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Discussion Starter #1
Hello again
I've just got details from Scat Pistons in their kit.
KB 718 are Flat Top listed with 4.9cc volume
This will bump up my CR a little more... Will I still be OK with this ???
Using CC XFI 280 Cam (69 ABDC) gives
12.42 w 0.040 gasket - DynCR= 9.56 199.89 psi

IF i thicken the gasket I get the following
12.24 w 0.045 gasket DynCR= 9.42 196.16 psi
12.07 w 0.050 gasket DynCR= 9.30 192.98 psi

Is increasing gasket thickness a sensible option or is reducing quench making things worse ?

Thanks
Mitch
 

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I made a few assumptions here: zero deck, 55cc chambers, 6" rods ...??

I certainly wouldn't give up the quench. I consider it a bad move to give up any amount at all for a marginal decrease in CR.

Do you already have the cam? willing to re-consider? The XFI292 would work great with that 12.4 compression, and bring the DCR down to about 9.0.
I can understand the XFI, with it's lower overlap, if you need good manners, less exhaust stench, etc. But, if not, consider a custom grind with more overlap (unless you have some outrageous cylinder heads that wouldn't respond well to the increased overlap). Just remind them one of your objectives is to limit low/mid rpm cylinder pressure because of fuel/detonation concerns.
And......by all means, get it dyno tuned when running that much compression :)
 

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Discussion Starter #3
I made a few assumptions here: zero deck, 55cc chambers, 6" rods ...??
Yes
Nothing purchased yet, still researching.
AFR 195 heads
I certainly wouldn't give up the quench. I consider it a bad move to give up any amount at all for a marginal decrease in CR.
That's my instinct but can't find anything to back it up

Do you already have the cam? willing to re-consider? The XFI292 would work great with that 12.4 compression, and bring the DCR down to about 9.0.
I can understand the XFI, with it's lower overlap, if you need good manners, less exhaust stench, etc. But, if not, consider a custom grind with more overlap (unless you have some outrageous cylinder heads that wouldn't respond well to the increased overlap). Just remind them one of your objectives is to limit low/mid rpm cylinder pressure because of fuel/detonation concerns.
And......by all means, get it dyno tuned when running that much compression :)
No cam yet. I erred on the side of caution and bought the XFI 268 last year but I feel the dynamic CR would be too much.
I'm keen on the XFI 292 but still trying to research tuning to pass the UK emission (sniffer) test. I havn't found any definitive information that would limit my cam specs.Gm High Tech (Aug 09) did cylinder Head tests with a XFI280 and made 500HP

Thanks
Mitch
 

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The AFR195's can be ordered with between 55cc-65cc, so that's where I'd be fine tuning my DCR, and keep my quench at 0.040" (or lower).

I started a thread a year or so ago on here about maximum DCR on pump gas and the result was inconclusive. Although there was one report well over 9:1, that turned out to be an error on his part (he was using 0.050" valve events) and his actually turned out to be below 8:1. Most SBC designs would caution anything over 8.5:1, but I'm sure you can get higher with reverse cooling, just not sure exactly how much.


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Firehawk is right. You've not purchased the heads yet, so your options are still wide open.
In fact, unless you're getting a heck of a deal on the AFR's, I'd look elsewhere to get better bang for the buck. "Out of the box" AFR's are not too impressive, and their fully ported heads are E X P E N S I V E !
Have you checked out head & cam package options from Advanced Induction or Lloyd Elliott? Especially on the heads, you can't beat their "HP per Dollar".
Be careful of the magazine reports of AFR heads; AFR is a big paying advertiser ;)

If you have emissions testing concerns there, I'd steer away from the XFI292cam. If you have good tuning resources, the XFI280 in a 383 would pass most States' emissions requirements over here.
 
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