exhaust valves/headwork

[fiznat]

Every time I talk about my KA-T plans (15-20lbs on a built KA 8.5:1 compression, T3/T04E .50 trim, emanage, etc) with someone who knows about cars I seem to hear the same thing: I need to get some better valves. I hear all these horror stories about burning up exhaust valves and having them chew up the pistons etc etc...

But on the other hand, most KA-Ts that I know of are using pretty much stock heads, maybe with mild P+Ps...

What do you guys think? Is it worth the money to do valves now also while Im building the engine? Thats money that could be spent on a LSD right there...

[andrave]

I would not do valves, but then again 20 lbs is a lot for a KA and I don't know a lot of people that have run that high of boost.

I haven't heard too many stories of exhaust valves failing and being ingested, but I could be msitaken.

[Ubernoober]

With increased boost comes increased exaust temperatures. If the factory exaust valve becomes overheated, it could re-temper the stem causing the head to pop-off and cause catastrophic damage. Be thankful that Nissan does not use two-piece valves (stem with the head welded on) like my old Fords. These things will simply fly off by using heavier springs and pushing the RPM out a few hundred. Nasty.

Ok, that being said, I would look into the material and design used on the KA and see if it was the same for the SR. I bet it is, in which case, the SR is already boosted and is a known factor. It would confirm that the factory valves are perfectly fine as long as you keep an eye on your pyrometer. If you were made of money, you could always go stainless or sodium-filled, but I doubt it is worth the cost. I will look into it though, since I am in the planning stages of my own KA-T with the same engine specs and boost as you.

BTW, "burning" a valve is kinda a misnomer. An exaust valve dissipates heat by contact with its seat, and a very small amount by the cooling action of the intake charge. As the exaust temps rise, the valve head or stem can start to warp as it approaches design limits. When this happens, incomplete contact with the seat occurs making it harder for the valve to cool off. This continues the climb in temp of the valve until either the rather common, permanent warpage (leakdown failure due to incomplete sealing) or the far less common, catastrophic physical failure.

[TrunkMonkey]

if your exhaust gases get hot enough to do damage to your valves (even @ 20psi), then you have other issues to address other than the valves themselves.

-demetrius

[Checkered-Member]

God bless for duel solid roller cams, and a timing chain. the valve train on a KA is simply not an issue of concern.

[C-Kwik]

If you are heating the valves that much, chances are you have a lot of backpressure in the exhaust. Trying to run 20 psi on a smaller turbo will have that kind of effect. With a well chosen turbo, this will be of little concern. At least as far as causes that stem from the turbo itself.

[C-Kwik]

God bless for duel solid roller cams, and a timing chain. the valve train on a KA is simply not an issue of concern.

I'm not sure what you are getting at here. All the cams I know of "roll". If you are referring to roller lifters, then we don't have them and actually would have very little benefit from them. Roller lifters were developed first for pushrod V-8's that have very heavy valvetrains that necessitate very stiff valve springs. The roller rockers/lifters help to eliminate some of the friction and frees up some additional power. The DOHC KA uses a shim on bucket design which is currently the lightest mechanical design available. The spring rates do not need to be very high(relatively) so there is little friction in this area of the motor.

And a timing chain or a timing belt adds little benefit either way. Both have compromises. The Chain is stronger, but also heavier and noisier. Belts need replacement periodically, but are lighter and quieter.

[95_240sx]

You bring up an intersting point. My boosted KA runs 16psi all the time, and 22 when i am racing it. I have a mild port and polish, and a set of stiffer valvesprings (for a few hundred more RPM) but stock valves, which have been machine cut along with the valve seat. I just had the entire head apart last week and the valves checked and they are all perfectly in spec and this is the second season on this motor. Last season however I was only running 14-16psi for high boost and like 10-12 around town. There is more wear and tear on the engine, but I want to see how she holds up to the abuse. I guess the real test will be this week when I goto the dyno (and hope to claim the highest HP record, to which is currently in AZ by someone, however I am exactly 100hp behind him at 8psi less, his is also on a dynojet SAE corrected, mine was on a Mustang and not correctly...anyways ) this coming week and see how she does. Lately I have just been having ignition problems, but I will report back after tuning

Rick

[C-Kwik]

You have a large enough turbo that burning valves should not be a concern for you at 22 psi. Depending on how you had the valves and seats cut, it may cause the valves to run a bit hotter(thinner seating surfaces transfers less heat from the valve to the head). But it still shouldn't be a measurable concern.

[fiznat]

c-kwik, do you think my chosen turbo (T3/T04E .50 trim) is big enough to run this boost without worrying about too much backpressure? I know that the compressor is within it's efficiency range more or less, but I havnt really looked into turbine (flow rates? backpressure problems?).

Watcha think?

[Nathan]

Fizzy...my egt's have NEVER broken 1250 f. under boost, I'm running that same turbo. While engine management differs, its possible to keep things cool enough I think not to worry

[fiznat]

at what temp should I worry about warping the valves?

[Nathan]

uhhh, I dunno

[C-Kwik]

EGT's aren't going to tell you the temperature of the valves. While they have an effect on it, heat will build up in the valves more if it operates at a higher pressure(backpressure). So, for a given EGT, you can have varying amount of heat in the exhaust valves.

As far as the T3/T4, running 15 psi should not be a problem. 20 psi might be getting pretty hot, but may still be okay. It can depend on what turbine trim and A/R you use. The bigger the better in the case of reducing exhaust valve heat.

[Projex240]

if your exhaust gases get hot enough to do damage to your valves (even @ 20psi), then you have other issues to address other than the valves themselves.

-demetrius

for once--i agree

The valves arent the problem. Valves will stand up to pretty extreme temps in the combustion chamber, so if you are running hot enough egt's/combustion chamber temps to burn up a vlave--most likely---theres something else that will break first. Just make sure to keep the temps down and youll be fine--BTW--just got done working on a ka that runs 20 psi on the street on pump gas--he has a totally stock head.