Thread: What gives the L36 Higher Compression?

I realize there are several different ways to achive higher compression. And seems how I never Got into the internals of the 3.8 (yet), How did GM go about this? And if the connecting rods are different lengths between the L36 and L67, How is the stroke the same? I thought It would have been different stoke lengths.

Originally Posted by asite57

i though it was in the PCM because you can just top swap to an L67 i mean this is a really good question and id love to know the real answer

Noo..

Compression is in the pistons and combustion chamber.

I think its due to the rod length. If it is, then:

Longer rods allow the piston to travel further toward the head, increasing compression. So why doesn't the stroke change? This is because the longer rods don't allow the pistons to receed as far into the bottom of the cylinder as the L67.

That too ^^

so the l67 goes down further and the l36 goes up further, therefore having the same stroke?

Originally Posted by asite57

so why can you just swap the top over and call it good?

I don't understand what you are asking.

A top swap retains the formerly NA block and heads.

Originally Posted by BlackGT97

so the l67 goes down further and the l36 goes up further, therefore having the same stroke?

Yes sir.

At least that is my intuition. I've never checked into it.

Originally Posted by asite57

so why can you just swap the top over and call it good?

You can't topswap and call it good. You have to tune for it. Having the higher compression and Boost causes more heat, which causes detonation. Flow mods, cooler plugs, and cooler air (intercooler) and tuning, are all ways to fight the detonation, AKA knock.

Originally Posted by asite57

ive never done a top swap, i though you swaped over the l67 heads up and left the NA block. im just trying to get a better understanding not sound like i know what im talking about

The heads, in the mechanicale aspect are the same. Only thing different is the port for the injectors. So your Pushrods, lifters, open the valves just as far as they would with the N/A heads.

Originally Posted by mechguy

I think its due to the rod length. If it is, then:

Longer rods allow the piston to travel further toward the head, increasing compression. So why doesn't the stroke change? This is because the longer rods don't allow the pistons to receed as far into the bottom of the cylinder as the L67.

As long as the stroke doesn't change then that doesn't change the compression. You will always start at 0 PSI on the intake stroke. Then no matter where the pistons stops if it makes the same stroke then it should make the same compression. Or my logic could be wrong.

Originally Posted by Iceman6669

As long as the stroke doesn't change then that doesn't change the compression. You will always start at 0 PSI on the intake stroke. Then no matter where the pistons stops if it makes the same stroke then it should make the same compression. Or my logic could be wrong.

No sir.

Imagine this: A 10 foot long cylinder with the same stroke as your engine. There would be almost no compression in the former.

^^^True. This right here is a good comparison.

Piston wrist pin to top of piston length is different. L67/L32's have more valve clearance.

The rods may be same length but the wrist pin ends are different and specific to the piston.

Federal-Mogul Resource Center - Part Details

Federal-Mogul Resource Center - Part Details

The S/C pistons have larger pins.

Compression distance is greater for s/c pistons which means you'd have to have a shorter rod to have the same piston/valve clearance.