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How much compression is too much? #2464199
03/10/18 10:12 AM
03/10/18 10:12 AM
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Mooresburg, Tn
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'72CudaRacer Offline OP
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620ci RB, B1 MC heads, cam is .850" lift solid roller, duration 286/306 @ .050", carbureted on gas. Bore 4.600", stroke 4.665", piston 10.3cc dish volume, combustion chamber volume 52cc, head gasket compressed thickness .066", 4.600 bore size".
How much compression ratio would be considered too much? This is a bracket engine, to be raced on a weekly basis. The engine was brought to me with 16.4:1. I can change head gaskets to get it to 15:1 with out doing anything else.
The question is, how much is TOO much?

Thanks, Brian

Re: How much compression is too much? [Re: '72CudaRacer] #2464227
03/10/18 11:53 AM
03/10/18 11:53 AM
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NW Indiana
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fbs63 Offline
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What's the lobe separation on the cam?

Re: How much compression is too much? [Re: '72CudaRacer] #2464239
03/10/18 12:22 PM
03/10/18 12:22 PM
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New York
polyspheric Offline
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I can change head gaskets to get it to 15:1 with out doing anything else.
Losing your quench isn't a good idea unless your VE is very low.

What does your chamber wall look like adjacent to the intake valve?


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Re: How much compression is too much? [Re: '72CudaRacer] #2464252
03/10/18 12:51 PM
03/10/18 12:51 PM
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NE Ohio
DoubleD Offline
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Sounds pretty healthy - for a bracket race engine the compression probably is a little much unless you plan on running C16 for fuel - Personally I would be a little nervous increasing the head gasket thickness given its already .o66 - maybe the pistons could be milled a little deeper if there is room - but with an engine that big a new set of pistons might be the best solution if you want to lower the compression

Re: How much compression is too much? [Re: '72CudaRacer] #2464270
03/10/18 01:44 PM
03/10/18 01:44 PM
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So. Burlington, Vt.
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fast68plymouth Offline
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I'd run the thicker gaskets and not worry about the quench.


68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
Re: How much compression is too much? [Re: '72CudaRacer] #2464272
03/10/18 01:52 PM
03/10/18 01:52 PM
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I gotta ask how do you have a 4.600 bore size? What block is it?

Re: How much compression is too much? [Re: '72CudaRacer] #2464285
03/10/18 02:32 PM
03/10/18 02:32 PM
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GomangoCuda Offline
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Leave it as is and run methanol.


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Re: How much compression is too much? [Re: fast68plymouth] #2464288
03/10/18 02:39 PM
03/10/18 02:39 PM
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Frostbitefalls MN (Rocky&Bullw...
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Originally Posted By fast68plymouth
I'd run the thicker gaskets and not worry about the quench.

I have heard that quench effectivness diminishes as rpm goes way up?


8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
Re: How much compression is too much? [Re: '72CudaRacer] #2464306
03/10/18 03:23 PM
03/10/18 03:23 PM
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WA
JD Dart Offline
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Buddy of mine had the same problem with C/R. He called Koffels and they said the B1 heads don't tolerate much over 15.3. I'd go with the thicker head gasket too. And I must ask what block is that to get 4.600 bore?


best so far 8.53 @ 158.59 MPH 60'1.240
Re: How much compression is too much? [Re: '72CudaRacer] #2464308
03/10/18 03:32 PM
03/10/18 03:32 PM
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Las Vegas
Al_Alguire Offline
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No reason for a basic bracket engine to be at that kind of compression ratio for sure. My little B1 was at 15.5 and worked pretty well for what it was. The one from the dragster is 15.3-1 and works pretty well too.

Id like to see the piston that gets you that kind of compression number for sure. I though I saw it was a world block somewhere but maybe not. 4.600 aint gonna leave much room between the cylinders on a 4.800" BS block for sure.


"I am not ashamed to confess I am ignorant of what I do not know."

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Re: How much compression is too much? [Re: gregsdart] #2464319
03/10/18 03:42 PM
03/10/18 03:42 PM
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Kalispell Mt.
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HotRodDave Offline
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Originally Posted By gregsdart
Originally Posted By fast68plymouth
I'd run the thicker gaskets and not worry about the quench.

I have heard that quench effectivness diminishes as rpm goes way up?


One of the guys on here did an experiment with a 440 and went from bad quench to tight quench and saw the biggest gains as the RPM went up, big time, like 50 HP. I think it was jesse lackman but don't quote me on that.


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Re: How much compression is too much? [Re: '72CudaRacer] #2464328
03/10/18 04:08 PM
03/10/18 04:08 PM
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Charlotte, NC
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LSP Offline
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Show a picture of the piston, most aren't finished before installation IMO

Re: How much compression is too much? [Re: '72CudaRacer] #2464342
03/10/18 04:38 PM
03/10/18 04:38 PM
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Monrovia, So-Cal, USA
racerhog Offline
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If it's been working?? Just use it as is... Just dont lug it in some heavy azz turd car and changing the combo to much... Just my 2 cents...

Or drag the junk over to Greeneville TN and have the Johnsons dyno it... smile

Last edited by racerhog; 03/10/18 04:38 PM.

Bob(Cowboy)Hogan
Monrovia So-Cal
Re: How much compression is too much? [Re: '72CudaRacer] #2464344
03/10/18 04:48 PM
03/10/18 04:48 PM
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Frostbitefalls MN (Rocky&Bullw...
gregsdart Offline
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If you do consider running methanol, a very sharp engine builder told me 15/1 is about the max that works well. Over that, more fuel is needed to run well, eliminating the advantages, and it is hard on parts.


8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
Re: How much compression is too much? [Re: WedgeFED] #2464374
03/10/18 05:49 PM
03/10/18 05:49 PM
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Mooresburg, Tn
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Originally Posted By WedgeFED
I gotta ask how do you have a 4.600 bore size? What block is it?


Koleno

Re: How much compression is too much? [Re: fbs63] #2464377
03/10/18 05:52 PM
03/10/18 05:52 PM
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Mooresburg, Tn
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'72CudaRacer Offline OP
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Originally Posted By fbs63
What's the lobe separation on the cam?


Crane cam, lobe separation 114.

Re: How much compression is too much? [Re: LSP] #2464382
03/10/18 05:57 PM
03/10/18 05:57 PM
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Mooresburg, Tn
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Originally Posted By LSP
Show a picture of the piston, most aren't finished before installation IMO


I can post a picture later tonight, but it is finished. Diamond custom piston, comp height 1.530", intake pocket depth .340" (2.600" titanium valve), exh .390". Piston weighs 654 grams. Not much left to take off of it.

Re: How much compression is too much? [Re: '72CudaRacer] #2464402
03/10/18 06:45 PM
03/10/18 06:45 PM
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Originally Posted By '72CudaRacer
620ci RB, B1 MC heads, cam is .850" lift solid roller, duration 286/306 @ .050", carbureted on gas. Bore 4.600", stroke 4.665", piston 10.3cc dish volume, combustion chamber volume 52cc, head gasket compressed thickness .066", 4.600 bore size".
How much compression ratio would be considered too much? This is a bracket engine, to be raced on a weekly basis. The engine was brought to me with 16.4:1. I can change head gaskets to get it to 15:1 with out doing anything else.
The question is, how much is TOO much?

Thanks, Brian


The only way to know if you have too much compression is to test it. Flame travel inside the combustion chamber is very complex. There probably isn't anyone who can read a post on the internet and then tell you if you have too little or too much compression. Even if you had a bunch of pictures of the piston and the combustion chamber I doubt anyone could give you a positive answer.

Personally I think that is way too much compression for a bracket engine. The tuning window will be narrow and you'll have to run expensive race fuel. If the car is being raced just for fun then what is the point of the sky high compression ratio?

Re: How much compression is too much? [Re: AndyF] #2464453
03/10/18 08:35 PM
03/10/18 08:35 PM
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Mooresburg, Tn
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'72CudaRacer Offline OP
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Personally I think that is way too much compression for a bracket engine. The tuning window will be narrow and you'll have to run expensive race fuel. If the car is being raced just for fun then what is the point of the sky high compression ratio? [/quote]

This is my line of thinking also Andy. I'm thinking any advantage (IF there is any) to having this much compression is out wayed by the toll that it will take on the engine parts (rings, rods, bearings piston pins, head studs, ect.).

And just so everyone understands, this engine was running when it was brought to me. The last two passes in a 2200lb door car was 5.13 @ 139mph (1/8 mile), even with the problems that I have uncovered. In working through the issues on this engine, I'm hoping to head off as many future issues as I possibly can. I feel like this much compression is just not needed.

Thanks, Brian

Re: How much compression is too much? [Re: '72CudaRacer] #2464458
03/10/18 09:00 PM
03/10/18 09:00 PM
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Mt Morris Michigan
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I believe it was David Vizard that had a compression pyramid in one of his books. It seems the beginning of diminishing returns start at 13.5-14:1. After that, not much to gain and fuel gets real expensive.

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