Opinions from those that have built high CR L28's

[skib]

Hey guys I thought Id get some opinions and maybe some of your experiences with running high CR L28's

 

 

Im using an F54 block from a turbo car, L28 crank and rods, the OEM "dome top" pistons (see pic) all balanced already, E31 that will get ported, valves unshouded and bigger valves, cam still undecided, and I plan on running ITB's and a Wolf ECU.

 

 

 

 

 

 

this is going in a track car that will see limited street use (driving to the track and back)

Edited July 17, 2010 by skib

[hoov100]

The shape of the "dome" on that piston looks like it would create hot spots on the crown from it having such a fine edge on the raised portion. If you have the budget for the engine build, you could lower the pin height a little to bring the top of the piston closer to the deck, while maintaining a flat piston crown.

[skib]

The shape of the "dome" on that piston looks like it would create hot spots on the crown from it having such a fine edge on the raised portion. I

 

 

not sure on that, they are factory Nissan pistons tho.

 

still trying to find some more detailed info in them

[hoov100]

I have always been told by every engine builder I know to avoid sharp ridges in the combustion chamber/top of the piston, as they can get hotter and can aid in detonation. (My wording sucks, but did you get the point?)

[JMortensen]

Those pistons aren't that unusual. I can't recall the name of them, whether it's the Euro piston or the Nissan Comp domed piston, but they're a factory item and if you look you shouldn't have trouble finding more info on them. They don't look like they have a super sharp edge on the raised part, which would be a detonation issue. If they did you could knock that edge down with some sandpaper and call it a day. I think you're going to be around where I was in terms of compression with that combo. My slightly shaved head and flat top combo with flat top valves was about 11:1 compression and required 95 octane gas to keep from pinging with a .490/280 camshaft. I would suggest a larger cam than I ran if you want to run pump gas. If you're going to run race gas you can run a smaller cam if that fits with the goals for the engine in terms of power and peakiness of the hp curve. It's better to lower the compression a bit and be able to run the optimum amount of advance than it is to retard the timing to prevent pinging. There are a lot of ponies in those last few degrees of timing.

[skib]

Thanks for the imput Jon, thats the kinda stuff im looking for.

 

The pistons are the Nissan OEM ones from Europe, I got them from BRAAP and Ill be going and bugging him when its time to pick a cam as well

[skib]

anyone run water/meth to reduce pinging?

[JMortensen]

anyone run water/meth to reduce pinging?

IMHO, going down that road is a BIG waste of time/money. Get a bigger combustion chamber and lower the compression, or run the race gas. On a recent thread I had found a calculator that will estimate compression vs hp. If you plug in what you hope to get out of this motor and then change the compression to what you would have with a bigger chamber, you'll find that the difference is not that huge. Given a compression of 11:1 and a hp of 250, a change down to 9:1 gives hp of 237. Not worth hassling with water or meth inj for me. YMMV. That calculator is nothing more than a general guideline, but it helps to prove the point.

 

http://www.bgsoflex.com/crchange.html

 

Another way to put it is: is there something else you could do (headwork, cam, ignition upgrade, exhaust, etc) that might give an equal or better amount of hp with the lower compression? I would guess that if the difference is 13 hp, the answer is yes.

[skib]

think I should swap the domes for late L28 flat tops to drop the CR to 10.1?

 

Id kinda like it to run off pump gas if I can

[JMortensen]

If it were my engine to build I'd use a different head. I built an L28/E31 and had the octane problems. If I had it to do over I'd go with a P90. Those pistons with a P90 would give you something like 9:1 I'm thinking. If that wasn't enough you could shave a bit off the head and shim the cam towers up. There is no magic in the E31, and the other head should be easy to find at $100 or $150.

[Mack]

get yourself a maxima N47. its just like a shaved P90, but without all the hassle. you get a 39cc, heart shaped combustion chamber. running F54 flat tops, the CR works out to be about 11.4:1, with a fel-pro gasket.

 

If you want to drop it a little, unshrouding the valves always works wonders to improve flow and drop the CR a bit.

 

I had this set up running in my 280ZX with megasquirt. I had to run premium, but even with the retarded timing and a very slight port job (basically just port matching), a 2.5 cat back exhaust and a 60mmTB with a crap cone filter, my car was good for very low 15 sec 1/4 miles. this is in a 280ZX 2+2 mind you.

[JMortensen]

get yourself a maxima N47. its just like a shaved P90, but without all the hassle. you get a 39cc, heart shaped combustion chamber. running F54 flat tops, the CR works out to be about 11.4:1, with a fel-pro gasket.

 

If you want to drop it a little, unshrouding the valves always works wonders to improve flow and drop the CR a bit.

 

I had this set up running in my 280ZX with megasquirt. I had to run premium, but even with the retarded timing and a very slight port job (basically just port matching), a 2.5 cat back exhaust and a 60mmTB with a crap cone filter, my car was good for very low 15 sec 1/4 miles. this is in a 280ZX 2+2 mind you.

Just how much do you think you can drop the CR by unshrouding the valves?

 

EDIT--Let me restate. I had an E31 and I unshrouded the valves myself right to the fire ring on the headgasket, knocked down the machining ridges around the valve seats and the bumps of aluminum on either side of the area where the valves are closest together, knocked down the spark plug threads, really cleaned up the chamber and took quite a bit of metal out. Then I put in an almost flat headed valve, without the dish as in the stock valves. The combo of losing the dish and the new valve sitting higher in the head made me GAIN compression overall.

 

I used to have the numbers of how big the chambers were before and after for each cylinder, but that paperwork is long since gone. I would say for a guy who wants to run on pump gas the P90 with the domed pistons he has is a better choice than the MN47.

Edited July 23, 2010 by JMortensen

[Mack]

you couldnt drop it much, maybe half a point at most. but, every little bit helps. If he doesnt want to run that high of compression, then the domed pistons with an UNMOLESTED P90 would be a good bet for him.

 

I was simply stating that a maxima N47 (with a set of regular F54 flat tops) would be a better starting point for him than a shaved P90 with all the machine work and cam tower shims and all that business.

[Zmanco]

The combustion chambers on my .080" shaved P90 with unshrouded valves measured 45.5 cc. That might help you with your CR calculations.

[JMortensen]

you couldnt drop it much, maybe half a point at most. but, every little bit helps. If he doesnt want to run that high of compression, then the domed pistons with an UNMOLESTED P90 would be a good bet for him.

Agree.

 

I was simply stating that a maxima N47 (with a set of regular F54 flat tops) would be a better starting point for him than a shaved P90 with all the machine work and cam tower shims and all that business.

Disagree. If you want to run pump gas, domed pistons and MN47 is not a good choice, unshrouded valves or not. If you wanted a bit more compression than the P90 would offer, you could shave .020 or .040 and probably get a much safer 10:1 or so without too much trouble (would have to cc the chambers to verify). If he's going with an aftermarket cam he'll be changing lash pads, so the only "extra" thing he would need is cam tower shims.

[woldson]

Meth injection a waste of money?

[skib]

Meth injection a waste of money?

 

 

is it?

[Tony D]

Meth injection a waste of money?

 

In the VW world, Gene Berg would run 7:1 CR's while all others would run 3 points higher. Of course his engines made nearly the same power...but lasted 150K miles.

 

As for wasting money, depends on what your costs are I suppose. If 100 octane is $6 a gallon, then running a full tank is $84.

 

If it costs you $3 for 91 octane, and $5 for a one gallon tankful of Methanol 50/50 wiper fluid (the most common mix) and gets you the same octane performance, or allows even more advance for a cost of $47 a tankful ($37 a tank saved...)

 

That means in 16 tanks you recover the cost of the methanol injection unit. Let me see, I run through two tanks a weekend while home, so in 2 months of driving I get my costs recovered.

 

Being I can drive (potentially) 52 weekends a year. then that means 44 weekends at $37 saved per weekend: $1628 a year versus running racing gas.

 

I need to see how this is defined as 'wasting money'...

 

(Keep in mind I'm inveterately cheap and run 87 all the time, so that means I save 0.20 a gallon over premium which STILL PINGS in my L26, but 87 and washer fluid doesn't... I had it on the Corvair for the same reason---to run REGULAR which didn't ping, versus running Premium which DID STILL PING.)

Edited July 25, 2010 by Tony D

[JMortensen]

For the benefit (again, what 13 hp? 15?) I think complexity you don't need and yes, a waste of money. If you're going to DIY the whole thing, maybe you can do it fairly cheaply, here is a pre-built kit for $360. Plus the cost of the methanol. And whatever you do, don't run out on a WOT pull.

 

I'd rather do something else to the engine to get 13 hp. YMMV.

 

http://www.amazon.com/AEM-30-3000-Gallon-Methanol-Injection/dp/B001DX7K74

Edited July 25, 2010 by JMortensen

[Tony D]

Look at the power under the curve, it's not manifested by peak horsepower gain. It's manifested by more torque down low where a street car is driven.

 

Like I said, I have it installed where my BONE STOCK ENGINE will not run WITHOUT PINGING on premium fuel.

 

The cost of installing the methanol system allows me to run REGULAR fuel (not premium) for daily driving whitout having to retard my spark (and the terrible drop in power of the already gutless L26 in lower rpm driving) and n fact allws me to run 15 degrees initial like I could in the old days when premium fuel was premium fuel.

 

There is no reason to compromise on torque when the methanol/water mix will allow you to run REGULAR pump gas, as well as get increased drivability.

 

I mean, if you want a gutless wonder go for it. But for the cost the economics are hard to argue. In the case of a stock L26 (or in most cases the stock HIGHER compression L24 with an E31 head!!!) it allows you to run the car on regular fuel without retarding timing (performance loss) for a modest investment.

 

On a turbo car, the torque increase down low is massive over simple race gas in the same application. The prejudice against it as an 'improper detonation aid' should vanish as the realization is made the current systems do more than simply blow liquid into the intake in hopes of suppressing detonation through simple chamber cooling. This is a supplemental fueling system which affects both power delivery curve as well as allow usage of more economical fuel for a daily driver. Driving hard I might go through half a gallon a tank. The turbo car uses a lot more. That system shown would likely work fine and is about 2/3 the cost of what the Snow system costs, so that would only improve the economic performance I experienced in terms of payback also realize my economic model was about 2X (conservative) on what real-world consumption really is... During the winter I can scale back on injection as it's not needed as much.

 

Try that with a compression change.

 

IMO dropping compression is a radically complex answer for what likely will be a part-time situation. Getting 100-octane equivalency is not hard with one of these systems and the performance you will get returned will be surprising. If you are making WOT runs and worrying about running out of methanol and giving that warning, then you also have to warn people about clogging a jet on their Mikuinis or Webers. I've seen FAR more detonation to death scenarios from a clogged jet than someone disregarding the low tank level light and then going on a bone-headed run. You will hear it baby ping far before you will do damage. I can't drive my L26 up a grade at more than 1/2 throttle without hearing it ping (even with the header and opened up exhaust now...) If you keep in it after audible pinging, you deserve what you get. I believe that warning is as much a Red Herring as a clogged jet warning for a Mikuini or Weber owner!

Edited July 25, 2010 by Tony D