Why to have your Valves back cut

[BogusSVO]

Back cutting valves for improved air flow


This is a procedure done to help flow the air charge across the back of the valve and smooth the transition into or out of the combustion chamber.
Normally don on valves that are not fully machined or swirl polished.


Once the valve has had the face re-ground. Most common is a 44* or 45*
The valve machine is reset, and another angle is ground onto the back side of the valve towards the stem, most common is 30*

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The valve on the left has the basic face ground to mate back into the valve seat.
The valve on the right has had the basic seat inked,(black) and the added angle ground. (sliver)

Most OE valves are not fully symmetrical, so the width of the valve face to seat varies around the valve, the added angle will help correct this and smooth the transition of air flow across the back of the valve.

 

[delta448]

What is the necessary width of the face grind and how thin can you go? The exhaust valves especially would need more thickness for heat transfer, but how thin is too thin?

 

[BogusSVO]

the general "rule of thumb" is if you can shave with the edge of the valve... it is junk.
I would say on the 4G63 Intake a min. Valve margin would be .030 and the Exhaust would be .045

The face with depends on a few factors, whats the width of the seat? is the valve job a "stright" valve job? 45* seat with a 45* valve face, or an "interferace" valve job? 45* seat with a 44* valve face?

an intrferance will have a narrower contact area.

I have not come across any stock 4G63 valves that have been ground too thin, Most get bent before they reach that point.

 

[cardot33]

is this a serious increase in power or does it just free up a few more horses?

 

[Killa_dsm]

I think it would be cool if a Moderator or someone could move all your threads into a tech guide section or something so all this good machine work information is in one place.

Thanks for posting and education some of us.

 

[Lumix315]

I agree...it would be very nice. From the few threads of BogusSVO's that I've read, he definately seems like he knows his stuff. It's also been a pleasure to read his posts. It's all beneficial info that I'll use in the future...so thank you x2!!

However, even though he doesn't seem to be a "new member/lurker," he is, unfortunately, confined to posting in the "newbie" forum until he sheds that particular status...or gets grandfathered in due to possessing an extensive abundance of knowledge of our cars. I'm pretty sure that's Defiant's call, though.

 

[ragdollstalon]

i have a head that had a 3 angle valve job done. the change is amazing, flow wise the numbers are more, but pull wise its alot more! basically it allows for a better flow into the head velocity wise. instead of pushing air through a coffee straw your now pushing air through a big gulp straw!

 

[Rob10_99]

Yeah I aggree, bogus' isn't some dbag n00b trying to act cool, he knows his stuff.

 

[BogusSVO]

is this a serious increase in power or does it just free up a few more horses?

It helps improve airflow, so this will vary from head to head, Most the time it is worth a few more cfm which will equal more HP

It would be hard to say how much of a gain you would see, with out some dyno time.
build the head with the single cut valves, then remove the head, remove the valves and back cut them, then reinstall the valves and do another dyno pull.

But I would suggest having this done at the time of having a valve job done.


To all the guys with the nice words....... Thanks!! It lets me know that you are all learing and going to be a more knowlagable and have a better understanding of what happens with your engines whan they are machined.