Increasing the CR

[Stallion]

I was wondering, what would happen if you, say, increased the L48s compression ration from 8.2:1 to, say, 9.5:1? Would you get a noticeable power increase (hp? tq? both?)?

And how would you go about to increase the CR? What's the science behind it? Just able to get more air into the engine? Or is it more than that?

Thanks!!

 

[MaineShark]

I was wondering, what would happen if you, say, increased the L48s compression ration from 8.2:1 to, say, 9.5:1? Would you get a noticeable power increase (hp? tq? both?)?

Both. On the downside, if you push the compression too high, you can end up with detonation. 9.5:1 would probably require 93 octane to run comfortably. Running only 9:1, I can operate on 87 octane.

And how would you go about to increase the CR?

You reduce the closed chamber volume. This is determined by the stroke, cylinder bore, head gasket bore, head gasket thickness, piston volume, deck height, and chamber volume.

Let's say you don't want to rebuild your engine right now, so you want to keep the same pistons, stroke, deck height, and cylinder bore. That leaves you with two areas you can work on: heads, and head gaskets.

I'm not sure what the actual numbers for the L48, so let me invent an engine with a 8.2:1 CR, and we'll make some changes. My invented engine has the same 4" bore and 3.5" stroke as your engine. We're going to set a deck height (the distance that the pistons are below the top of the block, when at top-dead-center) of 0.050", a piston volume (the piston is not flat - it has clearance dents for the valves, and they have this much volume) of 9.5 cubic centimeters, a combustion chamber volume of 72 cubic centimeters, and a head gasket thickness of 0.040"

That results in a 8.2:1 CR. In other words, the volume of air that can be placed in there when the piston is all the way down, is 8.2 times the volume of air that can be placed in there when the piston is all the way up. So, since the valve closes, and keeps the "down" air in there, the air is squeezed (compressed) to a smaller size.

If we pull the heads off this engine, we can do several things to increase the CR. We can simply put in a new, thinner gasket, and put the heads back on. Let's replace that stock 0.040" gasket with a 0.020" gasket. Run the numbers, and the CR is now 8.51:1. Drop to 0.010", and the CR jumps to 8.67:1. So, that's one option.

The other option is to change the chamber volume. The chamber is part of the head, so you can do this by modifying the heads, or replacing them entirely. Let's say we put back that 0.040" gasket, and take the heads to a machine shop, and mill off some of the metal (essentially moving the head lower by a small amount). Let's say we mill off just enough to drop the chamber volume from 72cc to 70cc. The CR will go up to 8.35:1. Or, if you decide to use the 0.010" gasket in combination with the modified heads, the CR will go up to 8.84:1.

The other option is to put on new heads. Let's say you pick up a set of 64cc heads, and put back the 0.040" gasket. You'll have a CR of 8.83:1. Go with the 0.010" gasket, and you'll have a CR of 9.39:1.

Of course, all this is for the imaginary motor that I thought up. I'm not sure what the actual piston volume, chamber volume, and deck height are for a stock L48, so there's no way of knowing exactly how much each change would affect things.

What's the science behind it? Just able to get more air into the engine? Or is it more than that?

The idea is to take the same amount of air (roughly related to the displacement) and squeeze it into a smaller space. This improves combustion (the closer all the air can be to the spark plug, the better, since it will take less time for the flame front to move outward from the spark to the last it of unburnt fuel) and high-rpm power (in order to fill the cylinder, the cam must remain open for a certain amount of time, but the actual opening duration decreases as the rpm goes up - higher CR allows the use of a longer-duration cam, which will stay open longer at any given rpm).

Those are the basics, anyway.

Joe

 

[Stallion]

Hey, thanks a lot, Joe. Very good information. I understand what you are talking about. Thanks a lot, bro.

So performance cars would want a really high CR (maybe 11:1?), right? Just to squeeze out the most power and mechanical energy, I assume. Or would/could they get a higher CR than that?

It seems like a job that could be easy (changing the head gasket) or hard (changing the bore and stroke of the combustion chamber).

But, if you do decide to change the bore and stroke, you would have to make it less? Then wouldn't that change the displacement of the engine? Is that desirable?

Thanks again, Joe!!

TR

 

[Stallion]

I just looked up detonation:

detonation:
The action of the fuel charge firing or burning too violently, almost exploding. It sometimes results in a noise called "pinging." Detonation is caused by autoignition of the "end gas" i.e., that part of the charge not yet consumed in the normal flame-front reaction. Detonation occurs because piston motion and compression of the end gas raise its temperature and pressure to the point where the end gas autoignites. The pinging or knocking noise is the result of intense pressure waves in the charge which cause the cylinder walls to vibrate. Also called "fuel knock."

So if this happens, would you blow out the engine? That's a very disastrous thing, I think.

 

[JohnZ]

Raising compression can also be done with new pistons with "domes" at the top (instead of the normal flat tops), which reduces combustion chamber volume at TDC, increasing the compression ratio without changing the bore or stroke; this is the most popular method of increasing compression significantly. However, there's no free lunch - as compression increases, so does the octane requirement for the fuel, or detonation can result; detonation, especially under heavy load at high rpm, can destroy an engine in an instant (pistons break, rods bend, rod bearings are wiped out, etc.). Detonation is actually the "collision" of the two flame fronts in the combustion chamber - the first normal one initiated by the spark plug, and the second one initiated by excessive pressure and heat prematurely igniting the end-gases of fuel with an inadequate octane rating for the conditions.

These days, many folks replace their original "domed" pistons during rebuilds (if they have them, only used in optional high-performance engines) with "flat-tops" or even "dished" pistons to REDUCE their compression ratio so they can run on today's lower-octane pump premium gas without detonation.

Back in the 60's, compression ratios of 11:1 and 11.25:1 were common in optional solid-lifter high-performance engines like L-71 and LT-1 Corvettes (the L-88 was 12:1) and Z/28's, but those engines don't run well on today's gas without either retarded timing or blending race gas with pump premium (or both). A good general rule of thumb for maximum practical compression ratio with today's fuels is 9.5:1 with iron heads and 10.5:1 with aluminum heads.

 

[MaineShark]

Stallion: John got most of them, but to address the question about bore&stroke, you would want to increase them to increase the CR, since the greater volume of air would be drawn in, then compressed to the same size.

If you go back, you'll see that the CR is the ratio of the volume when the piston is down, to the volume when the piston is up. If you increase the bore and/or stroke, you increase the "down" volume, without significantly changing the "up" volume (since vast majority of the "up" volume is contained within the head, not the block, changes to the block don't affect it much).

Of course, changing either the bore or the stroke changes the displacement, so you'd end up talking about a different engine (eg, a 355 or a 383 instead of a 350), rather than the same engine with a different ratio, which is why I left those two out.

What actually matters, as far as detonation goes, will be the maximum pressure (too much pressure can detonate the fuel without a spark, similar to a diesel), the heat (too much heat just helps the pressure along in that respect), and the mixture.

The CR has a direct effect on the maximum pressure. The material of the heads has a direct effect on the heat (aluminum transfers heat more efficiently, keeping the chamber cooler, which is why you can go to higher compression with aluminum heads). The proximity of the piston to the head at top-dead-center (called "quench") directly effects the mixture. Basically, the chamber only covers part of the top of the piston - part of the head is just flat. As the flat portion of the piston approachs the flat portion of the head at high speed, it squeezes the air/fuel charge sideways into the chamber. That aids in mixing the charge, so there are no lean pockets (lean areas will detonate easier than rich areas - a well-mixed charge won't have pockets that detonate early). Quench in performance engines is typically 0.040" to 0.060", often determined by the intended use of the engine (at high revs, the connecting arms actually stretch significantly - if the arm stretches by more than the quench distance, the piston will strike the head, leading to all sorts of bad things)(some applications also require more play in the fit of the pistons, allowing them to rock a bit - if the piston rocks, one side can come up closer than the quench and strike the head).

And that's just starters...

Joe

 

[Stallion]

Thanks for the information, guys!! I understand. Again, thanks for everything.

TR