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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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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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dv, that would give me 9.31:1. what would that do for me...being 015 down vs. 030?
 

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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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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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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/techarticles/94138_piston_head_clearance_guide/

Jordon
 

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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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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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Discussion Starter #10
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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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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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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Discussion Starter #13
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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ap72 said:
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 :D

K
 

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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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Discussion Starter #16 (Edited)
i could but.... you know me.

i hear a LOT of people talkin about how they are running like 9.7 or 9.5 on pump gas. yes i know they are talking premium or 89 octane. if not i DONT want to hear about it cause i aint gonna believe it. rated factory c.r. has NOTHING to do with REALITY.

that being said. if i zero deck my block with my 13cc pistons and 75cc heads i get, 9.6:1 c.r. i have been told by many others who i trust that i can't run that kind of c.r. with iron heads on pump gas. i know it wont run on regular, but if it will run on primo i will do it. if i have my 9.6, it will be with a 040 quench, which is optimum, so it should not detonate, esp if i go with a cam with alot of duration, and overlap, to bleed off the dynamic c.r.

since you guys are some of the sites heavy hitters in my book, i would like to know what you think about this 9.6 with 75cc heads. i have used the c.r. calculator at www.sdperformance.com to come up with my calculations. i have only measured one side of one piston to get my depth. my machine shop guy, who think's i'm crazy btw, wants me to measure all four corners of my block first before i tell him how much to take off.

know what? i just pretty much got convinced that 9.6:1 WILL run just fine on pump gas, be it premo or possibly less, so that's what i'm doing. i am zero decking my block to get 9.6, using my cast iron 061 castings at 75cc's, maybe less after they get trued, and going with it. i don't have any idea why people tell somebody ELSE that they can run higher c.r.'s than i'm asking about, and then tell my i cant run 9.6 (not this site) but still, i am po'd. i know everybody and their DOG runs a sbc with true flat tops and a 76 cc chamber and is right at 9.2 : 1 and running crap 87 octane, i don't see why 9.6 cant run great on 91 octane or less
 

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Just a general statement.. Not directed at anyone.

There is a lot of hype about compressions and fuel cost.

How much is the difference between 87 and 91 in fuel cost per year of your driving? $.20 per gallon for how many gallons per year?

Let see..... 10 mpg for 4000 miles is 400 gallons, times 20 cents......equals..... $ 80 ....,, yes eighty dollars per YEAR..... :thumbup:

Be realistic. If fuel price makes a difference maybe you should consider a non-hot rod.

I'd look into this link..... and use .040-.050 quench.

http://www.hotrodders.com/forum/singhs-grooves-ported-milled-vortec-heads-stock-350-a-102958.html
 

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Personally I would run as thin a head gasket as I could with the pistons as-is. A steel shim gasket of .015 might be possible and get you into good quench.

Remember, anytime you mill the block deck height and or the head surface, the intake manifold will be too wide. Dropping off .032 or more might be an issue.
 

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Discussion Starter #19 (Edited)
you have a good point, both of you. and a good idea with the gasket although i have been told my many that 455's were most all zero decked from the factory. same intakes, same heads for the most part, so everything should fit fine.

that being said there is a guy i trust who works for pontiac told me today he can run 10 to one on the street depending on alot of things like stall and gear so there is more to it that i know about, i spozed anything is possible. it could be above posters could have bought their car from someone who went out of their way to prep the hell out of their engine. i know i bought a chevelle from a drag racer and nobody could figure out why it was so damned fast. and yet it ran if i wanted, on regular too and i know it would slaughter my 9.2:1 350 i had in my camaro with a 262 cam. but his had a big 510 lift cam in it. i know a very high duration will bleed off alot of the dynamic c.r. and that is one of the tricks to do. about 88 degrees of overlap and 112 lobe center will be like the ra iv cam i decided i am putting in my 400
 

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I totally agree with the steel shim head gasket idea. Zero decking the block and a thick gasket compared to stock deck and a shim ends up being near identical piston to head clearance, why spend the money if not needed?

I run 10:1 compression in my 95 Suburban with a 0.030" deck height, 58 cc chamber iron head and a 0.26" thick gasket. Running on 87 regular with the stock camshaft the extra compression helps performance and fuel economy. I'm using the groove idea xntrik references on post number 17 of this thread.
 
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