guide to valve clearance a bigger issue in a turbo'd engine?

[dave brode]

All,

I have a Cadillac powered '71 chevy street truck. It currently has a high compression [11.7-1] engine that I ran N/A'd [11.66 at 114.8 street trim].

I have now fitted it with two 60-1, P wheel .96 A/R turbochargers. Although the turbo setup is on and functioning, I am driving it gently, until I can swap in a lower compression engine, more suited to supercharging. Mild goals [800 crank horse], 5500 rpm max. Will be run mostly on street, a trip or two to the track.

My question is on valve to guide clearance, and guide material. The heads I'll use need new guides. I have Ferrea 5000 series intake and 6000 series exhaust valves. I'm not a bronze guide guy, but they could be fitted tighter, I'll assume .0005" or so. Is boost pressure travelling up the guides an issue? Turbine side will probably see 2X the pressure as in intake tract.

Should I use bronze? Would iron guides last longer?

TIA

Dave

[donc]

Bronze will work,used to do some turbo cyl. heads I dont remember the guide clearence I used but I dont think it was that tight.

[nickmckinney]

I use the SI bronze guides which are actually Manganese-Silicon-Aluminum-Bronze according to the tags on the bags. I have found that they will "sacrifice" themselves should they be too tight for the combo and basically self clearance without hurting the valve. I send them out with 0.0015" of clearance and have had turbo heads come back after hard use with the same clearance and the largest growth I found was 0.003" at the 2 year range (700RWHP, 22lbs of boost, 30K street miles) Personally I have seen zero problems running these guides nice and tight, but I have had some other bronze guides come through here that were very different. One companies bronze is definitely not the same as anothers................

Honestly I think the heat in the valve and thus the guide and the clearances is majorly dependent on the tune and whether or not you also want to run a meth/water injection at the same time. If its going to get lean without any other injection system, its going to get hot.

[SWR]

No matter the clearance, the boost pressure will affect such a small area it will not an issue. Getting the heat out of the valves is more important..

[David Redszus]

Any engine modification that increases BMEP will cause an increase in combustion temperature and pressure. That will result in an increased valve temperature proportionate to valve face area and rpms. This heat must be transferred to the valve seat and also through the valve stem into the valve guide. If a guide material is selected with a higher thermal transfer coefficent, and closer clearance, more valve heat can be shed to the head.

There are many types of valve guide materials, each having unique thermal and wear characteristics. Unfortunately, these technical details are not often available to the racer or engine builder.

You might consider a visit to:
performancealloys.net

as a source for reliable and correct technical information regarding suitability of guide (and seat) materials.

[dave brode]

Thanks All,

Will get back with questions after some study.

Dave

[GregGood]

Phosphorous bronze would be my choice, and about .002" clearance. (If it is a *little* too loose only you know about it. If it's too tight everyone knows)

[David Redszus]

Phosphorous bronze would be my choice, and about .002" clearance. (If it is a *little* too loose only you know about it. If it's too tight everyone knows)

Which phosphorous bronze would you specify and what are the hot running clearances?

[GregGood]

I use phosphorous bronze guides in two forms. A solid guide from brodix or dart, or a K-Liner in a custom made steel guide (the best way to do it). I've had manganese guides that protruded into the port crack and break off. Not good for turbos.

I don't know what the running clearances are under boost. Hostile environment.

[David Redszus]

You might consider a visit to:
performancealloys.net

as a source for reliable and correct technical information regarding suitability of guide (and seat) materials.

[ptuomov]

If a guide material is selected with a higher thermal transfer coefficent, and closer clearance, more valve heat can be shed to the head.

Considering the oil between the exhaust valve guide and exhaust valve stem, does right clearance or loose clearance cool the valve better? Is there an optimum exhaust valve guide clearance from cooling perspective?

[modok]

the optimum will probably be a taper. Looser at the hot end.
I agree you can run bronze tighter, or rather you can run it too tight and get away with it, tho that won't make it last longer. Although it may make more power. Phosphor bronze IMO is the best combo of wear resistance and ease of machining. Sometimes they don't even tell you what kind it is tho

[ProPower engines]

the real issue is those valves mentioned do not have hard chromed stems. They gall in the plain or hardened guide as well as the material is not up to it either.

You need to step up to a Inconel exhaust valve material. The cheapo stainless valves will tulip under the heat based on the power goals.

I have seen a few street turbo guys buy bare heads and use econo valves of various brands ruin a set of guides in no time
just on a chassis dyno after a couple runs. And that is with .0015 clearance and a non restrictive seal. Some even tried no seal and still ate up stems and had valve faces distorted.

Being a caddy not sure what stem size you plan to run but in the better quality lines of valves 8mm should get the job done fine or even an 11/32" stem would be fine and maybe easier to get in a workable size. Less cost as well.

[ptuomov]

I think (but do not know) that the taper is supposed to be in the valve, not in the guide. For a typical exhaust valve, I think the thermal expansion would be something like 0.001" greater at the valve head end compared to the tip end. Vague recollection says that the valve stems have been tapered since 1980's.

But back to the clearance: Obviously, too tight will be hottest of them all. But what clearance cools the best? Is "too loose" just fine from cooling perspective, but will just consume more oil? Or is too loose a recipe for burning the valve? Does the type of valve stem oil seal factor into this? This is a total novice question, so I'd be grateful if someone would explain this to me like they'd explain it to a smart fifth grader.

[ptuomov]

the real issue is those valves mentioned do not have hard chromed stems. They gall in the plain or hardened guide as well as the material is not up to it either.

You need to step up to a Inconel exhaust valve material. The cheapo stainless valves will tulip under the heat based on the power goals.

I have seen a few street turbo guys buy bare heads and use econo valves of various brands ruin a set of guides in no time
just on a chassis dyno after a couple runs. And that is with .0015 clearance and a non restrictive seal. Some even tried no seal and still ate up stems and had valve faces distorted.

Being a caddy not sure what stem size you plan to run but in the better quality lines of valves 8mm should get the job done fine or even an 11/32" stem would be fine and maybe easier to get in a workable size. Less cost as well.

This is for an obsolete Porsche engine. For the engine that I'm playing with, the original stock exhaust valves from late 1980's appear to be very high quality. After a quarter century plus, John Kuhn has been converting those engines to turbo. The stock exhaust valves don't burn with 2,5x power in turbo conversions, although it's not like the cars get driven for many minutes at those peak power levels at the time. What you buy from the internet right as "stock" parts now doesn't have much resemblance to the quality part they put in the engine in 1987, and "genuine Porsche" exhaust valves are at the same price range as top of the line sodium filled superalloy aftermarket valves. Pretty much the only negative with the old used valves is that they wear out the valve guides a little fast (not valve stems) so I am thinking about what implications that wear has on reliability.