Valve polishing?

[kmack]

I'm getting started tearing down my original motor for my 240Z. L24 w/E31 head. I'm going to refresh it and possibly do a little porting on the head (maybe). My plan is to run SU's, stock cam, 6-2 headers, Crane XR700 ignition.

My question then: Is there any benefit to polishing the valves (intake and exhaust)? In theory I should get better flow across the valve along with lessening the possibility of carbon being deposited (hard to stick to a smooth surface). But is there any real measurable increase in performance? Should I even bother?

[Zedrally]

Never heard of anyone doing this.

I would think the effect would be mininmal as the valves stems are already machined.

my 2c

[2ManyZs]

Well, it can be done, but on a mostly stock engine anything other than just a good cleaning is going to be a waste of time and money.

You could have them "swirl polished" but it won't do much good unless you did a lot of work on the head and intake ports. The swirl polishing is supposed to give the incoming air a swirling motion as it enters the combustion chamber thereby filling the combustion chamber completely for a better air/fuel charge.

Unless you had the head ported and polished and the intake polished or perhaps Extrude Honed you won't realize any benefit by just doing the valves.

One thing you should do is to lap the valves to get the best seal at the valve seats.

I'd like to see how much benefit you could gain having the head and intake Extrude Honed. Wonder if anyone has ever done a before/after dyno test on an L-series after having the head and intake Extrude Honed to see if it really does what it claims. It probably doesn't do much more than a good machinist could do, but it sure is less labor intensive and should be quite a bit more economical....

[EScanlon]

I leave the power tuning to the mechanics, but here's what I've heard about Porting and Polishing the intake and exhaust portion of the head.

By smoothing out the ports, you avoid the subtle but annoying air turbulence within the "pipe". This turbulence can slow down the intake / exhaust such that the engine is actually losing efficiency. That is, it isn't going to run faster / better / smoother so much as that it will be able to fully utilize what it has.

The other point about it is that with the rough walls, it is possible for the incoming air / fuel mixture to "wet" the surface. This wetness is fuel. When it builds up enough it sloughs off and enters the chamber as a drop and not atomized. This sudden "squirt" of fuel causes that detonation cycle to be uneven in relation to the others. Just like if it "burps". This in turn is undesireable.

This is what I was told years back by someone who purportedly knew what he was doing. (Another Air Force member who was porting and polishing the heads on a Firebird engine.) If it is in error, please let me know so I can correct the memory bank.

2¢

[Zedrally]

Guys,

Isn't the question whether to polish the valve's, not polishing porting the head?

[2ManyZs]

Welllllll, that was my point...

No sense in polishing the valves if you aren't going to do the complete job. Head and intake come before the valves, so if the head and intake are stock, the valves have no need for polishing.

A good port matching should be one of the first things to be done IMO. If you decided to do a complete head and intake polishing later, then the valves would need a bit of attention to complete the package.

[Zedrally]

Absolutlety agree, no point in polishing valves if you don't do the port & polish.

Although I really can't see you picking up another horse & 2 just by polishing the valve stems, or am I missing something?

[kmack]

Well, I wasn't really talking about polishing the stems, just the face of the valve that is inside the port (not the cylinder side).

Biggest advantage I can see is that you would have a smoother surface and would have less carbon deposits. Thereby keeping the size of the opening (at the valve) the same. (I hope I explained that well.)

I realize there is no advantage, horsepower-wise, to just polish the valves and not the rest of the head. I was just curious if it made a big enough difference if included in a port and polish situation.

I will port match the head and manifolds when working on this motor, but at this time I don't think I'll polish the head yet. I've got a tired (somewhat) motor in the car right now, and I'd really like to get this one running and in the car.

I'll leave the port and polish job until I get an L28 for the car.

[Mudge]

I dont believe it would do a darn thing for flow, but you would keep the valves cooler, in theory. Polished valves OR coated valves, will be used in a high heat app like turbo/blown cars.

[Ed]

I've also heard that porting & polishing the intake runner will disturb the turbulance that is required to maintain air/fuel mixture. Thus causing the fuel to accumulate on the port wall and ending up as drops. You want to keep a rough surface on the intake side (about a 60 grit). On the other hand the exhaust side you could polish to a mirror finish to improve flow.

The valves would benefit the most by doing a 3 angle valve job. This would increase flow. I would think that polishing the valve would be beneficial if this engine was torn down frequently like in racing applications. If you polish the valve then run the engine for normal street use eventually you would have build up.

[Mudge]

True, cleaning up an intake runner is fine, but a rough surface to some degree is your friend. Get rid of casting flash but dont polish an intake runner, exaust yes, intake nope.

[2ManyZs]

Of course, the type of induction system is the biggest indicator of how much polishing you can do to get the most out of the engine.

MPI fuel injection such as the later 280's would benefit greatly from a fully polished intake and exhaust since the fuel is injected into the intake port itself. The more air velocity into the ports would really wake up an L-28 with FI.

Carbs are a different story, a lot depends on the runner length, diameters and such. It all depends on the induction system on the vehicle in question.