Guide clearance / EGT

[68corvette]

How do you estimate the required valve guide clearances?

I am trying to calculate required valve stem to guide clearance for turbo engine which spents time at 1-mile track and EGT can be up to 900C/1650F.
I know that EGT is just an average, but probe sits in the middle of the pipe / hot gases and not dissipatingheat as well as exhaust valve, so probably valve runs colder than probe.

With 7mm valve stem, 0.0000188 linear expansion coefficient (21-4N valve material), 880 C deg temperature differential I get 0.115mm / 0.0045" expansion.
From experience that quite a much of guide clearance, so I would assume that stem is running closer to 500-600C deg / 930-1100F?

With typical street driven engines or 1/4-mile race cars there has not been issues with 0.03mm / 0.0012" clearances. Probably in these conditions valve spends so little time on the high EGT area that it does not have time to accumulate heat and expand.
As this is a diesel, I would expect that during shifts and deckeleration there is much better cooling effect compared to throttled gasoline engines.
What issues I have seen is that guides wear more from the valve face side, which is normal, but in these cases very rapidly, like during one summer and increasing the guide clearance has been helping to mitigate the problem.

Good comparison on this board might be SBC engine running with .3413" stem and "typical" 1200F EGT needing 0.004" guide clearance with 21-4N valve material, which typically run happily with 0.0012" clearance or even less.

Would you have any tips or experience if guide clearance can be reliably estimated based on EGT?

[Newold1]

You should probably direct some of this conversation to Zeke Uttria at Ferrea Valves. Their company and family has had years of experience with Inconel and Nimonic 80 exhaust valves and guides for turbocharged racing diesels and I think you can probably take what they can share with you as "Gospel".

www.ferrea.com

[Racing68]

On the diesel pull tractor heads i do the egt is about 1750 but only pulls 20 seconds. I'm running 3/8 stem inconel valves at .003 and have had no issues.

[Keith Morganstein]

Is the temperature probe in the manifold or after the turbo? That would be very high temp for continuous duty after the turbo.
Either way, the exhaust stems inside the guide are going to be cooler than you are estimating.
Opening the clearance a little more can help.
Another thing to try is to counterbore the end of the guide 8-10mm deep so a carbon dam forms on the stem.

[MadBill]

Here's a few hundred images re valve temps and distribution: https://www.google.ca/search?q=exhaust+ ... 36&bih=844

[modok]

The usual higher clearance limits are enough to run at the limits of the usual guide materials.
So if you need go go beyond that, then you need different materials and or designs, more clearance will not help. But I'd say the usual high limit would be more like .0025-.003

IMo you are running very tight.
Even though it is a 7mm stem, there aren't a lot of exhaust valves in the world running that tight.
It might be better if we look at it in reverse. How can you run that tight and others can't? Such as, ford or whoever built it to begin with.

Another question is.... if going extra tight is doing some good for you....maybe what you want is... a tapered stem?

[Keith Morganstein]

I was recently measuring valve stem to valve guide clearance on a high output diesel.
7” bore engine with .375” valve stems.
The factory spec was .0015” +/- .0005” (intake and exhaust).

[68corvette]

On tractor pulling engines with 9.48mm / 0.38323" I have also used successfully 0.0024" clearance with Kliners.
Unfortnately I do not have the EGT details. I would assume those are high, but the pull is short.
Currently I have one set of tractor pulling heads that I filled with harblock, actually the reason why I started to investigate the valve temperatures and heat expansion as I know these have issues scuffing valves.

Modok, I have plenty of cylinder heads out there running 0.0012" with 11/32 stem or 8mm stem.
Several 1600hp-2000hp SBF/SBC/BBC engines drag racing for years without any issues and people are using stock LS heads on blown engines with iron guides and same clearance from factory.
Actually I have one 1600hp SBC under re-conditioning, I checked exhaust valve stems being 8.67-8.68mm and guides are 8.71-8.72mm, this build is over 10-years old and guides are worn evenly across lenght.
More than mandatory guide clearance increases the guide and seat wear and reduces power, but too tight can lead into bigger issues.
I have seen aftermarket heads running "happily" as N/A in street engines with 0.01mm / 0.0004" before I have opened the guides "for safety" when porting the heads.
I have been using as small as 0.0004" clearance with PS heads, but they are cooled before run, titanium has low heat expansion ratio and valves are DLC coated, so not directly comparable.
In small block world I would say that most of the engines are running similar clearances as what I have now used with 7mm stem.

Keith, Stock diesels with 7mm have ~0.06mm / 0.00236" clearance build in to valve, guide is 7.00mm and valves are 6.94-6.95mm.
This "feels" a bit lose and these engines are known to wear the guides within 100tkm. Tightening up the clearance has helped for wear, but it seems like when people are close or exceeding 200hp/liter and running long periods with a full thottle there is too much heat load and extra clearance is needed.

I think I'll just open the clearance to the stock specs in situations where excesive heat load is expected and continue using tight clearances on the milder combinations.
Issue with diesels is that even with a small turbo and low HP its easy to get high EGT with bad fuel injection timing or excessive fueling for given air mass.

Thanks for the support

[modok]

I think you have it right.
What came to mind was I've had some guys try more clearance....and it didn't help, doing whatever terrible thing they were doing, but they were running stock clearances to begin with.

[Newold1]

The OP is talking about 21-4N stainless exhaust valves for super high EGT's on this diesel racing application. Obviously valve stem to guide clearance will be important as well as valve guide material, but if his sustained EGT's are really that high the use of a 21-4N exhaust valve in his engine will be a bigger and more dangerous problem than exact valve guide clearance ranges we are talking about here! JMHO

[68corvette]

The only issue has been with guides, actually valve seats look really good and do not show wear at all.
I use wide 1.5mm seats on 33mm valves to move the heat.
Around 15 years ago I made first oversize valves from steel intake valves and those showd fast wear.
After moving to 21-4N there was not issues and I have been using that since.
That would also indicate that material is not really running close to its forming temperature.

Same applies to all above mentioned engines, none of those have had any exotic valve materials in use.

I have only used incomel or nimonic with gasoline engines using antilag or extreme RPMs with high boost pressures, like 1200+hp / 10000rpm with 2.2 liter engine.

Obvoiusly I would upgrade the material if there is issues with sole engine types, but my experience has been that 21-4N has been very durable material when valves are in spring control and antilag is not used.

What kind of issues you have Newold1 seen with 21-4N and what have been the conditions?

[68corvette]

Great article about materials from gsvalves:
http://www.gsvalves.co.uk/assets/g-s-te ... mation.pdf

[Newold1]

68corvette

My experience with exhaust valve materials comes from quite a few years of endurance and marine gas engine performance where sustained higher EGT's over long er periods of operation created valve fatigue, valve breakage and burned valves with 21-4N exhaust valves. I have been using high quality Inconel exhaust valves in those applications with great success. I have no experience with Namonic exhaust valve materials or a lot of experience in turbocharged diesels so I would expect you have the answers in those areas you need. My comment was what I had gleaned from your post that might have led one to believe that you were experiencing 1650 degree F. exhaust temperatures at the valve and seat areas. As for bronze alloy valve guide clearances in my applications the engines always did well with .0015" intake and .002-.0025" clearances with viton seals and good oiling and oil quality. Glad your expereinces and results are putting you where you want to be.

[68corvette]

Marine use is probably a much harsher for engine as high load conditions are sustained for a long periods.
1-mile racing is most demanding for cylinder heads that I build.

With these diesel engines 1650F is something what many keep as max peak EGT, but when not monitored and using smoky tune the EGT may exceed 1800F.
But these tems are just some seconds peaks typically as speed exceeds legal limits on the road and at 1/4 mile RPMs are going up and down.

Your experience and help is highly valued, thank you.

edit: what valve stem diameter are you using with mentioned clearances and do you know at what range the EGT is?

[HDBD]

I have seen manganese bronze guides become unstable when overheated. What temperature, I don't know for sure. I work on Harleys and we use those guides. If a bike is going to be boosted I use cast iron as I have had no issues with those. I observed the original clearance gone and the press fit lost on a few that were overheated.