NewbVetteGuy

I saw a thread or two on people doing AFR head swaps in their L82s where they were asking for advice on CAM selection and there was a general consensus that given the "good" ratio between the intake and the exhaust that a dual profile cam was unnecessary for AFRs.


I've got a few questions on this subject, I'm hoping someone can help clarify:

• Roughly, what is the intake vs. exhaust flow ratio where a dual profile cam is no longer required? -Is there a rule-of thumb?
• If you have a head like the AFRs where a dual profile cam isn't required, yet you use a dual profile cam that favors the exhaust; is that a bad thing? -why?

Using what information I could find on the AFR 64cc 180cc and the 64cc 185cc, I've plotted the intake CFM to exhaust CFM ratios and then compared them to my own choice the ATK casting Profilers below.

Given the flow numbers would I still benefit from/ should I use a dual profile cam?



○ ATK@ 0.200", 1.215:1; AFR 180cc@ 0.200" 1.255:1; AFR 185cc@ 0.200" 1.203:1
○ ATK@ 0.400", 1.395:1; AFR 180cc@ 0.400" 1.263:1; AFR 185cc@ 0.400" 1.275:1
○ ATK@ 0.500", 1.395:1; AFR 180cc@ 0.500" 1.256:1; AFR 185cc@ 0.500" 1.357:1
○ ATK@ 0.600", 1.352:1; AFR 180cc ; AFR 185cc@ 0.600" 1.381:1


Adam

NewbVetteGuy

Ok, I think I calculated it backwards. I just found the thread below and a post from Billa where he said: "the ideal GEN I SBC I/E flow ratio is about 75% or better".


https://www.corvetteforum.com/forums...180-heads.html


I'll re-calculate all the numbers with the exhaust:int ratio instead of int:exhaust and I think I'll have my answer.
Have to sleep now though.



Adam

gkull

If you read the fine print from AFR the flow numbers are achieved with a test pipe on the exhaust side. Kind of hard to duplicate in the real world with headers and exhaust system that creates back pressure across the rpm range except where the tuned header lengths and diameter create ideal cylinder scavenging.

I used to work on 4 cam 4 valve per cylinder with the best merge collector exhaust systems 8 10 and 12 cylinder motors and they used split duration cams.

The ratio crap can be argued all day long

REELAV8R

A split duration cam is primarily to allow the exhaust sufficient time to exit the chamber when the port or down stream back pressure restricts it's exit.

If there is little restriction to it's exit (ie side pipes or well constructed evacuating exhaust system) and the exhaust port flows well then the need for the split duration is reduced and maybe eliminated.

Another consideration is the overlap period. This is the period of time during the engine's cycle in which the intake valve and the exhaust valve are both open.

Too much overlap and an excessive amount of the intake charge can be drawn out the exhaust valve due to the low pressure created by the exiting exhaust gasses.

Too little overlap and you don't create as much of the needed low pressure area to assist getting the fresh intake charge into the cylinder.

The primary force drawing the intake charge into the cylinder is not the piston descending the cylinder. It is the low pressure created by the exiting exhaust gasses during the overlap period.

Both too little and too much overlap end up in a loss of efficiency and thereby loss of power and fuel economy.

Using a split duration with all else being the same creates more overlap.

Vizard explains this in detail and gives you a guide as how to choose your overlap based on your use in his book.

https://www.amazon.com/David-Vizards...s=david+vizard

NewbVetteGuy

More good info; some overlap is good, too much is bad. I thought that overlap also help to push the last bit of exhaust air out of the cylinder so that you're only getting clean air in there, too.

I calculated the numbers for the 185cc ATKs vs. the AFR 180cc vs AFR 185cc correctly finally and the AFR ratios are better, but they're shockingly close for an as-cast head vs. the reigning champion fully CNCed head (it's amazing what you can do when money is no object); given that the ATKs are almost 50% cheaper, I'm shocked at how good they are for the money.

It seems like the newest 23 degree raised exhaust port heads hit the 75% or better exhaust to intake ratio guideline, while standard height heads, even the best flowing ones are still a bit under it and of course as stated that's disregarding exhaust restriction at all. -Again I think the standard height exhaust port probably hurts the ATKs here, but I do NOT want to deal with header fitment issues so I'd rather have the standard exhaust height ATKs vs another Profiler casting like the Jegs.

Totally agree that a dual profile cam that slightly favors exhaust still seems like the way to go (especially for my heads and I'm guessing any standard height SBC head).

○ ATK@ 0.200" 82%; AFR 180cc@ 0.200" 79.7%; AFR 185cc 83.1%
○ ATK@ 0.400" 71.6%; AFR 180cc@ 0.400" 79.2%; AFR 185cc 78.4%
○ ATK@ 0.500" 71.9%; AFR 180cc@ 0.500" 79.6%; AFR 185cc 73.7%



Adam

bluedawg

For a real test on there flow numbers, a test on the same bench by the same dude whom one would hope is nonbias and don't have a dog in the fight. Both heads are good.

gkull

Adam, You have some misconceptions about raised port. My dart headed motor has .600 raised ports and my afr 210 I think are also .600 raised.

We have V-8 motors. The V has the exhaust ports at an angle. so it is not a vertical .600 It also moves outboard. If anything it is great to gain some ground clearance .


I've had 3 different headers on my vette with raised ports.

That ratio is dumb. The head with the most intake cfm has the potential to make the highest HP. I have 3 inch exhaust and use a split duration of 6 degrees on both of my motors. My smaller roller motor uses 112 lsa to lower overlap

NewbVetteGuy

So you don't think that raised exhaust ports give you the potential for better exhaust flow numbers? -I thought that was the whole point.

I get that more CFM is better, but with two heads making the same intake CFM, I thought the idea was that a higher exhaust CFM was better / meant you could have less overlap in the cam, no?


This stuff is so complicated. I understand why people just pickup the phone, call a CAM company or use a tool to pick a cam for them and then go with it...


Adam

gkull

No No! Raised ports have the highest exhaust flow for a given port CC because you are not trying to make a 90 degree turn as soon. Both my heads are raised ports. My heads have 2.08/ 1.60 and 2.10/1.625 valve sizes. My AFR 210's are on a 383 and and my Dart 227cc are on my 434 Motown small block. I wish that I would have bought the AFR 245 cc for my 434

427Hotrod

It's not necessarily a bad head if the I/E ratio is low. It might just be that the intake is THAT good. Pro Stock heads sometimes had a 60'ish% I/E ratio for years. They are all about intake flow. The later ones are getting better on intake and exhaust...and cams are getting a little less in duration.

You have to use the cam the heads/engine want if you want max performance. Too much exhaust duration "usually" gets exhaust opening a little earlier and might take some TQ away. BUT....if you're going to spin some RPM....the added exhaust usually helps.

It will often show up as how well an engine "hangs on" past peak HP. You don't want it to hit a wall and fall off. The intake side is what makes all the power...but the exhaust side is what will allow it to hang on and make high power at high RPM. If you're not going that high in RPM....no need to add all the extra duration.

But as mentioned in one of your other threads. Look at the latest higher performing LS engines. There are HUGE I/E splits on cam duration.

JIM