Stevie1982

Currently 9:1 with 76cc heads. looking at going with 64cc heads. can someone tell me how much this will change the compression?

lars

You didn't provide any information to calculate this data, but assuming this is a 350 with 4" bore and a 3.48" stroke, and assuming head gasket thickness of .040" with a deck height of .035" and flat top pistons with valve reliefs of 10cc's, the comp ratio with your 76cc heads is 8.06:1. That assumes that your 76 cc heads are really 76 cc's, which they're probably not - they tend to be a little bigger, so comp ratio is even a little lower than 8.06.

Keep in mind that the "advertised" compression ratio is typically a full point higher than the actual compression ratio, so the actual 8.06:1 ratio will be an advertised ratio of 9:1. Are you assuming your advertised ratio to be an actual 9:1?

If you then switch from the 76 cc heads to 64 cc heads, the actual ratio will end up being 9.01:1, which equates to an "advertised" ratio of 10:1. The advertised ratios were based on "minimum allowable cylinder head volume" and the deck height at minimum spec, so there were never any production engines that came out of the factory with the advertised ratio.

If you have actually measured all of your parameters (you have cc'ed your piston volumes, cc'ed your heads, measured your deck height, and you know your actual head gasket crushed thickness), and you have done an accurate calculation based on these actual numbers, and you know for sure that your actual comp ratio is 9:1, a change to the 64 cc heads will raise the comp ratio by roughly a full point.

Lars

Stevie1982

I have an 82 Crossfire. the specs say 9:1. Am I understanding that changing the heads will change the compression from 9:1 to 10:1?

lars

No - your advertised compression ratio is 9:1. That means that your actual compression ratio is below 8:1, as noted above. A change to the 64cc heads will bring the actual ratio up to about 9:1, depending on all the other factors outlined in the equation above.

Lars

Stevie1982

Thanks!!!

Rescue Rogers

IF you have 9:1, it will increase to about 10.5:1 with no other changes. It depends on the calculator you use, the replacement gasket, etc....

You will notice the difference. Is it an after market head or an old chevy head?

The drop from 375hp from the 70 LT 1 to the 330 hp 71/72 LT1 was a drop in compression from 11:1 to 9:1 by changing from a domed piston and 64cc head to flat top and 76 cc head. The cams remained the same.

It will be exciting, you may need to readjust the carb, definitely run higher octane fuel to reduce detonation.

Keep us posted. Btw Im doing the same thing, I'm looking to dyno it before and after

lars

That's not the reason for the drop in horsepower rating between those two engines. In 1970, the horsepower ratings were gross. In 1971, GM changed to SAE net ratings. The two engines were actually very closely matched.

Lars

Bloodzone

Quick hijack question.

Is this true for crate engines as well?

lars

It depends what you mean by "crate engine." Any more, anyone who sells a rebuilt or newly built engine calls it a "crate engine." Some of the engine builders are very meticulous with their specifications, and provide accurate data. If you're buying an engine from GM, the heads and specs are at the standard factory tolerances, and are not at "minimum allowable chamber volume." Many engine builders (other than GM) will use GM components, including heads, and will advertise compression ratio assuming the "advertised" chamber volume rather than the actual measured volumes. Unless the builder has actually measured all specs and volumes, he is assuming a "blueprint" compression ratio, which in most cases will not be correct. I have torn down and measured quite a few "crate engines" (not GM engines) and I have never found the actual compression ratio to be as advertised. Obviously, the ones I have torn down have been torn down due to problems and issues, generally caused by poor workmanship in general, so it's not surprising that these second-rate engine builders have not accurately determined the actual compression ratio.

Lars

Rescue Rogers

yes 71/72 were the same

70=375hp
71=330hp component change
72=270hp net vs gross hp readings

lars

You're right.
Although, in all the dyno testing I've done, I've never seen a 1-point change in compression ratio equal 45 horsepower... there's more to that number drop than a modest change in compression... didn't GH Bush call it "fuzzy math...?"

Jebbysan

Lars is 100% correct on the advertised vs. actual compression as built on an assembly line at a factory.
76cc heads usually come in about 78.....
A good comparison is the old faithful Targetmaster/Goodwrench V8 that they have been selling for about 30 years.
This engine is suppose to come in at 8.5 to 1 but is actually about 7.9 to 1......
There several other things to consider......
1) The factory built engines with a steel shim .015 head gasket......most people install a .039 Fel Pro....this negates quite a bit of the increase you are seeking.....you can run a steel shim head gasket...but I do not like to with aluminum heads. You could get a Dart Iron Eagle 180 and run the steel shim but that is a premium flowing head.....the TBI crossfire induction does not flow near enough to support any of it....you will see gains...but not killer ones.
2) One point of compression is worth about 15 horsepower/torque at this level across the board.
3) You will need headers, a Renegade intake, 85 Vette pump and a custom chip to run really well......and it can.

Just bolting the heads on....I think you will be disappointed for the $1000 + expenditure.
Try the C4 forum for CrossFire build ups.......and ideas.

Jebby

lars

That's exactly what we've been seeing on the dyno: If all other items remain unchanged (valve sizes, port sizes, etc.), a full-point increase in compression will produce about 15 horsepower. In a car as heavy as a Vette, you need to make about a 20-horsepower change before you can actually notice anything "in the seat of your pants," so the money spent just adding a point of compression will usually be a disappointment... If it's done in conjunction with other complimentary changes, it can be a worthwhile venture, but not so much as a stand-alone upgrade.

Lars

Jebbysan

It will make a TBI engine like this more efficient, and crisper.....it will even sound different....but unless the VE is increased via cam, induction and exhaust......it is a diminishing return.....

A quality header type exhaust......and the Renegade intake with the improved head and compression could reap big dividends if combined with a mild cam........
Like I say...the C4 guys are more in tune (no pun intended) to these systems.....

Jebby

mikem350

Maybe factor in "lazy" advance curve and initial timing??

gerry72

He's correct, Lars. The gross to net change occurred in '72 with the LT-1 rated at 255 hp. Advertised compression changed in '71, which accounted for the gross hp rating drop from 370 to 330. The engines are, as you note, not that dissimilar in performance with the compression reduction particularly in today's world where you aren't pumping Sunoco 260 on every street corner.

lars

I acknowledged that in post #11, along with comment that the slight drop in comp ratio is not likely the sole source for a 45-hp drop in the rating... I've never seen that radical of a drop on the dyno from a slight drop in comp - it just doesn't happen. I'm guessing that there are some insurance numbers that played into the rating drop as well.

BlackC3vette

I am thinking that your statement is more of a fact than a guess. Along came 1973 with more changes and by 1974..... Oh, then we had the oil crisis, more fun times.

gerry72

I somehow missed your response. My apologies. The drop was two points from 11:1 to 9:1. I too doubt the numbers are absolute but they're probably not that far off. After all, the cam didn't change. Not that the LT-1 cam was optimized for the higher compression ratio but it would have been a bit less optimized for the 9:1 motor.

Stevie1982

well, just to let everybody know. I'm doing a little more that just changing heads. going to go with Edelbrock topend kit 2022. cam # 2103 instead of 2102. and a few other goodys! Should end up around 330 hp.