F54 Block Flat top Piston work with P90?

[MadMaxDallas]

Ok while taking care of my son's cars. I finally got a chance to work on mine. Knew I had a blown head gasket between 5 and 6.

This is on my L28ET with P90 head I had redone and ported a bit awhile back. In the mean time doing some swapping I got a

fresh built lower end with balanced crank, .050 over pistons but they were flat top. So I went and looked up the calculations

and with a P90 head compression will be about 8.6 which is ok, even for turbo. But when I was breaking down my engine I noticed

that the valves extend beyond the plane of the head surface. With flat top pistons it would be smacking the pistons hard!

 

Now I want to know if anybody has ran flat top pistons with a P90 head, I guess a 1 MM (.050) gasket may give it some clearance but I want to know as a fact if this works from somebody who has ran flat top pistons with a P90.

 

I guess the alternative is to sell this beautiful block, and go ahead and redo my block. But thought this combination with

less than 10 psi boost would be a good combination.

 

[MadMaxDallas]

Ok while taking care of my son's cars. I finally got a chance to work on mine. Knew I had a blown head gasket between 5 and 6.

This is on my L28ET with P90 head I had redone and ported a bit awhile back. In the mean time doing some swapping I got a

fresh built lower end with balanced crank, .050 over pistons but they were flat top. So I went and looked up the calculations

and with a P90 head compression will be about 8.6 which is ok, even for turbo. But when I was breaking down my engine I noticed

that the valves extend beyond the plane of the head surface. With flat top pistons it would be smacking the pistons hard!

 

Now I want to know if anybody has ran flat top pistons with a P90 head, I guess a 1 MM (.050) gasket may give it some clearance but I want to know as a fact if this works from somebody who has ran flat top pistons with a P90.

 

I guess the alternative is to sell this beautiful block, and go ahead and redo my block. But thought this combination with

less than 10 psi boost would be a good combination.

 

 

UPDATE: I must be getting old, guess I freaked out when I saw the valve full open. Of course they are not like that

in relationship to the pistons while running. Going to bed ... Ignore thread ..

[ozconnection]

With the very mild lift on a stock turbo cam, there will be no problems running your P90 on that flat top piston F54.

 

Mine is an "A" grind cam in a P90 on an N42 with VG30DETT "flat top" pistons...no problemo!

[Pyro]

Check the piston deck height. You will notice the piston sticks out of the block around 0.010" which will bring compression up to 8.8:1 with a 0.040" head gasket.

 

The L series engine is prone to detonate. Yes, you can add a turbo to the 8.8:1 setup but can not run much boost or timing with a stock cam. When I did it, could only run 17 degrees of total timing on stock boost (7 to 8 psi).

 

You will make much more power with a 7.4;1 cr setup (stock turbo cr) and more boost and timing.

 

In any case, use a NA cam.

[MadMaxDallas]

Check the piston deck height. You will notice the piston sticks out of the block around 0.010" which will bring compression up to 8.8:1 with a 0.040" head gasket.

 

Ya, making me think I might sell this block, and go ahead and rebuild my bottom end. New block sure looks nice. But looking

at my crosshatch and pistons in my block with 220k miles on it, it looks good too. So I should be able to sell this block and

cover all my expenses for new rings, bearings, honing, and having it balanced.

[Datsun Deron]

I've been running flat tops with a p90 head for the last two years without any problems what so ever. I was too cheap to rebuild the bottom end so I just swapped the top end. Currently I'm running 7 lbs of boost using a stock 300zx turbo (same size as the factory 280zx turbo) with a Spearco intercooler. I'm also running Megasquirt II with EDIS. For timing I'm running 19 degrees at full throttle and the car pulls very nicely. I run the car slightly rich at full throttle 11.2:1 ish AFR just for the extra insurance. I also run 94 octane in the car. I tried running 21 or 22 degrees but I started getting some pinging. I'm hoping to turn up the boost to 10 psi later this summer and see what happens...

 

My advice would be to go for it and run flat tops with the p90 head but get the THICKEST head gasket you can to bring the compression down. I really wish I would have done this. If you can I might also suggest getting a larger turbo charger then factory which should run more efficient at higher RPMs and might bring your intake temperatures down somewhat and give you more power.

 

Another option if you want to run more boost would be meth injection as well but that could always be added later.

 

Overall ive been very happy with my setup, the car has great power on and off boost.

 

I hope that helps, if you have any other questions just let me know.

[Gollum]

You don't want to run MORE gasket thickness, you want to run AS LITTLE as POSSIBLE. The flat tops have more quench available than the OEM turbo pistons and you want to take advantage of this fact. There have been plenty of people to make good power on this combo, and personally I think it's probably the best mild sub-300hp setup for a L motor.

 

And people. please don't bash a combo based upon how much total timing you could run. That's like saying my car was slower because it wasn't as loud... Just because one combo can't run as much timing doesn't mean it's making less power. It takes dyno time with programmable fuel and ignition on both engines to really deduce which combo makes more power where. When you run a P90 on flat tops, you're not just reducing the overall size of the chamber volume, you're also reducing the flame distance as you cut off the outsides of the chamber due to the quench, giving you a more spherical shaped chamber. So you actually have TWO aspects that will cause the engine to NEED less timing. Note that needed timing is a very different aspect than timing limitations due to detonation.

 

If you want to do it right, I suggest using a OEM or felpro head gasket and then loading up the tank with race gas. Then you can play around on the dyno and see were peak power actually is in regards to timing. Once you have it tuned nicely, and safely, put in some premium and see where the chips land. If you only have to pull 3 degrees or less then the power "loss" from the added compression should be minimal. And remember that the main place that the timing needs to be pulled generally will be lower and mid RPM under heavy load. You shouldn't have to pull much at all at the very top of the RPM range, which is where the added compression will really shine.

 

Don't settle for blanket statements that say something can't or shouldn't be done. There's plenty of people that say the MN47 shouldn't be used with flat tops on street driven NA motors using pump gas, yet there's the people doing it. What doesn't work for one person can very likely work for someone else. I have my theories, but I'm here to say "go for it".

[Datsun Deron]

Thanks for the info Gollum you make some very good points, especially regarding timing. I suppose comparing flat tops to dished pistons is indeed apples and oranges.

 

The million dollar question for me is if I keep turning the boost up and retarding the timing at what point does it become worthless. The only real solution is to spend some serious time and money playing on dyno to find the answer.

 

As a side note I also ran an MN47 head with flat tops for a few years. The flat tops with p90 head and turbo has been far more fun .

[Gollum]

Thanks for the info Gollum you make some very good points, especially regarding timing. I suppose comparing flat tops to dished pistons is indeed apples and oranges.

 

The million dollar question for me is if I keep turning the boost up and retarding the timing at what point does it become worthless. The only real solution is to spend some serious time and money playing on dyno to find the answer.

 

As a side note I also ran an MN47 head with flat tops for a few years. The flat tops with p90 head and turbo has been far more fun .

 

If all out power is the goal, running more boost with lower compression is just about ALWAYS the answer. But in the street driven reality, very few guys own weekend warriors with sub 7:1 compression, it's just not practical. If you want and out the arse guess, I'd recommend flat tops + p90 to anything bellow 350whp. Above that I'd say you'd want the extra insurance of the lower compression, but at that point if you're spending any decent money I'd say get custom pistons that have a smaller diameter dish section, allowing the quench of the flat tops with the compression of the dish pistons. Also at that point you'd be wanting to have headwork done with chamber reshaping, and match the pistons to that shape.

 

Or you just do what many people do, buy a junkyard motor and turn up the boost and don't cry when it breaks. The 400whp HAS been borken on junkyard longblocks. I personally don't know I'd do that myself, but hey, there's no "right" approach here.

Edited May 28, 2012 by Gollum