Back-Cutting Valves

[Anonymous]

About back-cutting the exhaust valves . . .
is there a better material to make the valves out of so that they can withstand being back-cut?

 

[xctasy]

Yes there is. Inconel, a specification of metal (often used on Pontiac post emissions engines) which is very good for high heat situations. There were Nimonic metal valves, and several other types used as well, but I think Inconel was the most cost-effective. Ford may use it all ready.

Since the advent of unleaded gas, most auto makers, especially Chrysler, resorted to having thick valve stems. Latter on, they improved the metal specifaction and dropped the valve stem diameter. Like what happened with LA 318/360 engines in the 1990's. Early engines had the customary large diameter vlave stems, latter ones where Ford/Chev size.

 

[Anonymous]

So it's the stem that's breaking? Judging from the pic I assumed the problem was the cracking around the seat area. Or was that a crack that I thought I saw--?

 

[goinbroke2]

Are you talking about reducing the stem diametre, or back cutting the face?
I do head work and engines in my shop for local drag and stock car racers, and in my experience I've found it better to buy the valves with the narrowed stems than to try and do it myself.(numerous reasons I won't go into here).
As far as back cutting the face, I have found that it is best to only back cut intakes as this makes a wedge or venturi and picks up low lift flow on the bench.(SF-200) Back cutting the exhaust doesn't seem to do anything or at best is inconclusive.
I back cut the intakes 30 deg and run interference seat angles 45/46.
Altogether, I start with a 70 throat cut, 15 top cut ,30, then a final cut of 46 for the seat .060 wide on intake and .075 exh. The valve gets a 45 and a 30 back cut on intake only.
SS and Manley both make good valves with cut back stems.

Also, all engines do not neccesarily like the same angles. Pontiacs seem to like 30-35 deg seat angles due to chamber design and Comp and SuperStock engines like 60. But this brings up MORE problems with valve life due to increased stress with a flat seat pulling the head off and valve bounce. Or with the 60 angle it beats the seat in the head out quickly due to the increased "drag" across the seat by the valve.(but they sure flow better!)

Anyway, BUY valves already with reduced stems don't turn them down on a lathe.
Backcut valve face 30 deg(same width as seat .060) on intake only.
Like I said though, this is my personel experience only, others may have different opinions.......even if they are wrong!

 

[xctasy]

True. Back cutting exhast valves without a great quality valve is a head looser. The valve is a heat sink which engineers spend most of there development time working on. I understand that there is a performance gain if the valve is the restriction point, but if the risk is loosing a valve head or going to a smaller guide without proof that it's not going to grenade, then the risk is too much.


If valve seat angles are wrong you can loose a lot of HP. I've heard, forman old Hot Rod issue, that Windsor heads, with the wrong kind of 3-angle seat cut, can loose 5 hp on a 300 hp engine. You heard that?

 

[Anonymous]

Okay, I guess I need to back up a little and explain. I originally posted my question in the "sticky" at the top about 3-angle valve jobs but my post got moved here. The sticky post stated that back-cutting the exhaust valves caused them to break or crack, and from the pic, I assumed the damaged area was the valve face as the post said nothing about the stems.

Since a proper 3-angle job on the exhaust valves helps exhaust flow (which is one of the first places to look for more power) I was wondering if better valves were available that would not break or crack.

OR, did I completely miss the point of the sticky :

 

[addo]

No, you didn't miss the point. But Dennis had asked that related questions be started as a new topic, to keep the bare bones info clean and simple. In moving it, I tried to choose a topic line that was relevant.

Adam.

 

[Anonymous]

Thinking about trying to maximise power under 4600rpm in my 250 2V. Like the idea of dual pattern cam with more on exhaust but think 10 duration on a Crow 666 (1500-4500) cam is still a bit too much. If I I go ahead with this I can then afford to leave the stock valves in exhaust and go much bigger on inlet with bowl porting to match the 1.78 increase (think this should work very well). Execute suggested 1.78 valves possibly the 253 V8 valves might fit but would need to unshroud them.
Does unshrouding involve removing material from the chambers and would it lower compression much? Does anyone know if the 1.78 valves will fit or something similar.

 

[Hot 6t Falcon]

Hi Mike,

Actually, what I said was to not cut exhaust valves on a "street" engine because the heat is very hard on them, and they need their mass as a heat sink.

I tried back cutting a set of exhaust valves and needed to replace them after about 3000 miles. All of the valves had burned seats and leaked severly. Taking apart my head every 3000 miles and chucking approx. $60 into new exhaust valves is not a "good" street engine. However, those numbers would be acceptable for a race only engine.

My next experimental head will use stainless exhaust valves from SI Industries in Simi Valley, CA (805) 582-8285. They originally made them for Clifford. The 1.75 intake is #SEV 3007S (stainless). The 1.50 exhaust is #SEV 2530S (stainless). The cost is $9.90 each intake and exhaust.

My understanding is that the are made thinner in the first place so no back cutting is necessary - and they cost about the same as a cast iron replacement valve from Manley. How long will they last in a street engine remains to be learned.

Good Luck

 

[Anonymous]

Excellent! That answers my questions! Thanks!

 

[goinbroke2]

I checked on a flow bench and intakes that were back cut picked up flow at low lift. Exh however did not. I tried a d-8, e-7, 302 heads, and c-8 429 heads.
You are the first person I've heard with a concern for losing a valve head do to back cutting.
Since a back cut is only .070 or so wide, and that's off the seat area, why would you think it would weaken the valve? I can see it possibly letting the valve run hotter do to less mass, but that would be nothing in comparison to cutting the seat too narrow/having too small of a margin. And with both of those problems the valve runs so hot it burns.
If you are losing the heads off of valves, you either have wicked valve float, or massive spring pressures, or wrong seat angles(55+). For example, if your running a solid roller cam, and spinning to 8500+rpm, the springs required are MASSIVE!
Not only are they hard on the valve when it slams closed, but to run that rpm you need to continually check pressures so you don't kiss a piston.
That could lose a head, but back cutting? can't see it.

just my $.02

 

[goinbroke2]

Hot falcon, if you burnt the exhaust valves, I would like to ask did you run interference angles? 45 head seat-46 valve seat? (for self cleaning) Also, how wide was your seat? and how wide was your back cut?
All of these plus margin width will determine valve life. And you said you had to replace them? Were they burnt THAT bad?
Again, the mass you lose in a simple back cut would not be the sole problem. Perhaps your running high exh temps due to lean carb/retarded timing/etc?

 

[addo]

gb2, could poor fuel quality contribute to unexpected valve damage in these circumstances? All other things being the same, that is.

Cheers, Adam.

 

[goinbroke2]

Addo, I've thought about this all afternoon. I honestly can't see how fuel would do it. If you ran too low of octane then the detonation would knock a hole in the piston or break a ring land/bust top ring/pound out rod bearings, even knock porceline out of the plugs....but I can't see a steel valve being hurt before the aluminum piston.
Perhaps custom or "homemade" fuels with different oxygen content or other enhancers would possibly do something.
But generally speaking, driving down the road and get a tank of bad gas or two, no I don't think that would do anything to the valves. Especially if the rest of the engine is fine.
I think I would stick to the more common or more realistic problems. Late timing is a possiblity but the most common thing I would think is a veh set up properly then somebody does the headers/free flowing exh thing and doesn't compensate by jetting up the carb. Now it's lean, and running hot but it's not "noticable" so it's presumed to be fine.
Also, somebody doing a valve job and not getting some measurements right. Do a super thin seat or bad angle and see how long it lasts!

 

[goinbroke2]

Also just clear the air, Hot Falcon, I mean no disrespect in any way, but I find it" interesting that all the valves were burnt. I know you back cut all the valves, but since I don't think that was the problem, it makes me wonder what was the other common denominator? Obviously you did all the angles the same(cyl to cyl) How wide did you do the seat? Normally if it was a simple case of the seat being too narrow in the first place,they wouldn't of lasted that long. How wide was the margin on the valves,they might of lasted that long with thin margins.
Like I said no disrespect to you or your ability, I'm just very curious.

 

[Hot 6t Falcon]

Hello,

You've got me going back three or four years - without notes as to specs, so I'll jus do generalities.

They were the stock 1.38 valves with an interference cut 45/46 and were almost passed use (small margin) when the head was done. I have the pistons sticking out of the block .006 in an effort to minimize squinch with a thick head gasket and a 9.7 compression on 91 octane. I don't have preignition problems at all. I set my advance to 34 degrees total, and I don't even know what the inital advance is

Initally, the two-barrel ran TOO lean off of the idle screws (1 1/4 turn) to the point of putting a slight glow into the header's pipes. The Autolite was TOO rich when off of the idle circuit. We now tell people doing the Autolite two barrel swap to start at two and a half turns on the idle bleed screws and to work from there. The high speed richness was helped just by adjusting the float. Valves one, two, and six were burned beyond use, and the others were too close to their margin to reuse.

Besides, it was a good opening to try 144 ci intake valves as over-sized exhaust valves!!! FYI, we called Manley Tech and asked about the difference between material for STOCK 144 intake and 144 exhaust replacement valves. The answer was "Nothing. They are made of the same material"

I NEVER worried about losing the heads off the valves. I'm not sure how that got read into what I wrote. My only worry is about burning the valves prematurely.

I take no offense at your questioning. After all, I am still just learning as I go (or break or burn up). The only difference is that I happen to write it into a book - that people need to trust - So, I always end up on the conserative side looking for a solid "street" six.

Good Luck

 

[Anonymous]

Hello,



Initally, the two-barrel ran TOO lean off of the idle screws (1 1/4 turn) to the point of putting a slight glow into the header's pipes. The Autolite was TOO rich when off of the idle circuit. We now tell people doing the Autolite two barrel swap to start at two and a half turns on the idle bleed screws and to work from there. The high speed richness was helped just by adjusting the float. Valves one, two, and six were burned beyond use, and the others were too close to their margin to reuse.

Good Luck

A couple things I would input here. Adjusting the idle screw on most carbs increases or decreases the amount of idle mixture it does not adjust the idle jetting. Idle jetting is changed by changing the Idle Feed Restriction. The feed restriction is located in several different locations depending on carb design. A drill or plug and drill modification must be used to accomplish this. If increasing or reducing the amout of mixture a half or quarter turn the mixture is usally good enough. If a half of a turn or or more changing the Idle Feed Restriction may be necessary. Also the Idle Transfer slot relationship to the throttle plate (controled by the Idle Speed Screw) must be maintained. This is usally about 1/4 to 1/3 of the slot is below the throttle plate. If the Idle Transfer Slot has to much uncovered a hole must be drilled in the throttle plate to reduce the throttle blade angle (some carbs already have these) or to increase a champher must be filed on the throttle blade in the Idle Transfer Slot area allowing more of the slot to be uncovered. Hole sizes usally start at 1/8 inch If you go to far solder or JB weld the holes shut and start over.

 

[Anonymous]

One other concern is to be shure with the log style small six be shure the throttle shaft runs parallel with the log. The closer the throttle shaft gets to perpendicular to the manifold the worse the fuel distribution will become.

 

[Anonymous]

As far as back cutting the valves I have had no problem with it. But I have never taken it to some extreme. I have seen valves burn because of to narrow of a seat, to lean of jetting, and timing usally too far advanced.

 

[Anonymous]

My uncle used to tulip valves in his Sunbeam Alpine using two drills one held in a vice with a valve in it and one held by hand with a ball stone in it.
In the little 4 banger with isky 505 valve springs often the valve was pulled through the guide before the end of the race. Car ran very fast before it would break some said they had never seen an Alpine run like that.