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cris well 09-28-2011 04:09 PM

SBC 383 with 64cc Vortec heads and flat-tops
 
I'm thinking about buying an 84 C-10 SWB with a 383 and a turbo 350 trans and a 3.42 rear end.

He told me that he used flat tops and 64cc RHS Pro-Torker Vortec heads with 1.94's and 1.50's. He said the compression is 10.5 to 1 but that seems low to me with this combo.

I'm thinking it would be closer to 11 or 11.5 to 1 without knowing the deck height and overbore. It has an Isky flat hydraulic cam with 236 duration. I looked it up and it's an oval track cam with a range of about 3000-7000 rpms.

He said it runs on 92 octane pump gas.

My question is did he have to set the timing really low to avoid detonation therefore cutting the power way down?

Thanks

ssmonty 09-28-2011 04:38 PM

I'm no pro, but with a cam that big(I assume 236*@.050" lift) it should have an intake closing point about 70* ABDC giving a dynamic compression around 8-8.1:1 if I'm not mistaken. I'm figuring its got 4.030" bore, 0.039" gasket?, stock deck?, piston .025" in the hole? to get about 10.5:1CR. It should run ok without detonating if my math is correct? How well it runs in a truck is another story. I sure wouldn't recommend towing anything!
ssmonty

BigEd36 09-30-2011 11:04 AM

I figured with .041 head gasket thickness (the good ol' Felpro 1003) and got static CR of 10.29. Using 284 advertised duration and 106 LSA (specs from Isky's website) gives an intake closing point of 68* ABDC and an 8.05 dynamic CR, so this agrees pretty closely with ssmonte's figures. With a stock deck height of .025 and these thicker gaskets your quench will be .064 or .066. That would make it more detonation prone, so might need some timing pulled out. With the stock deck height but built with the thinner .015 shim gasket for a better quench, I get a static CR of 10.98 and 8.59 dynamic CR, which is pretty high for pump gas, there's a good chance it would need some timing pulled out. If the block was 0 decked with this combo your static CR would be 11.7 with a 9.13 dynamic CR, I wouldn't run that on pump gas.

With a setup that isn't "coming on" til 3000 it'll need a pretty high stall converter and lower gearing or it will be a pooch on the street. And then it'll need a good-sized tranny cooler to make it live. As ssmonte pointed out, towing anything with this setup will be a real exercise in frustration. A more street friendly combo would have been to use an 18-cc D-deck dish piston with a shim head gasket for a tight quench, your static CR would be right at 10:1. With this combo something like an Isky 270 Mega cam would put your dynamic CR at 8.21, should run great on 92 octane and be much more street friendly, especially if you're running a stock converter with your 3.42 gears.

ap72 09-30-2011 12:04 PM

Quote:

Originally Posted by cris well
I'm thinking about buying an 84 C-10 SWB with a 383 and a turbo 350 trans and a 3.42 rear end.

He told me that he used flat tops and 64cc RHS Pro-Torker Vortec heads with 1.94's and 1.50's. He said the compression is 10.5 to 1 but that seems low to me with this combo.

I'm thinking it would be closer to 11 or 11.5 to 1 without knowing the deck height and overbore. It has an Isky flat hydraulic cam with 236 duration. I looked it up and it's an oval track cam with a range of about 3000-7000 rpms.

He said it runs on 92 octane pump gas.

My question is did he have to set the timing really low to avoid detonation therefore cutting the power way down?

Thanks


those "magic dynamic compression calculators" aren't worth NEARLY as much as everyone gives them credit for. 11:1-ish in a heavy truck with a highway gear (assuming 28" or taller tires) is going to have problems running on pump gas. With some very good efi controls you can get away with it but I wouldn't want to risk it on a carbed engine with a mechanical dizzy. Knock it down to 10:1 and while you still need a good tune it is MUCH more doable.

And to think that extra point of compression and all of the headaches it brings is only worth about 12hp under perfect conditions...

cris well 09-30-2011 12:41 PM

Yea I agree. Too much of a headache.

I started out by looking at crate engines on ebay and then I found a whole truck for about the same price as the crate engine.

The more questions I asked about it though from the owner and you guys leads me to think I'll pass on it.

I'm in the same boat with a motor that I already have all the parts to put together that I was asking about in another thread.

350 4 bolt that is .030 over with 4-valve relief flat tops and 64 cc double humps. Fresh with screw in studs, guide plates, and Z28 springs good to about .550 lift (I think).

By my calculations, I'm gonna be over 10 to 1 with a zero deck height and thin head gaskets to get some quench. And from what I've read it's more prone to detonate with more deck height and a thicker gasket because then there is room for the gas to run off to the side of the flame and detonate.

Is this the case?


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