et gain's with heads ....
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.
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?
Heres my theory behind it, if I build a cam only setup im going to setup my cam for max effort to work in the rpm range of my intake and header then setup the converter to work with it, if I do a h/c setup I'm going to do the same thing. I did a 233/236 598 lift 108 +3 in mine. There's more left there yet if went all out, something closer to like a trex cam. Now you get a set Afr 215s have Afr set you a cam (I'm sure they did lot's of testing to see what there heads like) and you end up with something in the mid lower 230s on a 113-114 LSA to work in the same rpm range. Why is this? Because the head is more efficient and needs less duration and valve over lap to fill the cylinder. End result... Almost no more power cause you had to change the cam. You guys are right, that's why everyone always changes the cam with a head swap. But what I'm getting at is if you setup a cam correct (and the rest of the car) that you can go nearly as fast on a stock set of heads.
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 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
it does not matter if your head flow 100cfm or 1000cfm the longer you leave it open the more air you will flow
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Verdad Gallardo 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.
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.
The heads needs to set up for your car, I don't feel it's a "one head fits all" type of deal.
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.
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