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Old 08-16-2010, 03:02 AM
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Compression - Cam Dilemma

Hey, asking this on behalf of my mate whom I'm helping build his car. Its all done but the motor. He is currently not working so funds are an issue. He bought this motor complete, supposed to be rebuilt hot 350 chevy. It has brand new performer rpm heads 64cc 170cc runner. RPM airgap intake and 750dp holley. Unknown camshaft. ( was the reason he stripped it down) stock bottom end, dished std pistons. 0.041 gasket, block hasn't been milled. Now the dilemma.
He bought a gasket set with steel shim gaskets not realising they aren't recomended with alloy heads. He has a xe274 cam he wanted to put in it (230/235 duration .490ish lift)but from my calculations the static compression is low to mid eights and that cam prefers 9-10 static comp. Squish/quench area is .066. How safe is that? Is there anyway he can use steel shim gaskets? He doesnt have the money to deck the block at the moment. I told him to put a smaller cam in it to suit compression better as I have a wee Howards 214 duration cam here. So how would the xe274 cam run with low 8's compression compared to running the 214 duration howards cam with the same compression. Also can standard flattop pistons out of another engine be fitted? Any help with cheap tricks to either build around the low compression to make it run good, or cheap easy ways to build compression with alloy heads. Or at the end of the day does the block need to be milled? Hopefully that makes enough sense. Thanks
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Old 08-16-2010, 03:45 AM
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Anytime you use a steel shim gasket- even the rubber coated ones- the decks have to be flat, w/a smooth finish. The steel shim gasket doesn't provide any "give" for the different rates of expansion between Al and Fe- so I wouldn't recommend using them.

I think you need to recompute the compression ratio- even w/a 0.025" piston in the hole, a 0.041" gasket and a 8cc dish he still has about 9.5:1 CR, more w/a thinner gasket. A 0.031" gasket will be about 9.7:1.

There are composite gaskets thinner than 0.041", if I HAD to work w/the parts he has on hand I would find the thinnest composite gasket I could and use it.

Bogie has said: "GM 10105117, this baby is a multi-layered stainless steel gasket with a 4.1 bore, it's .028 thick, works with iron or aluminum heads, good for holding back high compression and tolerates some surface irregularities in the deck and head surfaces." Someone also said this gasket is closer to 0.021" compressed (9.9:1 CR)- of this I have no empirical knowledge, but would be worth researching.
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Old 08-16-2010, 03:50 AM
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Hi thanks for that. Problem over here is limited range of gaskets, normally we would have to buy them from over there on jegs or ebay and end up costing hundreds by time they arrive. Are the dished pistons only 8cc? My flat tops are 7cc and the dished pistons look like they would be pushing 20cc thats what I used in calculation, and got 9.4 if we could find some standard 4 relief valve flat tops. So no to the shim gasket with alloy heads but will start hunting for thinnest compatable gasket.
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Old 08-16-2010, 05:22 AM
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I have no idea of what dish volume you have. 8cc was the volume I had down for a shallow dish w/4 reliefs. It is simple enough to measure what he has so there's no guess work, but in any event, it will take a deep dish to be 20 cc.

A Vortec dished piston has an ~11cc dish volume from 4 reliefs and 0.080" depth as an example.

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My flat tops are 7cc and the dished pistons look like they would be pushing 20cc thats what I used in calculation, and got 9.4
I think you mean 8.4:1, not 9.4:1 in your CR calculations.

If it turns out that the choices are a bad quench and low compression, use the smaller cam.
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Old 08-16-2010, 03:12 PM
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Install the heads with the shim gaskets clean and dry. Retorque after two or 3 full heat/cool down cycles. Do not go WOT untill you have done the retorque on the head bolts. Install the XE274 cam further advanced from cam card specs to compensate for the low cr. Degree it in and install on a 103 intake c/L.
Thats advancing it 3 more degrees that what the cam card calls for.
Don;t guess, degree it, then move it.
Intake will now close at 38 ABDC (.050" lifter rise) instead of 41.
After the retorque and break in it willl be fine.

recurve the distributor for 24deg at idle 36deg at full advance
(shorten the mech adv curve to 12deg)

and 10-12deg of vacuum advance at high cruise.

Typical stock (other than the L-31 vortec motor) gm 350 sbc dished pistons are 15cc. Usually .025" down the bore.
Rebuilder pistons may be further down the bore at tdc.

Use a good coolant/antifreeze and colling system corrosion treatment when using steel head gaskets with aluminum heads.
You'll be fine.
the cr willl be 9.2:1 stock bore and .025" piston deck clearance
9.3 if its 4.030" bore.

use a 3000 stall converter 3.73-4.10 gears with this cam.
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Old 08-21-2010, 05:51 PM
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Thanks for all that info. Although it does sound a lot more complecated. We fully stripped tthe motor yesterday and done full measurements. Not good. First things we noticed was a pushrod that was stuck in the head/guideplates. It wasn't moving anywhere. Had to take the head off with the pushrod still in it. Is this a common problem with the rpm heads? It had a roughly 50 though head gasket with roughly 15cc dished pistons and standard deck height. Pistons didnt sit flush with deck surface either. Rust in the bores. All bores except one measure between 3 and 7 thou wear, but the number eight measured 30 odd though. This sin't normal and I don't know what would cause this. This bore also had what I would describe as buldges in the bore. As I said, bores were slightly rusty from sitting but these buldges were shiny silver and could easily feel them with finger. Was as if the metal had swallon? Now he is stuck on what to do? Would anyone out there know what would cause such excessive wear in one cylinder only and the high spots in the bore (roughly 2 1/2 inches longhorizontally 1/2 inch tall? was also on another bore but not as extremeas this bore. Thanks
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