I'm old enough to remember what happened to the 454 Chevelle from 1970 to 1972. It went from 460hp to 265hp when they had to drop the compression ratio to 8.5 to 1.
I know it can add throttle response. I know it cam improve daily driving manners in a cammed car. I know it cam allow a earlier EVO without loosing torque. I know people go to great (expensive) lengths to get it; fly cutting, milling. E85 and $160.00 head gaskets.
I read FRiCK's post and saw he was running 11.3 compression and thought that was the reason for his 460hp from a 230 cam. Then I plug numbers into the Wallace Racing compression app and am shocked. That calculator can not be right.

 

What was the before and after with Frick? That post is too long lol.

Compression adds power. That calculator is messed up.

The general rule (and this is highly dependent) is 1 full point adds around 3-4% across the range with everything else being equal. So a jump from 10.5:1 to 11.5:1 would net, on a 400rwhp engine, approx 15rwhp across the range.

As you can see, that's FAST territory at that power level. Get to 500rwhp and it gets closer to 20rwhp.

Now you can see why the optimization of compression is important. Bigger cams with later valve closing points need compression to retain torque and part-throttle transitions. The added benefit is more power and better driving dynamics.

The changes in the 70s were more than compression. They neutered those cars all over.

 

Basically compression is how much power you can get from a given amount of air/fuel mixture. Cams, intakes, heads etc. all allow *more* air and fuel into the engine thus making more power. More compression however takes that same exact volume of air and fuel and makes a bigger bang out of it, so you can make more power without increasing the amount of air going into the engine.

Thus as you can see it's a very efficient way of gaining power. It's like making more money at a job without working any harder or putting in more hours.

Compression gives a snappier throttle response and a more torquey engine. Like mentioned, it also allows you to run bigger cams without sacrificing low end as much.

 

This may be a dumb question, but why wouldn't everyone run higher compression then? What's the drawback?

 

Octane.

You can only run as much octane as your fuel will allow. 11.5:1 is about as far as 93 will take you. You can stretch it out some more with tuning, head design, cam design, quench, etc. But 11.5:1 is generally about as much as 93 will allow.

Moving beyond the limits of your octane will cause damage or require an awful lot of timing retard, which kills power. There is just a limit as to how much specific energy a given grade of fuel contains. The way around this power ceiling is more cubic inches or boost to force more air and fuel into the combustion process. But 93 still has limits with a 500cid motor or 14PSI with a 370, etc. Higher octane allows more compression and timing.

 

When you work the calculator, are you looking at static compression, or dynamic compression. Two very different things.

 

Thanks for the info, appreciate it

 

That calculator looks like it calls for static. The 3-4% number is for static. I don't know if there is a rule of thumb for dynamic, since it in itself, is a calculation derived from static.

 

Static. When I got tuned I put down 321 hp with 241 heads. I have 243 I plan to put on and the calculator said of the power I would gain 1.55 hp of it would come from the 0.4 compression increase.
For the 72 Chevelle, it was oxides of nitrogen. A cooler burn reduced the polutant.

I guess we can't talk about compression without referencing the Pat G Torque comming out of my ears thread.

 

Static CR is just that, static. It's an easy to understand number that has variable results depending on the rest of the combo. Dynamic CR is not derived from SCR, but uses SCR along with IV closing point, altitude, and some other stuff. You get a much more accurate idea of how the engine will behave. It can be confusing, though, since you want a street engine to be close to 8.0:1.

 

Ok, that simplifies how it works to me. Thanx.
I'm not sure what FRiCK's combination was before but he went from 400 to 460 HP with Livernois III heads and a 230/230 111+2 : 11.3 static with that cam converts to 8.5 Dynamic . With his LS6 intake peaking with a 42 IVC he was getting 2 extra degrees of cylinder fill with that cam's IVC of 44 so he was getting more cylinder pressure than the 11.3 would suggest at higher RPM.

Jake, I read where you and Kip said compression adds to exhaust loudness. Does it also put more demand on the cooling system ? My cooling is a bit marginal.

 

Dynamic is just a calculation. So is static. Both formulas call for Cylinder vol. + clearance vol. + piston Comp. vol. + gasket vol. + chamber vol. divided by Clearance vol. + piston vol. + gasket vol. + chamber vol.

The difference is in the cylinder volume portion. With Static, cylinder volume = 0.7853982 x bore2 x stroke. For DCR, you plug in DST for stroke here. DST = stroke - ((PR2 + 1/2ST) - RL), which is a series of other calculations. You get the idea.

Most aim for around 8.5:1 calculated DCR for close to an ideal setup on 93.

Even still, if you have 12.5:1 and a DCR of 8.5:1 (calculated), you're probably going to have knock with 93. There is a practical limit to how much DCR plays into the octane vs timing vs CR discussion. Just as you can run well over 9:1 DCR with a small cam and low compression. But this is where the idea of "compression bleeds off" comes from. The bigger cam doesn't bleed compression. It just doesn't build it due to a later IVC and overlap that keeps both valves open longer. But as the RPMs rise, this is where VE comes into play and that bigger cam builds more and more cylinder pressure, where 93 may not be enough to keep up, forcing the tuner to remove some timing.

 

Jake,

The 3-4% "rule" only applies at very low static CR.....meaning foolishly low such as an 8 to 1 engine getting bumped to 9 to 1.....every point higher is the law of dimininshing returns all else being equal.....BUT...more compression helps you cam it a bit more so the combination of more compression and more cam could easily cover that and more.

A 13 to 1 engine going to 14 to 1 picks up very little.....but as I mentioned it allows you to cam it a little deeper and you still retain the same DCR with an obvious gain in power upstairs from both moves.

Compression is the foundation of cylinder pressure (its the rating of static pressure after all) and cylinder pressure or should I say increasing it is what we all aim to do when we swap parts to hopefully make more power.

It helps economy, efficiency, torque and power output....everything....and raises the engine BMEP #'s as well (Brake Means Effective Pressure)....you want as much static compression as your combination will tolerate with the fuel your planning on running (application comes in to play here also....you cant be on the ragged edge in a car that sees extended WOT sessions or a lot of heat from other reasons such as climate etc.).

-Tony

 

I agree, Tony. The usual 10:1 to 12:1 will see a pickup on a heads/cam LS1. Not huge, but something. But 10.5:1 to 11:1 doesn't help much. It just helps in other areas as you mention.

 

So then, when deciding on the combination of parts for a build, should you aim for a certain DCR and then let the SCR be whatever it will be? Or are all of these numbers secondary to something else?

I've tended to focus on DCR, because I see aftermarket cams make large jumps from the stock cam in DCR, and assumed that was a big reason for the improved performance. Was that a mistake on my part - or at least over-simplified?

 

Best way would probably be to have 1) a target DCR in mind and 2) a target cam in mind, then setup the SCR accordingly for that cam to meet that DCR requirement.

 

Well, it's a combo right. So first you set up your cam - what street manners do you want and what kind of power? Okay good, so now you have valve events. You know a certain IVC will need a given SCR to achieve an optimal DCR. So you build the compression around the valve events to provide optimal DCR.

 

Taking into account the car's intended purpose and budget of the builder. My purpose is a fast daily driver that can do many things well. My budget is abysmal. The car is a 93 RX7 with a LS1 and M6. It has a Fluidyne radiator in the stock location with stock fans controlled by the PCM. I live close to Jake so it gets hot here but the car does not run hot now. I have 1 7/8 LTs and am working on a 4" ORY. I have 243 heads (not installed yet) and am trying to decide on gaskets for them. My cam flavor of the week is a 230/230 112+3 so with these heads and MLS I would be at 8.05 DCR, with .040 Cometic 8.28 but that would be $100 more for not much gain. To go to 8.5 DCR would be another $100 to mill plus the cost to flycut. That money could be spent on springs and pushrods.
So on the plus side, It helps economy, efficiency, torque and power output.
On the minus, It would be louder, run hotter, PCM more likely to pull timing and add well over $200 to the cost of the build.
Are there any other pluses or minuses I'm unaware of ?