For me the "provable" improvements were in trap speeds. Launch (and with me shifting) has so much to do with ETs that changing heads gained me 3-4 MPH traps but almost incidental ETs. FWIW it's hard to launch a M6 IRS GTO.

 

3-4 mph is vary respectable imo, that's a solid 40hp in that car, I'd be interested in a few more details, 5.7 or 6.0? Cam/Bolton car before?

 

My car gained 48rwhp / 21tq going from 806 heads to prc 2.5 5.3's. Runs better than when it was cam only but I dont think the 228R was meant to be ran on stock heads anyways.

 

This theory is wrong.
Clearly, ported heads flow better than stock castings. Therefore, while youre on stock castings, you dont need to run as much durarion and lift, with as aggressive valve events because at a certain point, the heads simply cant keep up and youll get diminishing returns.

Now when you go to ported/aftermarket castings, that limit gets raised by a considerable amount. The velocity and cfm of the head increases, along with efficiency in the combustion chamber among other things. Now, if youre going for max effort, you need to cam more. The cam you had with the stock heads is now not sufficient for the ported heads because the have the ability to flow more. The more air and fuel you can cram into the cylinder, the more power youll make. Now going with a smaller cam with ported heads like you say is a great idea for a street car, but max effort, youre going in the wrong direction.

 

^^^This x1000. I totally picked way too big of a cam and my dyno numbers suck when compared to my mods. Until I port the heads they will continue to suck no matter what I change lol.

 

I don't completely agree with this, duration is how long the valve is open, it does not matter if your head flow 100cfm or 1000cfm the longer you leave it open the more air you will flow, the factory head also flows great up to 650 lift. The more air movement you have in the head the more cylinder pressure you will bleed off in the lower rpm being the reason you actually have to run less duration. The added low end and throttle response comes from the added compression and more efficient chamber most heads will come with. 10cfm of actual added air movement will get you pretty close to 10hp, an ls6 intake can't really support much over 260. If your ported past that your start slowing down air speed which decrease's efficiency. As I stated in my vary first post I'm a believer in the ls6 head over the factory ls1 but anything past that just haven't really proven to be a benefit at the track. If the engine was bigger or being revved higher there would be a much bigger increase in performance with these ported heads

 

I see a lot of people spinning SBEs to 68-7000. Now i don't have any real life experience, but you would think the better your heads flow, the more air you'll flow for a given amount of duration. Even if the intake can't support that much air flow, you will still gain for the increased amount of flow from the ported heads. Sadly, i don't have the knowledge in the field to support my theory, but we have seen cars pick up time and time again on the forums. Im not sure why youre not seeing gains when going from 243s to ported/aftermarket castings.

 

It very much does matter because for the same duration, a ported head will allow more air in the chamber. So two engines with the same exact cam and intake setup but one has heads that flow 300cfm and ones that flow 250cfm, the one with the better heads will allow more air into the chamber during the intake stroke thus make more power. The engine with the stock heads will have to have a cam with increased duration to allow the same kind of air as the other engine with ported heads. But then you can put the same duration cam with the ported heads and make more power than the same cam but on stock heads.

 

Yeah I would agree, what most people don't think about is how you setup your runner will also affect were you make more power, for instance if you port down the short turn on the runner it will add high rpm power and take away low end torque, it's kind of trade off. Most companies will do that cause they will look better on a flow bench and on a dyno, but in the real world the loose in torque can hender your 1/8 mile and all you see is a gain in mph. That's why i feel most heads won't pick up much et. The heads needs to set up for your car, I don't feel it's a "one head fits all" type of deal.

 

Well that's when you look at dyno graphs of said heads and see if there is any torque gains down low or is all the power just found up top. Which is why most people on here recommended to search for the many dynographs posted because those will show you where the heads are designed for (torque all over or just topend power).

That's true to a certain point, you have to get the heads ported or runner size given for what you need. You don't want to put AFR 245s on a mild 346 just as you don't want to put AFR 205s on a 454 monster. However, with the relatively small amount of customization available on heads (as most places have a set runner program for their heads) most people do it the other way eg. get the heads and then customize the cam and intake setup around those. Then you have a well sorted setup.

If you're looking for the ultimate customization for your own needs, then you should look into the many porters out there that do custom hand work on off the shelf heads that deal with certain aspects of those that need more attention. For example, you hear many good things of "Mamofied" heads where he takes an already good AFR casting and puts in a custom hand touch for added benefits wherever the owner needs them. So there's really no excuse for a ported head to be put onto a well setup engine and make the car slower. If you do your homework and buy the right combo a nice set of heads will definitely make your car faster.

 

Huh?