Exaust Port Shape...

[texasz]

beandip, how did you get a 9.5 compression ratio out of a P79 on an F54 with flat top pistons? I have read that the this should be much lower, in the 8's somewhere. This is a stock configuration, I have one sitting in my garage now. The only difference is that you've converted to carbs and the one I just got last week has FI (again, this engine came stock with FI which I will be removing to go carbs).

[z-ya]

Unless you have shaved the head, and shimmed the cam towers, your CR will be around 8.5:1. If you use an N42 or N47 head, you CR will be around 9.8:1.

Pete

[texasz]

Ok, I have another thread going on compression ratios for the P90 here: http://www.classiczcars.com/forums/showthread.php?s=&threadid=7109

I have read that more than about 10:1 is not drivable on the street and I want to keep mine streetable. So...what is my best bet? I've read all the information on this and other sites and taking everything into consideration it seems as though the P90 head is my best bet (I will be using an F54 block) but the compression ratio is around 8.5:1. Is this good enough or should I try for something in the 9's? If I should try for a higher ratio then how?

Car use: I plan on autox and occasionally driving to work just to enjoy it and keep things moving properly...NOT my daily driver, just a fun car that would be used for autox.

Cost: CHEAP! I have to keep my expenses down and have remedial mechanical skills so any shaving etc would require to part going to a machine shop (I could probably remove and reinstall it but maybe not much more).

[z-ya]

I would use the F54 blcok, and pick up an N42 head. If you have the P90, you can sell it easy to a turbo guy.

If you can't find an N42, then look for an N47. They are the same but the N47 has round exhasut port liners. N47s are cheap and easy to find.

You can run 10:1 on the street. I know someone running 10.5:1 without problems. Yes, you must only run 93 octane or better fuel. The timing needs to be set correctly, and a high performance ignition and coild should also be used.

Pete

[texasz]

Currently I have the F54 block (flat-top pistons) with a P79 head on it (was pulled from a running '82 280Z). So either way I would have to buy the new head, which ever turns out to be the best.

What's the difference between the N42 and the P90? They both are from the 280 so the sizes should be the same (right?) and they both have square exaust ports. I have read that the P90 is supposed to flow better yet the N42 does show to give higher compression with no mods. Now the N42 has bronze valve seats which I have read make this head expensive to rebuild and as I recall require you to run a lead additive in your fuel. I have also read that you should use the N42 and N47 only with dished pistons and the E31, early E88, P79, P90, and P90a with flat-top pistons. What about this information?

Then there is the E88 head which shows to have the same compression as the N42 on an F54 block. So why not this one?

I'm really quite new to all this and am trying to learn as much as I can but there is just SO much to know! So please forgive my lack of knowledge and numerous questions.

[beandip]

the head was milled .03 but the towers were not shimmed. that is he limit any more off the head and it must be shimmed . the geometry has been checked and the engine has 38k on it and all looks perfect, doing the measureing and checking the volume of the combustion chamber gives me 9.4 actually but higher would have been more to my likeing. This is strictly a street cruser.

[beandip]

here is a web page that will give you all the answers. I am giving you all of it as it appears on my screen when I bring up the page . I tried to pass on another address from the same master page and it wouldent work for that person so let me know if this dosen't work and I ll print it and mail it to you . I dont want to unless there is no other way the last one was over 35 pages. Here goes, http://www.geocities.com/row4navy/head2.html good luck Gary