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Old 10-04-2006, 07:06 PM
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Built a High Comp chevy by mistake ... need help

hi all

my buddy built a high compression by mistake

the engine is 1996 LT1 << we got it really cheap
LT1 aluminum heads
4.02 bore
Speed Pro H345NP20 pistons ( Flat-top 5cc )
GMPP LT1 carb intake
EDL-1407 750cfm performer carb
comp cams Xtreme 4x4 cam 12-239-3 ( Yes it is Hydraulic flat tappet )

before building the engine he sent the heads to be resurfaced
but he didn't know how much they shaved so lets assume it is 50cc-54cc ( since stock LT1s have 58cc chambers)
those pistons were used because he usualy use them on normal SBCs ( not LT1 like this one )
this yeilds a static compression between 11.4 - 10.8 and a dynamic compression higher than 9.5 which definitly won't work with local pump gas (95 octan)

he doesn't want to change the pistons
but he will remove the heads and will slightly enlarge the chambers by porting
also he wants to change the cam to get a lower dynamic compression

the engine is on a 1986 Chevy k10
stock 10-bolts (rear+front) 3.08 gears
32 tyres
and is used mainly in sand and dunes

what is your advice?
and what cam do you recommend ?

your help is appreciated
and sorry for my crappy English

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Old 10-04-2006, 07:21 PM
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with the compression, have you considered shims? they replace the factory metal that was milled away when you shaved the heads. what you get is the starting point of the heads. with the cam, since its in a k10, i would go with as much torque as possible, get a good cam for off-roading and you should be set. with trucks bottom end is the way to go! ADAM
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Old 10-04-2006, 09:57 PM
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more better spark

put lots o money in spark delivery. problem solved. or change heads biger cc lower comp.
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Old 10-05-2006, 12:56 AM
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If it were an automatic I'd say run it with a very conservative timing. Since its a manual you'll probably run into trouble. There are several head gasket thicknesses available, try running a thicker head gasket. Another expensive option would be to run a high-end EFI setup that retained the knock sensors. Stock LT1s run as high as 11:1 on 87 octane. My iron-head LT1 runs 10.5:1 with a 191/196 cam making low RPM cylinder pressures super high and I never hear detonation on 87 octane.

Two things going for you are the modern combustion chamber helps quench and swirl which (on those heads) helps allow it to run another .5 compression or so. Then the aluminum helps with another .4 -.5. The knock sensors, MPFI, and very sophisticated spark map take care of the rest. But when comparing an LT1 to a typical old school V8, count on a buffer of at least .8 more compression for a given octane. Whereas it used to be a max of 10:1 on old school iron heads and 95 octane, consider 10.5 or 10.8 a little more realistic on an LT1. You might be fine with just a careful timing curve.

There are other options. Retard the cam a little, run water injection, run a bit cooler plug (which you can get away with at that compression anyway) use a 160 thermostat (which I don't recommend ANY other time except on LT1s since they are reverse cooling), and jet it up just enough to make it a tiny bit rich. I think you'll be fine if you use all or some of these techniques.
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Old 10-05-2006, 01:05 PM
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I think you are closer to 11:1 cr with your combo
(723 + 5 + 4 + 9 + 55) / (5 + 4 + 9 + 55) = 796/73 = 10.9

When figuring cr on a sbc the piston is typically below deck 0.020" or 4cc and the head gasket is about 9cc. Hard to machine 4 cc's out of a head, I'm thinking maybe 3cc off from resurfacing. You really need to measure that to be sure. 1 cc of water weighs 1 gm

Get a 0.060" or 0.080" thick soft copper head gasket set.

A 0.060" head gasket will add around 4.5 to 5cc more head volume over the standard 0.040" head gasket. A 0.080 will add 9 to 10cc.

Place the head gasket on the head and scribe a line around the gasket. Then unshroud the valves all the way to the head gasket. Should add about 2cc more to the chamber and improve head flow.

A 11.0:1 cr 350 will drop to 10.1:1 with 7 more cc's and 9.6:1 with 12 more cc's.
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