what size valves p90 head

[jay28]

Hi all,

Right I have the head off my engine, I was changing to a schneider stage 3 cam but thought ide take the head off and freshen everything while im at it.

My p90 head was shaved 2mm to higer the compression, but I used the standard valves that came with the head, Im I right in saying I should have bought longer valves seen as the head is shaved?

I'm also changing the valve stem seals, are the standard l28 seals ok or are there upgrades?

Many thanks

 

[Rob Gaskin]

My p90 head was shaved 2mm to higer the compression, but I used the standard valves that came with the head, Im I right in saying I should have bought longer valves seen as the head is shaved?

If anything I would have thought you would ask if you needed shorter valves.

IMO it would be good to check with a 'dummy' fit of the head first I reckon.

 

[jay28]

I have the 2mm shims under the cam towers, I read somewhere that I should use valves from a n47 head as the vavle stem is longer, this helps with the valve lash?

My lash old lash pads are FCKED because the lash pattern was way off. Didn't know any better back then but wont be making the same mistake again!

 

[Mr.F]

The valves are head specific. P90 valves are shorter stem because the roof of the chamber was raised (compared to N42) to lower compression for the turbo engine.

 

[jay28]

But because my head is shaved would I not need the longer stemed valves now?

Ill try see if I can find that thread to see if it makes sense to anyone.

 

[jay28]

Ok found it,

The valve stems are .080 longer in the n-47 head. These valves are used in p-90 heads by folks who are doing the head shave/cam tower shim modification that is very popular with p-79 or p-90 heads: the head is shaved 080 to raise the compression ratio, the cam towers are shimmed up 075 to keep timing chain tension correct,, and the long-stemmed n47 valves are used to keep the lash pad thickness and cam lobe wipe patterns correct

 

[jay28]

got it sorted

 

[SKiddell]

You can use either you will just need to either make custom shims or "pack" up the spring to get the correct installed height if you use the longer N42 valve

 

[jay28]

You can use either you will just need to either make custom shims or "pack" up the spring to get the correct installed height if you use the longer N42 valve

You mean shim the springs themselves? The cam towers are shimmed, so the valve stem is lower, I though by getting the valves with the longer stem that and the correct lash pads that it would be sorted. I dont understand why the springs need to be shimmed?

The head is going away for porting and polishing so I'll let the machinist sort it out

 

[Mr.F]

Valve springs have a correct installed height. Aftermarket suppliers will usually provide this information with their spring sets. Height adjustment is usually made by shims under the spring base or by machining the spring seat by up to 0.100" / 2.5mm. Your installed height will be affected if you move to different valve stem length...
Installed height for standard outer spring is 1.575"; inner 1.378".

If you increase valve stem length you will increase spring installed height and reduce the effective poundage of the spring...

 

[SKiddell]

Jay just a few points

Shimming the towers is primarily to correct for a shaved head (which can create chain slack)

Shimming the towers will effectively alter the cam/rocker geometry (the available shims are usually only 15 thou each not sure where you get 2mm from ?? unless you use 5 shimes on each tower .....not good) The shims are designed to compensate and ensure that the tensioner does not come out too far.The amount of skim does not necassarily equate to the amount of shims as you can get away with less, and too many shims create an unstable base to bolt the cam tower to.

Shimming the springs does NOTHING to alter the valve height, I only mentioned it because you asked what valves can be used and I said both P90 and N42

ALL springs have a specific "height" ie the length of the spring once its installed
This is CRITICAL to the seat pressure
Search "spring, installed height" in google

I use longer N42 valves as race/competition P90 valves are harder to get hold of

If you use a "longer" valve the distance between the "floor" that the valve sits on and the cap which is held in place by the collets at the top of the valve will be longer so the spring will need to be "shimmed" up, thats what the washers under the springs are for (I had custome spring seats made)

Whenever a head is rebuilt the installed spring height should be checked

If you alter anything it will almost certainly affect the wipe pattern on the rockers.....check and check again

 

[SKiddell]

Darn it ....lightning Mr F beat me to it

 

[Mr.F]

lightning Mr F beat me to it

That's because you wrote more - anyway, our replies complement each other beautifully...

 

[SKiddell]

 

[jay28]

Thanks for the explination guys, I undersatnd now,

I was talking to the machinist today, he also explained everything to me. he's said he will make single solid shim for the towers.
He's also going to do cnc cut Valve Seats, with performance profile? Port and polish and also do the valve guides. He will assemble the head then and check everything to make sure its ok.

 

[SKiddell]

Yeah thats more like it