engine management

[az240z]

I think I titled this right. I am in the process of a 5 year rebuild of a 1978 280z. Note this isn't a frame up rebuild rather I start and stop as life gets in the way. It will be a week end driver not show quality but will look good.

The question I have is the engine. My father in law did most of the work as he was a 40 year ford mechanic. I had done some research and had the engine bored 30 over, we installed flat top pistons and then a mn47 head. There was some mild porting and polishing done to the head when it was rebuilt.

After talking with a number of folks I'm told that the compression will 10.5 to

11:1. This to high as I'm not gonna run race gas and 93 octane isn't available her in Phoenix, Az. I'm told that I may be able to run on 91 octane if I use megasquirt or some other engine management system. I feel that this way beyond my capabilities and would like more of a stock experience.

So I guess what I'm asking is, what to do now. I am not going to use fuel injection and will be installing su's. It has been suggested that I could change out the head with a p79/90 or buy another engine.

What would you do??

[Zed Head]

There's a chart on this page that gives examples of CR for various block/piston/head combos. http://www.zcarz.us/TechnicalInformationPage.htm

I can't vouch for accuracy on the non-stock combinations. Small chart right below the first big block of data. It shows that just the flat tops get you up to ~10.4. The smaller Maxima N47 chamber would be even higher.

Here's a link to a calculator. I don't know if it works. - http://www.classiczcars.com/forums/showthread.php?s=&threadid=1292&highlight=L+engine+calculator

With carbs, you'll just be back to normal carb and timing tuning, and get what you get. You could try the Maxima N47 but you'll probably have to run low advance on your timing to avoid detonation. I imagine that engine response and fuel economy will suffer but you could do a head swap later if so.

An interesting dilemma. I've pondered carbs with some performance mods, since it seems like the cheapest way to get there once you have to leave the stock AFM and ECU behind.

[az240z]

The calc works however it doesn't have the mn47 head just the n47. Using this the comp showed 10:19:1

[ZCurves]

After using the Calculator, you should be able to find out where you presently stand. Then you could have your Head CC'd at a machine shop to bring the ratio back down to a workable level.

[Pop's Z]

FWIW I have an N42 on an F54 flat top piston block. The head was milled to a 10.5-1 compresson ratio. It has a stage 2 Delta cam and a header. I run 91 octane gas and have NO detonation issues, and most of my driving (spirited) is done at 5,000' or more.

Cheers, Mike

[Diseazd]

Same here....I have 2 F54 block L28'S with the p79 head shaved .080 inches with .040 over flat-tops running 10 to 1 CR with no sign of a ping. I've heard the more duration the less problem. Both run stage III Isky cams with .490 lift. Both are a blast to drive!