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Tyler06

Comp Ratio changing differences?

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Hiya all, just wondering what sort of difference i will see/feel by dropping my comp.. My engine had 11.8/1cr but i am dropping it down to about 11.1/1cr with the new block. Only reason being that i want the cast piston to last a bit longer whilst i build a new bottom end which will take 12-18mnths. Still wanna get times from this one then compare ;-) much of a difference ya reckon?

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I personally think there will be stuff-all. I reckon you're still in the danger zone for cast pistons, that is if you're not running a lumpy enough cam to bleed off some compression. I'm guessing though that you don't have such a problem with detonation as you would have mentioned it.

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I do have a pretty lumpyish cam lol but when it comes to casts and there limit it has more to do with wat comp and the detonation factor as far as i get it. Inc running too lean etc. my engine ran beautiful with my last tune but i had dramas where it would only start on 26* advance. Had it ramping up to 32* and it didnt ping. I stripped my wiring harness and found that my dizzy power supply was only supplying 9v. Change that and then it would start on 12* but hated anything after 26* and pinged real bad but up until it pinged it felt so much more healthier and smoked my tyres like they were 155s. My stall is rated at 4400 but it just went from 2000- 4200 in a heartbeat and smoked. I am changing my castys in my new bottom end but for now they need to last so im only dropping my comp to stop the pinging.

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out of interest what are the limits on casts?

It's all relative, as in what cam and fuel you're running but for a mild-medium cammed streeter running shitty 91 fuel I wouldn't go above 10.5:1 with cast pistons.

 

That's just me, as I like my engine to hang together for 300,000 k's and not do any surgery. I think i'm getting old. Some are happy freshening up an engine every 50,000 so and don't mind running on the ragged edge when it comes to cams and compression, etc. Horses for courses really.

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It will all come to a head wen i get it all back together. It seems ive been failing since the first time i started this thread. Def hope it works out this time!

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