Quick PTV/DCR question
Thread Starter
Melting Slicks
Joined: Oct 2007
Posts: 2,569
Likes: 170
Quick PTV/DCR question
Stock bottom LS2
.040 Cometic
63cc AFR 205s
224/230 113+3 cam (67 IVC)
Should be about 11.56:1 SCR, 8.98:1 DCR (assuming pistons are .006 out of the hole)
I run 94 octane
Can anyone verify that this cam will work PTV wise and run on 94 octane with that DCR (if my calculations are correct)?
.040 Cometic
63cc AFR 205s
224/230 113+3 cam (67 IVC)
Should be about 11.56:1 SCR, 8.98:1 DCR (assuming pistons are .006 out of the hole)
I run 94 octane
Can anyone verify that this cam will work PTV wise and run on 94 octane with that DCR (if my calculations are correct)?
Last edited by lt1z; Jun 11, 2009 at 07:48 PM.
Le Mans Master
Joined: Jul 2005
Posts: 5,328
Likes: 405
Stock bottom LS2
.040 Cometic
63cc AFR 205s
224/230 113+3 cam (67 IVC)
Should be about 11.56:1 SCR, 8.98:1 DCR (assuming pistons are .006 out of the hole)
I run 94 octane
Can anyone verify that this cam will work PTV wise and run on 94 octane with that DCR (if my calculations are correct)?
.040 Cometic
63cc AFR 205s
224/230 113+3 cam (67 IVC)
Should be about 11.56:1 SCR, 8.98:1 DCR (assuming pistons are .006 out of the hole)
I run 94 octane
Can anyone verify that this cam will work PTV wise and run on 94 octane with that DCR (if my calculations are correct)?
Don't assume the pistons are .006" out of the hole. When you're running this close to the edge on a high performance engine, you need to verify every dimension twice. Having said that, there are two issues with your request for PTV clearance opinions. The first is you didn't give the lift specs on your cam/valve reliefs in your pistons, and the second is what I just said about measuring piston deck height. Anything you get here about PTV clearance in your engine with this cam is going to be just an opinion unless they've run your exact combination...just measure PTV clearance and you'll know for sure.
Last edited by glass slipper; Jun 12, 2009 at 09:09 AM.
Thread Starter
Melting Slicks
Joined: Oct 2007
Posts: 2,569
Likes: 170
My 67 IVC was correct for a 113+3 LSA. 113+0 would be a 70 IVC. I would need +6 degress of advance to get to a 64 IVC and that would send my DCR even higher. I gave the IVC using the .006 number so I didnt have to list all the specs. Lift has very little to do with ptv compared with duration since at max lift the piston is at BDC. If I had usedthe .050 number It would be 42
Le Mans Master
Joined: Jul 2005
Posts: 5,328
Likes: 405
My 67 IVC was correct for a 113+3 LSA. 113+0 would be a 70 IVC. I would need +6 degress of advance to get to a 64 IVC and that would send my DCR even higher. I gave the IVC using the .006 number so I didnt have to list all the specs. Lift has very little to do with ptv compared with duration since at max lift the piston is at BDC. If I had usedthe .050 number It would be 42
While lift has little to do with PTV, it is a factor. A .650" lift cam will be a bigger concern than a .525" lift cam. Actually, max lift occurs at 116° ATDC for the intake and 110° BTDC for the exhaust both of which are pretty far from BDC. The lifter is at .006" at 27° BTDC and .050" at 4° ATDC, if you still have the 1.7 ratio rocker arms, that puts you at .085" valve lift at 4° ATDC. Assuming linear valve movement (which we know it isn't, but close enough for demonstration), valve lift would be ~.075" at TDC. Of course the smallest PTV clearance doesn't necessarily happen at TDC since the valve is moving faster than the piston but the numbers around TDC indicate no problems...just check it to make sure if you're concerned.
Thread Starter
Melting Slicks
Joined: Oct 2007
Posts: 2,569
Likes: 170
You're correct on the .050" number. I subtracted 113° from 180° and came up with 67° which was the IVC point you gave as a coincidence. After I checked to see if you gave the seat-to-seat timing, I forgot I hadn't continued my calculations by subtracting 67° from 112° and then subtracting 3° from that to come up with 42° which is clearly not 67°. Sorry about that...doesn't take much for me to get off track sometimes.:o
While lift has little to do with PTV, it is a factor. A .650" lift cam will be a bigger concern than a .525" lift cam. Actually, max lift occurs at 116° ATDC for the intake and 110° BTDC for the exhaust both of which are pretty far from BDC. The lifter is at .006" at 27° BTDC and .050" at 4° ATDC, if you still have the 1.7 ratio rocker arms, that puts you at .085" valve lift at 4° ATDC. Assuming linear valve movement (which we know it isn't, but close enough for demonstration), valve lift would be ~.075" at TDC. Of course the smallest PTV clearance doesn't necessarily happen at TDC since the valve is moving faster than the piston but the numbers around TDC indicate no problems...just check it to make sure if you're concerned.
While lift has little to do with PTV, it is a factor. A .650" lift cam will be a bigger concern than a .525" lift cam. Actually, max lift occurs at 116° ATDC for the intake and 110° BTDC for the exhaust both of which are pretty far from BDC. The lifter is at .006" at 27° BTDC and .050" at 4° ATDC, if you still have the 1.7 ratio rocker arms, that puts you at .085" valve lift at 4° ATDC. Assuming linear valve movement (which we know it isn't, but close enough for demonstration), valve lift would be ~.075" at TDC. Of course the smallest PTV clearance doesn't necessarily happen at TDC since the valve is moving faster than the piston but the numbers around TDC indicate no problems...just check it to make sure if you're concerned.
The cam has .610 intake lift using a 1.7:1 rocker and is actually the same intake lobe I already have in the motor and running good. Only thing is that the IVC on the current cam is 70* and I am moving to a 67* IVC on the same intake lobe and I know that has an effect on PTV. Thanks for your reply.
