Well, THERE'S the problem...

[spikeyamir]

I never expected to see this when I pulled it out. I knew I had a dead cylinder but I wouldn't have guessed it this bad.

What could have caused this carnage? All the PO told me was "yeah, she misses a bit..."

Ya think?!

Anywho, luckily the cylinder isn't scored bad at all and I was planning on changing out rings and honing the cylinders anyway.

You should have seen the nice surprise when I looked at the bottom of the oil pan!

So any guessing as to possible reasons? Maybe there is something that I'm missing that might cause this to happen again. Thanks!

[Nissanman]

Usually, caused by severe detonation and/or uncontrolled boost:rolleyes::rolleyes:

Was the engine boosted or not?

Pic. shows a piston from the engine of a Nissan N14 Pulsar GTiR which was driven with the Electronic Boost Control "inadvertantly" disconnected:ermm:

Oh, that piston will need replacing:nervous:

[Zak's Z]

Ya, that would be the problem. How do the rest of them look?

[spikeyamir]

Well, as far as I know it wasn't boosted. Everything looked bone stock when I got it. The rest of the pistons look great. No scoring or anything. One thing odd though, the cap at the connecting rod end of that piston was on backwards. Don't know if that would have done anything. Obviously the engine has been worked on before...

As far as the piston, whenever I go to a parts store, they only have the "dished" pistons for my car. I have a 76 and it has "flat top" pistons.

Another thing, I just got back from the junk yard after pulling 2 heads, one a 75 and the other a 78 just to discover that they both had "dished" pistons! Am I missing something here or aren't these years supposed to have the flats?

Best place to get the stock diameter flat top pistons? I only need one.

Agh, this project is dragging... thanks.

[skunkteeth]

Look at the first image:

http://datsunzgarage.com/heads/

If you have your stock N42 Head with flat tops, your compression ratio is about 10.4 according to the above site. There could have been severe detonation...

[sblake01]

Your 76 should have dished pistons. PO engine swap, most likely. What block do you have, N42 or F54?

[spikeyamir]

Well this isn't good. I don't know why I didn't realize this before. I have an F54 block and it has an N47 head... There is no way I can run this combination without dire consequences is there..?

So lets say, to avoid severe detonation, if I change from flat tops to dished running the same combination block/head, I should be getting 8.3:1 CR. That CR isn't too bad is it? Just a daily driver.

Does that seem like a good idea?

eep.

[sblake01]

I've never put the dished pistons in a F54 block but it should come out with the same CR as a N42 block with the same pistons. That's fine for a daily driver. Or maybe you could just find a P79 head. Plenty of those in the junkyards (out here anyway). It goes with the F54 block and flat top pistons and gives about 8.5:1 which would also be fine for a daily driver.

[Jeff G 78]

Classic detonation failure. If it wasn't boosted or on the spray, then the CR was too high for the fuel grade.

An F54 block and N47 head combo will give you about 10:1 CR but only if the head has not been shaved. Either the head has been milled and/or the PO ran too much spark and not the right fuel. I have the flat top/N47 combo and have zero problems with detonation. My head has not been milled and I run a Nissan head gasket which gives me just under 10:1 CR. I use only 93 octane gas and made sure I had enough fuel based on spark plug readings. I ran the car at various engine speeds and shut it down, coasted to a stop and pulled each plug to check the color. I fine tuned it with a potentiometer in the water temp circuit (see Blue's site). Someday I will put it on the dyno and tune it right, but for now it looks pretty close.

As far as flat top pistons go, buy them for an '81 ZX non turbo. The F54/N47 is a good choice and should be kept. No reason to go back to 8.3:1

[Jeff G 78]

On more thing, if only one piston was bad, you need to check the fuel injectors to make sure they are all firing and have a good spray pattern. There is a chance that you have one partially firing giving you a dangerously lean condition in that cylinder.

[spikeyamir]

So you think keeping that head/block combo is ok as long as I use higher octane fuel, say 93 octane?

As for the injectors, it had them, but I got me some SU's and was gonna put those on. Will this combo still be ok for the SU's?

I was thinking about the P79 head but I saw none at my local yard.. :-(

Thanks so much for all the help and insight.

[Jeff G 78]

Yes, the flat top piston/N47(or N42) combo works great and should be kept. This is a very common and simple way to increase your power output.

SU's can be used, but I don't have a lot of experience with them. I have had triple Webers on a Z, SU's on another that I never messed with, and FI on many others. FI works great if it is all in working order. If yours has issues or known bad parts, then swapping isn't a bad idea. Another option is to go with Megasquirt FI. Swapping from FI to unknown condition SU's could cause problems as well. Others here can tell you what you need for the swap to carbs. I know the fuel pump will have to be swapped out for a low pressure unit, but beyond that, I'm not sure.

I would take your N47 to a competent engine shop and have it measured for thickness and flatness before you go any farther. Since the engine has been worked on with suspect knowledge and assembly procedures, it's hard to say what condition the head is in. The original thickness from the top surface to the bottom surface is 108mm, I believe (do a search to verify). If it has been milled, it's possible that it's either too thin for flat tops (CR too high) or was warped and is now only flat on the bottom. Make sure they check both surfaces for flatness. I had one head that was apparently milled flat by a PO, but the head itself was badly warped causing the cam to be in a bind. My machine shop didn't catch the problem and I had major issues as soon as I installed it. I had to throw the head away and start over. I was young and dumb and didn't clay the pistons to check clearances prior to final assembly. The results were two damaged pistons, four bent valves, and a freshly machined lump of aluminum that was completely useless. :mad::mad: That was when I decided to go with flat top pistons.

With the stock FI, stock exhaust pipe diameter, a header and flat top pintons, I get 23-25mpg on 93 octane gas. After I go with a 2-1/4" or 2-1/2" exhaust system and install my performance cam, I expect the mileage to go up a bit. I wanted to break my motor in with the stock cam to eliminate a tuning variable. I now have about 2500 miles on my motor and will install the cam and associated parts this summer.