Back cut valves...

[travis]

I’m curious if y’all have seen any performance or flow improvements from back cutting valves in poor flowing or under valved applications?

I’ve got a set of pretty poor flowing mildly ported E5AE Ford heads with stock 1.78/1.45 valves, and I have a handful of extra valves that have been refaced but won’t be used. I am going to try back cutting them just to see what difference it makes on the flow bench.

[ProPower engines]

It will increase the low lift flow 99% of the time as long as the head is not total junk.
Even some stock stuff will pick up some rpm with a decent VJ and a back cut on the valves.
Put a slight radius on the margin that helps as well

[mag2555]

Back cutting the Intake will pick up like 12 cfm total across the .050" to .300" lift range and the total on the Exh side may even be more even though your dealing with a smaller valve .

The added flow mass passing thru the Intake Throat can induce sheer problems if the port size and shape was already on the edge of doing so which can be see by the test pressure Manometer bouncing up and down at the start of loosing conttol.

[RevTheory]

I was researching backcuts for hours a couple of weekends ago and saw several Pro Stock intake valves that had multiple cuts, some other classes with 2, some with 1 at .080 wide... all kinds of stuff. Larry Meaux had a bunch of old posts from forums I've never heard of. I can't imagine Pro Stock wanting anything to do with increasing flow below say .400 but, obviously, nothing was mentioned about what any of them were trying to accomplish beyond the vague "what you want to do is optimize your venturi."

I bet this topic is another one where what looks good on the bench gets worse on the dyno, sometimes, under certain conditions like a full moon or El Nino. I was about one more cup of coffee away from starting a thread but talked myself right out of it, lol.

[HDBD]

Sure why not if the reverse flow remains in check

[RevTheory]

Lost my chance to edit.

I suspect it has quite a bit to do with what top cut angle you have on your seat, how much total lift you'll have and whether or not an increase in low lift flow is the right move for your particular application.

If you're dealing with heads that don't flow very well, small valves and a mildish street application, a backcut that enhances low/mid lift flow is probably a good thing assuming the LSA isn't too narrow. Now what backcut angle and width is needed to compliment a 45* seat with a 30* topcut, I surely can't say.

[dannobee]

Start with a 22 deg back cut if you don't have access to a bench. Cut it back until it comes within 0.030" of the witness mark with the seat. And if the port is lousy to start with and you're forced to run stock valves, get rid of that ski jump on the intake valve. Low and mid lift will almost always go up, sometimes quite a bit. High lift flow usually won't change much one way or another (percentage-wise). If you have access to a valve grinder AND a bench, by all means, cut a few intakes at different angles and widths, then have at it. On the exhaust side it's not so clear cut. The ski jump might actually help flow, and it certainly doesn't hurt in keeping the exhaust from reverting back into the chamber. Again, cut a few old valves and try. Reverse flow the head to make sure you're not going the wrong way with reversion.

[Walter R. Malik]

You can use the back-cut angle and its width on the intake valve to somewhat tailor the flow throughout the lift curve.
Otherwise it can be used to simply narrow the seating area but, the angle used will have an effect somewhere.

[Joe-71]

Your results may vary, but I have never had a SBF exhaust valve with a back cut increase flow. Even the CNC heads I have flowed with the back cut on the exhaust lost flow over a simple 45* seat with identical valve. As much as 12-15 cfm loss. Joe-71

[rebelrouser]

check out this link viewtopic.php?f=15&t=58997

[PRH]

For those really mild street builds that retain the use of the OE castings, especially the ones that aren’t on any kind of “preferred” list...... I go after whatever low hanging fruit there is...... and whatever that yields........ iiwii.
I would pretty much always back cut the valves, since the cam associated with that type of build is rather small, and excessive back flow isn’t usually a problem, especially if the valves are pretty small.

20+ years ago I might have flowed with/without a back cut...... nowadays more likely to not even bother flowing it at all.
That’s a “one and done” type of job..... so whatever it flows...... that’s it.
If the customer wants to pay for a flow test, that’s fine.

Some of the OE heads yield pretty nice gains from minimal work..... some give you almost nothing.

[Alaskaracer]

You mean like this:

[HDBD]

Mark begs the question if the top angle at the margin is the seat or is the second angle the 45 or whatever?

[Alaskaracer]

Mark begs the question if the top angle at the margin is the seat or is the second angle the 45 or whatever?

I can't get into too many details about my valves, they are a custom design by my head guy. I can assure you that there isn't a single 45* cut either on the valves or the heads though......I just posted the pic as an example of top cuts, etc.....If you're curious, that's a 6.6" long, 2.475" Ti intake.....with the ports my guy designed to with them, they went just over 600 cfm with this valve....next freshen we are going to a 2.52" intake, cutting the exhaust down to 1.8", and reworking the seats and ports. I'm also going to see about moving the intake lifters over a bit to make room for a little more port work and bigger pushrods....

[3V Performance]

Mark begs the question if the top angle at the margin is the seat or is the second angle the 45 or whatever?

I can't get into too many details about my valves, they are a custom design by my head guy. I can assure you that there isn't a single 45* cut either on the valves or the heads though......I just posted the pic as an example of top cuts, etc.....If you're curious, that's a 6.6" long, 2.475" Ti intake.....with the ports my guy designed to with them, they went just over 600 cfm with this valve....next freshen we are going to a 2.52" intake, cutting the exhaust down to 1.8", and reworking the seats and ports. I'm also going to see about moving the intake lifters over a bit to make room for a little more port work and bigger pushrods....

146cfm per square inch? Did you mean a 2.575 valve? Figuring 91-92% throat