Mez

In last month's Hot Rod Magazine, they compared 11 cylinder heads on a 408 ci LS engine. I brought this up a few weeks ago when the magazine first showed up in my mail box.

So I did charts to help me better understand the results. Below are two charts. One that compares the Average HP & TQ vs. Intake/exhaust port CFM @ .600", and intake port cc and the second that compares the Peak HP & TQ vs. vs. Intake/exhaust port CFM @ .600", and intake port cc.

The results are a real eye opener but somewhat confusing. Note how close all of the results were. There appears to be little correlation between CFM and power as evidenced by the heads with the ~350 CFM did not make more power than heads in the ~320 CMF area. This raises the question "WHY?". One of the comments made in the article was perhaps the higher CFM heads were intended for bigger cubic inch motors.

In my opinion, the article missed the mark. The majority of guys are using these heads on 6.0L and 6.2L engines than a 408 stoker with a big cam.


Disclaimer:
These results are applicable for this engine, cam, headers, and intake manifold combination only. Your results may vary.

PRE-Z06

Interesting, thanks for putting that together and sharing.

Joe_G

Good post Mez.

I agree with you - the article would have been much better had they chosen a stock cube ls3 or even ls2 with a cam to check the differences. For all the guys building big cube engines, there must be 10x more just putting on heads and cam on their existing short blocks.

That said, Hot Rod remains one of my favorite reads each month and I think Frieberger is going a great job.

AR Headers

I especially love the headers they used to conduct all those tests.

Mez

Someone posted a comment elsewhere that the FAST 102 intake manifold is the limiting factor and if they had put a sheet metal intake on them, the results would be different. While this seems logical, since the test did not do that, we will never know. And where are these sheet metal intakes, what do they cost, and will they fit under the stock hood? The only one I know about is the Harrop which costs a whole lot. Who has one of these?

In any case, what I got from this article if you are building a 408ci with this particular cam, it really does not matter very much which head you choose. I mean, other than the Pro-Comp, they all were within a couple of HP which probably is the margin of error of the engine and test equipment.

This test seems to indicate that reports of 30 rwhp difference between two different aftermarket heads does not exist. I have personally seen a 25 rwhp improvement on an LS1 when the owner swapped out huge aftermarket port heads to smaller port heads. So, there must be an valid reason why the results were as close as they say. For now, I am buying the idea of the intake manifold is the limiting factor. So, for stock displacement an LS2 and LS3, does this mean you should save your money and just port the stock castings? I am confused...

AirBusPilot

I wish they'd tested a stock LS3 head.

It looks like you can conclude that adding cfm beyond a certain point isn't needed in this application.

Mez

Actually, in some cases higher CFM may hurt torque/power as it can decrease velocity which is what you want for cylinder filling.

This just reinforces that its assembly of the right combination of heads, intake, cam, and exhaust.

In another recent dyno I witnessed, they owner had built a 421ci Dart block for a Tahoe and it made only 340 rwhp. It was only 9:1 compression, shorty 1.5" headers, Edlebrock ram-jet type intake, and mild cam. The owner was extremely disappointed with the low HP numbers. We concluded even with 421ci, the compression and small shorty headers were choking it.

old motorhead

Agree...and it's kinda crazy when folks point to specific builds tested on various dynos, and try to make conclusions based on those numbers. A group of parts tested on one motor, with one dyno, with no agenda......has a whole lot more validity in my book.

ctusser

Robz in C5 section is running a Harrop on a destroked 427 block 388ci I think, and he makes about 700whp on a dynojet and runs low nines Natrually aspirated. You could PM him if interested. Of course he makes that kind of power b/c he is able to rev to over 9000rpm!

http://www.youtube.com/watch?v=PaxE6dsfb_o

csnow

Thanks for putting all of those side by side.

I read that article and went with Texas Speed ported stock castings. I was able to sell my heads on Craigslist within 24 hours to an LS1 owner, so the ported heads only ended up costing me $800. The extra 15 - 20hp wasnt worth 1.5k more to me over ported stock heads. For $800 I picked up some good power and turned the mileage clock from 35k back to 0 on the top end of my motor.

Mez

Oh, that must be fun. 9000 rpm. What kind of manifold design works at 9000 rpm? Must be a short runner.

Mez

Depends on you cam, too.

ctusser

Mez

Sick.