P90 head questions

[Riceburner]

Ok I have a few questions about my L28 f54 p90 combo... I run six 42mm bike carbs with custom 40mm intake manifold, extractors, scorcher ignition, 72deg cam. It's in good condition doesn't use oil and has good oil pressure but, I want more power without going too crazy. 

 

1) Does it have flat tops or dished pistons? I've come across conflicting info.

 

2) I was thinking about shaving the head 80thou to bump up comp to 10:1 and get some porting done. I understand this will cause the chain to be loose, I have an adjustable timing chain tensioner (looks like a standard one but has retaining bolt on the back to stop it falling out) will this be enough to take up the slack or would it require something more serious like the kameari twin idler?

 

3) What sort of power increase will a good head job alone give?

[C.A.R.]

1) Does it have flat tops or dished pistons? I've come across conflicting info.

Most likely not. Check the cylinder pressures with a compression tester.

2) I was thinking about shaving the head 80thou to bump up comp to 10:1 and get some porting done. I understand this will cause the chain to be loose, I have an adjustable timing chain tensioner (looks like a standard one but has retaining bolt on the back to stop it falling out) will this be enough to take up the slack or would it require something more serious like the kameari twin idler?

Bad idea. Get some flat top or domed pistions to suit. OR chenge to a head with smaller combustion chambers.

3) What sort of power increase will a good head job alone give?

How long is a peice of string? Or ask your missus

[garvice]

1) Does it have flat tops or dished pistons? I've come across conflicting info.

 

I was going to say pull the head, your going to shave it anyway.

Check the volume of the head chambers first, might already have been shaved.

[Riceburner]

A good one puts me too sleep afterwards 

 

I've measured the head thickness and it hasn't been shaved. The motor seems good so I don't want to do a full rebuild, yet.

[Zedman240®]

My first L28 that was a F54/P90 had flat top pistons but unless someone has tampered or altered anything, you will have to remove the head and find out. If you have flat tops, installing a N42 head will certainly bump up compression a fair bit. Time to remove the head and find out!

[garvice]

Look through the spark plug hole then (forgotten about this option).

I could see the dish on my pistons with a well placed torch on the cylinder that was on TDC.

[ozconnection]

A P90 is a good flowing head with a combustion chamber that is far more modern than say the N42 open chamber design.  Swapping over to the N42 head is a mistake IMO.

 

Look through your spark plug hole with a penlight to see what pistons you have as suggested by garvice. This will then tell you what you have and what you can do to the bottom end, if anything.

 

Shaving the P90 head is an option and head saver shims will restore the critical nature of the cam drive setup on the L series. The head is the power maker. It seems only natural that all the attention be paid to this area in the search for more power!  (Kameari sell 'em, Ive got a set ready for this mod on my L28.)

 

Good luck with your project mate.

 

[Riceburner]

I have an N42 but its been HEAVILY shaved and thought the P90 was better. Comp ratios with the current head will be low with flat tops or dished. What easy power upgrades will work and be best bang for buck. Bigger cam or port/head work or altering comp ratio?

[ozconnection]

I have an N42 but its been HEAVILY shaved and thought the P90 was better. Comp ratios with the current head will be low with flat tops or dished. What easy power upgrades will work and be best bang for buck. Bigger cam or port/head work or altering comp ratio?

 

A bigger cam doesn't require head removal, so that's the easiest. (Valve springs, if larger ones are needed to match the cam profile can be changed with the head still on the engine.)

 

Cams and comp ratios go hand in hand, stock comp. ratios shouldn't have big cams because they bleed too much cylinder pressure at low rpms (no torque) and high comp engines should have bigger cams because they can afford to bleed some pressure (due to cam profile overlap).

 

This can be tricky to get right and is influenced by things such as cylinder head modifications, cooling system mods, fuel octane, induction type (efi or carb.) engine management sophistication (how fine is the tuning resolution for both fuel and ignition) exhaust, drivetrain ratios and driving style to name a few.

 

 

 

 

[GV260]

You need to ensure that you have the right combination of all components to ensure you get maximum performance from the complete package. It's no use having a head that can flow huge numbers if you don't the cam, carbies etc to allow it to do so, as well as compression and vice versa. Hence best to take the head off, bench flow it to see what horsepower it can produce and then go from there to match it with the right components. This will then enable you to properly cost the build and pick the quality of parts you can afford versus the parts you already have.

[Riceburner]

Ok so maybe I could use the N42 head to up the comp, get it ported a bit and flow tested, fit my 72deg cam. Any I deas on the minimum thickness for an N42 head to give reasonable compression ratio on f54 block?

[garvice]

Have you checked the ozdat engine design utility

 

http://www.ozdat.com/ozdatonline/enginedesign/

[Riceburner]

Thats handy, thanks Garvice. Does anyone know the minimum thickness a head can be machined too without requiring shimming of the valve train?

[PZG302]

Thats handy, thanks Garvice. Does anyone know the minimum thickness a head can be machined too without requiring shimming of the valve train?

 

Depends, do you mean cam tower shims or lash pads and cam tower shims.

 

I had a L18 with an A87 head that was shaved by around 5mm. We just got away without placing shims under the cam towers, but the timing chain tensioner was at the limit to keep everything in place.

 

The lash pads were definately different to standard, but I can't remember by how much.

 

A lot of what is needed depends on the actual work done to the actual components, you maybe able to shave a couple of mm and not have to change a thing, you may also end up having to use shims and new lash pads.

 

The only way to really know is to bite the bullet and do the full rebuild replacing whta needs to replaced as it needs to be replaced depending on the final spec you want.

 

Also, do you want torque or top end? This will make a huge difference to what machining you get done to the head in terms of ports and combustion chambers.

[Riceburner]

I have a mild cam fitted with thicker lash pads, I'm just wondering how much I can shave off the head before I have to shim cam tower and spring seats.... :-\  Searching for top end, so would porting and shaving the head be worth while?