Exhaust Valves and dowel pins

Competition engines and ancillaries - general discussion
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jimnielsen
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Joined: June 19th, 2007, 11:58 am
Location: Junortoun, Australia (A)
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Exhaust Valves and dowel pins

Post by jimnielsen »

I have a competition 16vt engine in an Alfa Romeo 155 Q4. It is using an Autronic SM4 ECU and runs on E85 fuel. Currently it makes approx 300 wheel hp with 1.35bar boost. I am about to implement new inlet and exhaust cams which will lift its output to approx 350 hp @ 1.2 bar. I have two questions.

1. Can the standard sodium filled exhaust valves cope with this level of power upgrade - or should I be looking at something else? Note that the engine exhaust temp never gets over 400 degrees C. (measured just after the turbo).

2. The dowel pins seem undersized (in terms of diameter) - especially where it goes into the aluminum head. I have a couple of blocks and heads and they all seem the same. Is this just the way it is supposed to be?

Kind Regards, etc.

jim nielsen.
tricky
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Joined: July 6th, 2010, 5:41 pm

Re: Exhaust Valves and dowel pins

Post by tricky »

Hello Jim, We've not spoke but I've been following your progress a little on your normal forum, bad luck with the cracked liner by the way.

I do think the std valves will be fine, although I have no quantitive proof but Guy assures me they will be fine for my own engine build at a fair bit more intended hp than yours. I also see a few other 16v engines from the years with very high levels of power used in drag racing and track with the sodium valves installed, so they are proven in that sense. I pondered myself over using some inconel ones offered to me, but new sodium were installed for the above reasons. Also aftermarket stuff can be hit and miss and at least with a Fiat Lancia item made to a nice tollerance your safe in that sense too. Your method of measuring egt is'nt the normal, the turbo will easilly drop half of the temps off due to it's natural restriction. Try measuring in the collector if you have a nice manifold or better still 4 probes, one for each cylinder which, even then the readings will vary wildly depending on the exact placement / distance from the head.

Precicely what temps the sodium valves can manage, I'd like to know myself ! But I recon good header design will go a lot further to keep your valves from burning up than exotic stainless steel aftermarket ones.

I'm not sure what you mean by 'dowel pins' Jim, could you clarify ?
Twice as many valves
jimnielsen
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Joined: June 19th, 2007, 11:58 am
Location: Junortoun, Australia (A)
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Re: Exhaust Valves and dowel pins

Post by jimnielsen »

Thank you for your comprehensive and informative response, Tricky. I guess i will go with the exhaust valves that I already have then!

by Dowell Pins, I mean the cylindrical hollow steel locating pins that are supposed to locate the head on the block. They pass through the head gasket.

thank you, jim.
tricky
Posts: 101
Joined: July 6th, 2010, 5:41 pm

Re: Exhaust Valves and dowel pins

Post by tricky »

Oh, I see what you mean. Those dowels don't really take any serious load, they are just for location and to stop any shearing movement of the head and certainly do thier job and don't need changing for anything more robust. A decent gasket and decent uprated 10mm bolts / 12mm stud conversion if you prefer should see it out.

Don't take my word for it re valves, I am but a mere mortal ! Someone else may be able to offer some facts, which I would be keen to hear too.

Richard
Twice as many valves
Guy Croft
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Re: Exhaust Valves and dowel pins

Post by Guy Croft »

In general terms in a setup with stainless ex valve stems and ferrous seat inserts about 25% of the head absorbed by the ex valve is transmitted to the cooling system by the stem.

Here is the short story on s/c ex valves:

1. The sodium liquefies and helps transfer heat from the valve head to the guides giving (I am sure) a much higher proportion of heat transfer there than 25%. This certainly helps the longevity of the valve and seat.
2. To optimise the setup always use copper-alloy (bronze) guides with them,
and..
3. Use of valve seats with excellent thermal properties (conductivitiy and diffusivity) helps to keep valve head temperatures down.


Dispersion of head in the ex valves is important because the much-elevated temperatures compared with atmo units, added to the stress from valvetrain and even the cylinder pressure itself can (and does) cause distortion to the valve head and potentially the valve stem too. The effect of the cam trying to open the valves coupled with very high boost pressure and/or cylinder pressures at EVO, at the very time when the material is in a far-weaker state - can cause the ex valves to bend. Strange but true and also true of the inlet valves which with their bigger heads are subjected to even higher forces during the valve event.

It is the core material of the valve that determines how it will last. All good ex valves have either high grade alloy steel or austenitic stainless (21-4n) stems and certainly the best have 21-4n heads (some lower-cost valve are bi-metallic - stainless heads and aloy steel stems). For the mechancial properties refer to:
http://www.gsvalves.co.uk/mechanical-properties.html

There you will see that nimonic has higher strength at temperature in creep and tensile, but balanced against that is the fact that high chrome-nickel steels ('stainless') are very poor conductors of heat and I venture to suggest (and this is based on experience) that given a choice I would use sodium cooled valves rather than nimonic ones simpy because they are virtually guaranteed to run, well, cooler because of the superior heat transfer.

Now, regarding your engine - the level of boost is not that high - s/c is what I would certainly recommend. You have stated the ex gas exit temp but of course the entry temp will be maybe twice that. Nonetheless I doubt that the turbine entry temperature is going to be so high at that power & boost level to have a long term detrimental effect on s/c ex valves per-se. But even with an apparently modest setup it is rather important to look at the local temperatures in the manifold and one can see alarmingly high variations port-port with a system that is 'under par' for the duty it's being put to compared any single mean value measured at the turbine housing. The very best setups will have had ex gas temperature monitoring in all 4 primary pipes.

By the same token one can raise the cylinder temperature very excessively with high ex back-pressure (usually the A/R ratio will be too small) or bad ignition calibration, over-lean running and especially by commencing the ex valve event too early in the power stroke - where the cylinder pressure and temperature are very high: there it will be the ex valve that will see the consequences. Late EVO with short ex cam duration and the bare minimum of overlap event (on the ex side) are the best guidelines.

Hope this helps some.

G
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