Static v Dynamic compression ratio
Turbo Mini Forums

my engine - 1380 n/a 286 sp cam stage 4 36 x 31 head, current static compression ratio 10.5:1. The calculated dynamic compression ratio is 8.7:1. my question is could I raise the static compression ration to approx 13.5:1 and therefore the dynamic to approx 11.5:1 without detonation problems. I currently use Shell v power fuel with octane booster mixed at 3% approx 103 RON. My ignition is a mappable Omex setup. I realise that I will need to have the fueling reset and the ignition remapped to suit.

From experience.

it will pink like a bastard and be a pain the ass to start.

i run mine on a mix of 50% v shell and 50% sunoco 119 octane.

i run mine at 11.5:1 static, not sure on the dynamic, but its fine runs on v-power or bp ultimate, no problems, running a max of 28 deg ign timing, i can raise this, without det, but it creates no power increase.

i know this is on old thread but i think its better to keep this in one spot

so how are you guys getting on with your setups?

please have a read and unleash your thoughts

as some of you know im doing a 1098 based turbo now i have spent some time this arvo looking at compression, cams, boost and all that good stuff

so what i have planed is
1098cc 40 over bore (1150cc i think?)
lightened internals
home brew head cc unknown at this point in time (professional vale job!)
kent 274 cam intake valve closes at 69 degrease quite late
water to air inter cooler
E85 fuel (V8 supercar stuff 85% alcohol) supposedly the best next to expensive race gas..
now the info ive used to calculate with

bore : 2.6025
stroke : 3.2960
rod : 5.75
static compression : 13.25:1
dynamic compression : 10:1
dynamic with boost : 15:1
cranking PSI : 214
boost PSI : 7.25

from what i have seen on other car forums up to 20:1 if setup correct is fine on E85
so 15:1 should be a good buffer if all is well i may up to boost to something like 14.5psi giving 20:1 dynamic cr on E85 and probably water inj
also from what ive seen around the place you can get away with high cr on e85 due to the lower charge temps and slower burn compensated with a bit more ignition advance to bring the peek cylinder pressure back where it needs to be something like 30 dergese after top?

its known that exchanging 1 or more cr points for several psi in boost is the way to go for all out power
but im looking for low/mid range so a high cr is a must and then as much boost as i can basically i think

Firstly, your cam data is incorrect. The MD274 intake is published to close at 56 degrees ABDC.

I've attached a plot of the measured cam profile for the 274 and the Ph2.

I would not get too hung up on this subject. The published theory assumes that the cylinder pressure is at an average inlet manifold pressure at intake valve closure. This is far from reality.

The pressure in the cylinder at intake valve closure is dependant on manifold design, volumetric efficiency and therefore scavenging, or otherwise, at overlap. With a well designed manifold setup, the pressure could be far higher than assumed.

Besides, you can make more torque with a lower compression ratio with long intake runners and a LCB style turbo manifold.

Edited by Paul S on 13th Oct, 2013.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

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Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

my bad lol 56 degrees... that will change my calcs a fair bit!
i cant see the plot? scratch that lol it apeared after i posted
would that low cr be compensated with more boost?
my head hurts time for bed and a fresh start..

Edited by Turbo This.. on 13th Oct, 2013.

The theoretical improvements in engine efficiency by increasing the compression ratio are best explained by the Otto cycle:

http://en.wikipedia.org/wiki/Otto_cycle

Equation 6 in the above is the useful bit.

Basically, the smaller the combustion chamber, the higher the efficiency.

BUT this assumes that the combustion chamber does not contribute to the volume available for the the trapping of the air-fuel mass. It assumes zero scavenging between the exhaust and inlet strokes. Volumetric efficiency, based on swept volume, cannot exceed 100% under these circumstances.

Modern engines with performance engineered manifold systems can achieve higher than 100% volumetric efficiency by using pulses in the manifolds to achieve optimum scavenging and subsequent filling of the cylinder. Timing of the pulses to occur at the back of the inlet valve during overlap and intake stroke can be achieved by careful selection of the length and diameter of the intake runners. Similarly the exhaust manifold can be tuned in the same way (that's why a LCB is a simple performance improvement to the A-Series.

In relation to our cars, the Metro turbo manifolds were designed purely for packaging and make absolutely no use of the available pulses. The intake manifold runners are too short.

So, following that through, if you can achieve good scavenging at overlap and then keep filling the cylinder throughout the intake stroke and the start of the compression stroke, the volume available is the swept volume plus the combustion chamber. Hence, it is possible to achieve higher than 100% VE and a bigger combustion chamber ie. a lower compression ratio, could provide a higher torque output.

So, in a turbo application, for maximum torque, you do not need to get your compression ratio as high as you can if you use tuned length manifold runners.

Just look at what Dave Walker achieved with long intake runners:

http://www.emeraldm3d.com/articles/emr-adj-length-intake/

Edited by Paul S on 14th Oct, 2013.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."