nate_82zx Posted January 30, 2011 Share Posted January 30, 2011 I'm just wanting to make sure this seems like a good combination before I start tearing down the engine. '81 ZX bottom end with flat top pistons MN47 head with P79 intake valves installed. (I'm told they will work and are larger) I've read it bumps the compression but by how much I am unsure as i've read many different numbers. SU carbs with a 260Z manifold. Lightweight flywheel and new clutch Header and 2.5" exhaust What I'm looking for in this car is a fun weekend driver for spirited driving in mountains, back roads etc. I want nice throttle response and a fast rev. Also above all else, it has to be reliable. Does it seem like I'm going in the right direction? Guess I'll add that if my budget allows at the time, I also want a mild cam and some porting done as well. I'm open to ideas on this and will appreciate any constructive ideas anyone has. Thanks in advance. Quote Link to comment Share on other sites More sharing options...
BluDestiny Posted January 30, 2011 Share Posted January 30, 2011 Why don't you just use an N42 head? the compression won't be as high (pump gas friendly) and it will work nicely with a mild cam. It would also be cheaper. Quote Link to comment Share on other sites More sharing options...
nate_82zx Posted January 30, 2011 Author Share Posted January 30, 2011 Why don't you just use an N42 head? the compression won't be as high (pump gas friendly) and it will work nicely with a mild cam. It would also be cheaper. I already have the MN47 head laying here. I spend $30 for it at Pull a Part. I know the compression will be high with the MN47 head but how high I have no idea. I'll do some research right now about N42 vs MN47 heads. Thanks for the input. Anything else sound like it may not work? Quote Link to comment Share on other sites More sharing options...
rejracer Posted January 30, 2011 Share Posted January 30, 2011 I would ditch the MN47 for the P79 or P90 head. The chambers on the 47 and 79/90's are very similar, plus you don't incur the added cost of putting in the larger valves. With the lower compression ratio of the P79, it will provide additional reliability. If you do go with the MN47 make sure you get the smallest piston to head clearance as possible this is where detonation resistance will come from. Quote Link to comment Share on other sites More sharing options...
2eighTZ4me Posted January 31, 2011 Share Posted January 31, 2011 " The chambers on the 47 and 79/90's are very similar" Not quite..... the 79/90 has a 53.6cc combustion chamber - the MN47 has around 39.5. They're worlds apart. Smallest piston to head clearance?? Last I checked, that bumps compression ratio and compounds detonation. You're talking about quench. When you start getting in to using a MN47 on a flat-top motor, you've almost batted yourself out of the pump gas field - unless you have a monster cam. I'd have to run the numbers through OzDat's engine calculator - but I bet compression ratio would be at least 10+:1. There is scant little room for larger valves in the MN47 as well. I don't have the specs on the valves on each head in front of me, but the MN47 was a bit smaller - I believe 42 and 34mm. The 79/90 valves "may" fit though. Nonetheless - I agree with rejracer, that a P90 head on flattops would be plenty good for what you're trying to do - and the head would flow better in general. Throw a decent cam in there and you're even mo' better off. I've got a MN47 on my stroker motor (dished pistons) and am seriously considering swapping over to a P90. Even with dished pistons, I can only get max 33 degrees total advance without pinging. A P90 would drop compression ratio a bit, and allow me to bump more timing and probably build more power over a broader curve. Even though the head is race ported, I think I'd gain more through the added timing and flow of the head than a couple points of compression ratio would give me. Here's a good link to find all the differences in the L series heads (as well as a boatload of other useful Z-car information) http://xenons30.com/Heads.html Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted February 1, 2011 Administrators Share Posted February 1, 2011 [*]MN47 head with P79 intake valves installed. (I'm told they will work and are larger) I've read it bumps the compression but by how much I am unsure as i've read many different numbers. Wherever you got the advice of using P79 valves in an MN47 head, be wary of taking any more advice from that source. Yes the P79 valves have a larger head diameter at 1.73", but they are also much shorter in length, bad advice when there is another valve readily available with the same larger 1.73" head diameter, same cost, and its the same length as the MN47. The N42/N47 valves, same 1.73" diameter and same stem length as the MN47. 2eighTZ4me covered the MN47 vs P-series head. Quote Link to comment Share on other sites More sharing options...
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