Compression ratio what to do??

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JKrebs

JKrebs

68 GTS
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I'm building a new 340 for my 68 GTS. I have a 340 from a 69 gts. Steel crank .030 over with silvolite 4 dimple pistons. I plan on using the .028 head gasket with X heads. Using Lunati 703 cam. I cc one on the head chambers and it came in at 74 cc. After running the calculations I come up with about 9:1. I want to be at 10:1. Question: would would you do, shave the heads or use different pistons to get to 10:1? Or should I care? The difference between 9:1 and 10:1 is negligible.
 
1 pt. of compression is approximately 3% in power on average builds.

If you need this point in ratio due to a large cam, to keep the dynamic ratio up, then mill. Otherwise, don’t worry about it. Take head flow over compression anytime.
 
Difference between 9 and 10 to one compression on a 300 hp engine is about 10 to 15 hp. Do a little head work and make that up and then some.
 
JKrebs said:
I'm building a new 340 for my 68 GTS. I have a 340 from a 69 gts. Steel crank .030 over with silvolite 4 dimple pistons. I plan on using the .028 head gasket with X heads. Using Lunati 703 cam. I cc one on the head chambers and it came in at 74 cc. After running the calculations I come up with about 9:1. I want to be at 10:1. Question: would would you do, shave the heads or use different pistons to get to 10:1? Or should I care? The difference between 9:1 and 10:1 is negligible.
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If you want the compression ratio up, the piston need to be above the deck.

The difference between 9:1 and 10:1 is, as a percentage is greater than going from 13:1 to 14:1.
That said, I don't like running the CR that low. With the right cam (not a BIG cam but the RIGHT cam which most likely won't be on the shelf) and a proper tune up more CR is always better unless you do it by shrouding the valves with a bad chamber. An early closed chamber BBC head is a good example of a piss poor closed chamber head.
 
Is that compression ratio taking into account that the pistons are above the deck? Because in that year of 340, the pistons are proud of the deck, usually by .018".
 
72bluNblu said:
Is that compression ratio taking into account that the pistons are above the deck? Because in that year of 340, the pistons are proud of the deck, usually by .018".
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I spoke to the piston manufacturer and the pistons I currently have in the bock are zero deck height and -10 cc's.
 
Bumping the compression really wakes the car up. Not sure if all you want is a somewhat doggy driver or something with some muscle. Shift recovery, torque, everything comes around.
74cc chambers are huge, my eddie heads were 65, and i milled them 50 to get from 9.2 to 10.4 along with going from really thick head gaskets to 028. Heads ended up 58cc
Whats your budget? I wouldnt want those heads on anything like they are. It will be a dog at 9-1 with that short stroke
It will run like a 1973 340
 
B3422w5 said:
Bumping the compression really wakes the car up. Not sure if all you want is a somewhat doggy driver or something with some muscle. Shift recovery, torque, everything comes around.
74cc chambers are huge, my eddie heads were 65, and i milled them 50 to get from 9.2 to 10.4 along with going from really thick head gaskets to 028. Heads ended up 58cc
Whats your budget? I wouldnt want those heads on anything like they are. It will be a dog at 9-1 with that short stroke
It will run like a 1973 340
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Budget not really an issue within reason. I did want to run the x heads that I have and I'm just not sure if milling the heads or replacing the pistons is the answer. I'm leaning towards piston replacement as I assume that milling the heads will lead to milling the intake as well?
 
JKrebs said:
Budget not really an issue within reason. I did want to run the x heads that I have and I'm just not sure if milling the heads or replacing the pistons is the answer. I'm leaning towards piston replacement as I assume that milling the heads will lead to milling the intake as well?
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Yep. Roughly .95 for every 10 off the heads.

Not sure how much those heads can be milled, be cheapest way though. Milling intake and heads will be cheaper than piston swap and motor doesnt need to come apart..... rebalance?
 
Keith Black 243 will sit proud out of the deck, but they are hypers.
It gives me 8.98 static, and 8.06 dynamic using the same cam.
It also has uncut X heads, the chambers are huge on those heads, mine were 77cc's. but I chose them because I am tired of buying high test with race gas to cruise around in, and I also used the thick felpro gaskets.

I want a cruiser out of this car, that might be easy on fuel.
 
The Silvolite pistons are cast and so are the least durable of what you can have. So putting in some hypereutectics or forged is a good step anyway, and you can get the CR up at the same time. Do you have the Silvolite S1267 pistons? If so, the zero deck makes sense.
Silvolite Piston - 5.6L Chrysler (V8) 1968-71 [1267] - $446.57 : United Engine & Machine Co. Incorporated, Performance Pistons

With those pistons, I too am getting around 8.9 SCR. Note that the stock 340 hi-comp pistons weren't really a lot more than that.... maybe up around 9.3 SCR.

1) Easy forged to get you to that old number of 9.3-ish is Speed Pro L2316F. Stock weight, so very little work needed for balancing if at all.
2) The KB243 hypers put you at the same SCR spot with less weight so a rebalance is needed.
3) Ross 99626 bumps you up close to 11 so that is too much.
4) JE's SRP line gets you to the same place as #2 and #1, and need a balance job. Ditto with Diamond.
5) Nothing else jumps out at me....

So, with the head as is, you are kinda limited to around 9.3. I'd be milling and new, better pistons. Or new pistons and different heads is really where I would go (and went).

As noted DCR is more important to look at and SCR is a component in that. Also the cam and its timing works into DCR and the application is a driver to the cam. So what are the uses for this car/engine? The cam selection kinda says what you intend, but it would be nice to hear from the source.

BTW.....That 3% number comes from the waaay oversimplified Wallace calculator. It only derives some misleading results that looks only at PEAK HP with one parameter in the thermodynamic equation. The effects of CR on torque at low and mid RPM's is much, much greater than indicated by that calculator, and it helps extend the usable RPM range. That improves mid-range HP more than is being understood; that is not being accounted for in anything that only considers peak HP. CR also contributes to items like cylinder clearing of spent gases and helps filling, and that is all overlooked in that online calculator. Working with less CR than you can pushes the engine operation further down in one corner of the map and limits the possible results. That particular calculator needs to go away.
 
nm9stheham said:
The Silvolite pistons are cast and so are the least durable of what you can have. So putting in some hypereutectics or forged is a good step anyway, and you can get the CR up at the same time. Do you have the Silvolite S1267 pistons? If so, the zero deck makes sense.
Silvolite Piston - 5.6L Chrysler (V8) 1968-71 [1267] - $446.57 : United Engine & Machine Co. Incorporated, Performance Pistons

With those pistons, I too am getting around 8.9 SCR. Note that the stock 340 hi-comp pistons weren't really a lot more than that.... maybe up around 9.3 SCR.

1) Easy forged to get you to that old number of 9.3-ish is Speed Pro L2316F. Stock weight, so very little work needed for balancing if at all.
2) The KB243 hypers put you at the same SCR spot with less weight so a rebalance is needed.
3) Ross 99626 bumps you up close to 11 so that is too much.
4) JE's SRP line gets you to the same place as #2 and #1, and need a balance job. Ditto with Diamond.
5) Nothing else jumps out at me....

So, with the head as is, you are kinda limited to around 9.3. I'd be milling and new, better pistons. Or new pistons and different heads is really where I would go (and went).

As noted DCR is more important to look at and SCR is a component in that. Also the cam and its timing works into DCR and the application is a driver to the cam. So what are the uses for this car/engine? The cam selection kinda says what you intend, but it would be nice to hear from the source.

BTW.....That 3% number comes from the waaay oversimplified Wallace calculator. It only derives some misleading results that looks only at PEAK HP with one parameter in the thermodynamic equation. The effects of CR on torque at low and mid RPM's is much, much greater than indicated by that calculator, and it helps extend the usable RPM range. That improves mid-range HP more than is being understood; that is not being accounted for in anything that only considers peak HP. CR also contributes to items like cylinder clearing of spent gases and helps filling, and that is all overlooked in that online calculator. Working with less CR than you can pushes the engine operation further down in one corner of the map and limits the possible results. That particular calculator needs to go away.
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1267 is in fact the pistons I currently have. I plan on using the car for cruising and the occasional tire roast. It is a 4 speed so getting the rpms up is easy enough. It looks like I would need to get the chambers down to 62 cc to get the 10:1 I'm looking for. Can I even get the heads down that low? would I be sacrificing anything with the flow by taking the head down to achieve 62 cc? If not is there any documentation that may give me an idea on how much to take off? And thank you for you detailed response. I really do appreciate the effort that everyone makes in trying to help.
 
JKrebs said:
1267 is in fact the pistons I currently have. I plan on using the car for cruising and the occasional tire roast. It is a 4 speed so getting the rpms up is easy enough. It looks like I would need to get the chambers down to 62 cc to get the 10:1 I'm looking for. Can I even get the heads down that low? would I be sacrificing anything with the flow by taking the head down to achieve 62 cc? If not is there any documentation that may give me an idea on how much to take off? And thank you for you detailed response. I really do appreciate the effort that everyone makes in trying to help.
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Figure about 7 is one cc. So you would have to cut the heads almost 85 thou to get there. Cant cut them that much.
Why not put an aluminum head on it with smaller chambers and gain power with no driveability loss.
Eddies are a great upgrade. They come roughly 64-65cc. 4 speed or not, a 9-1 340 isnt going to make much steam with factory head.
That all said, i drive and have driven all my 9,10, 11 second small block combo's on the street, so anything slower than that just plain aint fun to me.......lol
But definately get some compression in that thing.
 
You can shave the heads, or to get part of the compression back, use a Cometic .027 gasket. The power percentages quoted don't take into account the loss of low speed torque, which is greater than the stated gain or loss from compression change. IMHO, get the compression up to where it can run on available gas. You won't be sorry. Remember, a motor has to struggle through the first half of the rpm range between gears longer than it does the second half. The taller the gears, or tighter the converter , just magnifies the torque difference and over all performance.
 
Ok, so what I'm reading here is that my 1971 340 stock motor was around 9.3>9.5:1 at best from the factory. I used the Fel-pro .039 gasket on a rebuild with the TRW 2316's so I was down even more. With just a .484/284 hyd., dual plane, 750DP, small hdrs stock 2.02/1.60 heads that motor showed 301 on track crank hp, and later with bigger carb/hdrs/cam 368hp, same CR.
The OP won't be much less CR than I was as is, the 703 Lunati Voodoo is more lift/less dur than the purple if I read it correct for better DCR so I cannot see its worth the expense of going to 10:1 for a few hp myself, obviously CR has all the benefits mentioned but its not everything, the Heads are, not so important on a street build...my2c's
 
Torque is more useful for cruising and tire fryin' than HP. If you push up near to 10:1, you will have plenty of torque. It won't be awful at 9; you just want to not be below 8 if you can help it. But, ultimately you need look at your DCR because the other end of too much CR is to get into detonation. That is the point of discussing the cam; a smaller cam will increase DCR.

Correction on the milling amount: You drop the open chambers 1 cc for each approximate .005" of milling. So dropping 8-10 cc's is easy by milling .040 or .050".

If you drop to 64 cc, then your SCR is around 9.8 and your DCR with that cam is 7.9 with the ICL at 106. You might squeeze more on the DCR but you have to be careful on tuning and will have especially keep the timing curve limited. I assume you can get 93 octane there, as you will need to use it.

AL heads have an advantage in avoiding detonation. And closed chambers like the Edelbrocks or Indy heads have better detonation resistance. So those are options. Both breathe much better than the stock head.
 
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I personally would go for a c/chamber Eddy head as he's at zero to raise CR and hp all in 1 go, cfm will help breathing/hp but ally needs more CR so full benefit won't necessarily be reached. X heads are the flavor so gotta work with what the OP has. I had plenty of torque with my set up, 3500 stall and later 4200, but he's 4 speed so no issue. But as I said what he has will work and make tyre shredding power/torque.
 
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nm9stheham said:
Torque is more useful for cruising and tire fryin' than HP. If you push up near to 10:1, you will have plenty of torque. It won't be awful at 9; you just want to not be below 8 if you can help it. But, ultimately you need look at your DCR because the other end of too much CR is to get into detonation. That is the point of discussing the cam; a smaller cam will increase DCR.

Correction on the milling amount: You drop the open chambers 1 cc for each approximate .005" of milling. So dropping 8-10 cc's is easy by milling .040 or .050".

If you drop to 64 cc, then your SCR is around 9.8 and your DCR with that cam is 7.9 with the ICL at 106. You might squeeze more on the DCR but you have to be careful on tuning and will have especially keep the timing curve limited. I assume you can get 93 octane there, as you will need to use it.

AL heads have an advantage in avoiding detonation. And closed chambers like the Edelbrocks or Indy heads have better detonation resistance. So those are options. Both breathe much better than the stock head.
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OK well. Being I'm looking at $700 to have my heads done up before milling and i will need to have my intake milled more $$. Then I will need to replace my pistons and rings. More money. After all the input from everyone i believe the closed chamber aluminum heads are the most efficient way to get 10:1. I did want to use the X heads but i really don't want to carve them up either.
 
nm9stheham said:
Torque is more useful for cruising and tire fryin' than HP. If you push up near to 10:1, you will have plenty of torque. It won't be awful at 9; you just want to not be below 8 if you can help it. But, ultimately you need look at your DCR because the other end of too much CR is to get into detonation. That is the point of discussing the cam; a smaller cam will increase DCR.

Correction on the milling amount: You drop the open chambers 1 cc for each approximate .005" of milling. So dropping 8-10 cc's is easy by milling .040 or .050".

If you drop to 64 cc, then your SCR is around 9.8 and your DCR with that cam is 7.9 with the ICL at 106. You might squeeze more on the DCR but you have to be careful on tuning and will have especially keep the timing curve limited. I assume you can get 93 octane there, as you will need to use it.

AL heads have an advantage in avoiding detonation. And closed chambers like the Edelbrocks or Indy heads have better detonation resistance. So those are options. Both breathe much better than the stock head.
Click to expand...


Your right, thanks for the correction. Should be 53 per cc, not 7 as i posted
 
JKrebs said:
OK well. Being I'm looking at $700 to have my heads done up before milling and i will need to have my intake milled more $$. Then I will need to replace my pistons and rings. More money. After all the input from everyone i believe the closed chamber aluminum heads are the most efficient way to get 10:1. I did want to use the X heads but i really don't want to carve them up either.
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That's why I said to mill the block some more. You can put the piston .050 out of the deck and get exactly what you want. You may have to cut the care of the head but that is still less money than another set of heads.
 
ok well I raced in an era of iron heads so take in some info today from supposedly knowledgeable people and assume they know what they are talking about, perhaps thats just to sell ally heads then?>

#11---Horsepressure
 
Hot Metal said:
ok well I raced in an era of iron heads so take in some info today from supposedly knowledgeable people and assume they know what they are talking about, perhaps thats just to sell ally heads then?>

#11---Horsepressure
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That is an interesting article on heat and pressure. Just when i thought i had a plan forward. Got me thinking again.........
 
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