Multiple back-cuts on 45*-seat intake valve?

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BradH
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Multiple back-cuts on 45*-seat intake valve?

Post by BradH » Tue Nov 27, 2018 12:18 pm

<< I posted something about this 4 years ago, but it didn't get much in topic-specific replies. Rather than bump the old one back from the dead, I figured I'd jump-start a new thread. >>

Have people experimented with using multiple back-cuts on a 45*-seat intake valve, as opposed to a single cut? I know steeper seat angles like 55* often have multiple back-cuts on the intake valve, but don't recall this as a common mod for a 45* angle.

I wasn't necessarily thinking in terms of flow benefits as much as adding another shear point for the air/fuel mixture, similar to the benefits of modern valve jobs with more angles used than the traditional 3-angle deal.

How would you expect that differences in valve angles (e.g., 15 vs 23), raised intake runner vs standard, etc., change what might -- or might not -- be effective?

Also, would it be a reasonable assumption that doing an effective double back-cut on a 45*-seat valve would likely be something different than simply adding another cut behind the standard 30*? E.g., Instead of 45* / 30* / 20*, something like 45* / 32* / 25* might prove more effective (just a w.a.g. for discussion purposes)?

Thanks - Brad

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Re: Multiple back-cuts on 45*-seat intake valve?

Post by GARY C » Tue Nov 27, 2018 12:24 pm

It seems some are getting away from back cuts all together.

I always wondered about after porting if you were to cut the ID on the seat ring just below the aluminum to mimic a step booster, just a small amount, maybe 10 to 20 thousand deep?

I asked around, all I ever got was that it would hurt flow.
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Re: Multiple back-cuts on 45*-seat intake valve?

Post by Erland Cox » Tue Nov 27, 2018 5:11 pm

I have some valves that are thick above the seat with 15 degree back angle that I have cut with 35 and 25 degrees above the intake seat.
I have not done any back to back dyno tests with flatter valves though.
The valves with multiple back cuts flow better than flatter valves with a 30 degree back cut but they are slightly heavier.

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Re: Multiple back-cuts on 45*-seat intake valve?

Post by MadBill » Tue Nov 27, 2018 8:35 pm

Cuts above the seat reduce the effective valve diameter. Do they work better than the same OD valve with a '90°" above the seat? (Or are the cuts so narrow that there is little effective loss of diameter?)
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Re: Multiple back-cuts on 45*-seat intake valve?

Post by pcnsd » Tue Nov 27, 2018 10:29 pm

It depends and the only way to know if they will work in your specific application is to test. On my application, a 30* back cut on the intake valve loses airflow across the lift range as does a 50* seat and valve cut. Using a multiple back cut (45-37.5-30) lost more flow. I can't use the purposed (45-30-20) because it would flatten the valve profile and I already know it likes about 25* for the tulip angle. Best airflow is with a straight 45 on the valve and 60-45 on the seat. A 30* front seat cut loses flow.
On the exhaust side using a valve back cut (45-30) gains a few cfm. Using the 45-37.5-30 cuts neither helps nor hurts. The exhaust likes a 75-60-45-30 seat cut.
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Re: Multiple back-cuts on 45*-seat intake valve?

Post by Ks Fats » Tue Nov 27, 2018 11:36 pm

Is this a 4-valve or 2-valve head?

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Re: Multiple back-cuts on 45*-seat intake valve?

Post by BradH » Wed Nov 28, 2018 7:43 am

In my case, it's a 2-valve 15* config with what I'd describe as medium-raised intake runners (not raised nearly as much as more serious 15* SBC race heads I've seen).

No plans to change what I have now, but it's one of those things that I'd been interested knowing more about.

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Re: Multiple back-cuts on 45*-seat intake valve?

Post by Erland Cox » Fri Nov 30, 2018 4:02 pm

Have you tried flowing the head with only the valve stem through the throat?

Erland

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Re: Multiple back-cuts on 45*-seat intake valve?

Post by JoePorting » Fri Nov 30, 2018 5:39 pm

What I found important on back cuts is not to make the 45 degree cut too small. The 45 degree cut should be at least .080" wide. If you cut into it further with any top cut you'll lose flow. Therefore if you want to cut a top cut the first thing you need to do is make the 45 degree cut around .125" wide at least, then put in a 30 degree cut that leaves the 45 degree cut around .100". But overall, top cuts don't seem to make that much of a difference. I'd spend my time somewhere else.

About 10 years ago I spent about a week experimenting with different top cuts with a flow bench and was surprised by the lack of change.
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Re: Multiple back-cuts on 45*-seat intake valve?

Post by BradH » Fri Nov 30, 2018 6:10 pm

Simply because a couple of responses don't seem to match the question, I'd like to re-state that I'm asking about having more than one back cut on the valve itself, but specifically for applications with 45* seats. Thanks - Brad

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Re: Multiple back-cuts on 45*-seat intake valve?

Post by jcisworthy » Fri Nov 30, 2018 7:18 pm

I have two 45* seat profiles. I use one with 15* or less valve angles and the other I use on everything else. A single back cut works best with the 15* or less cutter and a double back cut works best with the other.

A single cut on the second cutter gained air to about .400 and typically lost air .400 up but with the addition of the second angle the low gains were still there and the .400 up was as good or a touch better than no back cut
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