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Hey guys, I wanted to get your opinions on something. I am building an all motor k24/k20 and was wondering what c/r do you think is the highest you can or recommend before having to switch over to e85. Also this krx will see alot of road racing, so what c/r pistons will be best suited for this build. I prefer to stay tuned on 93...

Thank in advanced.
Eli
 

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If you are in a state that uses winter blend fuel i would definitely not go higher than 12.5 to 1. You will be fine on the summer blend but winter is pushing safety. If Using e-85 you can run much higher compression.
 

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I honestly do not encourage anyone to run any higher than 11.5:1 cr on pump gas. Is it really worth it the extra 10whp for a street car vs ruining your motor? That is the question you should ask before deciding what cr you will run.

Stefan
 

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I'm in the same boat right now. I'm building a motor with 13.8:1 pistons but I want to run pump gas so ill be going down to 12.5 via a thicker head gasket. With the right tune it should be just fine.
 

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I got this motor for free and it has these pistons already. And you get shimmed gaskets it's fairly common practice. I need it for ptv clearance anyways.
 

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I honestly do not encourage anyone to run any higher than 11.5:1 cr on pump gas. Is it really worth it the extra 10whp for a street car vs ruining your motor? That is the question you should ask before deciding what cr you will run.

Stefan
Depends, I bet dynamic compression is the same or lower @ 12.5 static with some stage 3/4 cams over 11.5:1 static on little cams.
 

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I'm not saying a Ferrari is anything like a k-series...but the 458 runs 12.5 and most others are around 11.5. All on premium pump gas and NA. But the 458 is direct injection so it has high compression in mind anyways.
 

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Quite, I'm saying static CR is only half the story

e.g. With 12.5:1 pistons, dynamic CR on a motor with big cams could actually be lower than a car with 11.5:1 pistons & oem cams.
Lol...I'm saying static CR is "none of the story." Static CR is simply for people who are too lazy to properly plan and build their motor. And yes....I agree with you.

Motor builds have gotten really boring around here in the last couple years....i.e. stage 3-4 cams and 12.5:1 cr is a brainless approach to building a motor.
 

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Direct injection is very different. Your fuel can't pre-ignite if it isn't in the cylinder yet.

And like stated above, someone running stock a2 cams will have more trouble on pump then some running a bigger cam.
I'm aware that DI is very different, thus why I mentioned the 458 as an exception.

So would someone want to explain in greater detail why a motor with more agressive cams will have an easier time with pump gas than one with stock cams?
 

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I agree Onefstek 100% I say 11-11.5:1. I have seen several 12.5 motors (without direct injection) pound the rod bearings when 93 was used. My buddy's 12.5 with no timing, pings real bad at peak tq on the low cam. The reason for running higher compression is to make more power but if you have to pull timing to reduce chances of detonation, is it worth it... IMO no

FYI my friend has some large duration cams and it does not seem to do much. Aside from pulling ignition timing he runs less vtc at peak tq, which further lowers the power (which further helps reduce detonation)
 

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Doesnt most detonation occur under high load on the low cam (where the lobes are not "big")?

Another thing I think might be worth mentioning. Arent most piston dome volumes calculated using a CC of 50.7 where in reality the chambers are 52-53 CCs. So in theory a K-series piston advertised as 12.5:1 is really ~12:1
 

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Doesnt most detonation occur under high load on the low cam (where the lobes are not "big")?

Another thing I think might be worth mentioning. Arent most piston dome volumes calculated using a CC of 50.7 where in reality the chambers are 52-53 CCs. So in theory a K-series piston advertised as 12.5:1 is really ~12:1
This is exactly what I've heard too.

Will a 11-11.5cr motor with the ability to use more timing make more/same power than a 12.5cr motor that has to pull a lot of timing? Everything else (bolt ons etc) stays the same, just the difference in compression between 11cr and 12.5cr.
 
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