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MN47 head with dished pistons


Perfect240z

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So I've tried searching on hz and I can't seem to find a answer to my question. I read a a lot about the cr for flat top l28 with the mn47 being on or around 11:1 and 91 premium isn't good enough for that combo, especially since I'm in California and I hear our premium gas is crappy? Lol anyways if I were to used the dishes pistons from my turbo block would that bring the cr down enough to keep it higher compression, have old power, and not have the pinging? I'm hoping to find a simple answer that isn't over my head lol :). Thanks for your patience.

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why don't you just use flattops and an N42 head?

 

Just wondering why you have an MN42 head. Sounds like you have it laying and just want to slap it on or something.

 

I do :) I Picked it up with a p79 for a total of 35 bucks. Im wondering this cause I have a set of dished pistons for sale and if it'll bring the cr down enough to run premium I won't sell them. I don't really know where to buy or get higher octane fuel.

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With the dished pistons you would lose the quench effect you would have with the flatops, so it would be pointless check out zgarage.com. The p79 or p90 can be a great NA street head with flatop's there are a few stickies about it.

 

Ohh ok, I guess I never really thought about the dished pistons not making use of the quench pads then. So how can you use this head then without having to use anything over 91 octane? LOL it sounds more and more confusing that this head is basically best used on the track, or you have to find higher octane fuel is that correct? I have a P79, I thought it was one of the not desirable heads? And I only would run a mild cam too, want to drive it basically on the back roads and when I want to release some steam too. I have read some threads of people actually using flat tops with mn47 and premium 91 fuel and they had no issues. should I assume its a lie then? :)

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Ohh ok, I guess I never really thought about the dished pistons not making use of the quench pads then. So how can you use this head then without having to use anything over 91 octane? LOL it sounds more and more confusing that this head is basically best used on the track, or you have to find higher octane fuel is that correct? I have a P79, I thought it was one of the not desirable heads? And I only would run a mild cam too, want to drive it basically on the back roads and when I want to release some steam too. I have read some threads of people actually using flat tops with mn47 and premium 91 fuel and they had no issues. should I assume its a lie then? :)

 

Alot of factors go into if the head will work for you sure at sea level with a very fine tuned EFI and good 91 octane it could run well. At a higher altitude it could be a different story, the mn47 makes a good race head and BRAAP actually recommends the p79 and p90 both almost identical except for the exhaust ports.

 

Go read some of the FAQ and you'll find BRAAP's stuff and learn about the motor's and what you want. I ran a stock p79 on a stock flatop block with 240z SU's and loved it made 150rwtq at 6200 ft on

91 and running a bit rich. Granted the caxr was a gutted 260z coupe it was just as fast as my buddies foxbody mustang and torquey as heck.

 

If I had kept the car I would have had the head built and a nice ITB setup or similar and Megasquirt and would hace totally been happy if I could have broke 200rwtq

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Alot of factors go into if the head will work for you sure at sea level with a very fine tuned EFI and good 91 octane it could run well. At a higher altitude it could be a different story, the mn47 makes a good race head and BRAAP actually recommends the p79 and p90 both almost identical except for the exhaust ports.

 

Sounds good. I'll go look up the braap stuff. Since I want to run triple mikunis it's going to probably need so much damn tuning. (I like the way the triples look and sound, that's why I don want to go with injection I anyone was wondering)

 

I may have a friend that has a p90 he's willing to sell me. Thanks!

Go read some of the FAQ and you'll find BRAAP's stuff and learn about the motor's and what you want. I ran a stock p79 on a stock flatop block with 240z SU's and loved it made 150rwtq at 6200 ft on

91 and running a bit rich. Granted the caxr was a gutted 260z coupe it was just as fast as my buddies foxbody mustang and torquey as heck.

 

If I had kept the car I would have had the head built and a nice ITB setup or similar and Megasquirt and would hace totally been happy if I could have broke 200rwtq

 

Ok, I'll look more into it. Just wondering cause I do have this spare mn47 head. If I need better gas the. Since I'll be going triple mikunis I'll have to look into further or sell the head since I don't know it I'll use it.

Edited by Alex731
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I have an MN47 running on a flat top block and with crappy gas, it will ping. But, up here in MN, I can find 92 octane all day long and 93 if I really want it, is at a few select stations.

 

I am running megasquirt n spark, so I have been able to tune MOST of the ping out, but some still remains on lower quality gas. This is all done with a stock cam (well, a 260z 256 degree cam anyway) and stock intake/exhaust manifolds. There was mild clean up work done to the valve bowls and also port matching and shaving of the injector bumps at the end of the intake runners.

 

I have a 40mm itb set up in the works with 40mm runners all the way to the head and also a delta regrind that will be going in this summer. That should put an end to the ping, and I would imagine I will probably break or at least get very close to the 200rwhp barrier. As it sits now, the car is a low 15 sec car and the engine is in a STOCK 1980 280ZX 2+2 slick top. So, the car is no lightweight and the engine is probably pushing around 150 to 160rwhp and a boat load of low end torque due to the smaller MN47 intake valves. LOTS of velocity.

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I started reading this thread and though "okay, where's mack when you need him?" and there he is!

 

Just run flat tops + the MN47. Throw in a nice and aggressive cam. Run carbs or aftermarket EFI, just be warned that with carbs and a high duration and overlap cam that it'll idle quite a bit rougher/sporadic than with a programmable EFI setup.

 

Also don't forget that you could also convert to E85 and run the stock cam just fine. E85 is much easier to find than race fuel, and works better in some ways in my experience. It just means you need to full re-do the fuel system, which same would be the case if you were converting to EFI from carbs. Make sure to use all alcohol rated components if you do. Don't pay for the "E85" tax that some part suppliers want to charge. Alcohol has been used as fuel since at least the 60's, if not much sooner, and those parts are cheap and readily available and will run E85 just fine.

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You could have some custom pistons made with a dish that is only cut out under the valves in the cylinder head, leaving it flat under the quench area. This would give you the option of lowering the C/R to whatever you want based on the volume of the dish without losing the advantage of the large quench area.

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All good info. Ordering pistons may seem a bit drastic since I don't know when I'll be heading to the track, that's the plan. Only time will tell, that is a good thought though.

 

I was lookin into getting a cam for this motor build, just didn't know yet how aggressive I wanted it. I think converting to the e85 might be more troublesome for me then finding the race gas. I haven't tried searching it yet, but I don't think I'll come up with much. I know I might have one by my work...40 miles away but to drive there, fill

Up and drive back would just be stupid. Gas can once a day to fill and bring home after work would take forever lol. The station would know me by name by week 2 lol. I'll have to mull everything over.

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But don't forget that a E-85 setup can run petroleum based gasoline just fine, while vice versa isn't always true. If you're using a programmable EFI setup, then you just run two different maps, one for E85, one for gasoline. Or do you the real trick setup and get the E85 mixture sensor from a GM vehicle and use it as a overall mixture modifier in megasquirt so the engine auto-balances to an extent.

 

The point is to get it running without ping on cheap pump gas, then when you want more power you can fill up on E85 and pick up an extra 30+hp

 

Remember, most cars that suggest premium fuel today will run on 87 just fine, but will make less power. This is the same detonation limitation you'd be looking at with 11:1 compression on the stock cam. As Mack is attesting to, it'll can be done on 93 octane, but i'm sure the engine becomes a very good gas quality tester.

 

 

There's many solutions to the detonation problem, and E85 is just one idea. I personally don't think it's the BEST option for most people, but I feel it's an often overlooked idea.

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I am sure Tony will chime in soon to tell you the benefits of water/meth injection. That would be another way to combat the detonation.

 

Don't forget that water/meth injection is really just a better version of E85. The same dual injection setup you'd use for meth could also be done with E85, and would be cheaper per gallon. The big difference is that 1.0 Lambda for water/meth is around 6.5:1 while E85 is up around 9.5:1, meaning you generally get more power bump from methanol. Both will do the job of removing detonation though, and should allow for much more aggressive timing/all around tune.

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