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Valve question

Started by Trevor Williams, August 26, 2008, 08:14:09

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Pete Sparrow

Bas, now you're giving my secrets away, people will talk if they thought we spent the night together  :o
The older I get, the more rubbish I talk
(and the more pills I take)

Nigel Hollis

By changing to very hard valve material could this mean shifting the problem somewhere else i.e. valve seats?

Paul Robertson

From my talks with the valve company nigel ,no .

Roy Eastwood

#48
Quote from: Trevor Williams on August 26, 2008, 08:14:09
Just interested in whether anyone else has had either the heads of the valves coming off, or little nicks out of the valve head surface? One head and piston destroyed in my race engine this year by the latter, as well as the same happening in an engine I leant out last year. Both had the waisted stem valves
Trevor

Myself & Richard Dalton, got precisely that problem (little nicks out of the valve head surface) when we started racing in Belgium, can't remember which year but we used a 'standard' Piper 285 or possibly a Newman cam, we never had the problem using a standard Citroen cam during all the time we raced up until 2003.

Having done most of the UK 24 hour races until 2003, more than one even with the same engine used to win the previous year's event, I can say that we never had valve problems with that original camshaft. As far as team driving restrictions on the engine - we did set rev. limits but I would admit to always being an abuser of the rev. limit - I would keep the pedal to the floor almost until the valves bounced - hardly the best way perhaps, my lap times never the best - but we never wrecked a single engine in all those years.

The only problems we found in engines built by ourselves for the UK series (up to 2003) were down to poor component selection, the use of non original valves (we got burrs on the collet grooves), if we were too ambitious at bringing the piston closer to the head we then had problems with the piston 'kissing' the cylinder head land.

One really useful exercise would be to remove a quantity of the club cam from well used engines and check what the profile has become, you may well find a totally unsuitable profile has developed simply because of wear on the ramp. The Citroen camshaft is chilled cast iron and the racing club profile is a very vigorous grind from the original, taking the surface way below the hardening induced by the effect of chilling.

The waisted valves commonly available in UK are widely used in French 2cv cross racing with (standard Citroen cam) no problems.



Roy Eastwood

#49
Quote from: Mary Lindsay on September 01, 2008, 10:13:23
We did wonder if it was because the valve appears to be made of two different metals. We found that part of the valve that broke on our engine last year was magnetic and part was not.

This is normal and is the principle reason why valves are NOT made one piece, the head needs to perform in quite a different function to the stem, the valve has a noncorrosive steel head welded to a wear-resisting steel stem.

Please note that when I wrote the above I did not know that Paul was looking into the production of one piece valves - so this is not intended as criticism - simply an explanation of why valves are usually bimetal.


Paul Robertson

I remember the first 24hr race we ever did seeing graham harpers engine with a hole in the piston where the valve head fell off ,and that was way back in 1994, i have also seen the inlet valve chipping on numerous standard engines over the years ,as well as the cam lobe wear that is always blamed on the club cam regrind.
Andy Bull has just finished plotting the standard profile against the club profile so we can scrutineer the cam without an engine strip and it looks almost identical, other than a flatter peak on the club cam giving longer full lift.

Sean

Hi Sean here (looks after Mr Coghill, and helps out with Harper ;) )been trying to join in for ages...anyhow

Intresing stuff on cams & valves

http://www.gsvalves.co.uk/general_faults.htm any look familiar? nice simple expalnations for failures.

Sean

Quote from: Mary Lindsay on September 01, 2008, 10:13:23
We did wonder if it was because the valve appears to be made of two different metals. We found that part of the valve that broke on our engine last year was magnetic and part was not.

They dont brake at the join they seem to go at the start of the flare on the stem so that would suggest there is proplems with either the machining causing a stress or metallurgy during the forging process

The join is about 1/3 of the way up the stem and is roughly at the point where the stem emerges from the guide easy to find with a small magnet and if you polish the valve you can see the change in colour too.

Trouble is a dropped head mashes whats left so badly its impossible to get a clear picture of the original fracture

Sean

Roy Eastwood

#53
Quote from: Sean on September 20, 2008, 18:45:31
Quote from: Mary Lindsay on September 01, 2008, 10:13:23
We did wonder if it was because the valve appears to be made of two different metals. We found that part of the valve that broke on our engine last year was magnetic and part was not.

They dont brake at the join they seem to go at the start of the flare on the stem so that would suggest there is proplems with either the machining causing a stress or metallurgy during the forging process

The join is about 1/3 of the way up the stem and is roughly at the point where the stem emerges from the guide easy to find with a small magnet and if you polish the valve you can see the change in colour too.

Trouble is a dropped head mashes whats left so badly its impossible to get a clear picture of the original fracture

Sean

Breaking or bending of inlet valves has been a problem only since the introduction of the club controlled cam, before that the only breakages were of exhaust valves which got 'buzzed' on a changedown from 4th to 2nd.

The problem that needs curing is how the valve relates to the camshaft/piston/head, which might involve getting valve spring rates absolutely matched to the cam with a self-imposed rev. limit.


Sean

Quote from: Roy Eastwood on September 22, 2008, 08:46:44
Breaking or bending of inlet valves has been a problem only since the introduction of the club controlled cam, before that the only breakages were of exhaust valves which got 'buzzed' on a changedown from 4th to 2nd.

The problem that needs curing is how the valve relates to the camshaft/piston/head, which might involve getting valve spring rates absolutely matched to the cam with a self-imposed rev. limit.


Agree with you Roy something about that cam means the cam, valve train and valves dont seem suited to each other. whether its profile, springs or what...
How does the re grind affect the angles at the rockers, has this an effect on the rates the valves move at?
Sean

Derek Coghill

It doesn't, you take that up with the adjusters....but if there's more lift over the same duration (see Bully's post elsewhere), valve speed will be greater and there'll be a difference of rocker-to-valve angles at maximum lift.

David Bain

Assuming it's not a trade secret, how is the greater dwell obtained? Remetalling the ramps to increase dwell while maintaining the standard lobe height or cutting down the lobe and rehardening? There seems to be a geometry aspect to this drama and knowing this might bring more understanding - or simply increase the number of back-of-envelope sketches.

Francis Rottenburg

The problem went away to some extent and has reemerged at this year's event.  It would be interesting to know how many of the cratered engines were using new valve springs?  Were people rebuilding engines with springs that were new two or three years ago?  Did anyone take the springs out of the remains and check the rates?  I remember Paul did some very revealing work on old vs. new spring rates that showed a marked degradation of some springs.  How many were using double springs?  At the least it would indicate whether new or checked springs have to be fitted before every season/24 hour.

Paul Robertson

One of mine let go and has good double valve springs francis.

Marc Fenner

As many of you know i am doing a degree in motorsport technology. I have been asking about different ideas as to why we are lossing valves.

A few things have been suggested.

1. The cam is too steep on closing causing more strain on the valve itself.