'New' Ebay engine...a keeper?

[inmocean]

Aquired a spare engine yesterday and I'm in the process of tearing it down. So far I know it's an N42 Block with flat-top pistons, marked 'p9006' and

'.75'

N47 head with externally oiled cam(E30).

-Both of these were surprises...

The engine is very clean inside, bores are freshly honed, good oil, and the valve train looks all new...it also came with a 5-speed trans, which will be a great winter project/upgrade on my 78 280Z...

Have I lucked into something cool here, or just someone's crazy project?

Told it was from a 2+2 ZX, this is supported by the W24 Intake and F54 exhaust, as well as AC, Cruise, Power steering, manifold fan, etc that were on the engine. $100.

Comments, Insights, and Input welcomed.

[robftw]

sounds like an abandoned project or someone tried to give you screwy parts on ebay to make a "complete" engine

[Zed Head]

Flat top pistons came out with the P79 head. N42 blocks came with dished pistons so it looks like someone installed flat-tops in a typical N42/N47 engine. With an N47 head you will have a higher compression ratio, and if the predominant word on the internet is correct, detonation problems on pump gas.

There are some out there who install the flat-tops just for the higher compression ratio,then retard the timing to avoid detonation. Some have reported that they don't have to retard the timing and don't have problems. Others have reported constant problems. It's hard to tell what really works.

One option, if you keep it the way it is, would be to use a thicker head gasket to drop the CR. Check out this calculator - http://www.ozdat.com/ozdatonline/enginedesign/ Under Pistons, "L28 Late" would be flat tops, "L28 Early and Turbo" would be dished.

Or you could get a P79 or P90 head and be back to a normal late model L28 NA style engine.

Or run the higher CR and see what happens.

[inmocean]

My plan A on this venture was to aquire a spare head for futures along with anything else I might be able to use for my Z, then sell or pass on to Z fans in need everything else. Everything I'v been reading and these intital replies make me think that's the best plan. This engine's value, if any, will be in keeping others on the road, not as a complete engine. But I have one more evening of disassembly ahead of me, and then lots of checking and measurements before I find out if any of it's any good...Thanks for the interest so far.

[robftw]

Well if you plan on selling parts, i could really use the extra block for my stroker build

[inmocean]

robftw, I'm going to tear it all the way down and look everything over for obvious cracks/damage, before I think about passing it on. But I'll PM you if it looks OK.

[Jeff G 78]

The N47/flat top combo is only a problem if the head or block have been milled. I run that combo on 93 octane and have never had any detonation issues at all. I haven't tried 91 octane, so I don't know where the threshold is, but 93 works fine.

[Zed Head]

You should really give more info about the collection of parts. Stock cam, stock heads, stock head gasket, initial timing, total timing, EFI, stock intake manifold, etc? I think that that is where the problems come from when people try to run high compression. Without the details it's just a head designation number and a piston. And your 10:1 CR doesn't quite match what the stock N47 flat top combo should give, unless you rounded up, according to available information. 9.75:1 is what I've seen.

Not to mention the gasoline quality. In Oregon we only have up to 92 octane 10% ethanol fuel at the common gas station.

The detonation is in the details.

[Jeff G 78]

Sure Zed Head. I run an unmilled, unmodified (see note below) N47 head, a stock cam, a Nissan head gasket, ITM flat top pistons, and a stock EFI system. The timing is set to 10 degrees BTDC. I have run all brands of 93 octane gas - all with 10% ethanol and driven in temps from 40F to 95F. I did not check the actual CR, but from my own calculations, I get 9.82:1 CR. This will vary a tiny bit based on head gasket brand, but with everything else stock, it will come in just under 10:1. Here in the Michigan and Ohio area, we have 87, 91, and 93 octane. I have also seen 89 and 94, but like I said, I have only tried 93. The only engine mods I have are a AZC 6 into 1 header, a turbo oil pump, new flow-matched injectors and a potentiometer on the coolant circuit to adjust A/F. My plugs burn a perfect light tan color and I get 23 - 25 mpg regularly. I have checked my odometer for 100 miles against the Ohio Turnpike mile markers and it was within a few tenths of a mile TOTAL. The speedo is way off, but the odo is dead accurate with my 225/50R16 tires.

Countless people have been running this engine combo successfully for decades. The main problem with detonation is that nobody bothers to check their head thickness before they go with flat tops. They assume that their 30 - 40 year old head has never been machined, but it's very common for shops to take a cut while the head is being worked on.

Note: My N47 head is bone stock, and now only has about 15,000 miles on it. When I bought the head, it was brand new and had been sitting on a shelf since it was removed from a new engine back in 1977. It was owned by a race team and the head was never used. I only replaced the valve seals, but all else was in factory new condition.