I've been fighting a dip in power with my 396 for some time now and may have stumbled onto something. I have flow numbers from a set of heads done by AI which I've compared to my flow numbers. They match almost perfectly, but NOT the EXHAUST number under .300". Here is a comparo of the relavent flow numbers :

Lift Mine AI E/I - me E/I - AI
.100 33 51 50.8% 79.2%
.200 84 111 61.8% 78.2%
.300 142 148 74.0% 73.3%

The low lift exhaust numbers are TERRIBLE!

I was talking with a buddy and we started to think how this would effect overlap. I would imagine this is very important for high RPM torque (one of the reasons why large duration cams make more top end torque). If my heads don't flow the low lift numbers, could this seriously effect top end power considering most of the overlap would be at those lift numbers?

http://compcams.com/Base/Images/Tech...ration-002.gif

If your answer is yes, would it look like this? :

http://www.afrashteh.com/dyno4.jpg

Just a little bit of history - as a 350 with the head sames heads a different cam and an A4, the dyno looked like this :

http://www.afrashteh.com/350dyno.jpg

Note - The intake side was original GTP for the 350 dyno. The heads I have on my 396 had the INTAKE side reworked with better numbers across the board - matching AI heads.

If you look, the same dip is there...even at the same RPMs. The cams are somewhat similar - 218/224 114 vs 220/230 112.

Comments?? Most people say low lift numbers effect part throttle drivablility and low end torque (good low lift numbers would indicate good port velocity). I've not heard comment about high RPM torque and low lift numbers though.

-Alex

 

I know this gets boring to hear but if you are comparing to #'s that AI is providing you need to flow YOUR heads on their bench and compare. They might not be flowing the same at other lift points like you think. Then again yours could have been tested on a stingy bench but if you are gonna compare #'s they need to be on the same bench and FWIW I would consider AI's #'s correct.

If you had a AI head flowed on the same bench that you had yours flowed on than the #'s can be compared.

I would look at your cam and see what piston is doing liftwise at these low lifts. Generally the piston is barely moving at these low lifts and there isn't much suction (intake) or pressure (exhaust) but that doesn't mean that the low (.150 and below) #'s are not important. I have always heard that better low lift #'s will make the powerband hang on longer before falling off but I am sure Chuck, Bret or Phil could explain this better.

NightTrain66

 

Just looked at the flow #'s a lil closer and they are way off.

From what I have found a valve job with a large venturi and thin angles flows OK at high lift (.600 and up) when the valve is out of the way and the diameter of the hole is determinimng the flow but if the valve is below .500 (and especially below .250) lift the radiused valve job with a smaller venturi works much better. In my opinion it is not the diameter of the hole but the ability to go around the valve at these lifts that make it a better valvejob. It is not only cfm but the port sounds much better on the bench as well. Not saying it is the valve job, could be the shortside layed back too much. there seems to be a fine line in getting good low/mid/peak #'s and FWIW most people can get an awesome .600 lift flow # if they are not worried about anything below it. Valve jobs and port shapes that help flow at .600 will hurt down low so when ya see a port with good #'s (on an accurate bench) at all lift points it makes ya realize how good they really are.

Some of the "smarter" porters are probably saying" hmmm, pretty much basics that I learned years ago" or "he still hasn't figured it out" but some of the people not as informed are reading this and saying "wow, good info".

I learn things all the time and just about the time I think I have got things figured out I realize how much I do not know.

And some of the things I learned turned out to be wrong, LOL.

As long as I keep learning I guess.

Maybe some of the others can post some info that can truely help your question but that is pretty much all I can help ya with.


NightTrain66

 

Don't really have time to get into it, but a bud emailed me a link to this...

Really, if you use the formula Riddeck posted for a quickie area under the curve calculation, you'll see that...

A stock head scores right at 110, yours are 129, and the set will had were 135. Quickly comparing.. 135-110 = 25, and 129-110 = 19... 19/25 = 76% the gain we show over stock.... so really, if those #'s from more are accurate, the intake port isnt even close.

Again on the exhaust, stock is ~82.3, yours are 90.1, and will's were 96.7... and the ratio of the differences is 7.8/14.4 or 54% the gain we saw.

That may help shed a little light as to why seemingly negligible differences make a pretty sizeable difference. Your #'s should be within 1% on our bench as well using the same setup. If you're curious and ever have them off, send them down and I'll flow them if you want. Like lloyd said.. there are alot of things you can do to affect things one way or another.. and I'd rather not guess at it, so I'd just have to say that I'd have to see it to tell you whats up.

If I have time later maybe we can talk about what you're actual question was about, but I'd better finish lunch and get outta here .

BTW, Christian asked me to post this if I posted anything...

Flow Graphs

-Phil

 

What you are saying above doesn't seem correct. You can't take the difference from stock, divide them and say the AI head is 76% better then the GTP head or that the AI head is 76% better then stock. Just look at the numbers, it doesn't even *look* 76% better than stock.

If I'm 120 (17.3% better than stock), and th AI is 135 (22.7% better than stock), then the AI head is +5% (or 34%) better then the GTP head. However, this is not the overall issue I'm working with here.

Quote:

Again on the exhaust, stock is ~82.3, yours are 90.1, and will's were 96.7... and the ratio of the differences is 7.8/14.4 or 54% the gain we saw.

Ok, here is where a bigger difference can be seen. Mine is ~9.5% better then stock while th AI head is 17.5%. This is a much larger difference and is part of my overall question.

Quote:

If I have time later maybe we can talk about what you're actual question was about, but I'd better finish lunch and get outta here .

Real simple, the question is :

How does flow numbers below .300" lift effect overall power?

This question raises more specific ones, like :

How does it effect tq/hp @ WOT?
What if it's only on the exhaust side, how does it change tq/hp?
What if it's only on the intake side, how does it change tq/hp?
How does it effect part throttle drivability?
Why would a set of heads have such poor low lift numbers, but have good upper numbers?
If the E/I is out of whack at these lowr lift numbers, what does that mean?

Flow Graphs

Look at this page again, he added a lot of good stuff.

-Alex

 

Can't answer every question but I have some opinion on the matter.

If you look at race heads in general they have a poorer e/i ratio but there's a reason for that. I'd say that the valve size lays a big part but also the seat. Most of the race heads use a 50* seat on the intake.
Wrt, low lift flow.... think about it. You need strong flow at the low lift for two reasons. One, the intake valve is at low lift at the last portion of the intake stroke, good low lift flow is gonna help cram that last little bit of charge into the cylinder as is the port size. If the port's too big then it aint gonna matter til the revs are really high.
The same can be said of the exhaust port... low lift is important right at the beginning of the blowdown period..... right when the exhaust valve opens. The port size is also important there. So, if you had a poor exhaust port and/or bad low lift, you might end up with a bit more exhaust residual left in the chamber. Not exactly good for performance and I'd think that things would just get worse with more revs. So yeah, low lift is important.
Just my .02

-Mindgame

 

Good stuff, Mindgame. Let's narrow it down and apply it to me now.

As can be seen above, my low lift exhaust numbers are bad (NOT the intake numbers). Would this cause a heavy drop off in torque at higher RPM as is currently displayed by my motor?

http://www.afrashteh.com/dyno4.jpg

-Alex

 

I gave it alot of thought and I have come up with a formula to help you find out the diff. in HP between the 2 engines at high RPM when the only diff is the .300 lift and below flow #'s. This formula will find the HP diff at 6500 RPM. As long as the .300 lift and below exhaust flow #'s are the only diff. in the motors.

Ya need 42" flow #'s so multiply all 28" #'s by 1.27.

Now add the .050, .100, .150, .200, .250 and .300 exhaust flow #'s and divide by 6.

Take that # and multiply by # of cylinders.

Take that # and divide by exhaust valve diameter X Pi.

Take that # and multiply by degrees of crank rotation the valve is below .300 lift.

Take that # and multiply by 6500

Take that # and multiply by zero.

Now take the HP # at 6500 on your motor and add 20 HP.

That is my guess 20 HP. I am amazed that Bret or Chuck have not "educated" us on this yet. Oh well, I guess they are busy but I would like to hear what each of them say about this.

NightTrain66

 

Alex, perhaps I should have worded it differently, but when I say the head in question showed approximately 76% the increase we did, I mean that ours was 24% higher, not 76% .

Oh I understand, it was just a note because most people don't fully understand the entire picture. They see similar numbers and think something is comparable. Even if something is similar at one instance, as a whole they're quite different.. and since it functions as a whole, I always tend to put it into those terms.

The best way I can briefly describe what's going on to you is this... it's all about efficiency. At peak volumetric efficiency your intake and exhaust ports are working together as a system. Where the two valves are open at the same time during the overlap period, you want the scavenging to start drawing in the fresh charge. Also, as the piston is nearing/passing TDC and the exh valve is closing, because your low lift flow is so poor, it likely wont empty the spent gasses as completely either. That's obviously bad because it dilutes the next charge of fresh air/fuel drawn in. Worse yet, its underscavenging, but the burnt gasses that arent exiting may be reverting into the intake as it cracks open if it poses less of a restriction. From what I've seen, I'd probably rather have it overscavenge a little and dilute less, than revert and dilute more. In the end, I think you'll end up with a less efficient package, and in turn less cylinder pressure... so less torque, and applied over time, obviously less power. As far as where that comes into play at part throttle etc., I'd not be as concerned, but IMO it isnt going to be as good as it could be. Poor E/I ratios etc. are just bad all around from what I've seen .

Hope that halfway helps, Alex. Didn't really get into what happens in the blowdown period and valve timing like 'game alluded too, but I've got to acquire some sort of sustinance before I pass out. I'm sure Chuck could give us all some valuable info, but free time seems to be more and more scarce for alot of us lately. I suppose that's good and bad

Oh, and Christian showed me the other stuff he added to that temp page. He's a graph making fool, I'll have to steal that at some point


-Phil

 

Hey Alex.. Who's your buddy?

I don't remember what got me started on this, but while making a comparison chart with Alex's head flow I noticed the poor low lift exhaust flow.

This struck a note because I thought of the importance of the exhaust valve closing event, which helps to syphon intake air during the overlap period. Without the benefit of overlap to get the intake air mass flowing there is obviously a power hit. It was mentioned that overlap occurs at low piston velocity, it's actually 38 degrees before top dead center on the exhaust stroke, the valve is open .222" at that point. I made a valve timing chart of alex's motor here:

http://www.carprogrammer.com/Z28/EA/...lve_everts.gif

I didn't even think of what Game mentioned, which is the blown down event, in this case Alex's exhaust valve opens at 90 degrees before botton dead center. The entire blow down event occurs below .360 valve lift with his setup. This presents a second really big drawdown on have poor low lift exhaust flow. Blow down is important because it helps to bleed pressure out of the chamber so the piston isn't fighting to move againest a vaccum.

Alex, let me know when you want to take the heads off, I'll be there to help. Owe you a few man.


Hope this helps,
-Christian
www.carprogrammer.com

 

All this theory sounds great, but I really need a concrete "yes" or "no" to my problem. Pulling off the heads is no fun task and it is even less fun to have them fixed up.

The info here is excellent...but just not direct enough for me. ANY comments will be much appreciated!

-Alex

 

Alex,
Based on what I've seen over the years and my discussions with Chuck Riddeck, Don Losito and a couple of other very smart dudes.... yes, it does play a part. I hate to comment because I have a very basic understanding of all that contributes.... I'm a static guy, not a dynamic one and I don't pretend to know stuff I don't.
What I have learned over the years is this.... yes, low lift exhaust matters. If you have an incremental trapping of exhaust residual, you are gonna lose power in the combustion process with more revs.
The port has to be the "right" size. What most pro head guys look for is uniform velocity across the port and an efficient port "form". I also know for a fact that the guys are really interested in low-mid lift flow (particularly in the .2-.5 range). The goal as I understand it, is to produce ideal chamber scavenging everywhere in the operating range of the engine. All of that equals a greater power band and more output. We all know that cars accelerate proportionally to their torque curve, so all this stuff plays a big part.

-Mindgame

 

Thanks all. I will pull the heads some time soon and have the exhaust ports/valves and the chambers looked at.

-Alex