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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: onelow84 on August 29, 2012, 08:54:30 am
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Have some questions about an engine I am building and hope someone here can help me out! OK here is what I am working with...1979 350 block, I have taken to the machine shop. They checked the bore (4.00) honed, cam bearings and hot tanked it. i have a new crank, recon rods, comp xe268 cam, vortec heads, performer rpm intake and a holley 600 dual feed double pumper. the block and heads have not been cut at all. my question/problem is the pistons.... I have a set of silv-o-lite flat top pistons and noticed they have a compression height of 1.54, what I have been reading is that this will put the piston .045 down the hole. can anyone tell me what my compression ratio and quench will be? should I send these back and get some with the correct 1.56 height? I wont be turning more than 5000 rpms and wont be using any power adders but I dont want to build this engine and have it be a turd.
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The stock Chevy 350 has a deck height from the crank centerline of 9.025". The stock rods are 5.7" long. Stroke is 3.48" and the stock Pin Height is 1.545". if you take the Deck Height - 1/2 the Stroke - Rod Length - Pin Height you will get the distance the piston should be in the hole. In the stock case it is 0.040". The stock head gaskets were embossed metal and had a 0.015" thickness. This put the Quench at 0.055" stock form.
About the smallest quench distance that you can make without knowing exactly what you are doing is right around 0.040". You have to take into account what thickness of head gasket you are going to use and what it's compressed thickness is. A FelPro PermatorqueMLS gasket is about 0.026" compressed thickness. Gaskets can range from 0.015" to like 0.071" compressed thickness. I would recommend an MLS (multi-layered steel) gasket.
I would weight the cost of the new pistons vs. the cost of having deck of the block decked about 0.030".
What you would have is Deck Height (9.025-.030) - Rod Length (5.7") - half Stroke (1.74") - Pin Height (1.54") + Gasket Compressed Thickness (0.026") = 0.041" Quench.
or
Deck Height (9.025") - Rod Length (5.7") - half Stroke (1.74") - New Pin Height (1.56") + Gasket Compressed Thickness (0.026") = 0.51" Quench.
Either way I would probably have the block decked to the proper height based on the gasket thickness you are planning on using.
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thanks for the reply, the pistons are still in the box and I can return them, will a quench of .041 be cutting it too close ? what should optimal quench be?
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you reaally want to get as close to that 0.040" as you can. It doesn't make you much more power, but it reduces the octane requirement of the engine, allows you to pull-out some timing and the intake charge is more homogeneous which helps the former items and can produce better efficiency.
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Thanks for the help everyone, I have decided to return the pistons and went with speed pro flat tops with four valve relieves and a 1.56 height. To get a lower than .051 quench with a .026 head gasket can I use a .015 shim to get my desired .040 quench or would that be too risky with clearances? thanks again for all the help