Wanting to pick up low lift flow numbers on my heads and have always heard of the 30* backcut helping low lift flow significantly.

Wondering if I could/should do this with my manley race flo valves. car is a street vehicle and low lift flow is important to me. thanks

 

Yes - there is a significant difference on the flow bench. Depending on the valve shape(degree) and bottom cut angle you will vary the backcut from 29*-35*, but you can just go ahead and backcut at 30* all the way to the lap line(seat contact patch) and be good to go.

 

Actually while it does make a difference on a flow bench at lower lifts I have found (at least w/ my engine specs) using EA Pro that more flow @.200 and lower hurts torque and does not help peak power. that is to say more flow @
.200 and lower doesn't help but rather hurts average power on my setup. might be different for a given engine or a combo. I know Meaux has said the same. anyone else care to comment

 

So with your setup port velocity is a problem.

 

No, I was of the same mind that more flow in the .200 and under was good until I did more research on it on my setup and read an article on speedtalk by Larry Meaux. Again this is with my setup and 195cc heads on a 350 lt1 (might be beneficial on a bigger motor). Heads flow very well already but when I played around with altering the .200 and under lift cfm (raising flow) I lost torque and gained nothing anywhere. Lowering flow in the same area produced more torque which was interesting but only for that range of flow. Other lifts obviously had an adverse effect.
Just trying to stimulate some healthy debate on this.

Meaux also thinks that this is why valves weren't backcut from the factory as it would have been a cheap performance increase for auto makers

 

I missed where you supported saying that port velocity at low lift was not the problem.....

But the factory valves ARE backcut - take a look at an early model valve, then look at the late LT/Vortec valves.

Not disagreeing with Larry at all(he knows way more than I do), but in the motors that we have done back to back comparisons on in regards to valve job angles, valve sizing, and backcut angles - a backcut valve with our valve job(custom radius profile) flows better, and shows a difference on the dyno.

 

I didn't mean to say Larry said our valves were't backcut -he was referencing a different engine - thats my mistake. curious what lifts it helps improve with the backcut you use and hp difference? As far as supporting my having effective port velocity what exactly are you wanting? the heads I use flow as well as LE3 heads but use a 195cc intake port (was expressly designed to be a high velocity port) and a 2 inch valve and make great power with high mid flow numbers 288cfm @ 28" H20 .600 lift sf600 serdi valve job manley race flow valves on a 350 lt1 with a mid sized custom ground cam. To be clear I don't have issues with the car not performing or power not being where it should - car performs beutifully just looking for a little extra.

Not trying to discredit you or anything..Hell I'm not not claiming what I'm saying is correct- it may be a fluke and it could be my heads themselves thats why I was hoping some of the other head porters would jump in on this thread with their .02.

 

Well backcut valves are not going to make a seat of the pants difference lol.

Did the guy who ported your heads not backcut the valves to begin with and you are looking to see if that will pick up some power?

 

I searched high and low and couldn't find the pic to save my life, but I remember seeing this pic awhile back of a valve that had been back cut, except it had 10 or 12 small "steps" cut into it. Anybody know what I'm talking about?

 

Perhaps you are thinking of my old website. I shut it down at the end of the year. However I think I might still have a picture you are thinking of. I did it as part of an AFR-sponsored study on air flow when porting my 195's. It improved airflow under .200" lift by three cfm, if I remember correctly. That's a couple percent.

This is a low-res version of that picture:

 

I appreciate you posting the pic, but that's not the same as the one I saw. If this makes sense, the whole backside of the valve looked like it was "terraced," like how Japanese farmers cut into the side of mountains to create more flat acreage to plant on.

 

I've chucked up valves in the lathe and actually cut a radius down to the seat width, didn't make enough difference to warrant the time spent.

 

I think the valves were for a carb application. The steps being an atomization enhancer.

 

something like this?

 

BINGO, we have a winner! Thanks for posting that. You wouldn't happen to know if there's a specific term used to describe that kind've backcut would you?