Thread:

lost rpm with top end change.

motor is a 396 with a 454 crank (434). i had small oval port semi-closed chamber heads, 10.5:1, with an edelbrock performer rpm and a 650 holey dp. I came across some iron merlin rectangle port open chamber heads w/ 1.88/2.30 valves, 9.5:1, i put a victor jr and a 750 holey dp on it. cam is a solid flat tapet, 258 duration at .050, .615 lift, 108 lsa. im running logs and 4" risers. pump is a fresch jg with a detailed bowl and b+ stainless impeller, loader and diverter.

i was turning 5200 rpm now im down to 5000 rpm. im thinking i need to go with a smaller impeller to make up for the bigger ports and loss in compression. i know its running a little rich but not enough to loose 200 rpm in my opinion. any input is greatly appreciated.

Did you degree the cam? 2 degrees adv? whats the timing set at?

cam is degreed in ar 106* lobe center and timing is 34* total

Originally Posted by wpmxwb

cam is degreed in ar 106* lobe center and timing is 34* total

Bump the timing to 38 total all in by 3000rpm. Your logs are not helping but that should do it for ya!

Originally Posted by jetboatperformance

Ran this (similar Combo (434) in my RR boat with 2 600 Holleys on a low rize , MSD , 10 1/2 to 1 , iron rec ports , balanced , solid roller turned a AA 5100 , for comparison for ya Tom

ive been told to go to an 850 maybe that could be part of the problem. but dam that makes my motor sound like its got a big problem.

Originally Posted by PS&B1

Bump the timing to 38 total all in by 3000rpm. Your logs are not helping but that should do it for ya!

ill try putting some more timing in it. i really wanna get rid of the hei and get an msd at the same time. ive got a set of bassetts that are going on as soon as i get the money to get them coated

Just for grins I would try the smaller carb again just to see if the loss was carb tuning. The open chamber heads should work better than the old school heads even with less compression. Wouldnt take a lot of time or money to swap carb.

Just my .02 Every move you made either reduced peak torque or moved the torque curve to higher rpm. The bigger heads really reduce flow velocity. I would borrow a larger carburetor and see if you get the rpm back before messing with the pump. Easier to do and less costly. I do not see the logs being the point of restriction necessarily. I have a 540 that turns 5,400 rpm with "A" berkely impeller with logs similar to those you are describing so pretty sure you can get the exhaust out. Sounds like you may be hurting on getting air and fuel in. As I keep reminding my wife, bigger is not always better.

Originally Posted by jetboatperformance

Ran this (similar Combo (434) in my RR boat with 2 600 Holleys on a low rize , MSD , 10 1/2 to 1 , iron rec ports , balanced , solid roller turned a AA 5100 , for comparison for ya Tom

Tom, Dont mean to jack thread, but what manifold is that on that bbc, thanks much

I don't mean this to sound like I am being a smart ass...but...

you aren't running NOS, you aren't running huge cubes or compression, and you aren't running big RPM....

so why the desire to ditch the HEI?