Compression Ratio Vs Boost

[Roy]

Ok,i know some of the theory, and generally ppl seem to suggest you want to run std compression or a tad higher, as it helps response etc etc.

But say you dont care about power off boost, and you have a turbo that is undersized for your application, and wont have trouble spooling it.

So if you take two identical engines...one has 8.5:1 compression and it starts to ping at 22psi and plenty of ignition. Engine 2 is 7.5 compression, but with the sane amount of ignition can handle 28psi before pinging

It seems pretty obvious that the 8.5:1 engine will be more responsive, efficient etc, but will it make the same power. Oh and assume the turbo is still efficeient at a pressure ratio of 3.

So who is going to make more power? I would think its the low compression big boost engine????

And how good is the generalisation that if you go a bigger compressor housing it helps the thing make power at higher boost levels?

[Busky2k]

Yeah I would predict the low comp high boost would make more power. This is because the loss in thermodynamic efficiency from reduction in compression is would most likely be well offset by the extra charge in the cylinder from higher boost. How much more power it'll make is anyones guess though.

Have you messed around with a compression ratio estimator? Its intersting to see that the dynamic compression ratio of a 7.5:1 @ 28psi engine is _roughly_ the same as with a motor at 8.5:1 @ 22psi.

Also compressor housing changes only really alter the pressure ratio behaviour I believe, not the actual compressors flow unless its way off.

[Abo Bob]

I suspect that they will make the same power going by the rule that 0.5 drop in CR requires 0.2 bar more boost to acheive the same power. (I think I have that right)

Are you using the same turbo on both these engines? Cos you probably wouldn't given the choice.

[Cubes]

All though it may be different due to fuel used..

Do remember the way the Gibson GTR's were setup. 9:1 comp ratio.

I would personally stick down around the 8.5:1 ratio, I wouldn't be game to venture higher.

What comp ratios are you looking at if you use off the shelf items?

[Roy]

I was just looking at compressor maps, and there are a few turbos that seem to like 2.5bar of boost. I cant see many engines with std compression ratios being able to run anywhere near that compression ratio with pump fuel and 30psi.

And if that ril of thimb CR to psi equi is right, then cool. Was just curious to see...and yes they woudl be using the same turbo. Odds are im goign to go 8.5:1....given up on the Yanks getting me reasonably priced solid lifters. Can get greta prices on titanium retainers, springs, valve quides etc etc, but not the lifters

As for Gibson GTRs, that may be right...but they were hit with boost restrictions by CAMS. All the Sierras ran crazy low compression with boost up the brazoo

Is it because Gibson was so right and the others missed something terribly obvious. Or is because Gibson couldnt run the boost he use to with the R31, DR30, Bluebird because CAMS were trying to even up the field? Why the change from model to model , more effecient head design allowed the compression??

Who knows, i suppose its an area that we never discuss, as the general thinking on SAU is never decompress an engine...but so many ppl cant be wrong when they drop a bit of compression to give it crasy hits of boost!?!?!?!!?

[Cubes]

The other thing to look at is the 100RON fuel that is slowly becoming available. Hopefully it becomes a little more mainstream.

When I built my motor the engine builder really wanted to go down to 8:1 if I were to run 20+psi on pump fuel.

He mentioned there was some fuel ron to boost graph on some supercharger web site.

Basically he said.. higher comp ratio with low boost on pump fuel won't make as much power as lower comp ratio with higher boost on pump fuel.

But that is once on boost..

He was from a supercharged v8 background so i'm not sure if the same principles really apply. I would assume they do.

I'm not sure if the 0.2bar of boost for every half a point drop in compression ratio would actually = the same power. I do know it equals the same dynamic compression, would this really equal more power?

See the new 911 Porsche runs a Variable nozzle Turbo and makes 350kw or there abouts.. 600 and some thing nm of torque from 1950rpm.

[Roy]

And if you have an engine with the extra displacement over the RB20, i suppose i wont care about a drop in compression, as its still going to be punchier off boost then an RB20....so where everyone cares about response, and getting onto boost...lol i dont care about response or a poor boost thresh hold. As long as its making over 200rwkws at 4,250rpm, then i think im ahppy to trade enything under that , to get the extra 25-30rwkws average between 4,500-8,500rpm.

Oh, and looking at race cars is a good idea, but need to carefully to look at the regs they ran under as well