mrx

Hallo,

searching on the net i found differenct formulas for calculating the effective compression ratio under boost, but they are sometimes totaly differenct, so i wan't to know which is correct?

1.) sqrt((boost+14.7)/14.7) * CR = ECR (found on a turbo website)
2.) CR*(boost+1)^0,714285714 = ECR (boost in bar in this formula, found in a engine builders book)
3.) Final Compression Ratio (FCR) = [ (Boost÷14.7) + 1 ] x CR

which one is correct???

Thanks
Malte.

The_Head

I always used #3, I'm willing to bet the other two come up with around the same results, they just use different math to accomplish it.

mrx

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by The_Head &raquo;</TD></TR><TR><TD CLASS="quote">I always used #3, I'm willing to bet the other two come up with around the same results, they just use different math to accomplish it.</TD></TR></TABLE>

Have you calculated the results of all three formulas?! then you can see that they ALL give TOTALY different results which are not within a small error... it makes a HUGE differenct.

i.e. CR=9:1 Boost 14psi

Formula 1) 12.6:1 ECR
Formula 2) 14.6:1 ECR
Formula 3) 17.6:1 ECR

i think Formula #3 is totaly false, because its IMPOSSIBLE to run an engine with 17.6:1 CR with pump gas, maybe with race gas but never with pump gas and there are many many 9:1 CR cars which run 14psi on pump gas.

Formula 1 and 2 make sense, but even 14.6:1 seems a bit high for my oppinion on pump gas. But i have it from a book used for educational propose here in germany so when its wrong this will be a big mistake...

Formula 1 makes much sense in my eyes because with a bit ignition retard i think it is possible to run 12:1 on pump gas but i have no information about this formula and only found it one time on the net so this is not a guarantee...

Hope to get some more oppinions.

Thanks
Malte.

The_Head

I didnt realize it was that much of a difference for the 3 formulas, it was just a hunch.

Static and dynamic compression are 2 different things, that's why you can run a turbo motor with 9:1 compression and 14 psi on pump gas (if tuned well)

Now if you had an all-motor car with 17.6:1 compression pistons in there... completely different story.

mrx

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by The_Head &raquo;</TD></TR><TR><TD CLASS="quote">I didnt realize it was that much of a difference for the 3 formulas, it was just a hunch.

Static and dynamic compression are 2 different things, that's why you can run a turbo motor with 9:1 compression and 14 psi on pump gas (if tuned well)

Now if you had an all-motor car with 17.6:1 compression pistons in there... completely different story.</TD></TR></TABLE>

hm, maybe you can explain the difference. Until now i though that Effective Compression Ration is more or less compareable with static compression ration in term of how much retard you need and octane is needed to prevent knock.
i know that a 17:1 Turbo Compression is not 100% the same like a 17:1 NA compression because of air mass, temperature etc. etc. but how makes effective compression ration sense if it is not compareable in a general meaning?
why can i run i.e. 17:1 on a Turbo setup and not 17:1 on a NA setup? if this is so then where is the difference?

Thanks
Malte.