I have a 1970 240Z I recently purchased. It is a regular 240 block with an E31 head. It runs but the #4 cylinder has little to no pressure. the rest are 155 - 170. So I'm considering a rebuild project. Here's my dilemma. I could rebuild the E31 cylinder head in case thats the problem (stuck valve etc). It also could be broken rings on the #4 piston. Or I could go the more power route which I have been reading about extensively. This is attractive to me since I have a 1978 280Z next to the 240 in the garage that I just stripped down bumper to bumper so I will use the 5 speed, diff., driveshaft & mustache bar to transplant over. Thats the easy part. The hard part is conflicting engine and head combos. I like the idea of a 1983 F54 block with flattop pistons and a stronger block, what I can't seem to find agreement on is which head. One person says the P79 is fine and not to worry about the exhaust port liners (that heat red hot for emissions) that they don't restrict flow that much. Another guy says they do. There is also the N42 head from 1975 - 76 which has the bigger valves. Then there is my E31 which I have now. The person states you simply have it (the E31) rebuilt using 280 valves. What I'm looking for is a street engine with a nice punch but no super high octane fuel to make it perform, yet I want higher compression much like the original 240Z engine 9:1. It would also be nice to use some of my existing components that I already have. Do you have any suggestions or could you refer me to someone. Thanks for your help

 

i know this problem.......im thinking it is a common z problem....#4 exhaust valve....lol.....anyway......if you are planning on retaining the roundtop SU's then i would probably go a head and use the E31 head, bigger valves will do a couple things.......with stock or close to stock compression, you will lose low end torque...and gain upper rpm horsepower....more is air is better, but depending on cam choice, exhaust choice.........and lil change in compression...are you really gonna gain that much usable power spending the money for bigger valves. i myself would just have the head gone through, maybe clean up the casting marks in the runners and run it.......using the later 2.8 block with the earlier head and a mild camshaft....header and free flowing exhaust should net you a nice running, very dependable long lasting engine combination.

 

Quite simple really. I have been talking to norm for along time about this when i was still using my NA motor. As far as off the shelf heads go, definitely the n42. If you can afford the E31, you must change the valves to get equal performance of the n42. The e31 will be higher compression and you stand to gain around 20hp from a valve swap. Im not makin this up, but we ran many combos on desktop dyno. Definitely use the flattop lower end!

Evan

Well while i have it apart i might as well........

 

Thanks Evan for taking time to respond. Like I stated before, I have the E31 head now and I also have the N47 head off my 280Z that I stripped of my other Z. So I would have some components to transfer over as far as a valve swap on the E31 head. Are you saying after the swap the updated/modified E31 head (with 280Z vavles) will have 20 more hp than a regular "off the shelf N42? I like the E31 head idea largely due to it will look stock to my year Z, 1970. OK here is the other question which you eluded to at the end of your first comment-"well while I have it apart...". What do you think of the whole diesel crank thing? More cost more time and is it worth the cost? I do not want to go too nuts but would like to make smart choices while I have it all apart! I also don't want a high maintenance engine either. If you have time I would appreciate any insight you may have. Thanks

Jeff

 

Thanks for your help. One of the reasons I'm posting here is to ask people who have actually done it and can say for sure one way or another. Your suggestion of just having the E31 gone through is a good sound one and is definitely an option. I wish I knew for sure about the larger valve swap. How much low end do you loose? How much upper rpm do you gain? As with any project like this there are choices to be made along the way. Making them would be much easier if I knew for sure. I just want to open the hood after this is all over and know I made good decisions. Thanks again for all your help.

 

My 2 cents.

Definately go with the E31 with larger valves, and a flattop. You can run your Su's and a mild/moderate cam and be pretty happy I think. As far as losing low end torque with larger valves, this is the first time I've ever heard of that. Logically I can't see how it could possibly make you lose torque? I will go out on a limb and say that you will gain an enourmous amount of low end torque over your current setup by upgrading to the higher compression setup, and the extra 400cc's displacement.

As for going with the stroker crank... I wouldn't bother. Its a lot of money for a little gain. If you really want more power, take the money you've saved by not going with the stroker, and put it into a killer port & polish.

 

low end torque loss from running larger valves is quite simply explained like this.......at low rpm....the velosity of the air entering the combustion chaber is reduced due to the fact the large valves creat a larger "venturi"........smaller venturi makes more velousity entering the combustion chamber..........however at high rpm the airflow is restricted by the smaller venturi.........thus larger valves make the upper rpm airflow less restrictive.........making more power......in a street car that gets driven they way it was meant to be.......mid range and upper end power are what i look for......bottom end just means you have to contend with traction........and yes....the extra 400cc's of displacement will make more power on the bottom end..........mid range....and top end........especially with more compression.

 

Jeff, please email me if you would like to see all the specs i have run through desktop dyno. Ill be more then happy to share that with you. And it sounds like you are in the same boat i was back in the day.

As for the motor. Yeah you in theory will lose low end torque from bigger valves. The problem is the E31 valves are just too darn small to make good mid high end HP. If you wanna know more please email me. You really should register so i could email you directly

Evan

Well while i have it apart i might as well........