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RacerX

Highest Engine Compression Ratio on Pump Gas

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I was reading engine articles on NA engines and I've noticed that a couple of the high performance engines were running 11.0:1 or 11.5:1 compression ratios. This was the new Z06 Corvette LS7 engine with 11.0:1 while the 2006 BMW M3 is running 11.5:1 CR.

 

My question is that it's obvious these cars are able to run on Premium Pump Gas. With the technology of stand-alone computer systems that are now available for use when doing engine swaps - do you feel you could get away building a high compression engine for your next engine swap. Obvious there is much more power to be had.

 

With this in mind, do you feel 12.0:1 CR is streetable? I remember in the late 60's when guys were running 13.0:1 on pump gas - but back then the octane was much higher than todays pump gas.

 

Your thoughts please.

 

thanks,

 

RacerX

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It's all in the tuning. I bet that if you had to put something less then premium in those vehicles the computer would recognize that and adjust accordingly.

 

Even the 300zxtt boosts less when ran with lower grade gas, and the 350z pulls timing.

 

There are a few Zs running around with about 12.:1 or higher.

Mario

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Guest Mike

RacerX

 

Seriously... check out that link to the fuel additive. It has tetra ehtyl lead and for just $5 per 10 galons of pump gas, you can bump your octane by 16 POINTS. It's poisonous... just be CAREFUL.

 

If you only want to bump your octane 5 points, it will only cost you $1.80 per 10 galons!!

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Guest Mike

If you travel 100+ miles from home often, carrying extra additive might be necessary. Regarding carrying spare additive in an ice chest... I doubt you'd have to do that but I'm no expert.

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Aluminum heads - which both those motors feature - can take at least 1 point higher than iron. I'm sure somebody will pipe in here to clarify, but I understand heat dissipation, as well as heat distribution, make much of the difference for Al. Another point of importance, w/r/t detonation, is the dynamic compression ratio. There has been some talk about it on the forums here, but the gist is that the *actual, running* compression is always lower than the calculated number to the valves' being open during part of the compression stroke, and modified somewhat by inertia effects in the working fluid(s). So you can have a radical c/r calculated for static, but see a much, much lower actual number in operation. Grumpy is the one that clued me into this one. It's not hard to guesstimate, you just use the ratio of the volume at the valve closing point to volume at TDC, instead of from BDC to TDC.

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I'm not sure how this applies, but my 2005 Honda 600rr had a compression ratio of 12:1 and the manual recommended 87 octane. I never understood how they pulled that off. My RC-51 (1000cc Vtwin) requires premium and the CR is well below the 600, maybe 10.8:1. Does it have something to do with the cc's per cylinder?

 

chuck

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Guest Mike

chuckmorgan79

 

Dunno... maybe VVT and ECU management:icon43:

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Guest Mike

Oops... I've shown my ignorance again:redface: But yes, the ECU is preventing the ping. I'd run the highest octane I could get with that CR.

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It's all in the tuning. I bet that if you had to put something less then premium in those vehicles the computer would recognize that and adjust accordingly.

 

Even the 300zxtt boosts less when ran with lower grade gas, and the 350z pulls timing.

 

There are a few Zs running around with about 12.:1 or higher.

Mario

 

And this is why most 350z owners(like myself) are a littled p'd at the VQ. The engine is a little to smart.

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The compression ratio of the new BMW v-10(s85) in the M5 and M6 is 12.0:1.

It has very precise fuel and spark mapping(duh!) and a unique ignition system that uses the spark plug to measure in cylinder ionisation to determine combustion quality and detonation. It will run perfectly on cali. 89 octane, at a slightly lower maximum power I'm sure. It will still try to spin the tires at 70 m.p.h in third gear from 7000 r.p.m. to 8200 r.p.m.

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You don't need a stand alone computer to automatically adjust for low octane gas. All modern FI engine use knock sensors to automatically retard timing should you gt bad gas. My geo had that back in the 90's.

 

As for direct injection, that is getting much more common, even for domestic econoboxes. Detonation is not possible because there is no fuel in the compression chamber until it is ready to fire the spark. I think there are still octane limits on direct injection motors. I know the Germans used it in there aircraft back in WWII.

 

I would stay the F away from anything with concentrated TEL. Can't believe that is even legal to sell. Not to mention what it will do to your catalytic converter.

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I was under the impression that the shape of the combustion chamber, plus good quench area goes a long way to prevent detonation.

 

Our cars need like 34-36 degrees timing, whereas a honda with a combustion chamber that directs the mixture quickly to the center of the spark plug might only need 24 degrees total timing (just an example, I have no idea what timing hondas use). They become much less prone to detonation because the mixture burns so much quicker, there isn't as much time for the mixture to explode on its own.

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