91GMCSuburban
1/2 ton status
So what is the "right" cr for a TBI motor and how do I fine tune it? Right now, I am in between 9.7 cr (28cc) or 8.8 cr (18cc). I want to go as high compression-wise as I can with 87 oct.
CK5
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Highest CR for 87 Oct.
- Thread starter 91GMCSuburban
- Start date
As pretty much everyone has said, there are so many variables that when talking about compression, what works in one case, won't work in others.
My *opinion* and others are free to disagree, is that 9.7:1 with your setup is iffy enough to be too high. It *might* work, but then again, it might not. And if it doesn't work on 87, and you have to run 89 or 91, how is that going to make you feel knowing you will have to live with that? (or buy/replace parts)
On that same note, perhaps someone can answer how much power you will be "giving up" between 9.7 and 8.8:1. Compression does increase power, but I don't think the difference in one point of compression is huge when talking even in the 250-300HP range. If it's a small difference, is that going to sit well with you, knowing you aren't getting every last ounce of performance out of the motor?
My *opinion* and others are free to disagree, is that 9.7:1 with your setup is iffy enough to be too high. It *might* work, but then again, it might not. And if it doesn't work on 87, and you have to run 89 or 91, how is that going to make you feel knowing you will have to live with that? (or buy/replace parts)
On that same note, perhaps someone can answer how much power you will be "giving up" between 9.7 and 8.8:1. Compression does increase power, but I don't think the difference in one point of compression is huge when talking even in the 250-300HP range. If it's a small difference, is that going to sit well with you, knowing you aren't getting every last ounce of performance out of the motor?
Triaged
1/2 ton status
91GMCSuburban said:
Why are you only looking at 18 or 28cc dish pistons? That is a huge difference in size! Why not look for something more like a 20 or 22cc piston that might get you closer to 9:1. You can get just about any size/shape piston for a SBC off the shelf.
You also might look into cc'ing the combustion chambers and maybe do a bit of porting of the chambers to get them to all just the right size.
91GMCSuburban
1/2 ton status
I was looking at KB pistons, and thats what they offered. What other hypereutectic pistons would you suggest?
Is speed-pro still making pistons?
Badger is another I can think of off the top of my head.
Hypereutectics should be everywhere. Heck, summit or Jegs probably has a huge section on pistons alone.
Badger is another I can think of off the top of my head.
Hypereutectics should be everywhere. Heck, summit or Jegs probably has a huge section on pistons alone.
beater_k20
Banned
no.... for the Vortec combustion chamber to work properly, it needs the .035"-.045". this is precisely why you will find .015" head gaskets on MANY factory Vortec 5.7s.Mad-Dog said:
camshaft has nothing to do with static compression.
when dealing with a fullsize truck, horsepower is not the name of the game. they're too heavy. the name of the game is torque.
"Horsepower is how fast you hit the wall, Torque is how far you take the wall with you."
91GMCSuburban
1/2 ton status
Beater,
I understand your statements, but what I don't understand is how to avoid detination other than just lowering my static compression. And I believe it is with the cam and custom chip. this is the info I really need, what is thier true relationship, how do I use that info to choose correctly, etc.
I understand your statements, but what I don't understand is how to avoid detination other than just lowering my static compression. And I believe it is with the cam and custom chip. this is the info I really need, what is thier true relationship, how do I use that info to choose correctly, etc.
Triaged
1/2 ton status
To lower detonation (off the top of my head):
-Smooth every sharp corner in the combustion chamber and valve reliefs in the pistons.
-Take in cooler air (don't use an open element)
-Close the intake later (by either changing cam timing or duration). This will also reduce low end torque.
-Use a cooler t-stat (LT1's used reverse flow cooling sending the cool water to the heads first)
-Tighten up the quench as much as you can (even if it increases static compression ratio!). Some people say .035" is as close as you can go and you better measure everything for each cylinder to be sure.
-cc the cylinders (piston and head) to be sure they are all the exact same size (you don't want more compression in one cylinder).
-Polish the piston tops
-Ceramic coat the piston and chamber (some people think this is snake oil...I would still try it if I had the money)
-Put a radius on the "margin" of the exhaust valve (been a long time since I took my head rebuilding class so I might have called it the wrong thing?)
-I don't think getting rid of 'crevice' volume would hurt. Most head gaskets are larger then the bore and newer engines are moving the top ring closer to the top (like Kieth Black pistons).
-reduce spark timing (obviously)
I'm sure there are more I'm missing
-Smooth every sharp corner in the combustion chamber and valve reliefs in the pistons.
-Take in cooler air (don't use an open element)
-Close the intake later (by either changing cam timing or duration). This will also reduce low end torque.
-Use a cooler t-stat (LT1's used reverse flow cooling sending the cool water to the heads first)
-Tighten up the quench as much as you can (even if it increases static compression ratio!). Some people say .035" is as close as you can go and you better measure everything for each cylinder to be sure.
-cc the cylinders (piston and head) to be sure they are all the exact same size (you don't want more compression in one cylinder).
-Polish the piston tops
-Ceramic coat the piston and chamber (some people think this is snake oil...I would still try it if I had the money)
-Put a radius on the "margin" of the exhaust valve (been a long time since I took my head rebuilding class so I might have called it the wrong thing?)
-I don't think getting rid of 'crevice' volume would hurt. Most head gaskets are larger then the bore and newer engines are moving the top ring closer to the top (like Kieth Black pistons).
-reduce spark timing (obviously)
I'm sure there are more I'm missing
Mad-Dog
1/2 ton status
I'm sorry i did not realize we were discussing a vortec engine...
My bad...camshaft duration affects dynamic compression ratio, thanx
Torque is a direct result of horsepower....the more HP you got the more torque the engine produces, engine design dictates whether you got bottom end , midrange or topend or a little bit everywhere.
horsepower is still the name of the game at my house...!
My bad...camshaft duration affects dynamic compression ratio, thanx
Torque is a direct result of horsepower....the more HP you got the more torque the engine produces, engine design dictates whether you got bottom end , midrange or topend or a little bit everywhere.
horsepower is still the name of the game at my house...!
beater_k20
Banned
incorrect. horspower really is only an estimation, based on torque and engine RPM.Mad-Dog said:
HP = Torque x RPM / 5252
and when we're building an engine at your house, we can build it your way. until then, lets stick to the right way.
Mad-Dog
1/2 ton status
You win..... 
Triaged
1/2 ton status
One other thing that I forgot about in my list of things to keep detonation at bay. Some engines use oil squirters that squirt oil on the under side of the piston to keep the pistons cool. You can modify a SBC to do this. That is also about the only reason you would ever want a high volume oil pump in a SBC.
