My latest combo - mn47 L28

[Rebird]

I just wanted to share my latest combo as a whole bunch has been written about the mn47 maxima head. The motor is a low mileage L28 with flat top pistons, ported mn47 head, delta 280/460 camshaft and delorto triples. This combo replaced a stock L26 with the same delorto triples. Although i only have a couple hours on this combo, here are my thoughts if anyone is interested. 1) The power difference is huge over the stock L26!!! Night and day!! The drivability is also much better because of the increase in low end torque and throttle response. 2) The mn47 head and L28 flat tops with full timing (35 degrees) is certainly doable with pump gas with a couple caviots; 93 octane Chevron pump gas and I cleaned up the chambers. I flogged it for a while and then checked the plugs with no signs of detonation. Ran it hard, ran is soft and no signs of detonation. I intentionally lugged it in fourth and fifth gear at low speeds and it did detonate. I never drive it like that, but it will detonate if you lug it!! 3) the 280 cam is very streetable with great response down low and rev's strong up high. I'm unsure of rpm because my tach is not acurate. If you want a rumpity idle, go larger! (should have taken jmortensens advice on this one)..... 4) I bought this car three months ago and have spent waaaay too much money on it but I love it!!! I really enjoy the web site, i read it everyday!! thanks

[olie05]

what compression is that? 11:1?

 

What size are the triples, and do you know anything about your AFRs?

[Rebird]

olie05, i think the comp ratio is in the low 11's. the delorto's are 40 mm with only 30 chokes. yes the chokes are too small but this is what i had on the L26. i have not tuned the carbs at all with my new combo so i'm sure it will only get better with some tuning. I'm new to z's and have a lot to learn but am enjoying the ride!! I am certainly not a purist but i am sure having fun with this little L motor!

[madkaw]

Is that stock valve sizes?

Love the Silver!!!!

[Rebird]

Thanks!! yes, i'm using the stock diameter valves. I worked the chambers and bowls for the most part with just a little runner work. The exhaust liners are.... well... interesting!? Can't say i've ever seen anything like that before but as 1 fast z has written before they can still flow pretty well.

[HizAndHerz]

Rebird, its good to hear your having fun with it. It's great that pump gas is working well for you at that CR. I'm also curious what the bigger exhaust valves make.

 

I'm in the process of putting together my F54 flattops/MN47/Mild cam/Custom intake/60mm TB/MegaSquirt engine. All it takes is lots of time and money.

[z-ya]

If it is STD bore, and you didn't shave a lot off the head, the CR should be around 10:1. What head gasket are you using?

 

You should be in the 200-220 WHP range.

[HizAndHerz]

From my research: standard bore, flattop pistons, Nissan headgasket the CR should come to around 10.8 or 10.9. No?

[z-ya]

If you use the Lengine program, it reports 9.8:1. A 1mm gasket will get you to 10:1.

[Mack]

L engine is fooked. combustion chamber size on the MN47 ids about 39 to 39.5cc. then use a felpro. supposed to be .8mm instead of standard 1.0mm. works out to be a nice compression ratio.

[JMortensen]

Standard headgasket is 1.2mm. Felpro is .040 or 1mm. You can do a .8mm with a HKS by pulling the layers apart, I think each layer is .020 thick. If you start with the 2mm gasket, then you can get 2 .080 thick gaskets or 3 .060 gaskets out of it for the cost of one.

[Rebird]

hey pete, i ran the felpro gasket. i did not measure for an exact compression ratio but rather just went by what I read from mack, i fast z, braap and others in the know. should the power be around the 200 rwhp mark?

[Rebird]

p.s... i had to cut the head .020 to clear it... that was the first time it was cut off of factory specs. i do have a pretty good "flutter" when leaning hard on the throttle so i do have some tuning to do. i sinc'd them and adjusted the idle circuit but that is about it so far. first time with triples and i have a bunch to learn.

[Rebird]

hizandherz, your combo should be real strong with an MS and a cool intake... i guess injection is the way to go but i like the look of the triples!!!

[z-ya]

Yep, my mistake, the MN47 has a smaller combustion chamber than the N47. With the Felpro Gasket you should be just over 11:1 CR.

[BRAAP]

Guys, in computing your compression ratios, don’t forget that the L-series Flat Top pistons pop up OUT of the cylinder bore approx .020”-.025” at TDC so don’t forget to “subtract” that volume from your comp ratio calculations.

[Dtsnlvrs]

Guys, in computing your compression ratios, don’t forget that the L-series Flat Top pistons pop up OUT of the cylinder bore approx .020â€-.025†at TDC so don’t forget to “subtract†that volume from your comp ratio calculations.

 

Great point BRAAP. I too have considered doing a MN47 / F54 flattop combo, but am leaning more on having my existing E-88 worked over (ie unshroud the valves some more and open the ports up) and keep my triples (better, just spent 500bucks on NOS parts, spares, jets, emm tubes etc) and keep the old school / period correct performance modified look.

[Rebird]

Any guesses on what this car is going to run or does anyone have a similar combo with time slips? i plan on running it in the next few weeks and i'm just curious. i do plan on posting the et (if i'm not too embarrassed!) it is pretty quick so my best guess is low 14's to high 13's but i may be way off. also, any suggestions on what rpm to dump the clutch off the line (street tires) at the track? thanks a bunch!