P90 With Dished Pistons (Yes, N/a)

[HuD 91gt]

Hello all,

 

I'm looking for info if anyone has ever run an P90 head on a dished piston L28 block (Yes, a L28et without the turbo).  If you care for the details you can read the rambling story below.

 

Maybe a bit of a silly question, but some of you may have some quality input.  I've stumbled across a L28et engine in my area for a great deal.  I have been searching for a while, and i've had two ideas in my head which both involve said engine.  Either A, do the turbo engine swap, or B, get some flattop pistons, shave and shim the head and run a higher compression N/A engine.  Both have their complications and their costs but for now I am looking on a temporary solution.

 

I have a 1971 240z with an E31/L24 combo.  It runs fine, but storage is my top issue at this time (I live in a condo, and rent a garage to work on my Z, but I have no room to store a spare engine). 

 

If I purchase this engine, I need to do something with it.  NOW!  So, i'm thinking I may drop it in, put on my stock SU's and an electric fuel pump and I should be ok for the time being.    With such a low compression, is this going to be an absolute dog, or perhaps perform similarly to my smaller L24 (Although different characteristics of course)?

 

Has anyone experienced this combo?  I plan to do execute Part II of the engine work (Headwork and or turbo/megasquirt) over next winter.

[Diseazd]

You would be right about being an absolute dog. IMO, I would keep the L24/E31 in the car........store the L28 in your wife's closet until you can install flat tops and a shaved and shimmed P90 head with a nice Stage III cam. I don't know what h.p. the de tuned engine would turn, but my guess would be less than what you've got now.

[siteunseen]

If you get the turbo motor make sure and get the ecu too. They're different I've read. You'd want to disassemble and clean it up anyway so storing the parts would be easy. Head under a bed. Block standing on it's ends take up the same room as a vacuum cleaner. If there's no transmission you could find room I'm sure. Is it a p90a? I think they have hydraulic lifters.

 

http://datsunzgarage.com/p90/

[HuD 91gt]

I don't believe the ECU will come with the motor, but as I said before i'll most likely be doing a complete Megasquirt install eventually, I just can't dive into that project now.  I need to drive this thing this summer, and I know how these projects can go on and on.  I've never fumbled around with a bare L28 before, but I think i've read, complete they are somewhere in the 500lb range!!??!!  Tough to hide in the living room, but disassembled may be an option.

Doing some mild work on the motor prior to installation does seem like the better option as it's condition is relatively unknown. 

 

Siteunseen, i'll have to check out the head once ahead, I completely forgot about the hydraulic lifters!

[madkaw]

I would rearrange the furniture or have a garage sale, or throw away some sh!t and find a way to store the motor, but wouldn't run it NA. You will be disappointed to say the least and in reality it will take a LOT longer than you think get turbo charge that motor or do anything else to the motor to keep you happy.

45$ buys you a cheapy engine stand and you can roll it around out of the way.

That L24/31 combo will run circles around that 54/90 combo NA.   

[S30Driver]

It's actually not quite a bad as you would think.  I purchased an 83 ZX with a seized turbo & drove it for about 5 months before replacing the turbo with a new one.  Performance of course was not exactly spectacular but not as bad as I thought it would be.  

Factor in that your car weighs about 700 or 800 lbs. less than the turbo ZX and it would be even better than I experienced.

 

However, doing a swap with an largely unknown motor would stop me from doing it, & would likely take the advice of those above.

Find a way to store it & best of luck...

[Zed Head]

There was a guy on the forum a while ago that put either a P79 or P90 head on a block with dished pistons.  Said it ran fine, but it was his daughter's car and he was just getting it running so he could sell it.  It was an EFI engine.  

 

Assuming the engine itself is in good shape, your biggest issue might be getting the SU's and intake mounted and the carbs tuned.  I don't know if the P90 heads come with carb manifold mounting holes.  I think that the other heads, N42, N47, for sure, and maybe P79, do, but they're designed for NA.  The P90 is a turbo design, so no need for carb manifold holes.  That's probably your decision-maker.

[rcb280z]

In reality, what would the power "loss" be if installing a P79 or P90 on a dished piston block? Would it really be that noticeable? I realize the compression ratio drops slightly but what does that mean in terms of power loss?

[Zed Head]

It would run like a turbo L28, off-boost.  Probably okay.  17% more displacement, and bigger valves, might make up for the lower CR.  There are general formulas out there that try to estimate the benefit of increasing CR.  Here's one - http://www.wallaceracing.com/hp-cr-chg.php

 

Doesn't look like much loss.  This is one of the basics of the argument people make that optimized ignition timing is far more important than a few extra CR points.  

 

The added displacement might actually increase power.  The engine will sound a little dull though.  The exhaust won't have that rippy sound that high CR and a performance cam tends to give.

[rcb280z]

I was just curious because I have a really good P79 head in my garage and my N47 needs attention that is on my block right now. I was thinking about throwing the P79 on while I rebuild my N47. It sounds as if I would only lose a couple of horse's if I install the P79. The CR of the N47 is 8.3 but I don't recall what the P79 is (?) Just thinking out loud here but I could throw a performance cam in the P79, and then when it's time for the emission testing I could put the stock cam in, maybe?

[siteunseen]

After reading this, http://datsunzgarage.com/p79/, you should rebuild the N47 for emission testing only and run the P79 year round. 

[rcb280z]

I like that idea but that is a lot of work. I'd rather change out the cam then pull the head.