Question: Deck clearance on L81?

[macx]

Planning some mild mostly top end engine and drivetrain upgrades to my 81 so I can take some longer trips after I retire and get good mileage while still improving mid range throttle response.

The one bit of information I can't find is the deck to piston clearance on a stock L81 engine.

I can calculate it, but I have read somewhat varying information on piston compression height, as well as installed head gasket thickness.

From what I read, there's not much in the way of a quench area on top of the stock pistons, let alone in the comb chamber. I had hoped to avoid pulling the engine and changing pistons because of the work and cost involved with parts & machining, and because the shortblock is in decent operating condition, but fear I won't have any decent quench area otherwise which is necessary for detonation control so I can up the compression about 3/4 of a point and still run 87 octane.
I will be going with aftermarket Vortec style heads but will probly need pistons with matching quench area.

Thanks!

 

[macx]

After more research & calculating

Found out I'm probly better off leaving the stock dished pistons in the shortblock so I don't have to disassemble it and pay machining and misc small parts costs to switch pistons which would run at the bare minimum $300 to get flat top hypereutectic's, plus avoiding the work of removing/replacing & disassembling/reassembling the engine which I wouldn't do if I don't do more to the shortblock than change cam. And with available reasonable cost hypereutectic pistons and Vortec style heads, I can't end up with low enuf comp ratio to run 87 anyway, so not much to gain realistically.

If I run with the stock pistons (1.54 compression height, 9.9cc dish, with actual deck height probly at least .05 with a calculated .045 at a blueprint block height of 9.025) and go with 64cc heads and an .020 x 4.1 coated head gasket (which should seal without decking the block) I'll end up with about 9.07 to 9.09 comp ratio and will have to use 91 octane as I won't have any effective quench area.

For a max expected total mileage on the car of 50k miles, and figuring 2 mpg increase with 91 octane due to a little higher compression ratio and advanced timing, and $.30 per gallon over 87 octane, that would only add up to about an extra $150.

So around $150 less total cost to leave shortblock alone and run 91 octane, with maybe a few lbs more low & mid range torque from more advanced timing.​

 

[lars]

with actual deck height probly at least .05 ...

You might want to measure the actual deck... I've never seen a stock block with quite that much material above the pistons. Most of the stock small blocks I tear down have about .015" - .020" of deck height, and I generally mill them .010" to get the deck to a "street-safe" clearance of .010-.005". Also, most good head gaskets are going to give you a compressed thickness of about .042". You might want to run these numbers in your compression calculator and see what you get.

Lars

 

[macx]

Hello! I think I remember you from several years ago when I first bought the car and started researching it online, incl this forum of course.

Anyway - when I had time to dig into it after I got it home, I found numerous major problems I wasn't able to view when I bought it as it was a good distance from home. Such as the rear suspension and hubs/brakes, body mount bushings, door post and floorpan rust, etc. And not being familiar with Corvettes and their unique problem areas, I didn't even know to look at the rear suspension.

As I work away from home I didn't have time to pursue it then. Now I'm getting closer to retirement and am trying to finalize my planning while I'm laid off so I'm ready to dig into it when I retire.

I got that .050 number by subtracting 1/2 of stroke, rod length, and the stock piston compression height of 1.54" from a probable actual block deck dimension of 9.03 as manufacturing tolerances, esp in those days, probly didn't result in the blueprint spec of 9.025. Not to mention likely variations from cyl to cyl. So 9.03 minus 8.98 gives me that .05 figure. That plus even an .020 head gasket puts me quite a ways away from any effective quench.

I did run the numbers thru the calculator on the KB Silvolite site which takes into account even the head gasket dia. Being I'm going to have to run 91 octane anyway, I might as well get the extra .01 or .02 compression by using a thinner head gasket, such as the coated .020 gaskets available at Summit. They have coated gaskets down to .015" but am a little hesitant to go that thin, even with the coating, on a non-decked block, and even tho the factory gasket is only .016 or .017 depending on who's figures you read.

So with 64cc heads and an .020 head gasket on top of the stock non-machined shortblock, figuring it at .05", will give me about 9.07 to 9.09 comp ratio depending on the actual deck height I find when I get the heads off.

Going with (probly) a Comp flat hydraulic cam in the 208 to not over 212 @ 050 range with intake closing at from 55 to 58 ABC (@ .006 lift) and lift in the lower mid 400's range that should give me in the neighborhood of 310-320 hp with a street rpm effective range of up to about 5200, and torque in the high 300's at 3500 to 3700. That's with Vortec style aftermarket heads with 170 intake that flow 235 at .500 lift, a Performer intake so it fits under the hood, and a 670 cfm throttle body tuneable speed density efi that I already have with a closed loop kit, which will support up to about 325 hp, and using my Innovate wide band to tune the open loop and monitor the closed loop. I'll retain the stock small diameter headers but increase the single pipe section to 3" and use a hi flow true 3" in/out performance cat.

Also going to replace the electric lockup TH 350 with a mildly modified and blueprinted 200-4R and 2200 lockup from Bowtie Overdrives, and being I have to rebuild the diff will go to probly 3.31 gears which should work out to nice street and economical hiway rpms and yet a little fun with the 2.84 low gear in the trans. The electric controller will be removed and a vacuum advance performance HEI distributor substituted. The trans is hydraulically controlled and of course the efi won't work with that controller any way. I've used
that efi on a mild small block in a 3/4 ton van some years ago and was amazed at the increased throttle response and gas mileage.

Gotta have something to keep me occupied when I retire!