If you had 2 heads with the same flow but one had a much smaller intake runner volume would one make more hp/tq than the other...

The reason I ask is my LT1 heads flow 270cfm Intake and 195cfm exhaust with a 188cc intake runner....was comparing them to alot of the heads advertised today and heads with compareable flow numbers are running ~200cc intake runners.

Was wondering how the 2 heads would compare on the same motor.

 

Dave, really ain't that simple.

Everyone wants to think if you have a smaller runner then it will have more velocity and therefore make more TQ. Too much velocity doesn't work well since the air/fuel become seperated.

The port very well could be right at 188cc, depends on what the local shapes and cross sections down the port are.

Bret

 

Here's mho. If the car is stock bore/stroke I would definately take the smaller runner assuming port quality was comparable. If the engine was a 383 or larger take the larger runner. It is really all about filling the cylinders efficiently and the more cubes you have the more size you want (within reason). Other factors should be taken into account and different depths show different things on a bench. A bigger cc can flow much better than one with a smaller cc at all lifts and make more torque throughout the powerband or it can lose throughout the band.

It really depends much more on the quality of the portwork than the size of the runner in short.

 

A small port may flow very well at the standard test dP of 28" H2O, but in practice, the dP can be greater or less than that depending on where in the cycle you are. When the dP is greater than that, the mach number of the small port really starts getting out of hand and the losses start stacking up.

 

If you shape the port to where it is longer, it will have more cc, butcould have the same cross section as a smaller cc head.

The way I understand it the smallest cross section much more determines the peak power point etc, more so than the heads cc.

 

so in my case what head would you guys reccomend for my 396 shortblock....AFR, TFS, ETC....

My LT1 castings were ported by GTP around 1999..not sure if it was stg2 or 3, but I think the only difference was the valve spring between the 2

The car is not a daily driver.

 

396... I've had good luck with TFS heads ported for that. Espeically with porting and a flat top piston.

396, 11.8:1, small street cam, single plane = 600hp SAE

Depends on the setup. You might want as small of chamber as possible so a LT4 casting ported might be the best trick.

Bret

 

The current setup is ~12.2:1 CR with a 48CC combustion chamber, 0'd block. Current cam is a 246/254 .613/.590 112lsa

Will be running a single plane when I upgrade the heads.

I am trying to see how far I can push a hydraulic roller/stock PCM'd LT1 into the 9's...I am currently shooting for 9.8's on the LT1 casting.
The car ran a 11.3@119 (1.56) NA & 10.4@130 (1.49) fighting traction on a 150 shot.
Adding QA1 rear shock, Wolfe sway bar, and bumping up to a 250-300 shot.

 

Several things that you have to consider in the big/small question and they are:

Where is the choke point vs the overall port volume
What is the velocity in FPS at that point (can be calculated)

Port velocity has a direct relationship to where the power curve begins and ends both as far as the BHP and the torque.

My experience is that "IF" we can get the port velocity correct and in the correct place the overall volume becomes irrelevent - hence the shape determines the efficiency of the port.

Denny

 

Ray,
If the heads were done by Craig himself..hard to say...but his STR, was always very nice...not enough room for what you are doing, but a small amount of weld on the shoulder, Bigger limiting factor (throat / pinch), these heads should run good.

For some reason, Craig's work got a bad name....although it was not Craig, it was his business practices. Craig is a very capable person, his staff was not.

Dennis

 

Been down to Craig's shop in Houston and agree 100% with the above. Craig did two sets of heads for me and when I had one set independently flowed, it was exactly the same as his sheets said. Truly very sorry that his business turned a bit sour as Craig is a great guy and terrific head porter.

Back at the ranch regarding you're real questions, not quite a technical but a little more pragmatic.... For all practical purposes your combo is very close to two different 396's that I've owned. One had GTP LT4 heads that flowed 183/203 and another has (my current) has a set of TFS that flow 304/218 (both @ .600).

The later engine has a bit more cam but is similar to yours (245/248 hydraulic vs 236/242 solid roller in the first engine) but the powerbands are very different. I'd whole hardily recommend going with larger heads or at least having your current set reported to current standards. Tried a bunch of cams in the past progressively getting larger and never really improved power wise so my thinking is it's in the heads.

Believe firmly in "average hp within intended rpm" and pay much more attention to the powerband and average within intended use than peak numbers. In this case the first 396 with the GTP LT4 heads peaked @ 6100 rpm and the best shift point was around 6600-6700 rpm as it quickly ran out of gas (being figurative) after 6500 rpm. The new combo with better heads makes the same power @ 7000 rpm as it does @ 6000 rpm peaking about 6650. Little over 30 peak hp difference with less torque, but with a much better upper powerband.

On paper the average hp advantage of the later combo is:
1st Gear 11 rwhp
2nd Gear 31 rwhp
3rd Gear 36 rwhp
4th Gear 33 rwhp
5th Gear 37 rwhp

A monkey on a tricycle can run 11's with enough nitrous so if you plan of putting a bunch of juice to it, very well could be a moot point but my experience says better heads. Forget the hyper technical stuff as theory is one thing, actual results are another.