M43 valves in M20 head

[SkiFree]

Let me explain a bit more why I'm not a proponent of this attempt.

1) The better profile of the valve is also found on any decent stock-replacement valves. Case in point, the SS IE valves (which cost the same as std. replacement valves).



2) The "weight savings of 13grams" is largely because of the valves height difference. Which means by adding in something to make up the gap between the spring/eccentric you have to add the piece's weight as well (given the height difference this would need to be thicker than a lash cap or over-sized eccentric). This could very likely could end up ADDING to the overall weight negating your primary reason for doing this. Not to mention you're needlessly adding one more component under pressure invariably increasing the assemblies chance that something goes wrong.

3) If you don't add something in to compensate the height difference then you're increasing the installed height pressure which is generally the OPPOSITE of what you want out of a stiffer valve spring (the installed-height pressure will increase with stiffer springs, but ideally at a much slower rate in comparison to the compressed-height pressure). In short, this means NEEDLESSLY adding pressure on the cam lobe, wearing it out noticeably quicker.

With all due respect to your tinkering (which is awesome), this particular instance is like taking a sledge hammer to set a dislocated joint. You're going to do more harm than good.

[LJ851]

Let me explain a bit more why I'm not a proponent of this attempt.

1) The better profile of the valve is also found on any decent stock-replacement valves. Case in point, the SS IE valves (which cost the same as std. replacement valves).



2) The "weight savings of 13grams" is largely because of the valves height difference. Which means by adding in something to make up the gap between the spring/eccentric you have to add the piece's weight as well (given the height difference this would need to be thicker than a lash cap or over-sized eccentric). This could very likely could end up ADDING to the overall weight negating your primary reason for doing this. Not to mention you're needlessly adding one more component under pressure invariably increasing the assemblies chance that something goes wrong.

3) If you don't add something in to compensate the height difference then you're increasing the installed height pressure which is generally the OPPOSITE of what you want out of a stiffer valve spring (the installed-height pressure will increase with stiffer springs, but ideally at a much slower rate in comparison to the compressed-height pressure). In short, this means NEEDLESSLY adding pressure on the cam lobe, wearing it out noticeably quicker.

With all due respect to your tinkering (which is awesome), this particular instance is like taking a sledge hammer to set a dislocated joint. You're going to do more harm than good.

The M43 valve is longer than the M20 valve.

[digger]

seems like there are easier alternatives.

[SkiFree]

The M43 valve is longer than the M20 valve.

whoop.... that's a whole 'nother can of worms, thanks. At the core though it still introduces more problems than it fixes.

[IronFreak]

Just throw more boost at it....it'll push air in quicker.....

[digger]

whoop.... that's a whole 'nother can of worms, thanks. At the core though it still introduces more problems than it fixes.

ive wondered what affect the longer valve has on the resulting valve motion but never investigated, similarly will it tend to wear out the guides a bit more because the eccentric is not in the optimal location and causes higher lateral forces on the stem?

[abit]

Thank you all for participating in this discussion, me like

Just throw more boost at it....it'll push air in quicker.....

yeah right :D

ive wondered what affect the longer valve has on the resulting valve motion but never investigated, similarly will it tend to wear out the guides a bit more because the eccentric is not in the optimal location and causes higher lateral forces on the stem?

...mmm but valve top will be shortened about 1mm, so at the end It would be almost the same length as stock and all rocker movement will stay like original :)

[digger]

is the valve tip properly hardened? you wouldnt want to expose a softer part of the valve which i have heard about on other engines.

people machine down valves all the time though

[Bullhead]

I started thinking on this as well. The longer valve would put more pressure on the camshaft via the rocker arm possibly keeping the valve open. I don't think just a spacer under the spring would solve the problem, yes it would keep the spring tension the same but it wouldn't help the fact the rocker arm was being depressed a few mm further.

[e21jps]

one advantage i can see is the fact you could run a fairly large profile reground cam with a reduced base circle and still get the eccentrics to be in range! The geometry would be quite good infact

ps but you MUST shim the springs back up to the original install height, mild steel washer works fine

[abit]

I will use 285/285 cam. So you say I could maybe even not shorten the valves, but just space the valve springs?

[Bullhead]

I will use 285/285 cam. So you say I could maybe even not shorten the valves, but just space the valve springs?

He is saying if you get a reground cam where it's base diameter is smaller than stock would compensate for the taller valve tip. This theory is sound if everything is in spec. You just don't want to have to shim everything all weird to achieve the desired tension. I would talk to the people you are getting the cam from and tell them your valves are 2-3mm taller and see what they say. If they are decent machinists they will grind the camshaft to where it will fit those tolerances and you won't have to worry about tip height. But you will still need to shim your springs up to keep the correct spring rate on your rockers. I agree some mild steel zinc coated or stainless steel washers would work fine.

[TobyB]

Actually,

will it tend to wear out the guides a bit more because the eccentric is not in the optimal location and causes higher lateral forces on the stem?

this, IF everything else is stock. But if it's not, all bets are off. We muck up the
valve tip/rocker geometry all sorts of ways, and it doesn't seem to make a drastic
mess of the top end.

Ideally, the rocker is exactly perpendicular to the valve stem when the valve's half- way
open so that lateral forces on the guide are minimized. But if you're off a little bit,
forces don't go up much. Travel might, some, but since it's already sliding, it's not going
to be all that much different- since it's already sliding.
I think where you'd run into problems is if the sliding force wasn't initiated with movement-
as in, if the rocker was so far off that it was perpendicular to the stem when it initially contacted
it. THEN it might make a real difference.

But that's WAY off...

t

[Bullhead]

Actually,

this, IF everything else is stock. But if it's not, all bets are off. We muck up the
valve tip/rocker geometry all sorts of ways, and it doesn't seem to make a drastic
mess of the top end.

Ideally, the rocker is exactly perpendicular to the valve stem when the valve's half- way
open so that lateral forces on the guide are minimized. But if you're off a little bit,
forces don't go up much. Travel might, some, but since it's already sliding, it's not going
to be all that much different- since it's already sliding.
I think where you'd run into problems is if the sliding force wasn't initiated with movement-
as in, if the rocker was so far off that it was perpendicular to the stem when it initially contacted
it. THEN it might make a real difference.

But that's WAY off...

t

He is right on the money here imo, you would have to change the angle 20-30 degrees for it to impact where it settles on the valve tip and where it follows the camshaft.

[Northern]

you could run a fairly large profile reground cam with a reduced base circle and still get the eccentrics to be in range! The geometry would be quite good infact

I was waiting for someone to mention this.

There has to be a spring that would fit with these, although I still don't see a huge advantage. The cost of replacing more and more of the valvetrain to fit a valve that isn't supposed to be there is going to be close to just buying proper aftermarket valves or undercutting/back cutting the stock valve and living with the weight difference.