Yeah, started on-track with the flow bench isn't a running engine, obvious but needs repeating apparently.....then He took a left turn to Dickville.......
Port velocity average target fps?
- Thread starter 273
- Start date
-
I played around with your formula, works out Chad's formula is a simplified version, cfm x 2.4 / csa = average fps. 144/60= 2.4 now I see where his constant comes from was wondering that.
Basically a 6683 rpm 367 cid engine needs 300 cfm and a 2.25" csa giving an average 320 fps according to their formulas.
Question is, is that an acceptable fps for a street strip my guess it is, but to get the fps to where Eric says it should be 285 fps would need to be a 2.53" csa. But funny as he said 285 fps should be the goal and one of the heads he was comparing calculated to be that but actually speeds were way higher then that leading to believe there actually average was way higher.
I'm not overly worried about it, obviously we only got a few heads to choose from anyways.
Phreakish
Well-Known Member
I'm no cylinder head expert, but it seems to me that velocity will be a result and not necessarily something to target.
On a street/strip setup people have good luck with "too small" of a head than too large. Probably because bigger really only benefits at the top of the tach, it makes things worse elsewhere.
The formulas and numbers are good to ballpark things and spot check choices, but I wouldn't treat them as gospel either. For a street strip engine, it's probably OK to let the velocity run higher than you would for a dedicated racer.
Air has to accelerate with each valve event, and so lower target velocity can help when extreme rpms reduce the valve open time by reducing how much the air has to speed up. Obviously there's also resonant tuning impacts from the intake and the exhaust and are also significantly impacted by the cam, but all things equal the lower velocity port will pass more air through at maximum rpm. Of course, rarely do all other things remain equal!
To Chad in the video fps and csa seemed to be the two most important aspects which means the flow is the one that needs to be adjusted. Not saying that's how it must be done, he just was vague on the one part he seems to consider most important.
I take all this stuff with a grain of salt, most us aren't building max effort engines, some seem to get super focus on one these max effort aspect of performance and lose site of all the rest and want to apply it to everything.
Especially out of place when people want to do a basic 4bbl cam exhaust and tune to their stock running engine.
For me it's more about the puzzle of it, I like adding pieces.
To be honest, I think your knowledge quest on this topic is good.
I however think, you’re over thinking it. There is more to this than what you’re bringing to the table to discuss on and it’s a bit deep. If your trying to gather up a sum of knowledge to build a super beast right out of the gate, I think your not only a long way off but in for a disappointment. Far be it from me to say don’t bother but certainly please do!
If your just interested in building a really nice street strip car, I would suggest setting a target goal in your head with a displacement you can afford/build yourself/easily see doing, etc…
So let’s say you’re interested in building a small block at 380 cubic inches. Your goal is a 9 flat in the 1/4 mile. Right now in your head the only stumbling block is the cylinder head but everything else, save maybe the cam, you know what you want to do.
Call a head porter, talk about what your plans are and what would be a wise cylinder head choice.
Decide on a cylinder head and have the porter check it out and work on it as needed. Find out the flow curve and where the head starts to loose flow. This will help you in valve lift.
The same method for anything you’re building from a grocery getter or hot rod to a street machine or street strip brute and beyond really applies. Since the cylinder head is ether a major limitation or not at all, when the plan is being developed and your head guy is informed you both can work on an angle of attack.
For most engines, the smallest port with the highest flowing cfm is what your looking at and this will carry you pretty darn far. The feet per second isn’t your biggest worry but just a good thing to further help everything.
As long as the fps is in a good and reasonable speed, there’s little worry your engine will create some really nice power, throttle response and mileage if that is also part of the goal.
@273 As a late thought entry, if you’re just building a regular street hot rod, IMO….. the trick flow head is about the best bang for the buck. It has a small port with high velocity and flow numbers. On paper, it’s a 600hp capable head.
I have it on my wife’s 11-1 - 360 with a dual plane and a small hyd roller and it’s really excellent for what it is. There are a lot of short comings coupled to the head the militants max out put. A dual plane intake, small cam duration and not taking advantage in valve lift vs the head flow. But it’s also designed to be a total driver anytime anywhere and it does great.
I have it on my wife’s 11-1 - 360 with a dual plane and a small hyd roller and it’s really excellent for what it is. There are a lot of short comings coupled to the head the militants max out put. A dual plane intake, small cam duration and not taking advantage in valve lift vs the head flow. But it’s also designed to be a total driver anytime anywhere and it does great.
And like I said, there working great on a much milder build. Great throttle response and reasonable mileage. Drives nice and easy even in traffic. Very good passing power on the Hwy.
What’s 450 capable of in the 1/4 w/a 3000 car ready to rock?
Engines and Music theory are the to main subjects I continually research
For my own car has the 5.9l crate engine with less than 10,000 miles on it no real desire to touch the longblock.
I do plan on doing some drag racing this year, beside working the bugs out and Gears and tires, fix the last owners wiring some interior and minor body work That's my main goal as of this point.
If I do upgrade the heads at some point I'm might run these calculations on them but wouldn't get bent out of shape over it if it didn't line up.
Eg. So say I decide to try to make peak power up to 6000 rpm and buy new heads to for it.
The Calculations say 265 cfm with a 1.97" csa which calculate to have an average 320 fps.
So ported stock or mild prep edelbrocks, but for the money I'd probably go tricks flows. Room to grow plus probably get peak at 6000 rpm with less cam, more streetable.
Really I'd probably try porting the stock heads see how that goes 1st then buy trick flow down the road if that was still my goal at that point.
Last edited:
low 11's @120 or so
Max effort on there stock flow should be like 625 hp.
The average guy is making 1.1 to 1.25 with some getting up to 1.35 lbs-ft per cid and 1.5-1.9 hp per cfm, I realize that most of ideas like this are for serious engines running 7500-9500 rpm making 2-2.5 hp per cfm and torque per cid in the 1.45-1.65 lbs-ft. Far from the engines we generally talk about beside the pic of the one your always posting the 1350 hp @ 7500 rpm
Phreakish
Well-Known Member
Chad is coming at it as a racer chasing every last tenth, so it makes sense to start with velocity and cross section since it will be defined by the rpm and induction allowed by parts allowed.
Like you're getting at, for a more casual build using mildly modified off the shelf parts it kinda goes the other way - working with the csa available or achievable and then making the most out of it.
Thank you Sir.
@273 Sounds great! Enjoy that. For sure. I was hoping to have been doing that already, but things don’t go as fast as I do and I wait a little longer. It is what it is….
There is not an easy answer to target velocity. It depends on the head. For instance, a head with a 10 degree valve angle and a high port should be able to handle higher velocity than a head with a 23 degree valve angle and a low port. The 10 degree head would have a more favorable short turn and better pressure recovery in the chamber.
Another difference between race heads and street heads is valve lift. A race head with 0.800” or greater lift is getting most of its cylinder fill after the venturi convergence point. A street head with 0.500” lift is getting most or all of its flow before the venturi convergence point.
For example, an 18 degree small block mopar head on the street needs to keep the air attached to the SSR for the entire lift curve, so the port will need to be designed for less than 300 fps. If the same head were to be used for racing, the cam would be more aggressive (faster opening) and higher lifting. Now, the SSR would be laid back more and the the port designed to shoot the air over the top of the valve, so higher air speeds can be tolerated.
I think when these head porting experts are put on the spot, they have to give a conservative answer for the max safe street velocities.
Even the pros don't agree, I was reading about all this on some engine forum and Eric, Chad and Darin among others arguing the validity of all this. They all basically agree but value it differently. Chads more is more in the 100% camp this way of going about it.
Hence why I said I think your over thinking it.
That's what I do best
It obviously works for him, everyone seems to have their own philosophies.
Hope your having a Merry Christmas.
I don't think so, it was on I think yellow bullet, I would of only recognize the IQ52 handle though.
-
