Unlike an NA Engine, a Centrifugal Blower is more than likely working in 'Compressible Flow'.
Sorry I should have been more specific. Vette is running a Centri blower with meth injection. Big difference compared to these heat pumps.
Better heads will still help.
And sometimes a good head can and will help a lot!
Simply quantifying cylinder heads via their CFM Flow rate does not tell one the complete story.
Correct size / area, shape of the port, as well as including fins that run the length of the runners floor, which help stop the flow from changing sides after convergence lift is reached, are not found with stock heads, or even ported stock heads.
***We want the flow to enter in the middle of the cylinder,
not on the shrouded side.
This does not happen on any of the LS3 style heads after the valve lift reaches convergence lift.
Before convergence lift is reached, the flow moves from entering the cylinder, in the middle of the cylinder. But after convergence lift, the flow moves to the other side, which is the shrouded side of the combustion chamber.
So then; is someone going to tell me an engine equipped with a SC don't give a damn about any of the above? Frankly porting a head to work in compressible flow is much more difficult than porting one for a simple NA Engine.
Couple the above with an incorrectly engineered head that begins to restrict the flow at higher lifts, as the intake valve is being moved towards the spark plug side of the combustion chamber, where it finds a wall where the chamber rises steeply. Here a head with an 11° valve angle is much better than one with say a 23° valve to deck angle. Simply laying back that side of the combustion chamber, as some do, might help a little, but most likely will end up hurting the flow on a live engine, headed down the track,. .. a lot!
To correct the above, one must start out with a cylinder head this is too small, but has enough material that can be safely ported to achieve all of the above, plus maybe more. . .Depending!
I prefer the 'Peanut Heads' offered for the 427 CID Engine.
What you start out with is a peanut sized runner, and for the
most part a small chamber which can have its size and shape
engineered by the porter.
Camshaft requirements are very, very different for a Centrifugal Blower. . . .
So essentially; one would require a head ported differently, as well as
a camshaft that fits not only the requirements of an NA Engine, but
now fits the cylinder head ported for compressible flow, at some(?),
but specific engine rpm, which is determined by the stroke of the
This is why cylinder head porters and camshaft grinders always ask. . . .
"At What Peak HP RPM" are we building this engine for. . . .
The next questions that needs to be answered is. . . .
At what Engine RPM will you leave the line, and how far down will the RPM drop at the shifts.
-regarding an NA engine, or one with a simple roots blower-
I desire for the cylinder heads and camshaft to be working at 135% VE at Peak HP RPM.
When the engine drops down after the shift, I look for 100% VE and the shift cannot drop the engine down below peak torque rpm.
A 376 CID engine having an 237° intake lobe duration, will have at
best, at 7200 RPM a VE of 119%. Minimum cylinder head flow
required then would be ~317 CFM.
***Our 376 cid engines have a 2.165" Intake valve. An intake lobe having a duration of 237° should be good for peak hp to be generated at about 7000 rpm. I used 7200 in order to raise the piston speed. Shifting 10% above peak hp engine rpm is not uncommon.
Now, here is the 'Kicker'. . . That flow is with the 'Intake Manifold'
bolted on. Let's assume that the manifold 'only'
drops the cfm flow
rate by 10%.
That means that the cylinder head must flow 350 cfm on the bench at 28" of water drop.
And since the flow moves to the wrong side of the port at or around
convergence lift, we can't run the higher lift camshaft profiles.
The flow needs to be 'All In' at or around 0.550" to maybe 0.600" of
net lift. So we can't lower the duration and add lift to generate more flow.
***A well engineered Hi-Perf engine will use a ratio of about 0.37, multiplied by the diameter of the valve, instead of 0.25 which is used to calculate the convergence lift. If we multiply 0.37 by 2.165" intake valve, the valve lift would then be 0.801", whereas convergence lift is found at 0.541" of valve lift.
The 237° lobe duration coupled with a 'Net Lift' of 0.600" should yield a fwhp of ~900 fwHP on quality gasoline, assuming that the blower fits the adiabatic efficiency defined by taking the square root of the absolute boost. I used a value of 2-BAR Absolute, which yields a density ratio of 1.41.
Finally, none of this can be simply reduced down to a statement such as;
"Just turn up the boost and let the blower do the work."