Power stroke and Compression Calculations

[SPIIRIT]

Gentlemen;

I am very lax in my math except for the basic algabreic calculations and ask you to bear with me...All statements are based on normal operating temperature.

I am building a cast iron Dart BBC (638.5cui.), 4.625 Bore x 4.750 stroke . "Digital Dyno" program is giving me torque/hp numbers of 1145/1207 respectively with 10.3:1 C/R and open headers (I am not even sure what fuel these figures are based on). The Comp cam is rated 274 dur./0.750 lift/114 deg. L/S. It is set in roller bearings. I have a few questions at hand.

1. I have read that "ALL" Otto Cycle engines find maximum torque at 14-15deg. ATDC is my understanding correct?

2. I have read that the optimum torque point of the power stroke is 14-15deg. ATDC. In conjunction with this I have also read that in an aluminum "fast burn" chamber, 92-93 octane pump gas has a flame front that requires ignition 20 deg. BTDC (34-35 total advance) to attain this optimum condition saftely (detonation). Do I understand this correctly?

3. I have read data that causes me to believe that the optimum safe C/R for my engine should be 10.3:1 on preimum pump gas with the spark advance settings indicated above (34 deg.)... Is this correct?

4. I understand that all this is just a base setup which will be dialed in on the dyno runs and if that is true then does the 14-15deg. ATDC optimum rule stand fast reguardless?

5. To me, C/R statements I have read tend to culminate as a veritable rats-nest of confusion in my mind! I functionally understand "compression pressure" (cranking), and I understand "dynamic pressure" (cam related "duration"), I also understand all the principals of pre-ignition and detonation...but the articles I have read leave gaping holes in their reinterations by neglecting to detail whither the C/R mentioned is static, cranking or dynamic. Now I am pretty good at reading between the lines but, in such critical cases as we consider here, I make no attempt to do so nor will I accept such thinking from another.

I always end up unsure of myself it this area. I have read that 165-170 lbs. cranking pressure is the max. (considering detonation). I have read that under certain conditions a C/R as high as 12:1 can be used with 92-93 octane gas (1500 lb. vehicle), is this all correct?

Is there a chart available that considers all these factors? And if there is, when they just say "C/R" without detail, what is the interpretation of the sum of their presentation supposed to be (considering that ratio)? Static, Cranking or Dynamic? I know if it's in RPM it's dynamic but the others are almost always left open and that is the rub.

T.D.W.

 

[magnograil]

Hi T.D.W.,
If you have a plot of cylinder pressure versus crank angle you can calculate the maximum torue point from the geometry.
Not sure how you are relating 20 BTDC and 34-35 total advance. 20 BTDC is a very retarded ignition. High octane fuel burns slower so needs more advance to get 5/9 combustion done by TDC. High turbulence chambers burn faster but unlikely that fast.
Premium in the US is (R+M)/2, elsewhere Research only. I do not know the specifics of the Dart engine.
Without a fast acting load cell and crank angle sensor you would not know where the maximum torque point is on a dyno. The flywheel dampens the torque over the cycle. Varying fuel and ignition you can tune for maximum torque during the dyno run.
Compression ratio is just the volumetric ratio of the cylinder with the piston at TDC and BDC. Cranking pressure varies with cam timing, CR, speed and leakage.
Japanese motorcycles typically run over 12:1 CR whereas american are much less due to their less efficient chamber design. It depends upon many factors. A poor chamber design will cause a partially burned mixture to spontaneously ignite the unburned fuel causing knock. The CR limit is essentially spontaneous ignition without spark which is how a diesel works.

 

[icrman]

Compression ratio - expansion ratio is locked by the dimensions. So the talk of cranking dynamic or otherwise is kind of a misnomer. But what will change is the pressures due to the variation of inlet manifold pressure. And that will effectivly ACT as a lower or higher compression ratio would in a pressure sense.
And to take an educated guess where the peak torque crank angle would be I would guess from between 45 and 90 ATC.

 

[s13sr20chris]

http://www.theoldone.com/

This guy claims he can get up to 15:1 (iirc) on pump gas in a small block. He says that it is due to his high squish heads.