Odd combination, question about compression.
Thread Starter
Drifting
Joined: Mar 2008
Posts: 1,756
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From: Wichita KS
Odd combination, question about compression.
A friend is asking me what kind of compression this combination has. He purchased a pair of 305 heads with 58cc combustion chambers before figuring compression. This is to be a street car, pump gas. He's also asking me what cam he should run.
"They are forged '70 LT1 standard bore pistons with a 16th of a inch milled off the top to leave a dish. It is a 307 block.The block has been punched out big enough to accept the 350 pistons and the bottom of the block has been ground on to allow the 400 crank to fit. The rods are 400 rods and the whole assembly has been balanced. No, the block has not been put together yet,,"
Anyone have a clue?
"They are forged '70 LT1 standard bore pistons with a 16th of a inch milled off the top to leave a dish. It is a 307 block.The block has been punched out big enough to accept the 350 pistons and the bottom of the block has been ground on to allow the 400 crank to fit. The rods are 400 rods and the whole assembly has been balanced. No, the block has not been put together yet,,"
Anyone have a clue?
Burning Brakes
Joined: Nov 2001
Posts: 1,189
Likes: 2
From: Austin TX
So, assuming a 4.00" bore, 3.75" stroke, 58cc combustion chamber head, flat top piston with a 6cc valve dish relief, the pistong .020" in the hole at TDC, and a .040" compressed head gasket, I'm getting about 11.1:1 compression ratio.
That 307 block has a 3.875" bore originally, which means it had to be bored .125" over for those pistons. I would be concerned about the thickness of the cylinder walls. Have they been sonic checked?
That 307 block has a 3.875" bore originally, which means it had to be bored .125" over for those pistons. I would be concerned about the thickness of the cylinder walls. Have they been sonic checked?
Thread Starter
Drifting
Joined: Mar 2008
Posts: 1,756
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From: Wichita KS
Thank you for the info.
I have no idea what he's doing and why he chose this path. I think he had a turbocharger planned for it and a pair of 441 heads (76cc chambers). I recall seeing that on a table one visit. I sold him a standard bore 454 block with a standard crank for $35 a few years back because he wanted to put a big block in this car. He traded it off for something else. Kind of wished he would have mentioned he wasn't going to use it so I could have it back. But he's a friend....
11-1 doesn't sound too bad with 91 octain. 36 degrees all-in @2800rpm should work for timing. Maybe he can use a thicker head gasket?
I have no idea what he's doing and why he chose this path. I think he had a turbocharger planned for it and a pair of 441 heads (76cc chambers). I recall seeing that on a table one visit. I sold him a standard bore 454 block with a standard crank for $35 a few years back because he wanted to put a big block in this car. He traded it off for something else. Kind of wished he would have mentioned he wasn't going to use it so I could have it back. But he's a friend....
11-1 doesn't sound too bad with 91 octain. 36 degrees all-in @2800rpm should work for timing. Maybe he can use a thicker head gasket?
Thread Starter
Drifting
Joined: Mar 2008
Posts: 1,756
Likes: 0
From: Wichita KS
Ok, more info....
"this engine already has 500 miles on it but had a bad viberation so I pulled it back apart. What the problem was,the engine builder didn't rebalance the crankshaft after turning down the mains to fit in the 307 block. SO,,,all this time that I've been telling you about my 377 stroker,,,"
He's not happy that I don't remember his combination and edited out the last paragraph. I do recall taking it for a test drive to see if I could sort out the vibration. 400 cranks are external balanced and I remember looking at the harmonic balancer.
"this engine already has 500 miles on it but had a bad viberation so I pulled it back apart. What the problem was,the engine builder didn't rebalance the crankshaft after turning down the mains to fit in the 307 block. SO,,,all this time that I've been telling you about my 377 stroker,,,"
He's not happy that I don't remember his combination and edited out the last paragraph. I do recall taking it for a test drive to see if I could sort out the vibration. 400 cranks are external balanced and I remember looking at the harmonic balancer.
