305? I feel like it is, but I have never had my 305 running perfect, so..... Was just curious. Also The 350 TPI's Are they the same ones that similar year vettes carry? If so whats the difference between the two if any? If not, than is my 350 Formula just as fast as the vette with the same engine. Just curious. Thanks.

 

A 305 is 87% of a 350, that's about it really.

 

NOT REALLY!
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
305=366hp
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

 

350>305. You could have either engine with either induction if you wanted.

If the 305 is a factory carb 305, its an lg4 and a dog compared to even a 305 tpi, more so compared to a 350 tpi. Almost no part of an lg4 is worth keeping or even upgrading where as a 350 tpi can take some bolt ons and perform rather well

Its very similar to the vette motor minus a few small differences like I believe the vettes had aluminum heads, better flowing stock exaust and possibly a slightly larger cam(?).

 

I've heard the valve argument before and it's not as significant. Typical 305 valves are 1.84 versus the 350 which usually gets 1.94. Relative to the engine's displacement, the 305 has more intake valve per cubic inch. The same is true if you take the 305 up to 1.94" valves (the practical max) and the 350 up to 2.02".

The bigger piston equals more torque is another thing that's blown out of proportion. Yes, the same pressure will equal more force, but all other things are not equal. The smaller bore of a 305 gives it an advantage in that it requires a little less spark advance (flame front doesn't have to travel as far in the small bore) which means it does less negative work. There's also the frictional loses of the larger piston. In the end the big piston advantage doesn't really make much of a difference.

So the 350 really can't make more hp per cubic inch, at least not without opting for the absurdly expensive aftermarket heads that are CNC ported to the extreme. The only advantage a 350 has is 45 cubic inches.

I'm not saying a 350 is bad or a 305 is better. A 350, with equivalent mods will make more power for the same cost, that's just a fact. But if you already have a 305, it can be made to scoot along decently. You need not scrap you 305 if you want to go fast. 305 vortec heads and a nice cam can make a 305 a decent performer.

 

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.

 

Why would I use the same CC heads on each engine? I'm going to use smaller CC heads that suit the 305 more, not off the shelf 350 heads.

If what you say is true, then why doesn't a ZZ502 BBC crate engine make significantly more torque per liter than a ZZ383 SBC crate engine? Both are crate engines made by GM to run on 92 octane, both have 9.6:1 compression, similar camshaft duration, and both make about 70 lb-ft per liter of displacement.

The same problem existed in 1989. The TPI 305 and TPI 350 both made about 60 lb-ft per liter. By what you're saying the 350 should make 68 lb-ft per liter to the 305's 60 lb-ft. This isn't the case though.

EDIT: Also, yes the 350 has 5% more curtain area at any lift, but it has 15% more displacement to feed with that increased curtain size.

Yes, a 4" bore block is better. I said that already, but if you don't have the money, time, or skills for a full engine swap, a 305 isn't bad. Also I think there are a lot more 305 third gen f-bodies running around than 350.

 

Yea i have to say, if im reading this correctly, people who are saying that the larger pistons with there greater surface area = more torque generated is just simply is wrong. What there forgetting is the length of the crank. Like a wrench the longer the wrench the greater the torque. So for a given displacement to increase the bore you must decrease the stroke meaning there is really no net advantage to having a larger piston or longer stroke because it it really just a matter of displacement. Thats not to say it doesn't matter what bore or stroke is used just that the surface area of the piston does not equate to more or less torque.

 

Uh well I had a mildly built 305 and it didn't do any justice for the money it had invested in it.

As far as stock goes, I think it's only a difference of like 20 hp but once you get past the bs like carb, intake manifold, headers and onto the heads and cam the 350 shines.

 

At the end of the day, cubes are going to be king, given that both engines have the same powerband. I'm not saying you should do a full build it on a 305 (though I might because I'm silly like that). But some bolts on, a set of 305 vortec heads, and a cam will really liven up a 305. Not as much as it would a 350, but if you want to work with what you have it's not anywhere near as bad as people make it out.

 

the old saying there is no replacement for displacement . I was thinking of building my 305 , they do have stroker kits that make them a 334 but the amount of money to build a good 305 is higher than the amount it takes to build a nice 350.

 

I used to run my 83 chevy4x4 pickup with a 305 i had ported heads, lunati voodoo cam, harland sharp roller rockers, edelbrock performer intake and carb. and it ran circles around my Brothers' trucks with similarly built 350s. every on was pretty surprised with it. So its not to say is displacement but more of quality of parts and how well they work with your setup.

 

I think your reading things incorrectly
if you said that provided the displacement remained constant and only the bore and stroke ratio changed, the difference in power potential will be similar Id agree
but 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)

 

absolutely you can ( not that I'm saying I have or have even seen it ) if someone who has no Idea what there doing can build a 305 to be faster than a big block if you know what your doing and that person doesn't. It's like all the s-10's at the track with 400 small blocks that only run 14's .Hell I ran 16 flat with a 2.8 with a little mods which is not fast by any means but it's better than spending all that money and time for a 400 in a s-10 for 14's.

 

I understand that, but why don't we see this manifested in an engine's torque per displacement figures? According to this, any increase in displacement should result in a increase of torque by a factor of the square of the increase in displacement.

 

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

 

Thats why i said FOR A GIVEN DISPLACEMENT. Obviously if the cranks stroke is remained constant and the pistons surface area is increased there will be more force generated because you have a larger displacement. However i said FOR A GIVEN DISPLACEMENT meaning the overall displacement remained the same the is really no advantage of having a large bore and short stroke over a small bore and a long stroke in terms of power generated. Again thats not to say there is no difference between a motor that has a big bore short stroke or a long stroke and a small bore but in terms of power there really is no difference. Once you start to change the displacement that really changes everything and increasing displacement (bore or stroke) given the same volumetric efficiency will obviously increase the total force generated.

 

to answer the OP question though there is no replacement for displacement. the 350 equally modded will out perform a 305. Is the difference very significant its definitely pretty noticeable but depends what motors your comparing. On factory F bodys yes its a good deal different. Furthermore factory vs factory TPI is better performance wise over a carbed motor after you start doing big time mods it reverses carb faster than TPI. Not to say either cant be good at anything but depends what you intend to do with it. However i wont get into carbed vs TPI as thats a huge discussion with many pros and cons for both sides. Now the formula 350 has a very similar motor to the corvette L98 (actually there both l98s) just things like i believe the cam is different, the heads although the same style are cast iron VS corvettes aluminum plus the castings are a bit rougher than the vettes, the exhaust manifolds are better on the vette, and the vette is a 4 bolt main block vs the camaro and firebirds 2 bolt main block. So is the 89 formula as fast as the vette with its similar engine? No the vette has a few advantages over the Formula above and beond the motor differences.

 

So the larger piston and the greater force it generates is just how the increase in displacement is manifested, correct? And we get all excited about this why? Sounds to me like it's just an extra 45 cubes.