103" build compression question
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Intermediate
Joined: Aug 2012
Posts: 47
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From: SW MO
103" build compression question
Hello. I am rebuilding my TC96 after an inner cam bearing failure, and had a question on compression. I am boring the cylinders out to 3.875". Stroke length is stock. Heads are stock. Head gasket will be .040" Cometic. I am putting in Andrew's TW50 cams which are speced for 9.5-9.8:1 comp (I am unsure if they mean static or corrected). I bought KB Forged pistons with a 6.9cc dish. KB lists compression with this piston at 9.5:1 in a stock TC96 with a 103 Big bore kit. I was inclined to believe this is what I would get on my build, but maybe that's not the case. After playing with some of the calculators on various motor builders websites, it looks like it will actually be lower. I am concerned with not having enough compression for these cams. Maybe I should have got the flat top pistons. Have I made a mistake with the dished pistons? I haven't bored the cylinders yet so there's still time to get a different set. I would appreciate any advice. Thank you
Thread Starter
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Intermediate
Joined: Aug 2012
Posts: 47
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From: SW MO
Are you running stock heads and stroke? That would make the KB specs accurate for compression
Tourer
Joined: Dec 2009
Posts: 440
Likes: 7
From: wisconsin
My calcs are 10.82to1 static comp with those pistons and with the cam figured in your at 9.58to1 I like this number at 9.2 to 9.3 and where your at is 200.7 cold cranking psi which is high. This number I like 185 to 190 and i would suggest comp releases.Your numbers with what your doing are to high and it could be a ping monster.IF you what 10.1 static comp. .30 headgasket s.e. flat tops 9.0 corected comp 185 cold cranking psi This is more what your looking for.
Thread Starter
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Intermediate
Joined: Aug 2012
Posts: 47
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From: SW MO
I must be figuring it wrong. I have been using the calculator on KB's site. I will look up the actual formulas and try again. Here is what the calculator looks like I have been using with my numbers entered:
ENTER YOUR DATA CALCULATED DATA
Cylinder Head Volume (cc) 85 Cylinder Head Vol (cubic in.) 5.185
Piston Head Volume (cc) 6.9 Piston Head Vol (cubic in.) 0.421
Gasket Thickness (in.) .040 Swept Volume (cubic in.) 51.595
Gasket Bore (in.) 3.875 T.D.C. Volume (cubic in.) 6.137
Cylinder Bore Diameter (in.) 3.875 Gasket Volume (cubic in.) 0.472
Deck Clearance (in.) .005 Deck Volume (cubic in.) 0.059
Stroke (in.) 4.375
STATIC COMPRESSION RATIO
9.407
OPTIONAL DATA
Rod Length (in.) 7.667 Adjusted Stroke (in.) 3.964
Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees 48
DYNAMIC EFFECTIVE COMPRESSION RATIO
8.617
ENTER YOUR DATA CALCULATED DATA
Cylinder Head Volume (cc) 85 Cylinder Head Vol (cubic in.) 5.185
Piston Head Volume (cc) 6.9 Piston Head Vol (cubic in.) 0.421
Gasket Thickness (in.) .040 Swept Volume (cubic in.) 51.595
Gasket Bore (in.) 3.875 T.D.C. Volume (cubic in.) 6.137
Cylinder Bore Diameter (in.) 3.875 Gasket Volume (cubic in.) 0.472
Deck Clearance (in.) .005 Deck Volume (cubic in.) 0.059
Stroke (in.) 4.375
STATIC COMPRESSION RATIO
9.407
OPTIONAL DATA
Rod Length (in.) 7.667 Adjusted Stroke (in.) 3.964
Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees 48
DYNAMIC EFFECTIVE COMPRESSION RATIO
8.617
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Tourer
Joined: Dec 2009
Posts: 440
Likes: 7
From: wisconsin
I must be figuring it wrong. I have been using the calculator on KB's site. I will look up the actual formulas and try again. Here is what the calculator looks like I have been using with my numbers entered:
ENTER YOUR DATA CALCULATED DATA
Cylinder Head Volume (cc) 85 Cylinder Head Vol (cubic in.) 5.185
Piston Head Volume (cc) 6.9 Piston Head Vol (cubic in.) 0.421
Gasket Thickness (in.) .040 Swept Volume (cubic in.) 51.595
Gasket Bore (in.) 3.875 T.D.C. Volume (cubic in.) 6.137
Cylinder Bore Diameter (in.) 3.875 Gasket Volume (cubic in.) 0.472
Deck Clearance (in.) .005 Deck Volume (cubic in.) 0.059
Stroke (in.) 4.375
STATIC COMPRESSION RATIO
9.407
OPTIONAL DATA
Rod Length (in.) 7.667 Adjusted Stroke (in.) 3.964
Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees 48
DYNAMIC EFFECTIVE COMPRESSION RATIO
8.617
ENTER YOUR DATA CALCULATED DATA
Cylinder Head Volume (cc) 85 Cylinder Head Vol (cubic in.) 5.185
Piston Head Volume (cc) 6.9 Piston Head Vol (cubic in.) 0.421
Gasket Thickness (in.) .040 Swept Volume (cubic in.) 51.595
Gasket Bore (in.) 3.875 T.D.C. Volume (cubic in.) 6.137
Cylinder Bore Diameter (in.) 3.875 Gasket Volume (cubic in.) 0.472
Deck Clearance (in.) .005 Deck Volume (cubic in.) 0.059
Stroke (in.) 4.375
STATIC COMPRESSION RATIO
9.407
OPTIONAL DATA
Rod Length (in.) 7.667 Adjusted Stroke (in.) 3.964
Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees 48
DYNAMIC EFFECTIVE COMPRESSION RATIO
8.617
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Thread Starter
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Intermediate
Joined: Aug 2012
Posts: 47
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From: SW MO
Big Boyz comes out almost identical. 9.38 static, 8.33 corrected, 163 CCC. I set "Piston Dome Volume" at -6.9 because I have a dish. If I set it at 6.9 the calcs come out similar to yours. On KB's site they have you enter a dish as a positive value and dome as negative. Looks like Big Boyz does it the other way around.
If this is the case I can go with a .030 head gasket which will gain me .2 static comp. Any opinions on whether or not .2 compression will make any difference at all? Based on conversations with experienced builders, I don't think so, but now is the time to get it right while everything is tore down. I have seen this cam do well in a stock TC103 which is supposed to have 9.6:1 comp, but I'm willing to bet it's a bit below that. I might be splitting hairs here. By the way, thanks for the input guys.
If this is the case I can go with a .030 head gasket which will gain me .2 static comp. Any opinions on whether or not .2 compression will make any difference at all? Based on conversations with experienced builders, I don't think so, but now is the time to get it right while everything is tore down. I have seen this cam do well in a stock TC103 which is supposed to have 9.6:1 comp, but I'm willing to bet it's a bit below that. I might be splitting hairs here. By the way, thanks for the input guys.
HDF Community Team
Joined: Jun 2006
Posts: 12,776
Likes: 2,607
From: san antonio
Just a couple of comments for the OP to consider.
The TW50 cams are mid and high end performers. If you are looking for that kind of performance and not expecting much on the low end, they will work well for you.
KB has been in the piston business a long time and the kind of variance you have seen between their calculator and the BigBoyz calculator should be sorted out. As he points out, now is not the time to be guessing or making assumptions. The variace is worth a call to KB to bottom out the variance; there shouldn't be any.
If there are valve reliefs in the top of the piston, ask KB to verify if the piston volume includes valve reliefs.
I like to check piston dome/dish volume so I know for sure what I am dealing with and that both are close enough that equalization is not necessary.
A good target for compression would be 190 to 200 psi (with compression releases) and/or 9.3-9.5 corrected. Needless to say, a good fuel management system and tune should be part of the plan.
The TW50 cams are mid and high end performers. If you are looking for that kind of performance and not expecting much on the low end, they will work well for you.
KB has been in the piston business a long time and the kind of variance you have seen between their calculator and the BigBoyz calculator should be sorted out. As he points out, now is not the time to be guessing or making assumptions. The variace is worth a call to KB to bottom out the variance; there shouldn't be any.
If there are valve reliefs in the top of the piston, ask KB to verify if the piston volume includes valve reliefs.
I like to check piston dome/dish volume so I know for sure what I am dealing with and that both are close enough that equalization is not necessary.
A good target for compression would be 190 to 200 psi (with compression releases) and/or 9.3-9.5 corrected. Needless to say, a good fuel management system and tune should be part of the plan.