300CFM wil get you 617FWHP,everything else being correct.
_______________________________________
Bret,
The prime words here are,"everything else being correct"
As you know that rule of thumb is the capabality of the "head" to support the given HP. You could make that intake "work"---- at 8500+RPM's He Heeeee Hey your's will work much better.

 

Which intake manifold were you relying on to get the air there? And which headers were you planning on using to get the exhuast out.

Cylinder heads are important, but a TUNED (note: that means more than gasket matched) intake manifold and headers make all the difference in the world.




Not to mention CFM is basically cross sectional area with a few variables. far from the whole story. And if you really think you can estimate power based on cylinder head CFM alone, I have some ocean front land out in oklahoma Id like to sell you.

If it wasn't a super secret, Id tell you the headflow of some heads at work and ask you what kind of power they made. Bechya wouldn't be close

 

Well the rule of thumb formula has been around for years and it is pretty close on the engines I build.It doesn't include anything but head air flow and "everything" else has to be correct.
There was no mention about cam,headers,ect,it's just air flow.

 

Ok......... if you can't go by valve size and flow, then exactly how do you judge cylinder heads?? Besides trial and error on a dyno. Let's assume the matching components are correct or can be corrected.

Take a AFR 227 head with a 2.10 valve and it flows 305-310. An LT4 head flowing in the 290's with a 2.02 valve will probably make as much if not more power. So what is the secret?

 

Port efficiency
a Head has to have a good Flow number
but just as important, the correct port velocity everywhere in the port

i've seen Cyl Heads make a bunch more HP & TQ with corrrect port velocity throughout the entire port, than a similiar head with much higher flow numbers

most all the SuperStock heads are around .285 Flow Factor
most of the Comp and ProStock around .29 to .31 Flow Factor

but some heads are about .23 to .257 Flow Factor
some of it depending on Port Efficiency & Engine Combo Efficiency

a SS350 11.4:1 CR, .700" Lift Roller, QJet carb, will make 575 to 600 HP
out of 250 to 260 CFM , run 9.80's to 10.0's -130-132 MPH at 3330 Lbs

 

Ok............. So just how does th average Joe Blow like myself figure "Port Efficiency"?

I do have the Performance Trends software and I notice anytime you put in a larger valve it obviously lowers the "port efficiency" factor. I sure there is more to it. Whomever wrote the software has built corrective factors, but what is a good rule of thumb for getting an idea of how efficient an particular head may be?

Is there anyone whom you can send heads to who for a fee who will give ya a hint as to how good they are before you bolt them on besides going to a flow bench and then running it through something like Performance Trends?

 

Formula for Flow Factor......

HP of engine at Flywheel/number of cylinders/CFM at peak lift=flow factor

Obviously you need to have run a set of cylinder heads on a motor before you could tell how good they are.

400rwhp * 1.2 (20% correction factor) = 480hp

480hp / 8 = 60 HP per cylinder

60/270cfm = .222 flow factor.

So LT1 motors are below average in flow factor rating and will be because of the lack of VE that good intake pulse tuning gives you.

If you chassis dyno them then you need a corrective factor for drivetrain loss which you would have to take an accurate guess at it.

The basic Performance Trends EA programs do allow for valve size to effect power output, where the more advanced programs use flow data and valve changes don't change the power curve as much. L/D data is usefull here (Lift vs. Diameter which is another good way to look at how good a port is relative to valve size)

With good programs you need to have good measurements to even tell you how much power it could make. If you really get crazy with it you can import velocity maps of the runners and use that data to help estimate power.

Actually I would say the other way, the valve size is relative to the flow. A small valve helps on valvetrain control because it usually has more mass but it will hurt you with a lack of curtain area. There are issues with bore wall schrouding but a 2.02 valve isin't even getting close to that issue on a 4.030" bore. Race heads will run a 52-54% Intake valve diameter relative to the bore size. Looking at LT1 heads 50% is where a 2.02 valve is in terms of bore to intake valve diameter size. Realizing that you could get a 2.08-2.100 valve in there and be on the low end of the range or with some serious work you could get a valve in the 2.15-2.18" in there (most likely with a canted valve head).

That explain it a little better?

Bret

 

Your pops claims you're closer to your Polish roots but I see a little of that German heritage coming out every once in a while Bret.

All joking aside, and I hope everyone can take a joke here, this is a good thread. Interesting reading and makes me wonder why some people haven't posted in other threads of similar subject matter.

-Mindgame

 

Bret

Thank You very much for your response and sorry for the delay in getting back as I’ve been out of town for a meeting.

Guess the bottom line is the software has me a bit confused because it weights the valve size so heavily in figuring hp. You say the software does not accurately reflect the true effects of valve size and a bigger valve does not have that much of a negative effect on making hp.

If the ports are optimized, then a larger 2.08 or 2.10 valve would be more most advantageous for making hp than a 2.02 then.

Hey, I never trusted computer geeks anyway. Even if they do have grease under their fingernails.

 

If the port is "maxed" with a 2.100 valve,then going to a 2.200 will not pick up much flow. If you drop back to a 2.02 and the bowl and seat are sized for it it will hurt flow.This also depends on lift and the curtain area of a given size valve.A rule of thumb is .25 x valve dia to get lift to uncurtain the port.The bigger the valve the more you have to lift it to get it out of the way of the flow.
EAP takes the port volume,port size and valve size to establish a given hp.It also uses flow with the intake ,which is generally 10% less than head flow,unless it's a GOOD sheet metal intake.Play around with the volume and valve size,leaving everything else the same and see.Then play with valve lift,It'll suprise you.Remember these numbers you get are not chiseled in stone,they are trends.

 

Thanks again for the input. Learning something here.

I'll play with the software as I'm not a big believer in software programs in general, but am surprised how many good shops use some form of Performance Trends software. Thre logic behind this particular software seems pretty good.

 

Yeah, you might be surprised at the high-end race teams that use PT stuff, along with other, generally much more expensive stuff. Some of them admit it, some don't.

Of course it's not just the software, it's the knowledgable user that makes it work.

 

I respectfully consider that explaination oversimplified and somewhat misleading. I consider your valve size to bore relationship an oversimplification. You've (or someone that collaborates with you) apparently have done flow studies/tests on this to substantiate/warrant that reasoning, however. Nonetheless, using any given valve orientation angle, to reflect on it's impact on head/intake port flow itself, AFAIC, does not constitute the total airflow equation, and doesn't include it's (valve orientation) impact on the total useable airflow, at it's destination. A comparo with the LSx series of engines, will shed lite on what I am attempting to convey.

Seeing what I do, when I bolt a set of heads on the block, and believing how the LTx chamber shrouds the valves, I am compelled to disagree with your statement. Maybe I'm seeing 'things' differently than others.

 

It's compeletely an oversimplification, no specifics mentioned. It's a general rule and what can be done if given a clean sheet of paper, not a casting that you have set limitations with. Both valve axis and the centerlines will have a huge effect on the size of the valve going in the bore. It's just general rules of what can happen and what guys look for.

Obviously valve placement is a big issue too. Moving the intake valve closer to the center of the cylinder gives you the ability to put a larger valve into the cylinder heads.

Obviously a 2.150-2.180 valve is not going to work in a LT1 head with the layout that it has. A 2.055 is about as big as you want to go there anyways and on a 4.030 bore does work well.

You can solve chamber shrouding problems many ways. The most obvious and widely used is opening a LT1 chamber up for the valves. Running something like a 2.055 valve is going to get you better results but the chamber has to be made accordingly. Then again the quote was about bore wall shrouding not chamber issues. Going to an aftermarket casting is going to give you more valve size do the port above it and the seat width and placement.

Bret