Ported GT-40x heads vs AFR185

[hi-tech cobra]

I need help with head selection.

I have a set of ported gt-40x heads with the Ford Racing GT40 VALVE TRAIN KIT for my 347.

I'm concerned the heads will limit what the engine is capable of. The local machine shop thinks my concerns are overblown. They don't think I will gain much with the AFR 185's, they say ported gt-40's with the smaller valves (1.94" Intake and 1.54" exhaust) will actually give more bottom end grunt. But I can't get the AFR185's out of my head.

Here are the specs for the bottom end:
*forged Eagle stroker crank (internal balance)
*forged Eagle H-beam rods
*block: stock roller 302 with beefier after market main caps and girdle
*Intake: Edelbrock Air Gap
*Cam: not selected yet
*Carb: Holley 650 but thinking I need a 750 DP
*Rockers: Comp Cam Magnum's with stud conversion and guideplates

The heads were ported by a nascar head 'guru' in Charlotte, NC, don't have any flow data. Rough guesstimate, around 270 cfm on intake side and 190 cfm on the exhaust. Will the Ford Racing valve train (single springs w/ internal damper) hold up with a high lift cam (aprox .550)?

Any thoughts? Money is tight on this project but I can swing the AFR's if they will make a noticeable impact on HP and torque.
Thanks!

[KevinM]

Lots of needed information is missing. At what lift are those flow numbers estimated and by who? Find out what the flow rates are at .300", .400" and .500" lifts, and post them. How much did he open the ports up? Were they CC'd and if so, how big are they?

Kevin

[JustOne]

I think the AFRs are a good investment, you may even want to look at the AFR 205, especially if the pistons do not need to be notched or still can be. Chances are you are looking at picking up 40 - 70 horsepower by changing to better heads. Is 40 - 70 HP worth spending the moey to you?


Justin

[hi-tech cobra]

Flow number are my best guess after talking with several people who port gt-40x heads regularly.

Intake: 270 cfm at .550" lift
Exhaust: 170 cfm at .500" lift

The heads were hand ported by a well known nascar guy in Charlotte. I asked the machine shop today and he can't recommend anyone in Jacksonville, FL to flow test the heads. He said it wasn't necessary. I would like to have them flow tested, just don't know where to get it done.

Kevin you asked how much the ports were opened up. How do I check, mic the intake & exhaust openings? Do you think the valve size is a problem?

[hi-tech cobra]

Justin,
What advantage will the 205's have over the 185's? The machine shop say's 10hp gain max from AFR 185 after seeing the port job on the GT-40x heads. Said I'd be throwing $ away. I have no idea at this point. But I need to know if the smaller valves will affect power.

[Excaliber]

The machine shop may be right! Consider this: What really makes AFR heads 'special' is their consistent flow throughout the rpm range. While other heads may flow better at peak CFM they tend to loose out on flow compared to AFR heads at lower to medium rpm. The advantage this offers is better 'streetability', more torque at a lower rpm range, crisper throttle response. It's not just valve size but the way AFR runners and ports are set up. Smaller ports, runners,valve size generally provide better throttle response at lower rpms in general. Thus the machinist take on the smaller valve GT40 heads may perform well. Better than AFR? Well thats a tough one. In my opinion AFR may well be the BEST head out there for overall performance because of their well thought out design. Worth the extra money over a nice ported set of GT40 heads? Tough call. You looking for PEAK horse power at max rpm? You could do better than the GT40's or the AFR's in that case!

[xlr8or]

Well there you have it. now you might want to consult someone like Keith Craft who specializes in CNC porting that has the flow numbers to back up the work so you can get the opinion of a real expert.

[Excaliber]

You know I forgot to consider the fact you haven't yet selected the cam. Sounds like you have a very nice strong motor capable of high rpm. I always match the CAM to the SPRINGS by buying a 'cam kit'. It's possible you could get to big a cam, go for the high rpm and find the single spring is not compatible with the setup. Whatever heads you get if they flow LESS than the cam will allow for then theres your 'bottle neck'. There is always a 'bottle neck' somewhere (headers, carb, valve size, cam profile, etc.). The key is matching the components to the end result you seek. Such as 'cruise rpm'. If the cam profile works best at 3000 and higher and you 'cruise' at 2600-2800 rpm the motor will not be working within design parameters. Maybe you don't care about 'street' use. So you get a cam that runs to 7000 and the heads limit performance at 6200, theres your bottle neck! Or the valves 'float' (single spring) at 6200 and the heads and cam are good to 7000.

Match the cam and springs to whatever head you decide to go with. Don't forget to at least consider 'cruise rpm' (cam profile and rear gear ratio) and how that impacts streetability with your setup. I rebuilt my motor for the street, when I got the car it was setup for high rpm drag race duty. REALLY unpleasant on the street! After rebuilding (more modest cam, reduced compression ratio, smaller vacuum secondary carbs) performance was BARELY down from what it was, gas mileage was DOUBLED and streetability was dramatically improved. I love to race but fact is, 90% of my driving was on the 'street'. Just say "NO" to radical race profiles unless your a serious racer! I STILL run in the 11's and gave up an estimated 150 horse power when I rebuilt for the street. Think about that, clearly my Cobra was 'traction limited' not horse power limited! If you can only hookup 500 horse, 600 IS truly a waste of time.

[hi-tech cobra]

I called KC. They said in no uncertain terms: AFR185's. It looks like that's the way I'm headed. Now for the next big question: Cam? Anyone have a fav Comp Cam grind?

Cobra will be street driven with occasional track time. I want an aggresive cam. I've read about the 266 and 274 but aren't they for EFI motors?

[Excaliber]

Don't get me wrong, when I say I went with a more 'mild' cam, it's all relative to where you been! I went from a solid roller cam 3500 to 7000 rpm range to a 2300 to 6200 rpm range cam. My intake and heads will certainly flow more than my cam, so the cam is my current 'bottle neck'. Nice 'lumpy' idle, sounds 'bad to the bone' and still pretty darn quick. The big ports (intake and heads) could be better optimized for 'street' use. Comp Cams 292S flat tappet, but that was for an FE.

Heres the key question: What peak rpm range do those AFR185 heads like? At what rpm do THEY start to 'fall off'? No point in getting a bigger cam than the carb\intake\heads\headers will support.

[KevinM]

hi-tech,

This is a lot more involved that I think you are aware of.

If the heads really do flow 270 / 170 @ .550 lift. then the intake is about the same as an AFR185 but the exhaust is down about 20 from the AFRs.

Even if those estimates are correct, we don't know the flow at .300" and .400" lifts, which are very important for understanding the real flow of the heads. The smaller valves are definitely likely to be an obstruction and are one reason I am a little dubious of those estimates.

Smaller valves do not improve flow at lower lifts. The AFRs outflow GT40s from .100" lift up. and have much better midrange performance.

I also have to question the "Nascar Guru" thing. Nascar uses a completely different kind of head with different ports, different valve locations and angles, and a completely different valvetrain. Even if the person was knowledgeable on Yates and C3 heads, that information wouldn't transfer to wedge type street heads.

There is also the matter that people who really do a lot of work on Nascar engines get paid a lot of money for that. It is hard to believe they would be waisting their time porting a set of GT40 heads.

As for cam, how high do you want to turn the engine, and what kind of low RPM bad behavior are you willing to put up with? For a street driven car the Comp XE266HR is a very good cam with the AFR heads. The Comp XE274HR would be better at the track, but will not be a great day to day drivability cam.

If you do go with your ported GT40s, based on the highly questionable numbers we have, you would want a cam with about 12 degrees more exhaust duration than intake. That would have to be a custom cam.

Kevin

[tbirdz12]

Tough call. 185's are great heads. You cant go wrong with em.

Put it together with what ya got and if your still not happy, spring for the 185's.

RPM Air Gap with a XE274HR and 750 speed demon.

[hi-tech cobra]

I have added pics of the heads on my gallery page. Can anyone tell me what the extra port is for on the exhaust side pic? It goes both ways, out to the exhaust side and a smaller hole that goes out to the intake side. It is on the middle exhaust port on both heads.

[tbirdz12]

Its the heat riser port. Some hot exhaust gets redirected to the intake where it heats up the choke mechanism, so your choke will "unchoke"? when the engine gets warm.

[hi-tech cobra]

Thanks tbirdz12,
Can you tell from the pics if the port job looks good. It looks like these will be the heads I go with after all (build budget). I have the single springs that came with the Ford Racing Valve train kit. What would be a good double spring upgrade?

[tbirdz12]

Originally Posted by hi-tech cobra

Thanks tbirdz12,
Can you tell from the pics if the port job looks good. It looks like these will be the heads I go with after all (build budget). I have the single springs that came with the Ford Racing Valve train kit. What would be a good double spring upgrade?

Cant tell on the porting. Maybe someone else can. You should get valve springs that are compatible with your cam requirements. Some require higher spring pressures than others, depending on rpm range, ramp angle and more.