I have tried to search here, google etc... nothing was honda specific.... mostly harleys...

How does one determine the compression ratio where you can run optimal spark advance being octane limited?

There is always the option of adding too much compression and pulling timing to make the same peak HP with the same octane, but that is a poorly designed setup.

For example:

I am building a B20 vtec with a PR3 head. Locally i have access to 91 octane at the pump. I am trying to maximize the power output.

How do i determine the optimal compression ratio to make the most power without compromising spark timing?


Cheers,
Matt

 

Most people run 11.5:1 with good results. I would suggest talking with your tuner and see what he feels comfortable with.

 

I forgot to mention... the example was not intended for people to tell me what to run... it is to discuss the theory.

Edit: There is an optimal zone or angle of crank angle to have the ignition fire, and this would be ideal. too high of compression and you cant hit this point of angle to maximize output... your merely making power from the fact your compression is too high.

 

I believe you want peak cylinder pressures to be something like 12-16 degrees after TDC. Getting the spark timed just right to get this is the tricky part...So in essence its not so important as the timing of the spark, but the timing of the peak cylinder pressures during combustion, aka MBT

Static compression is one thing, but the dynamic compression is the more important factor since it factors in camshaft timing...Hence why a 13:1 compression engine wont ping with large cams but may with smaller/ less duration cams...

(waiting for EG1834 to come in and school us)

 

I found a great link up by Q16racer/Blake/98vtec on another site...

http://www.superhonda.com/forum/f55/...across-268106/

 

I know all about dynamic compression but left that out for discussion purposes to help determine the way to choose a compression ratio to shoot for static. Machine work and piston selection will be based 100% on the calculations.

 

Correct about the first part. Peak cylinder pressure should be around 15* ATDC for MBT but timing will be backed off for longevity of the motor, with minimal power loss.

I keep hearing this debate about cams and dynamic compression, but cams don't bleed off compression like most people think. More info on the subject would be nice.

Static compression should be factored in imo. I'd think that static compression is just as important as dynamic. Without compression, the internal combustion engine would be useless. Less static compression should lead to more timing advance. But if you have inefficient compression than you should rebuild or replace your motor, not advance your timing.

 

I always thought you want to tune ignition for MBT on a dyno and thats that. Going past that point yields no more power and eventually if you advance far enough,you will encounter detonation.
So youre saying once you reach MBT, you should back off a little for engine longevity?
Im still a beginner at tuning and appreciate different opinions of the way to tune.

 

Last I heard to tune use timing as far as you can making power, then pull it back to 1HP less.

 

I attended an EFI 101 course a few years ago and the instructor/tuner demonstrated how advancing ignition timing affects torque/MBT on the dyno...

He then continued to advance timing slowly showing that you can go quite a bit past MBT until you reach the knock threshold. The window was surprisingly large. I believe it was said that going past MBT causes excessive and unnecessary stress on the engine even before you start to see MBT drop. Of course once MBT starts to drop, you know for sure you are too far advanced...

From my understanding, tune for MBT unless you are super close to the knock threshold, in which case you need better (higher octane) fuel.. If you are limited as to what fuel to run, this is when you must retard timing to give you a "safe zone" away from the knock threshold...
If your too high of compression for the fuel your running, you may have to retard timing excessively as to stay away from detonation even though you are far below where you should be for MBT...
Anyways, I guess Im kinda getting off topic here...

 

I didn't realize you can advance that much after MBT; depending on fuel obviously. Good stuff.

 

I highly recommend the EFI 101 class taught by Ben Strader. I learned alot about ICE and theoretically how to tune them. He goes over the basics like sensors, inputs, outputs, etc, and then gets into VE, cylinder pressures, MBT, tuning on a dyno, what to look for as far as EGT, AF ratios, etc.
After I left, I felt much better about "tuning" my own car. Of course you can only do so much w/o a dyno. You can get pretty close with a "street tune" but tuning for MBT is the key and the only way to measure that is with a dyno...

The car being demonstrated on was the (then) ~ 600 whp twin turbo 350z shop car at Mainstream Performance...

Lots of good info here http://www.efi101.com/forum/

 

I have his book at home, definitely a must read.

 

I don't have his book but I have a couple others. They're a must for anyone looking into tuning. I might take the class next time it's in the northwest.