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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
those are afr 235/245 real life numbers which have much bigger valve and much larger port area. That exceeds the theoretical limit of 146 cfm per square inch. I think something went wrong on the test bench
After comp valve job-untouched valves (prob something more to be gained by running them through the valve grinder for sure but these are the numbers regardless)
Don't get me wrong those #'s can be achieved. But usually with a larger runner.
Pro-filer has several 23° heads that come near 340cfm. And AFR has some larger port eliminators that do the same.
I kind of wish there were more as cast intakes for 13° SBC stuff. I wish the whole market would shift to 13° for SBC since the heads just bolt onto the engine, and then you just need an intake to bolt onto the heads.
Yes, used a crappy old Autolite spark plug on the flow bench. This was done back in April or May time frame but I do remember having to switch the Flow range selector from the usual position (3 suitable up to 300 cfm) to 4 (suitable up to 3xx cfm). Its tricky too because like any LS head I've ever flown or friends have, you always have to have 2 people on the bench (one watching & recording numbers on the dro, one watching the dial indicator, announcing loudly what lift you're currently at and holding the exh valve closed) as the vacuum motors pull so strong the checker springs aren't strong enough to keep it from simultaneously opening too.
*Not that it matters but this was done on a SuperFlow SF-600 bench too for those wonder. Also the bore adapter used I believe was a 4.060". The engine is a.040" over L31 so that would'nt make much difference. I too feel the numbers are a bit "unbelievable" but everything was double checked, and the head was gently secured to the deck with large clamps too for complete accuracy.
Last edited by Rob 97 Z28; 10-13-2020 at 11:37 AM.
Its just not possible so i would redo the test. Do you have a pitot tube to measure velocity? Did you use it?
if it was realistic then alot more people who professionally port heads for a living would be seeing similar results
when you need to get into a 235-250cc head with a 2.1+ valve in anyone elses heads including AFR, to see those numbers Then you know something is wrong lol. Why wouldnt afr use that valve job if it gave 50 cfm?
Its just not possible so i would redo the test. Do you have a pitot tube to measure velocity? Did you use it?
if it was realistic then alot more people who professionally port heads for a living would be seeing similar results
when you need to get into a 235-250cc head with a 2.1+ valve in anyone elses heads including AFR, to see those numbers Then you know something is wrong lol. Why wouldnt afr use that valve job if it gave 50 cfm?
Yeah, Pitot tube was all hooked up and working. The DRO was showing everything hooked up right, 28" hg etc.
The only other odd variable in this whole thing was the fact that I used a different cutter when I cut the intake seats they're actually reversed if I remember. I used a 4903 where it should've been a 4904 and vice versa.
It'd be great if that mistake was a stroke of genius but I highly doubt it
Yeah, Pitot tube was all hooked up and working. The DRO was showing everything hooked up right, 28" hg etc.
The only other odd variable in this whole thing was the fact that I used a different cutter when I cut the intake seats they're actually reversed if I remember. I used a 4903 where it should've been a 4904 and vice versa.
It'd be great if that mistake was a stroke of genius but I highly doubt it
Air was being pulled in somewhere other than the intake opening. Either that or someone is playing a cruel trick on you.
The numbers you posted are real close to a set of 256cc 18-degree CNC heads with a 2.18” valve from Chapman. In fact they only flowed 161cfm at .200 lift.
Just go out to Stan’s head flow page and check for .200 lift flow any where close to 170cfm for a SBC. They’re all large port 13-degree heads with 2.19 or bigger valves.
There are a couple out there pulling 177cfm at .200 lift and they’re all big block heads with 2.25” or bigger valves. AFR biggest 385 CNC BBC head with a 2.165” valve only pulls in 169cfm at .200.
I’m at work and can’t right now but someone needs to do the math on curtain area at .200 and see what that yields.
oh you said it was hooked up and working earlier? You could use those velocities and area of the port to factcheck the cfm readings
Yes. All parameters were set correctly. Cylinder head secured to deck, dummy spark plug installed, flow range adjusted correctly (after finding out the "3" setting was "running out" of air before .500"-.550" lift. Unfortunately the cars been down so long (since last fall) and I just want to get the new combo back together, in the car, make some passes before December and start dialing it in I know with work and limited time to mess with this stuff I won't get a chance to put them back on the flow bench.
I'm going to assume that the heads peak flow somewhere around .600" valve lift prob moving high 280's-ish
For anyone that read through all this and wondered the direction I'm taking, a ton of research, talks with head experts, tgo members own experiences, legit race and machine shop, etc has lead me to decide on...
My current 08-501-8 212-218 cam. I'm used to it, like it's idle, helps keep my powerband where I want it.
HSR intake based on BadSS recommendation, it certainly won't be a restriction, simplicity and cost.
AFR 180s due to a number of reasonings. Biggest reason over the afr195 is that those really shine above .5 lift and since I'm keeping the cam then that's cfm I'll never use. So cfm per cc in my lift range goes to the 180s. And not to mention the recommendations from a couple different builders.
Keeping the SLP 1 3/4 shorties, and if my 3.08 gear is just too high, then I'll worry about that when I cross that bridge. Hopefully the engine is a moderate enough build that I keep the gears but it's not a big deal to swap axle later.
If this sounds preposterous to you, then please feel free to make your case. Hopefully this thread has some useful information for someone in the future.
You'll like that combo. Mine has the manners of a factory car, which I really like. But tons of power. Certainly not the fastest car on the road these days, but plenty of power for me to have some fun with.
Keep in mind you'll probably need some recalibration on the chip - particularly with the accelerator enrichment (AE) now that you're running a short runner manifold. If you're able to do the physical work of burning chips, folks in the DIY PROM section (including myself) can help you with that.
The only future improvements I would suggest would be full length headers (shorties are not buying you anything in the rpm range of the cam and heads).
Also, 3.42 or 3.73 rear gears. The bang for the buck on that going from a 3.08 will be extremely worth it.
Keep in mind you'll probably need some recalibration on the chip - particularly with the accelerator enrichment (AE) now that you're running a short runner manifold. If you're able to do the physical work of burning chips, folks in the DIY PROM section (including myself) can help you with that.
The only future improvements I would suggest would be full length headers (shorties are not buying you anything in the rpm range of the cam and heads).
Also, 3.42 or 3.73 rear gears. The bang for the buck on that going from a 3.08 will be extremely worth it.
I can tune though when I have troubles or questions I'll definitely appreciate the help.
I can't do long tubes, slave cylinder in the way.
3.42 or 3.45 may be in the cards. I've had 3.73 gears in cars and I hated every one of them. Heck, there's a 3.70 in my garage now.
Not that it matters anymore, but for clarity purposes I believe I now know why I had such high flow numbers...
When I was still in school I flowed some other LS castings I had been working on and remembered that the flow bench dial setting for CFM rate has to be changed to accommodate the flow range of the heads BUT ALSO in the DRO part of the flow bench, that Also has to be changed to match the flow rate setting the dial is set to as well🤦🏻♂️🥴😞
10-13-2020, 07:34 AM
