let's talk compression
 

So, I searched and didn't come up with much.

I am planning on running a realatively high compression for my formy. i would be running an 11.66 scr and an 8.9 dcr. is this safe for 93 octane pump gas on a safe tune for a car that wil be driven on the street?

i should have good quench around .033. I'm just looking to pul the most power and torque from this cam.

anyone?

Yes. That's fine. I have a 12.8.1 scr 408 tuned on pump gas.

anyone else?

re
 

the debate on dcr and scr is a sore subject with to many variables.... quench, VE, temp of the motor, cam size blah blah blah supposedly for the average motor ( I am an LT1 guy so there is iron involved) 9.0 is safe for 93 with a good tune/ cooling sytem and air temps in the normal range

Nothing like a thank you....

well yes, thank you everyone for the input thus far. I was hoping for more than just an example or two. I didn't want this turning into one of those debates like flycutting with a big cam where everyone is like I did it, it will fit.

thats some compresion!!!! what sort of power is that motor making N/A??? also it that on 93??

thanks Chris.

I have heard that 8.7 DCR is about max for a 93 pump gas

Theres a point of diminishing returns with raising compression.

High compression on a NA motor will reduce residual gas left over after a cycle.. which improves VE.. It will also improve thermo efficiency.. It will however reduce mechanical efficiency.

My point.. well, its a lot like ignition timing.. Do you try to run as much timing you can without knocking? Or do you run what makes the most power. 2 different things

care to explain the reduction in mechanical efficiency? I didnt think an engine, in practical terms, would ever get to the point where the loss in Mech Efficiency would overcome the gains in thermal efficiency

Sure,

The mechanical efficiency in a typical street engine is obviously related to how much work is required to turn the engine. The amount of work required as compression goes also rises, fairly linearly.. This is easy to see by looking at the delta T at the start and end of compression. So what this is saying is mechanical efficiency starts off high at very low compression then goes to low at high compression. ... Negative slope line with increasing compression

Also we know that thermal efficiency climbs rapidly till about 9:1 where it tapers off. For instance going from 9 to 10:1 might net you an additional 5% thermal efficiency while going from 11 to 12.5 will only get you 1-2%

So combine the two efficiencies and you will have a parabola with the local max being the best compression for the combined efficiencies.


It actually gets a little more complicated than those two things with compression ratio and what it does to VE. at the end of the exhaust stroke and when the intake valve opens there is a left over mass the volume of the combustion chamber and at the pressure of the exhaust. Its called residual gas and its where reversion comes from. Anyway, this left over mass flies back into the intake manifold, then gets sucked back down into the cylinder during the intake stroke. This gas is used up and inert so it is basically a waste of space. That waste of space obviously negatively effects volumetric efficiency.

Reversion and residual gas can be eliminated with valve event timing and intake and exhaust tuning.

You heard wrong. There is no magical number.

Why do motorsports run such high compression ratios then? Something tells me its not worse BSFC or decreased power :confused:

i had 9.0 dcr on 93 but it was winter / spring so i didn't get a chance to see what it would have done in warm weather. as already said there is variables like say a very small runner velocity head will have more cylinder fill than a larger runner so it will have more pressure so i'm sure it would be more likely to not handle as much scr/dcr as a slightly lazier runner untill the upper rpms but that may be debated also so that is just my .02.

And why don't they run 13-14 on gasoline? Evidently about 12:1 is thier magic point.

because it knocks and because of emissions, not because gas milage would go down.

Don't worry about SCR; think about DCR and VE.

For that reason, I wouldn't start comparing race engines to street engines.
IVC (and all other valve events) are night and day.

The valve closing point and trapped charge is what determines the optimal
static compression for a given fuel and engine setup.

Would you rather:

1. Compress 100 cc of mixture 12:1 to yield "x" amount of energy minus the losses required to compress the mixture at this ratio

or

2. Compress 120 cc of mixture 11:1 to yield "x" amount of energy minus the losses required to compress the mixture at this ratio

The point of dimishing returns likely occurs when you raise compression beyond the point of maximum combustion efficiency and take more loss to
compress the mixture at a higher ratio.

One question: Expansion ratio is often forgotten in all this- which is based entirely off SCR, correct?

I guess if DCR is based off SCR and IVC, then "dynamic expansion ratio" (is that a real term?) would be based off SCR and EVO. Right?

Seems like for a given DCR, it would be thermodynamically more efficient achieve that DCR using a high SCR because you get every last bit of energy out of the exhaust stroke.