Troubleshooting performance

[dcraigbrown]

Does anyone know what the plug gap should be on a 240z with a zx dizzy upgrade? I've found a few threads mentioning you can open up the gap since the spark will be stronger- but no specifics. I opened mine from .032 to the zx spec of .044. The car seems to perform better but is it best? Should I also use the plug specified for a zx or open the gap even further? Thanks for any input.

I guess this isn't a HELP ME! type of question.Maybe there should be a "help me when convenient" forum:).

[240ZX]

I believe you have choosen the right path grasshopper!

[Bruce Palmer]

.045" was where we ended when we went electronic..... Set 'em and forget 'em......

[dcraigbrown]

Set and forgotten. I wish I had a calibrated butt-o meter. Its been hard to tell if this car is way below its stock performance specs or not. The engine is probably a little tired but I don't have the $$ to build it like I'd want to. So working with what I have, a few things have gotten it close- I think: 1. upgrade to zx dizzy and open plug gaps 2. replace elec fuel pump filter 3. Clean and balance SU's 4. Adjust valve lash. Not sure what else I can do absent a rebuild.

[MikeW]

Not sure what else I can do absent a rebuild.

Have you done a compression test to see if a rebuild is warranted?

[dcraigbrown]

Yes. All read 115 - 120. Most repair/service manuals merely mention to check that all cylinders are within ~10lbs of each other. 115-120 sounds very low but it occurred to me that I'm at 6400ft and maybe an adjustment for altitude needs to be considered. I found a couple websites with conversion formulas that bring 120 up to about 150, based on 6400ft altitude. I've never seen this point brought up on this forum and it seems to me it would be a pretty important piece of information to correctly analyze compression- but maybe thats why more attention is given to the difference between cylinders rather than the numbers?? I also bought a Harbor Freight leakdown tester which showed 0% leakage on all 6 cylinders- ya, right!

Well, to make a long "post" short, I've been pretty perplexed with the compression readings that seem to be good on one hand and bad on the other.

[MikeW]

I'm clearly in way over my head here but it seems like a simple ratio. If ambient pressure at your altitude is half of that at sea level then I'd expect your compression to be half as well. Based on the chart here it looks to me like your pressure at 6400ft (about 11.5 PSI) is about 80% of sea level (14.7 PSI). From that I'd guess that your engine compression at sea level would be closer to 150, exactly what you came up with yourself. The fact that all of the cylinders are close and the leakage tester is good tells me that you're in good shape.

On the other hand, performance at that altitude is still going to suffer without something like a turbocharger. P51 fans can attest to the difference at altitude between the Allison and supercharged Merlin engine.

[Bruce Palmer]

Ya know one thing we do for high altitude folks is we supply undersize SU fuel nozzles to compensate for the oxygen deprivation. Reducuing the fuel as a % of the mixture increases the oxygen as a % of the mixture going into the carbs. I'd submit this would make things less blubbery and more crisp. A "lean" mean fighting machine!!

[dcraigbrown]

Yep, pretty place to live, Colorado, but really sucks the life out of internal cumbustion engines. Well, thanks for your input- you pretty much verified what I was thinking- the engine is still tight- which is a relief. After opening the plugs to .045 its doing much better. It just seems to get a little ragged too soon at high rpm- 5000 - 5500 like not quite enough fuel, a slightly weak spark or out of balance carbs, etc. I'll keep tweaking.

[dcraigbrown]

Bruce,

What size needle, price?

[Bruce Palmer]

Not needles, nozzles. Stock fuel nozzles are .100" we can supply any size but typically .099" are enough smaller to take the rich out the mix..... $40 for the pair with a $50 core charge because we need the heads back.

[Carl Beck]

If you "up-grade" to the ZX distributor - you have to also supply it with a ZX spec. coil. Otherwise you can suffer from too weak a spark, to jump that larger gap consistantly.

The car will run with the 240-Z coil - it just won't run as well...

FWIW,

Carl