What compression............
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Joined: Nov 2001
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From: Centrifugal City
Re: What compression............ MTI is building my new motor at 9:1 and said all their FI motors are at 9:1, I was thinking a bit higher around 9.2-9.3:1, but I'm ok with it.
Looking for 12# + 50-75 shot.
Looking for 12# + 50-75 shot.
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From: Raleigh, NC
Re: What compression............ Even at 8.5:1, with that many cubes AND FI, the torque will be so high, it will have plenty of umph down low. You will be able to run BIG boost with that compression. It won't impress anyone NA, but it will be a killer at 18# or 20# or more boost! 
I don't know what your plans are, but you are going to need a lot of fuel to feed that!
If you are going to run only, say, max 15 PSI of boost, I'd make it 9.0:1. If it will run max 10 to 12 PSI, make it 9.5:1.
I don't know what your plans are, but you are going to need a lot of fuel to feed that!
If you are going to run only, say, max 15 PSI of boost, I'd make it 9.0:1. If it will run max 10 to 12 PSI, make it 9.5:1.
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From: College Station, Tx
Re: What compression............ Static compression is also only a part of the entire package - your dynamic compression ratio is going to have a much larger effect on what kind of gas your car requires, etc. You could have a 14:1 NA car, but if you close the intake valve late enough you can bleed away a good bit of that compression.
On a blower car you will see this as a greatly reduced boost pressure - just swapping out the cams could change your measured manifold boost at the same rpm, pulley combo, etc. by 5+ lbs.
If you are turning higher rpm you can get away with a lower DCR (later intake valve opening/static compression combination) - and take more "boost" and power on the same octane.
Ideally for a blower you would determine the maximum massflow point on your compressor and set that as your target. Decide what kind of rpm you want to turn, and for a give set of cubic inches that will delineate your cam choice fairly well. From there you will know what kind of DCR you will need to run based on the power/boost/rpm it will be made at - and going with your target DCR and your intake valve closing you can come up with the appropriate SCR.
When unsure it's probably better to err on the lower compression side unless it's an a serious race car/running race gas all the time isn't a problem.
If you want you can solve for DCR with
DCR = 1 + (((Stroke - ( Stroke * (1/2 - 1/2*(cos(IVC * pi / 180))))) * 2.54^3 * Bore ^2 * pi / 4) + CompressedVolume) / CompressedVolume
Compressed Volume is in CC's.
On a blower car you will see this as a greatly reduced boost pressure - just swapping out the cams could change your measured manifold boost at the same rpm, pulley combo, etc. by 5+ lbs.
If you are turning higher rpm you can get away with a lower DCR (later intake valve opening/static compression combination) - and take more "boost" and power on the same octane.
Ideally for a blower you would determine the maximum massflow point on your compressor and set that as your target. Decide what kind of rpm you want to turn, and for a give set of cubic inches that will delineate your cam choice fairly well. From there you will know what kind of DCR you will need to run based on the power/boost/rpm it will be made at - and going with your target DCR and your intake valve closing you can come up with the appropriate SCR.
When unsure it's probably better to err on the lower compression side unless it's an a serious race car/running race gas all the time isn't a problem.
If you want you can solve for DCR with
DCR = 1 + (((Stroke - ( Stroke * (1/2 - 1/2*(cos(IVC * pi / 180))))) * 2.54^3 * Bore ^2 * pi / 4) + CompressedVolume) / CompressedVolume
Compressed Volume is in CC's.
Re: What compression............
heads 64cc, head gaskets msl 6.0, flat top pistions with gas relief of -2? stock length rod, And I asume stock everything else? THANKS
Just be sure you measure it when the engine goes together. If your pistons don't look like a dish and has the stock rise (.01 out of the bore) then your compression is going to end up being around 10.3:1
The numbers I used:
Bore: 3.8976 in
Stroke: 3.662 in
Piston Dish: 2.8 cc
Comb Chamber: 64 cc
Gasket Thickness: .06 (works out to 11.7 cc)
Piston Rise: -.07 in
You might want to check and see what the piston manufacturer has in mind for your pistons.
Good Luck. It should be a missle when you get it all together.
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From: College Station, Tx
Re: What compression............ Black LS1 T/A - yep, thanks - sent you another one.
The biggest variable is going to be the heads here - since they are ported, etc. the actual cc's could be all over the board - you should have them cc'd if it's not to late already so you can know where you are starting.
The biggest variable is going to be the heads here - since they are ported, etc. the actual cc's could be all over the board - you should have them cc'd if it's not to late already so you can know where you are starting.
Thread Starter
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Joined: Oct 2002
Posts: 1,648
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From: F.H,Waterford,Port Huron, MI
Re: What compression............ NOGO Thanks that realy helped me understand. My heads ar CNC ported so I will check all the real numbers on the paperwork at the shop before it goes together. Sounds like it is ok. Thanks again guys!
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Joined: Sep 2002
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From: Redondo Beach, CA
Re: What compression............
Even at 8.5:1, with that many cubes AND FI, the torque will be so high, it will have plenty of umph down low. You will be able to run BIG boost with that compression. It won't impress anyone NA, but it will be a killer at 18# or 20# or more boost!
I don't know what your plans are, but you are going to need a lot of fuel to feed that!
If you are going to run only, say, max 15 PSI of boost, I'd make it 9.0:1. If it will run max 10 to 12 PSI, make it 9.5:1.
I don't know what your plans are, but you are going to need a lot of fuel to feed that!
If you are going to run only, say, max 15 PSI of boost, I'd make it 9.0:1. If it will run max 10 to 12 PSI, make it 9.5:1.
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Joined: Dec 2001
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From: Dublin, CA
Re: What compression............ ChrisB - interesting formula.
I'm stock bore & stroke 346 with 71cc heads and 8.6cc dished pistons. I have a 'Thunder racing blower cam' but just looked at it, and the specs have been ground off the end. (WHY?!?) I'm ordering Cometic gaskets and noticed they come in different thicknesses, gotta figure that one out too...
I'm stock bore & stroke 346 with 71cc heads and 8.6cc dished pistons. I have a 'Thunder racing blower cam' but just looked at it, and the specs have been ground off the end. (WHY?!?) I'm ordering Cometic gaskets and noticed they come in different thicknesses, gotta figure that one out too...
Thread Starter
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From: F.H,Waterford,Port Huron, MI
Re: What compression............ I checked at the shop and all the numbers are right but the heads, they are CNC 76cc. So no matter what it sounds like I am ok. thanks guys!!
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Joined: Dec 2001
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From: Dublin, CA
Re: What compression............ Let me double-check that. First for cylinder volume:
OK, that checks out. Next for CR:
Hmm, sounds a bit low for street before gaskets. I may have to up boost a bit. Or mill heads and shorten push rods.
Cylinder Volume = pi · r2 · height
Stock = 3.905" bore & 3.622" stroke
pi = 3.1415926535897932384626433832795
pi · (3.905/2)^2 · 3.622 = 43.37908665999562618694284242379ci
43.37908665999562618694284242379ci · 8 = 347.03269327996500949554273939032ci
347CID
Stock = 3.905" bore & 3.622" stroke
pi = 3.1415926535897932384626433832795
pi · (3.905/2)^2 · 3.622 = 43.37908665999562618694284242379ci
43.37908665999562618694284242379ci · 8 = 347.03269327996500949554273939032ci
347CID
head chamber = 73cc
piston dish = 8.6cc
gasket = ? (will edit when I figure this out)
total = 81.6cc
1.00 cu-in = 16.387 cc
43.37908665999562618694284242379ci · 16.387 = 710.85309309734832632543235879865cc
710.85309309734832632543235879865cc/81.6cc = 8.7114349644282883128116710637083:1
73cc chamber + 8.6cc dish on a stock size 347 = 8.7114349644282883128116710637083:1 =
8.7:1 compression ratio
71cc chamber + 8.6cc dish on stock size 347 = 8.9303152399164362603697532512387:1 =
8.9:1 compression ratio (my motor)
piston dish = 8.6cc
gasket = ? (will edit when I figure this out)
total = 81.6cc
1.00 cu-in = 16.387 cc
43.37908665999562618694284242379ci · 16.387 = 710.85309309734832632543235879865cc
710.85309309734832632543235879865cc/81.6cc = 8.7114349644282883128116710637083:1
73cc chamber + 8.6cc dish on a stock size 347 = 8.7114349644282883128116710637083:1 =
8.7:1 compression ratio
71cc chamber + 8.6cc dish on stock size 347 = 8.9303152399164362603697532512387:1 =
8.9:1 compression ratio (my motor)