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Thoughts on back-cuts, flow gains, & airflow stability

General engine tech -- Drag Racing to Circle Track

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BradH
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Thoughts on back-cuts, flow gains, & airflow stability

Post by BradH » Sun Apr 14, 2019 11:36 am

I'd been meaning to post this stuff and get some folks thoughts on the results.

I. The flow #s at 28" for a one of my heads using same brand of intake valve:
A - No back-cut
B - 34* short back-cut
C - 32* short back-cut
D - 30* long back cut starting from the edge of the seat

------- _A_ / _B_ / _C_ / _D_
.100 - 70 / - 77 / - 76 / - 77
.200 - 142 / 146 / 153 / 157
.300 - 212 / 215 / 221 / 227
.400 - 274 / 275 / 280 / 284
.500 - 321 / 320 / 325 / 327
.550 - 335 / 335 / 336 / 339
.600 - 344 / 342 / 347 / 343
.650 - 348 / 348 / 350 / 347
.700 - 352 / 355 / 359 / 345
.750 - 353 / 347 / 344 / 343
.800 - 346 / 344 / 342 / ---


II. The flow #s at 35" to see how the uncut valve and the 32* back-cut valve compared under higher pressure at high lifts:
A - No back-cut
C - 32* short back-cut

------- _A_ / _C_
.600 - 384 / 388
.650 - 394 / 392
.700 - 405 / 399
.750 - 396 / 385
.800 - 382 / 382

Same test results from 35" converted back to 28" from 35" w/ original 28" in ()

------- _A_(28")_ / _C_(28")_
.600 - 343 (344) / 347 (347)
.650 - 353 (348) / 350 (350)
.700 - 362 (352) / 359 (357)
.750 - 354 (353) / 344 (345)
.800 - 342 (346) / 342 (342)


Comments & observations from 35" tests:
1. Inclined manometer was noticeably steadier at all times with the uncut valve

2. Whereas the 35" tests converted back to 28" vs. the original 28" tests were very consistent for the 32* back-cut valve, the valve w/o the back-cut's results picked up at .650 (+5) & .700" (+10)

3. Higher test pressures caused the valve w/o the back-cut's results to back up sooner than at lower test pressures (at .750 vs .700"), but the drop-off wasn't as severe as the 32* back-cut valve's tests

4. At .800", the two configurations flowed the same, with more loss shown at 35" for the uncut valve than at 28".

5. The engine's net valve lift is right at .650", and I went with the 32* back-cut valve config since it showed a solid gain in the cam's lift range without as significant a loss at .600"+ as the 30* back-cut.

--------------------------------------

Your thoughts? Thanks - Brad
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steve cowan
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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by steve cowan » Sun Apr 14, 2019 5:30 pm

Brad,
i think it is good you can test at a higher pressures than 28'' from my limited experience on the bench it shows a lot - velocity gradients over the SSR as an example.
i have dozens of different test valves and some have factory back cut on them and the back cut valves always seem to flow better especially at low lifts but what does that really mean ??
there is two schools of thought -as i think you would know
good low lift flow
shallow angle seats
thick valve margins
back cuts on valves
abrupt SSR effect low lift flow,excess material taken out unshrouding valve etc.

Less low lift flow
no back cut valves
steeper seats with complementary angles
effective deshrouding of the valves
layed back SSR with corrected velocity profile.
i have seen pictures of pro-stock valves with multiple back cuts .........????????
Brad,
i dont want to derail or mess up your thread but i have a observation on your flow bench #s at higher lift.
your port goes backwards after .700''
you have a net valve lift of .650''
if port stalls i dont think thats to bad but if #s go backwards thats turbulence
i would be looking for more stability in flow and velocity up to and around 0.800'' lift
just my thoughts.
steve c

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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by Walter R. Malik » Sun Apr 14, 2019 8:21 pm

It simply proves to me that different back-cut angles and widths work differently for different applications and valve lifts.

The brand head and porting of one can require a totally different valve shape than a different brand head or engine.

That kind of testing needs to be done with every different combination being used.
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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by BradH » Sun Apr 14, 2019 8:47 pm

My own flow bench can't pull that sort of depression. I've been a customer & "collaborator" with member PRH, Dwayne at Porter Racing Heads, over the years and the flow testing is all from his shop.

There have been a number of threads here where one of the common themes is stable flow without the port backing up is more important than better flow #s. Even though the port still backs up and doesn't pull the same #s without the back-cut valve, Dwayne's observation that it was more stable across the whole curve at the higher test pressure makes me curious if / how the two different valve configs would change the engine's performance. But there's no plan to dyno the engine again, especially for one simple change.
Last edited by BradH on Sun Apr 14, 2019 9:32 pm, edited 1 time in total.
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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by steve cowan » Sun Apr 14, 2019 9:02 pm

BradH wrote:
Sun Apr 14, 2019 8:47 pm
My own flow bench can't pull that sort of depression. I've been a customer & "collaborator" with member PRH, Dwayne at Porter Racing Heads, over the years and the flow testing is all from his shop.
nice to have some one to help like Dwayne
he has helped me with some porting stuff as well on my own threads :D
steve c

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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by GARY C » Mon Apr 15, 2019 1:05 am

You would have to dyno one vs the other, the guy that does my valve jobs quit doing back cuts years ago because the cutters he came up with made more power with no back cut regardless of what the bench said.
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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by mag2555 » Mon Apr 15, 2019 7:29 am

Regardless of the flow loss at high lift what you want is those fat flow numbers at .300" to .400" lift where the Cam is already providing the most open valve time.
This is in large part why 4 valve heads make greater power even if there high lift / peak flow numbers only match that of a 2 valve head.

If you want to play around with reducing reversion and have the motor come on the Cam sooner you will need to tame the added low lift flow numbers at .050" to .100" that the back cut provided, this can be done by not adding as much 15 degree top cut to the chamber if such is used.

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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by RevTheory » Tue Apr 16, 2019 10:44 am

I'm of the *opinion* that the one that remained stable at higher lift and depression would show better on the dyno than one that picked up a little low-lift flow on a bench at 28".

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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by ClassAct » Tue Apr 16, 2019 12:15 pm

RevTheory wrote:
Tue Apr 16, 2019 10:44 am
I'm of the *opinion* that the one that remained stable at higher lift and depression would show better on the dyno than one that picked up a little low-lift flow on a bench at 28".
Not always the case. I can make most any port go turbulent if the bench will lower the depression enough.

The question is...does a port see 28" when its at .500 lift or more? I say probably not as I can't be dogmatic about it. I doubt even the best head sees that much pressure drop at higher lifts.

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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by BradH » Tue Apr 16, 2019 12:30 pm

ClassAct wrote:
Tue Apr 16, 2019 12:15 pm
RevTheory wrote:
Tue Apr 16, 2019 10:44 am
I'm of the *opinion* that the one that remained stable at higher lift and depression would show better on the dyno than one that picked up a little low-lift flow on a bench at 28".
Not always the case. I can make most any port go turbulent if the bench will lower the depression enough.

The question is...does a port see 28" when its at .500 lift or more? I say probably not as I can't be dogmatic about it. I doubt even the best head sees that much pressure drop at higher lifts.
I hate to date myself, but that might be The $64,000 Question!

However, I believe the data from people using in-cylinder pressure sensors has shown the peak depression can get much higher than even 50". You gotta have some serious flow bench HP to be able to test in that range, and that ain't me.
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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by ClassAct » Tue Apr 16, 2019 3:20 pm

BradH wrote:
Tue Apr 16, 2019 12:30 pm
ClassAct wrote:
Tue Apr 16, 2019 12:15 pm
RevTheory wrote:
Tue Apr 16, 2019 10:44 am
I'm of the *opinion* that the one that remained stable at higher lift and depression would show better on the dyno than one that picked up a little low-lift flow on a bench at 28".
Not always the case. I can make most any port go turbulent if the bench will lower the depression enough.

The question is...does a port see 28" when its at .500 lift or more? I say probably not as I can't be dogmatic about it. I doubt even the best head sees that much pressure drop at higher lifts.
I hate to date myself, but that might be The $64,000 Question!

However, I believe the data from people using in-cylinder pressure sensors has shown the peak depression can get much higher than even 50". You gotta have some serious flow bench HP to be able to test in that range, and that ain't me.
I agree it can go way over 50 inches, but what I've seen has always been at pretty low lift numbers. I'd love to see some data that can show if those test pressures are acheivable at lifts over say...maybe .500 and up. That would be interesting to say the least.

And you'd need a test bench like DV is building. And enough power to run that thing without causing a neighborhood black out when you turn it on.

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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by digger » Wed Apr 17, 2019 5:12 am

You only need to ask the question what depression on a flow bench would it take to generate port speeds of Mach 0.5 or so

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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by steve cowan » Wed Apr 17, 2019 5:33 am

digger wrote:
Wed Apr 17, 2019 5:12 am
You only need to ask the question what depression on a flow bench would it take to generate port speeds of Mach 0.5 or so
might depend on the port size and shape as well
i have seen nearly 490 ft/sec on my bench @ 28''
steve c

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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by digger » Wed Apr 17, 2019 5:59 am

steve cowan wrote:
Wed Apr 17, 2019 5:33 am
digger wrote:
Wed Apr 17, 2019 5:12 am
You only need to ask the question what depression on a flow bench would it take to generate port speeds of Mach 0.5 or so
might depend on the port size and shape as well
i have seen nearly 490 ft/sec on my bench @ 28''
i mean average across a cross section not local speed

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Re: Thoughts on back-cuts, flow gains, & airflow stability

Post by steve cowan » Wed Apr 17, 2019 6:00 am

:D thanks for the clarification
steve c

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