Posts : 93
Join date : 2012-03-28
|Subject: Re: Bangshift Boxy Tue Nov 20, 2012 8:01 pm|| |
Thanks for the info fellas!
The 083 castings are not swirl port heads. They don't have the vane in them. Great heads they aren't but they are better than the 193 swirl port castings. I spent the day at the machine shop today doing some bowl work on these, lightly cleaning up the ports, back cutting the valves, etc. There were cracks in two of the exhaust valve seats so Jon and Dana cut them out with the Serdi 60 machine they have and blasted in two new valve seat inserts and then they machined all the seats with the five angle cutter. Heads are all back together with new springs and I should be bolting them on tomorrow.
Old timey hot rod stuff for experimentation purposes. Will be interesting to see the power changes (for better or worse!) on the dyno. All in the name of science and dumb fun.
The short block is 100% stock and has not been decked, rebuilt, or otherwise messed with. I think I am going to stick a thin head gasket in there to eek some more compression out of it. After I run it on the dyno with this setup and the headers I'll swap in the new cam, and then probably test some budget basement aluminum heads to see what works best as far as bang for the buck goes. Then there's that old plate nitrous setup in my basement from years past that is calling my name.
Duck and cover!
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Join date : 2010-11-16
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|Subject: Re: Bangshift Boxy Tue Nov 20, 2012 8:37 pm|| |
- bfurches wrote:
- 1984twodoor wrote:
- bfurches wrote:
- My guess....with a smog pumper....8.5:1
1987 5.7 VIN CODE LM1 is an 8.2:1 Compression ratio so says the FSM. Buford is an 87 right? Hope that answers your question.
Ahh damn it all! So close LOL
It would be good to take advantage of as much compression as you can get, but up to a point.
There are methods that allow considerably more CR than you would otherwise think possible.
GM's SwirlPort intake head was an attempt to add turbulence to an incoming charge of air to help delay the occurrence of detonation.
It is very similar to the L98 head, but with the "vane" or "ski-jump" ramp cast in. Problem is, it kills air-flow for RPMs in a 350 CI engine.
I like to get the piston to nearly, or barely "kiss" the quench surface area of the chamber, this helps hold off detonation quite well.
Quench thicknesses of .045" - .080" are naturally prone to detonation, and this is what the OEMs gave us in the L03&5, LM1, L98 engines, among others.
They weren't able to (or willing to) affordably hold tolerances close enough to give us .020" - .030" quench thickness, and yet, they knew better!
My formula usually includes a flat top piston perfectly flush with the deck, or even a few thousands out of the hole, combined with a steel shim, marine-type application head gasket.
At my highest RPMs, the quench area of the piston might/could "kiss" the head lightly due to "piston-rock", but not impact it until the rod journals and bearings started to really "go away" in earnest.
Iv'e found that 11.25:1 CR in a cast iron head on pump gas was no problem, as long as the 92/3 octane gas was fresh, the A/F ratio was just a very small "touch" rich and the timing (and it's curve) were set with care.
There are a lot other little "tricks" to get more "free" power, driveability, and efficiency out of a simple SBC engine package for cheap!
Use of an aluminum cylinder head allows/requires nearly a full compression point more because of it's much higher coefficient of thermal conductivity that so effectively draws away heat from the combustion chamber.
Its a good result when an engine can overcome the "ills" of an otherwise "too-big cam" at even it's lower RPMs by coaxing the air in and about the engine in an intelligent way!
My next book will be David Vizard's "How to Port & Flow Test Cylinder Heads", I had read some of his earlier books for learning techniques that were advanced for me.
The book "How to Build & Modify Chevrolet Small Block V-8 Cylinder Heads" in 1991 was hot off the press when I started to learn how to get much better results for my low-dollar Hi-Perf street engines.
The beauty of a Higher static CR is that a bigger cam that bleeds off low RPM static compression is effectively offset, yet you can still let the cam work it's magic at much higher RPMs.
Afterthoughts (11/25/12): Sometimes I forget about the common engine building techniques & procedures most shops employ, particularly when using largely unchecked OEM parts and the bare minimum of measuring steps.
Above I had suggested a very tight quench area thicknesses, but this goal must be sought only if extensive pre-assembly checking & measuring is going to be done.
Generally I am also using very good aftermarket rods, pistons, crankshafts, coupled with the blueprinted machining of all parts for precise stack heights, running clearances and oil film thicknesses in the bearings.
There is a qualitative difference between an engine at 7,000 RPM that uses Crower billet rods, a top shelf Callies crank, forged & fully machined pistons, "H" grade bearings, etc., as compared to the typical unmeasured stock LT1 parts.
When I strive for such close tolerances, I have to pre-assemble that engine many, many times using clay and precision Starrett and Sunnen mics that are constantly verified against their "standards".
When the whole engine's rotating assembly is thought to be ready for the next steps, I place strips of clay on the piston top and pre-assemble one more time, using used head gaskets of the exact type I intend to use.
The engine is turned top-side down on the stand and I use a brass rod fed down through the bottom end, and sharply tap ALL 8 pistons on both sides of the rocking wrist pin @ TDC to actually measure the quench thickness.
After impacting the bottoms of the piston crowns up toward the head, I am looking for some specific clay thickness still to be present, because as RPMs rise, things actually stretch out in the rotating assembly.
When a well broken-in engine has made good power at is intended peak RPM range, and then upon cylinder head removal I see a slight tannish area where the piston just nearly or barely kissed the head, I know all is right.
By starting out with thin steel shim head gaskets, and then after a few good seasons things have loosened up, I can always revert back to the thicker LT1 gasket, or the even thicker LT4 head gasket for more quench clearance.
I believe good engine parts are a very smart investment if reliable power is desired, but very tedious pre-assembly is absolutely mandatory, you must leave nothing to chance.
Re-using stock parts & non-blueprinting engine re-building techniques, you can probably by-pass the claying of piston tops, but if you are assembling your own engine, the cost is practically free, and you will have peace of mind.
The stock quench thicknesses are normally expected to be between .045" & .065" on most LT1s. Many shops aim for .035" - 0.040" and the pursuit of this target can not be faulted whatsoever.
Last edited by Machine-De-Zine on Sun Nov 25, 2012 7:20 pm; edited 1 time in total
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|Subject: Re: Bangshift Boxy || |