Stock compression ratio??

[goingballistic]

I am trying to figure out how the compression ratio of the stock 350 in the 81 vette is rated or specd at 8.2:1???? I came out with a 8.9:1 compression ratio, something is wrong..........obviously....... with the info I used, so I'm looking for the "right" stuff, so to speak.

The info that I have found is somewhat controversial and I was wondering if anyone has the definitive answer.

Stroke--> 3.48
Bore--> 4.00
Gasket Thickness when compressed--> ??? ( I used 0.039)
Head Volume--> ???? ( I used 76cc)
Dish Dome Volume--> ???? ( I used 5 cc)
Gasket Bore-->??? ( I used 4.0)
Deck Clearance-->??? ( I used 0.005)

Anyone have the original info on the motor when it came stock from the factory????

Paul.

 

[Glensgages]

I'm just 'guessing' here, going strictly from memory, but:
#1: the volume of the 'dish' of a 'typical' late-'70s/early-'80s 350 SBC piston (with a -deep-dish & 4 valve 'eyelets') might be more than 5cc;
#2: the gasket bore would be slightly larger than 4.000" diameter (4.125" diameter ?), adding even more volume;
#3: it might have a deck-height of more than .005" (.015"/.020"???), increasing the volume again...

 

[goingballistic]

Yea this seems to be the problem that I've run into...........I can't get anything solid to work from. I've plugged various values into this and keep coming up short ( higher compression than advertised), the numbers ( I made my own calculator) never seem to be 8.2:1 unless I make drastic changes and then they seem wrong.

I've checked on the web with other calculators and get the same results, so I know it's not my math or spread sheet...........I'm obvously missing something related the "actuals" from the stock 81 350 and can't for the life of me figure out what it is.

I actually did this so I can figure out the right combo for a 383 I'm building and it works for that ( keeping the CR at 10:1), I can vary the gasket thickness etc and see the change, I'm just stumped about the "published" specs???

 

[Jack]

There are several OE dished pistons with VRs. Some have as much as -21cc. Yours probably has the smaller dish & 4 VRs and about 1.560" compression height ... those pistons have right at -11cc total of dish + 4 VRs ... in an OE spec block (9.025" crank CL-deck) they should be about 0.025" down-in-the-hole. OE gasket bore was "about" 4.166". I wouldn't sweat a coupla-three CR points on what's in there now. As you plan for the 383 (or any build) you'll need to know the block's exact deck height AFTER any machine work is done ... know this before you calculate CR / order parts. Same goes for heads' surfaces. Block/heads MAY need to be shaved simply to true em up ... or to help obtain desired CR. As you know, OE sbc 350 stroke = 3.48", rod length = 5.7" c-c.

-edit- BTW, a very nice head gasket that can be used with either iron or aluminum heads is a Victor Reinz P/N 5746. Compresses to about 0.039" and has the MLS construction that has proven blow-proof in our race cars ... has its own moly-graphite coating ... uses NO sealant. Get em at NAPA for about $30 each ... I mail order em from FL (ERW 1-800-940-4379) for about half that.
JACK:gap

 

[goingballistic]

Jack, thanks for the info about the gasket sounds like a plan.


Looking at the stock 350, and using a 3.48 stroke, 5.7 rod and 9.025 deck, you come out to 1.585 CH. I'm now making the assumption that the OE piston was a 9.5 domed as that gets you really close to the 8.2:1 CR with a 76cc head. I'm not too sure of the actual OE head on the stock 81 350??? I'm assuming at this point it was about 76cc w/0.039 gasket.

I'm taking the heads down to zero deck (9.000), 400 crank ( 3.75 stroke) and keeping the original rods ( new bolts). With a 0.039 gasket, 74cc heads and a +5cc dome (+30 over) I'm looking at about 9.9:1 CR. If I use a flat top, most are -4 or -3 dish, it rasies my CR above 10:1 to almost 11:1 and I'd REALLY like to use pump gas so I'm striving to keep it right at or just below 10:1CR.

One quick question you may be able to help with is a statement you made about "don't worry" about about a few CR points..............can you define a "few" slight changes in my spread sheet make the CR go from 9.5:1 to 10.6:1.

Any idea? Thanks for all your help by the way.

Paul.

 

[Glensgages]

One quick question you may be able to help with is a statement you made about "don't worry" about about a few CR points..............can you define a "few" slight changes in my spread sheet make the CR go from 9.5:1 to 10.6:1.

Not quite sure if this helps, but.....
My Z28 runs fine on SUNOCO94 with 10.38:1 cr with a 244* @ .050" cam (292H), and has for over 13 years, with a few thousand 1/4-mile 12-second runs...

 

[Jack]

Jack, thanks for the info about the gasket sounds like a plan. Looking at the stock 350, and using a 3.48 stroke, 5.7 rod and 9.025 deck, you come out to 1.585 CH. I'm now making the assumption that the OE piston was a 9.5 domed as that gets you really close to the 8.2:1 CR with a 76cc head. I'm not too sure of the actual OE head on the stock 81 350??? I'm assuming at this point it was about 76cc w/0.039 gasket. I'm taking the heads down to zero deck (9.000), 400 crank ( 3.75 stroke) and keeping the original rods ( new bolts). With a 0.039 gasket, 74cc heads and a +5cc dome (+30 over) I'm looking at about 9.9:1 CR. If I use a flat top, most are -4 or -3 dish, it rasies my CR above 10:1 to almost 11:1 and I'd REALLY like to use pump gas so I'm striving to keep it right at or just below 10:1CR. One quick question you may be able to help with is a statement you made about "don't worry" about about a few CR points..............can you define a "few" slight changes in my spread sheet make the CR go from 9.5:1 to 10.6:1.
Any idea? Thanks for all your help by the way. Paul.

Paul:
No ... the piston I refer to has 1.560" compression height and sits BELOW deck about 0.025". So, if you use the CR calculator here at CAC site http://www.corvetteactioncenter.com/tech/tools/displacement.html# and plug in 4.03" for bore, 3.48" for stroke, 0.064" for deck height (0.025" + 0.039" gasket), 87cc for combustion chamber volume (76cc head + 11cc piston) and 8 for cylinders ... calculator returns result of 8.2:1 CR ... shazam! If you use a calculator that also accounts for gasket bore of 4.166" ... the CR will probably be a tiny bit less than 8.2:1. Relatively small differences in deck height and/or gasket thickness and/or compression height can have big effect on CR. I wouldn't be too concerned if it's 9:1 or 9.5:1 ... a .5 spread's not gonna make a whole lotta difference in real street-car world.

Speed-Pro piston H602P is hypereutectic DISH top w/ 4 VRs, has compression height of 1.425" and volume of -12.5cc. This piston will zero when deck is cut about 0.025". With 3.75" stroke, 74cc heads on thirty-over, zero deck & 0.039" gasket ... CR about 9.2:1. (I run these w/76cc iron heads, 3.8" stroke ... my CR=9.2:1 also ... amazing coincidence ... no, just math).

Speed-Pro piston H600P is hypereutectic FLAT top w/ 4 VRs, has compression height of 1.425" and volume of -6cc. This piston will zero when deck is cut about 0.025". With 3.75" stroke, 74cc heads on thirty-over, zero deck & 0.039" gasket ... CR about 9.9:1.

Speed-Pro piston L2491F is forged FLAT top w/ 2 VRs, has compression height of 1.430" and volume of -3.4cc. This piston will zero when deck is cut about 0.020". With 3.75" stroke, 74cc heads on thirty-over, zero deck & 0.039" gasket ... CR about 10.2:1.

Glensgages experience indicates his cam's duration probably "bleeds-off" some of his "static" CR ... all cams do to some extent. Most all CR quotes use an index called "static CR" ... another CR index called "dynamic CR" also figures the cam's bleed-off into account ... but dynamic CR is rarely published.

Good job! ... You're definately on the right track by doing your homework on CR before you buy/build ... just make sure you measure correctly and get your math right.
JACK:gap

 

[Glensgages]

... the CR will probably be a tiny bit less than 8.2:1...

I'm not TOO sure the factories calculated their OWN c.r. very accurately; my brother-in-law once built an Olds 455 smog-motor, listed at 8:1. After whacking the heads, we figured our OWN c.r., going as far as lagging the chambers and dish/valve reliefs with modeling clay. AFTER cutting the heads, our TRUE c.r. was 'just' 7.88:1...
?

...Glensgages experience indicates his cam's duration probably "bleeds-off" some of his "static" CR ...

Correct. At the time, I had to drive the car to the strip, and we figgered the big-stick would 'bleed-off' enough to run pump gas...

 

[goingballistic]

Jack and Glensgages, thanks gents. I figured out what my problem was with my spread sheet..........operator error!!!!!!!! I was putting in dished as a negative volume, WRONG. Anyway now my spread sheet matches your numbers, Jack, exactly. So that now determines a flat top -4cc 2 VR's piston, 400 crank cut, 30 over, .039 gasket, zero deck, case cut for the stock rods and we're off.

Pick up the new "old" block this weekend ( before the wife changes her mind) and the crank and balancer. Got all summer so I'll be checking and rechecking.

Once I get all the math sorted on this, I think it's OK right now, I'm putting in a HP calculator as well and then put the whole thing into a Visual Basic SW package. I'll post it up here when I'm done and then others can use it. I'll make sure all the math is available so others can see "how" it works. Been quite the learning experience here, so far and am now on the way to the rebuild............until the wife finds out much money I'm spending...........yikes.
:eyerole

Glensgages: How did you figure out the cam, this part seems to be the rocket science part of all this. Any help??

 

[Glensgages]

Glensgages: How did you figure out the cam, this part seems to be the rocket science part of all this. Any help??

As much as I'd like to take 'credit' for choosing the 292H cam, it was just blind luck.
We had figured that the 292H would be just about as much cam as we'd dare run with the heads & bottom-end we had planned to assemble. Had it NOT run on 94-octane fuel, I'da mixed CAM2 at a 1:4 ratio, but the SUNOCO was good enough.
The next year, a fellow racer built a very similar motor, with a milder cam, and it, too, ran fine on Ultra94...

 

[goingballistic]

Glensgages, I've read some pretty nasty stories about vacuum problems running too high a cam. Have you noticed anything like that with the 292H? This is a daily runner, fun car, it may go the track once or twice but that's about it and am trying to avoid costly upgrades with vacuum additions etc.

 

[Glensgages]

Glensgages, I've read some pretty nasty stories about vacuum problems running too high a cam. Have you noticed anything like that with the 292H?

Vacuum ain't the greatest, but with a vacuum reservoir, the power brakes function very well;
never had a vacuum gauge on the Z28 once it hit the strip...