[Robert Imhof]

How come my buddy can run his aluminum headed small block chevy with 11.5 compression on 93 octane with no pinging but with a pontiac aluminum headed engine going even into the 11's on 93 octane is pushing it? Are the chevy heads that much better?

-Robert Imhof

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"It's all fun and games until somebody gets their doors blown off!" Sleepers Rule!
-Pontiac Lemans, The ultimate sleeper.

[Goatman]

The small block Chevy head is not "better", but its design allows more compression with "pump" gas. One of the benefits of running a small block.

[77TA]

smaller bore allows for more detonation resistance on the SBC.

700 HP with <500 cubes on pump gas posiable as Ive seen 2 built but neither were POntiac. Both Big block fords one 511cid and the other 540 cid though but hp was close to 800 on the 540. We do not have as good a head as these motors and thats the key to making the big hp numbers that and a decent bore/stroke ratio which both of those motors had and any pontiac over 400 cid does not. I am starting to beleave the best way to get 700-800 with a pump gas pontiac is a 400CID mill with a 10-12 psi super charger and EFI.
use 87 cc E-heads ported big time on the exhaust side with 2 inch headers and 3 inch exhaust.

[Goatman]

Bore has nothing to do it. The Pontiac 326/350 has a smaller bore than many small block Chevies.

[Dave A]

77TA was right, bore does factor into it...it affects how far the 'end-gas' region is from the spark plug region where combustion starts, and plays into how 'efficient' the combustion chamber is overall. The quicker the burn is completed, the less time for the end gas to build up pressure and temperature before the flame front reaches it, and the less chance for detonation.

The Pontiac 326 would enjoy some benefit for knock resistance from its smaller bore.

The shape of the combustion chamber and how it affects the in-cylinder gas movement, both pre-combustion (how it affects squish movement especially) and during combustion is important.

I have heard SBC combustion chambers are more 'efficient' (efficient as it relates to combustion speed and amount of ignition advance required) than Pontiac chambers - not being a Chevy person, I cannot say exactly why. If that is true, even a 400 CI SBC would enjoy some detonation resistance over a 400 Pontiac with the same bore and stroke.

I am sure about the bore thing, though, the question is, how much of a factor is bore versus the shape of the combustion chamber. My guess is the shape is more important in the case of the sbc versus the Pontiac.

[Ockhams Electric Razor]

The aftermarket aluminum SoBC heads are almost all far better then any Pontiac head available. They have not only a smaller, more efficient chamber but the shape of the chamber promotes better charge mixing and they don't have the awful valve-shrouding problem that we do. The SBC also has the benefit of a much shorter stroke, which in turn generates less frictional heat from dragging the rings up and down the cylinders, which reduces the tendency towards pre-ignition. The shorter rods also plays a large factor, as does the typical high RPM nature of a SBC, for at higher RPMs detonation is less of an issues as there is simply not enough time between cycles for any sort of abnormal ignition to occur. Most SoBC's are also in light cars with 4.xx something gears and a 8,000 RPM converter, all of which helps let them run more compression. There are many other things as well and plenty of theory behind it all, too much to list it all here, really….

[Goatman]

OER has it correct. Bore size is mainly insignificant in this discussion. BB Fords and Mopars with huge bores don't suffer from detonation problems like some of the Pontiacs do. Its in the heads. Cam specs also play a large part in the equation.

[Robert Imhof]

You gotta remember that Chevy has so many more years of R&D over the Pontiac. Only now is the Pontiac aftermarket starting to catch up. Pontiac will NEVER be on the same level as Chevy on an technological level but Pontiac aftermarket will get better. Everyone has a Chevy, that why I have a Pontiac.

-Robert Imhof

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"It's all fun and games until somebody gets their doors blown off!" Sleepers Rule!
-Pontiac Lemans, The ultimate sleeper.

[Dave A]

Thinking about it some more, I agree, combustion chamber design is more significant that bore size when comparing sbc to Pontiac (4" bore on 350 Chevy to 4.18" bore on Pontiac 455) - and the same probably holds true about the Ford and Mopar.

A larger bore generally means more risk of detonation irregardless of the chamber design, though - it still comes into play.

Ockam, I agree with most of what you said, but don't follow the bit about ring friction increasing risk of preignition (due to longer stroke on Pontiac)...ring contact occurs near the top and bottom of the stroke when piston speed is the slowest, other than that it is oil lubricated and probably doesn't exceed the temp of the oil, until combustion starts. I s'pose the piston can develop hot spots, but I'd imagine that's due to combustion heat, not friction heat.

Adding some more info: studies done by MIT on 'similar' (same combustion chamber design, rod length, stroke, etc) engines back in the 50's generated some curves of bore size versus octane requirement.

They found a linear relationship in their test engine, and octane requirement varied by about 10.5 points per inch of bore size in a range of 2.5" bore to 6" bore, measured at 1800 fps piston speed. For the 0.18" difference between the 350 sbc and 455 Pontiac, that's about a 2-point increase in the octane number required.

They found this relationship to hold true whether employing a squish-type combustion chamber or a non-squish type.

They also observed that cylinder wall and combustion chamber surface temperatures increased with increasing bore, one factor reducing the detonation tolerance, but with coolant temp adjusted to maintain a constant cylinder temp, the still observed an increased octane requirement for larger bores.

However, as agreed above, the Mopar and Ford engines may enjoy some detonation resistance versus the Pontiac on account of the chamber design.


[This message has been edited by Dave A (edited 12-19-2001).]

[Goatman]

Dave, that sounds like an interesting study. I wonder if they kept the cam the same and if so, what specs did it have. I ask because I would imagine that cylinder pressure, as we all know, would play a significant role in detonation as well. I would think that as the bore increases, with the same cam, that the cylinder pressure would also increase, thereby promoting detonation.

That may be of some significance as well. It would have been really cool if they could have changed the cams so that the cylinder pressure remained constant throughout the increased bore "trials" and then the detonation factor could have positively attributed to the increase in bore alone.

Just some thoughts.

[jyeager]

Bore size is important to this discussion. The big horsepower numbers require big RPMs and the frictional losses of the small bore/big stroke engines at higher rpms are very significant. Also, valve shrowding is an issue in our small-bore big-blocks. That is why the Mopar and big-chevy find it easier to outdo the pontiacs in peak HP numbers.

[Dave A]

Goatman, I am not entirely sure of all the details of the test - they are reported in 'The internal combustion engine in Theory and Practice' by Charles Fayette Taylor, and it in turn references a Thesis project by an MIT student, as well as a progress report to the Shell Oil company.

Some details of the test setup are provided, though:

3 test engines - 2.5" bore x 3.0" stroke, 4" bore x 4.8" stroke, 6" bore x 7.2" stroke, with a cylindrical combustion chamber and no squish. (although some tests were run with a squish piston).

My guess is that cam specs were kept the same, and obviously engine displacment varied with bore, but bore to stroke ratio was kept constant. The tests were all run at the same piston speed. I'm not sure exactly how the PEAK cylinder pressures differ between the different sized engines - but I do think that the larger bore engines would have a higher end-gas pressure, and consquently a higher end-gas temperature.

Trying to apply this directly to our comparison of a sbc to a pontiac engine is strictly acedemic - I would be hesitant to claim anything other than the bore size has some effect, but that the effect of the bore size is often overshadowed by the effect of different combustion chamber design. I think it would be very difficult to isolate the effect of bore size alone, since the bore size can affect several factors affecting detonation resistance, sometimes in a complex fashion (such as gas dynamics, and whether even with similar combustion chambers this effects scale predicatably).

jyeager - comparing two engines of the same displacement, one big-bore short stroke, and the other big-stroke small bore, the smaller bore engine does have smaller piston rings and less piston skirt area - for the same piston speed, the smaller bore engine may actually have less friction - but for the same RPM, I think you're right, the big bore engine will have less friction. Depends on other engine geometry too, I guess, such as rod length and how much side loading you get.

[This message has been edited by Dave A (edited 12-19-2001).]

[Robert Imhof]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by jyeager:
Bore size is important to this discussion. The big horsepower numbers require big RPMs and the frictional losses of the small bore/big stroke engines at higher rpms are very significant. Also, valve shrowding is an issue in our small-bore big-blocks. That is why the Mopar and big-chevy find it easier to outdo the pontiacs in peak HP numbers.<HR></BLOCKQUOTE>

Is that why NSCA Pro Nostalgia Pontiac cars Jim Trettle's GTO are made to turn High RPM's? (6,000-7,500 rpm)

-Robert Imhof

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"It's all fun and games until somebody gets their doors blown off!" Sleepers Rule!
-Pontiac Lemans, The ultimate sleeper.

[Goatman]

Those RPM's are low compared to almost everyone else in that class. The Chevies and small Fords are 7000RPM + all the time. I would imagine that after a certain point, that you must make more RPM to continue to produce HP, which, in a big stroke motor is a no-no.

However, this topic was originally about detonation and other than a slight increase in cylinder pressure, I would still have to say that bore doesn't mean much in that particular situation, all other things being equall. Pontiacs just don't do well with compression and pump gas.

[Half-Inch Stud]

yea huh