Intake Valve Flow Numbers

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Earlie A

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Here's a little flow bench data that may be of interest. I was curious about the effect of changing the back cut angle on intake valves, so here's the test:

Out of the box Speedmaster small block mopar head (cylinder position 1 or 8)
2.02 intake valve
Test pressure: 28 in Hg
Test 1: Stock Edelbrock valve, 45 degree seat 0.100" wide
Test 2: Ferrea valve, 45 degree seat 0.060" wide, 30 degree back cut
Test 3: Edelbrock valve, 45 degree seat 0.060" wide, 32.5 degree back cut
Test 4: Edelbrock valve, 45 degree seat 0.060" wide, 35 degree back cut
Test 5: Edelbrock valve, 45 degree seat 0.050" wide, back cuts of 25 and 35 degrees @ 0.040" each

Darin Morgan says a back cut on the intake valve is the easiest 10-15 hp you will ever make. It sure increased the low and mid lift flow numbers. In this situation the actual measure of the back cut angle doesn't seem to matter much.

IMG_2206.jpg


EPSON031.JPG
 

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Very well done, nice comparison of the various valves and backcuts. 65'
 
NICE! Thank you very much.

Can you add some details to the last valve test on the back cut widths?
 
Darin’s right but you have to remember the glow goes both ways, especially at low (relative) engine speeds. So reversion can become an issue if the cylinder pressure is still high when the intake valve opens during overlap. The cylinder needs intake charge. Not something diluted with residual exhaust gasses.
If it’s a factory design port and lift expectations are moderate the back cut will help a lot. With a properly designed aftermarket port and larger lifts the effect is less dramatic and reversion can be more of an issue.
 
moper said:
Darin’s right but you have to remember the glow goes both ways, especially at low (relative) engine speeds. So reversion can become an issue if the cylinder pressure is still high when the intake valve opens during overlap. The cylinder needs intake charge. Not something diluted with residual exhaust gasses.
If it’s a factory design port and lift expectations are moderate the back cut will help a lot. With a properly designed aftermarket port and larger lifts the effect is less dramatic and reversion can be more of an issue.
Click to expand...
Can you give a specific example (in a properly designed system) of back cuts on an intake valve being a bad idea? In general, if reversion is a problem then intake port velocities are wrong, cam is too long for the application, or exhaust system is inefficient. By the way, I'm trying to learn, not argue.
 
If you need examples do more research. Your on the right track but there’s a lot more to it.
My take on you post was that there is no downside to an intake valve back cut and you did not note any of the possible other issues in your initial post.
My point was to note the potential down side of it especially in a racing application. If a well designed port and back cut valve are used, the potential of dilution becomes more possible and a loss of low end usually results with that. It’s another case of flow bench “progress” not quite being reality. That means among other things the camshaft design becomes more critical as you note in your response to my post.

In short- like everything else the whole design has to be understood and the package put together with parts that exploits its advantages and minimizes the negatives.

Not sure who can view this 17 year old post from Larry Meaux on SpeedTalk.com so I’ll copy and paste his comments. I’d would suggest you read anything he writes. Also look at Don Terrill’s book The Horsepower Chain and pay attention to the areas regarding cylinder blow down, the intake cycle, and camshaft overlap.

maxracesoftwareVendor

Low-Lift Flow and Torque​

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  • Post Tue Jul 24, 2007 2:47 pm
    the "Engine Masters" Summer 2007 issue
    on Pages 74-81 just about "mirrors"
    my Dyno experience with increasing Low-Lift Flow
    or the effects of Low-Lift Flow on bottom end RPM Torque output.

    In that series of Tests...they used different Cyl Heads....
    and basically came up with same results as i found.

    in my Dyno Tests..i used same exact Head,
    didn't change the casting or CSA, or Volume CC's to any extent,
    just increased Low-Lift Flow.


    2-Valve Heads
    So far my Tests show increasing Low-Lift Flow always
    "Hurts" Torque "below" the RPM point of Peak TQ
    and sometimes increases top-end HP
    ...just the opposite of what most people think.

    most will "equate" increases in Low-Lift Flow = increased bottom-end
    or low RPM Torque......but i've never seen this to be True yet,
    even with a smaller duration Cam like in the series of Engine Master tests.

    The 2 Cyl Heads=> AFR and the TricFlow 's
    had the best .100" Lift Flow to .300" + Flow
    but the TQ output was not as good or any better at 2700 RPM to 3500
    as the worst low-lift Dart Head
    (especially at 2700 RPM)
    i wonder what was happening below 2700 RPM ??
    along with gas mileage MPG ?

    the 2 worst Low_lift Heads (Dart + Edelbrock)
    made as good or more low RPM TQ.

    the better test would have to change Low-Lift Flow on the same exact casting one at a time..and repeat those Tests studying Low_lift Flow effects.

    There are times you can increase Low_lift Flow then use an even shorter
    duration Cam (like 4-Valve designs)
    and Pickup HP and TQ at low to midrange RPM,
    but usually always looose top end HP cutting down duration to achieve that.

    Also Heads with CSA Choke problems respond favorably to increased
    Low to Mid-Lift Flow so far in my Tests.....but overall the best method
    is to fix the Choke ...and not rely on Low-Lift Flow so much.


    MaxRace Software
    PipeMax and ET_Analyst for DragRacers
 
moper said:
If you need examples do more research. Your on the right track but there’s a lot more to it.
My take on you post was that there is no downside to an intake valve back cut and you did not note any of the possible other issues in your initial post.
My point was to note the potential down side of it especially in a racing application. If a well designed port and back cut valve are used, the potential of dilution becomes more possible and a loss of low end usually results with that. It’s another case of flow bench “progress” not quite being reality. That means among other things the camshaft design becomes more critical as you note in your response to my post.

In short- like everything else the whole design has to be understood and the package put together with parts that exploits its advantages and minimizes the negatives.

Not sure who can view this 17 year old post from Larry Meaux on SpeedTalk.com so I’ll copy and paste his comments. I’d would suggest you read anything he writes. Also look at Don Terrill’s book The Horsepower Chain and pay attention to the areas regarding cylinder blow down, the intake cycle, and camshaft overlap.

maxracesoftwareVendor

Low-Lift Flow and Torque​

  • Report
  • Quote
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  • Post Tue Jul 24, 2007 2:47 pm
    the "Engine Masters" Summer 2007 issue
    on Pages 74-81 just about "mirrors"
    my Dyno experience with increasing Low-Lift Flow
    or the effects of Low-Lift Flow on bottom end RPM Torque output.

    In that series of Tests...they used different Cyl Heads....
    and basically came up with same results as i found.

    in my Dyno Tests..i used same exact Head,
    didn't change the casting or CSA, or Volume CC's to any extent,
    just increased Low-Lift Flow.


    2-Valve Heads
    So far my Tests show increasing Low-Lift Flow always
    "Hurts" Torque "below" the RPM point of Peak TQ
    and sometimes increases top-end HP
    ...just the opposite of what most people think.

    most will "equate" increases in Low-Lift Flow = increased bottom-end
    or low RPM Torque......but i've never seen this to be True yet,
    even with a smaller duration Cam like in the series of Engine Master tests.

    The 2 Cyl Heads=> AFR and the TricFlow 's
    had the best .100" Lift Flow to .300" + Flow
    but the TQ output was not as good or any better at 2700 RPM to 3500
    as the worst low-lift Dart Head
    (especially at 2700 RPM)
    i wonder what was happening below 2700 RPM ??
    along with gas mileage MPG ?

    the 2 worst Low_lift Heads (Dart + Edelbrock)
    made as good or more low RPM TQ.

    the better test would have to change Low-Lift Flow on the same exact casting one at a time..and repeat those Tests studying Low_lift Flow effects.

    There are times you can increase Low_lift Flow then use an even shorter
    duration Cam (like 4-Valve designs)
    and Pickup HP and TQ at low to midrange RPM,
    but usually always looose top end HP cutting down duration to achieve that.

    Also Heads with CSA Choke problems respond favorably to increased
    Low to Mid-Lift Flow so far in my Tests.....but overall the best method
    is to fix the Choke ...and not rely on Low-Lift Flow so much.


    MaxRace Software
    PipeMax and ET_Analyst for DragRacers
Click to expand...
Thanks for the response. I'll do some reading.

You also raise another point I did not consider - that others might read my post and take it for gospel. I'm simply doing some testing and posting some results for the sole purpose of sharing. I think I'll add a disclaimer comment in the future. I don't want to be spreading bad information. Thanks.
 
I read one of the trick flow engineers say the activity design lower flow numbers at low lifts and focus on .300-.500 lifts for a street strip head.
 
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