[Stan Weiss]

Originally Posted by Jay S

I read through the speed talk post’s by Vizard. A lot of good info there, but my conclusion is there is not a whole lot of info that can be moved to a d port Pontiac from his BBC info, including the 128 rule, or his reverse split cams. He alluded why that is the case in his post when he was talking about head flows at the low lifts and the effect it has on the cam specs, LCA, ect. If his BBC cam tech does not allow for those changes between the platforms the cam engine combo is likely not going to run right. Especially on pump gas. I think his tech does allow for those inputs, but his basic calculators do not. One obvious input that is not correct on the 128 graph for a Pontiac is the 30 intake valve. The 30 and 45 intake valve flow different, and that 128 rule is for 45* valves.

If the engine has big exhaust flow numbers and the cam has a smaller E/I split then there is a good chance Bullet will adjusted the LCA narrower to keep the overlap where it needs to be. The biggest change is at the EVO event. That is usually what Tim at Bullet will adjust for the E/I ratio. From what I have seen, Steve C’s current cam does exactly that.

With regard to a reverse split, my suggestion on the split were more geared toward the seat timing, not really at .050” like Vizards comments on ST. I do the nearly the same thing on engines with a V series Rhoads by more lash on the exhaust side.

I think the reverse split could be make to work in a d port 455 Pontiac, but I think it would take head work to make it work right. The 30* intakes would have to go and the bowls below the valve would need to be opened up a lot.

Better vacuum was mentioned for fuel economy. The 30* valves increase the low head flows a bunch, but hurt vacuum. If you were truly after economy and hp had good lift on the cam with the head flow to back it up they need to be switched to 45* IMO. All the dyno tests I have looked with decent heads and cam the 30* intake hurt the tq at very low rpm when compared to a 45 in the same engine. But once they start working they can turn the engine into a fire breathing tq monster. Problem is though that can also create some major issues on pump gas. Especially if the cam is small.

I have probably ran some reverse splits. I can’t think of to many other than from the factory cams like IHC used on there v8. Those big reverse splits can be difficult to tune on pump gas. I know with just a slight advance the IHC V8’s have to be run on higher octane gas. We have several old IHC trucks and pickups. I have done a lot of single patterns in Pontiacs that ran great, I have a hard time picturing the reverse split working very well on a d port. By the time the cam would be big enough to manage the compression the idle would be getting a bit snarly.

Jay,
Do you have a link to which thread on Speed Talk about David and 128 you are talking about?

While David may have said it when talking with me, I just don't remember him talking about reverse split cams. We did talk about single pattern cam.

David has said that BBC 's should use 132.

Stan