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Discussion Starter #1
Larry, Chris, Tim, et. al.

Can you enlighten me as to the pros and cons of increasing the compression ratio when building an engine for the MKIV. I've seen individuals that do and those that don't. Is there a conventional wisdom with regard to this option these days?

I can't profess to being very knowledgable about engine dynamics, but it would seem that increasing the CR would improve off boost performance. Question is, does it have detrimental effects when on boost? Does it increase the potential for knock, and if so, can you run a pump gas tune on an engine that has an increased CR, or are you committing to high octane only?

Lastly, if there are palpable benefits to an increased CR, what is considered to be the best compromise of risk/reward?

I thank you in advance for any light that you shine on this subject.

Ron
 

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Increased CR =

1) More "out of boost" response

2) Quicker spool.

The only real "con" is that you would have to use a higher octane (all things being equal) than a motor with a lower CR.

BUT the way i see it, if you are running a built motor you probably will be running race gas anyway. Many, Many people are going to 9:1 or 9.5:1. Some pioneers have actually upped it to 10:1
 

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Discussion Starter #3 (Edited)
MissSupra

Pretty much what I was thinking. I was just wondering where the limits were especially as pertains to the risk of detonation. I don't daily drive my car, but of the time that I do drive it, I'm not going deep into boost frequently, because I'm not out trolling the highways looking to instigate a race. At $12/gal C16 is being wasted much of the time that I am driving. Question is, can a 9-9.5:1 engine be tuned to run safely on pump gas that will allow for less expensive cruising (say when the destination is a local car show, not a race event, or en route to an event some distance away, i.e. Vegas or Texas)? My gut insticnt is that going to 9:1 or therabout represents some potential positives without great risk of the concerns that I have alluded to. I'm sure that the CR may need to be considered when tuning the car, and I don't know if that means that you would need to further restrict boost compared to the same set-up with a stock CR, pull timing, or both. Or am I worring alot about nothing, and should just let this be my tuners concern. (I have reconsidered this last statement after reading of the recent tragedy suffered by alpha6164. I am worring, in advance, about this issue.)

Anyway, thank you for the insights.

Ron
 
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