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Old 11-28-2006, 11:36 AM
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how bad really, to be 032 in the hole?

for my 66 tempest, 400 pistons are down 32 thous. will this affect me much? only going with a ra III or so cam, maybe a used 230/238 at 050 on 108 ls. not sure of the overlap. but with a lousey quench, 9:1 c.r. not very rad cam, would it really hurt if i didnt zero deck my block. zero decking would mean i would have to toss my heads (75cc)

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Old 11-28-2006, 12:19 PM
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If it were me, I would just have it decked about .015 this would knock it down to around .015 in the hole. Since it wouldn`t remove much, the compression wouldn`t go up much either.
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Old 11-28-2006, 12:55 PM
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dv, that would give me 9.31:1. what would that do for me...being 015 down vs. 030?
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Old 11-28-2006, 02:03 PM
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I would deck the block and then take a little out of the chambers if necessary.
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Old 11-28-2006, 07:15 PM
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Wouldn't it be less likely to detonate with the pistons up a little closer? I think I'd do like doublevision said........try to get about .015 closer......just my 2 cents
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Old 11-28-2006, 07:36 PM
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bull, if possible, try to get the quench to .040-.045. That will be best for detonation resistance. The 421 HO I built for a customers 64 GTO was 0 decked and used a .040 head gasket. He is running 10.98-1 on premium pump gas. If you can deck it .015 and use a .025-.035 gasket, you should be just fine with 9.3 and a RAIII cam. Good Luck on the build.

Barry
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Old 11-28-2006, 07:52 PM
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If your quench is tight you can actually increase your compression and reduce detonation at the same time. I decided to push the limit when I built my engine, My Quench is .037 at 9.3 compression. I wanted the tight quench to reduice the chances of detonation with Nitrous.

I would zero deck it and run a .039-.041 head gasket. This is pretty good article I found http://www.chevyhiperformance.com/te...earance_guide/

Jordon
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Old 11-29-2006, 09:01 AM
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Bull, it probably wouldn't ping either way with premium gas and proper tuning. A tight quench, 0.040", would give you a few lb.ft. through the bottom half of the curve. More squeeze and more efficient combustion = go faster.

Larry
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Old 11-29-2006, 10:45 AM
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premium? I wouldn't run more than 87 octane on it as long as you keep it cool.... It's only 9.3:1... I run 9.7:1 with my Olds 350 with iron heads year round without a problem.
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Old 11-29-2006, 11:04 AM
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okay, i'll zero deck the ***** and start grinding out the area between my pistons 8 eyebrows. leave my chambers alone, see what i end up with. i have to put this build on semi-back burner anyway cause now i feel the need to upgrade my 4x4's corporate 10 bolt front end to 30 spline axles and posi and steeper gears front and rear for my work truck. i have an ad out jan first in the yellow pages, got to have a truck that will go anywhere before i build this engine, that basically, i dont need. but i have had so many people tell me to zero deck it.
as for your olds having 9.7:1, ap72, is that what you figured ACTUAL compression out to, or is that factory ADVERTISED c.r. i hear they are not anywhere near the same. also, are you zero decked, and if not, how far down are you, how thick of a gaskets etc? thanks.
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Old 11-29-2006, 11:15 AM
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The olds probably has better quench, so it runs better at higher compression.

I keep up with Jeep stroker sixes. The budget build is throw a 72-91 4.2L (258) crank and rods in a 4.0L block. With stock bore this produces 4.5L, 0.030" over 4.6L (like mine), and 0.060" 4.7L (but you have to sonic check the block before boring this much!). The stock quench height on the 4.0L is around 0.060" (piston 0.020" or so in the hole, head gasket about 0.040" thick compressed). With the oversize 4.0L pistons and 4.2L crank/rods the quench is about 0.080". Static compression with this setup is around 9.7:1. The computer doesn't like anything over 9:1 or so (stock EFI computer). So people run high overlap cams to bleed off a bit of pressure, or they dish the pistons more. The quench height hasn't been nearly as important as compression. Some people dech the block to 0.0" deck height (top of piston at top of block), and run thinner head gaskets (0.035" or so) to get the maximum quench. This sends compression up to 10:1 or more, so they have to get custom made pistons -- can't dish a stock replacement that much! The guys who are running higher compression and zero deck height blocks get 10-15 hp more than someone like me with a budget engine. They spend another $1200 or so to get it though.

As far as running, we've found that a big quench number isn't detrimental to performance. To much compression is. If you have to have the most power you can squeeze out of the engine, deck the block and order custom pistons. For the V-8 that will be a bit more than for an I-6 -- $1500 or so over the cost of a rebuild leaving the quench alone. If you're keeping compression down to around 9:1, you should have no problems at all. If you want to run a little more compression, you need to spend the money on getting a better quench. At the lower compression the edges of the piston don't heat up enough to cause preignition. Higher probably will.
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Old 11-30-2006, 07:34 PM
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Would the fact that the Pontiac's rods are almost an inch longer than a SBC's mean you'd need a little more clearance with the Pontiac? Seems to me the Pontiac's rods would get proportionately longer when they're hot, necessitating a little more cold clearance than a Chevy would need.
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Old 11-30-2006, 08:37 PM
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that makes perfect sense, however, my understanding is that most every 455 pontiac is zero decked from the factory. i have heard the 400's range from .030 to .050 down. quite a diff
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Old 12-01-2006, 07:31 AM
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Quote:
Originally Posted by ap72
premium? I wouldn't run more than 87 octane on it as long as you keep it cool.... It's only 9.3:1... I run 9.7:1 with my Olds 350 with iron heads year round without a problem.
9.7 on regular gas is pretty far beyond the rule of thumb. There are a lot of variables here and I dont think its a good idea to suggest running 9.7 on 87 octane is ok as a general rule. The weight of the car, gearing, timing, carb tuning and engine type all play a part in this.

Also, sometimes folks talk about running 11:1 on 91 octane (we used to have a guy on here who would go round and round with you about how he could do it) and really you can't take their motor down and measure, but when you get in there 11:1 probably ends up being more like 9.6:1

K
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Old 12-01-2006, 07:54 AM
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I wouldn't sweat it too much Bull. I would measure both sides of the pistons to get a true measurement.

If you CC everything, and you are over 10.0:1, then I would work some out of the combustion chambers.

With the short camshaft timing of the RA 3 cam, will like a lower compression ratio. Pontiacs have smooth machined chambers and I think you'll be happier with the torque produced by the smaller cam.

I think it's a non-issue.

Put it together. You can always change the cam later if need be.


Nairb
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