Need Advice, Destroker L6

[Chewievette]

I know you see questions about engine buildups all the time, infact there are probably a couple right near this one. However I have been searching all night and have not found anyone asking about a setup similar to what I've been tinkering with. The best tool that I have to figure out setups is the Lengine calculator but I've been told its not always accurate so I'm a little skeptical. I think the below components will work together for a big bore, short stroke engine.

 

Block - L28

Con rod - LD28

Cyl Head - E31

Gasket - Felpro

Overbore - .120

Crank - L20A

Pistons - KA24

 

I also figured out that if I use my L24 crank and a P90 head with an HKS 2mm gasket I'll get similar results without having to track down and buy a L20 crank. How much are L20 cranks going for anyways?

 

Obviously I want a freely revving engine but the real question is will rebuilding my engine with these parts provide much difference over a more stock rebuild? The engine in my car right now is a stock L24 but I have an L28 to rebuild for it and I'm looking for options. I've never rebuilt one of these engines before so any help would be awesome. The setup I have listed should give a compression ratio of 9.03:1 which is in the pump gas range, exactly where I wanted it. If I wanted to turbo it later on would that ratio be too high? I guess what this all boils down to is, would this be a streetable engine or should I save my money for other things and do a stock rebuild?

[TheNeedForZ]

I don't think L20A is available here.

 

The spec you listed gives too little room between head and piston anyway(0.25mm) what up with that?

[Chewievette]

Hmm, I dont know why its that low, especially for 9:1 compression. If I put in LZ24 pistons instead of the KA24 ones it shows a drop in compression to 8.27:1. Are the LZ24s the dished turbo pistons?

 

I've got a line on a L20 crank already.

[Chewievette]

I got a couple more questions, In the calc program it has FJ20 rods which are 140mm. I did some research and found that the small end on those is 22mm instead of the desired 21mm. I did some more searching and found the LD28 rods to also be 140mm, a little bit heavier, but with 21mm small end. I was just doing more research and found reference to the LD28 having a 25mm small end! Which is it 21mm or 25mm? and if it is 25mm can I get them rebushed down to 21mm?

 

Maybe I should do a stock rebuild, at least then I know all the parts fit together.

[Dan Baldwin]

What do you mean by "freely revving"? If you mean the ability to increase revs quickly, then a stroker, not a destroker, is the way to go. If you mean maximum rev potential, you will *theoretically* gain ~300-500rpm headroom up top with an L24 crank vs. L28 or LD28 (i.o.w., not nearly enough increased rev potential to make up the displacement lost), but the word is that torsional harmonics limit a well-built L-series to ~8000-8500rpm regardless of stroke.

 

I don't think the destroker is a good idea at all, unless you're building to a displacement limit.

[Chewievette]

My idea is to decrease rotating mass to make a smoother running engine, using the big bore compensates for the power lost by destroking.

[JMortensen]

So you're going to take mass off the crank and add the heavier rods from the LD?

 

I'm in agreement with Dan, this is not a good idea. More displacement = more power, and unless you're willing to sacrifice power for less vibration, I'd build the biggest displacement L series you can afford to build.

[Hugh]

Shorter stroke makes the motor want to rev higher. (speaking of engines in general)

 

Is it out of the question to buy aftermarket (read: custom) rods to do this?

[TheNeedForZ]

Cewievette,

 

You sure you have a L20A crank, not L20B crank? Cuz L20B is a 4 cylinder crank. Don't count on it unless you already have it.

 

TheHelix wrote this, it's better than Lengine http://www.ozdat.com/ozdatonline/enginedesign/

[JMortensen]

Shorter stroke makes the motor want to rev higher. (speaking of engines in general)

 

Is it out of the question to buy aftermarket (read: custom) rods to do this?

The trick is to determine what the LIMITING factor is. If it isn't the stroke, then don't bother destroking. In this case it isn't the stroke, so destroking just costs hp.

[Dan Baldwin]

Shorter stroke makes the motor want to rev higher. (speaking of engines in general)

 

Tee hee, Jmort and I are in total agreement!

 

For one thing, to argue semantics , shorter stroke doesn't make the engine WANT to rev higher. In general, it can make for an engine that will TOLERATE higher revs, but not enough more revs to make up for the displacement lost.

 

Also, in another sense, instead of making for an engine that WANTS to rev higher, shorter stroke makes for an engine that *NEEDS* to rev higher to make the same power.

 

BUT,

While engine torque varies directly with stroke (displacement), theoretical rev potential (assuming the limiting factor is piston acceleration), only increases (roughly) with the inverse of the SQUARE ROOT of stroke. i.e., reduce stroke by 5%, you could GENERALLY expect to gain only ~2.5% more revs up top, but you will LOSE 5% torque. Basically, you can expect to lose power to the tune of ~2.5%.

 

And, again, word is the rpm is limited more by crank torsional harmonics than by peak piston acceleration anyway.

[Dan Baldwin]

My idea is to decrease rotating mass to make a smoother running engine, using the big bore compensates for the power lost by destroking.

 

The additional mass and polar moment of inertia of the L28 or LD28 crank vs. L24 crank is pretty much negligible relative to other loads on the engine. As for "smoother-running", if the L28 or LD28 is balanced to the same tolerance as the L24, they should run just as smoothly, no?

[Mack]

I too have considered going this route. Not quite the way you are going about it and not quite for the same reasons....

 

I was thinking of an L24 bottom end with rods to match and toyota 20R pistons. they have a 38.4mm pin height, a dome on top and the pin is .8665 offest. pretty close to our venerable L series and they come stock in an 88.5mm bore.

 

punched to 89mm, with .5mm overbore pistons with the L24 crank and RODS, you would have 2751cc's for about the cost of a stock rebuild.

 

the key factor here is the 1.8 rod/stroke ratio (ideal is said to be 1.75:1). something most all Lseries 6 cylinder motors lack (save for the L24, in stock form!)

 

dont know what if any performance would be gained, but this engine would rev like an L24, but with the torque of an L28.

 

I seem to recall that someone stated in an earlier thread, the highest HP NA Lseries engine on record was an L24 built by sunbelt, with 357 at the crank.

 

since HP is a function of torque AND rpm, and the L24 can rev higher (rod/stroke ratio combined with shorter stroke) I guess its possible. Odd that it would beat out a 3.2L with almost 50 cu/in on it.

 

just the ravings of a mad man...

[Dan Baldwin]

Read above. Power is a function of torque and rpm, as you say. But while torque is a LINEAR function of stroke, theoretical rpm potential is only a function of the SQUARE ROOT of stroke. Reduce stroke by 5%, and AT BEST gain 2.5% in revs (or possibly gain closer to NOTHING on the L6), but LOSE 5% torque everywhere for SURE! ~2.5% lost power potential.

A losing proposition.

 

Also, there may indeed be benefits to increasing R/S ratio, but not enough to make up for the lost displacement. I'll stick with 3.1 liters and "inferior" R/S and B/S!

 

*IF* (big if) the highest hp na Lseries ever is an L24, it is only because nobody's ever built up an L28 to the same extent.

[JMortensen]

That L24 you're quoting was running 15 or 16:1 compression, and hp it was measured at the flywheel, not the wheels. Seeing as how we've seen that you can get a L31 to run close to 300 whp, I think exceeding the 350 bhp with the L31 wouldn't be such a difficult task. Again, agreement with Dan. I pinched myself, and I am indeed awake...

 

FWIW I don't think that 1.75:1 r/s ratio is necessarily the perfect "ideal" either. I think that really depends on the RPMs you want to run. I know that NASCAR engines run 2:1, I'd like to find out what other types of racing engines run too.

[Hugh]

Tee hee' date=' Jmort and I are in total agreement!

 

For one thing, to argue semantics , shorter stroke doesn't make the engine WANT to rev higher. In general, it can make for an engine that will TOLERATE higher revs, but not enough more revs to make up for the displacement lost.

 

Also, in another sense, instead of making for an engine that WANTS to rev higher, shorter stroke makes for an engine that *NEEDS* to rev higher to make the same power.

 

BUT,

While engine torque varies directly with stroke (displacement), theoretical rev potential (assuming the limiting factor is piston acceleration), only increases (roughly) with the inverse of the SQUARE ROOT of stroke. i.e., reduce stroke by 5%, you could GENERALLY expect to gain only ~2.5% more revs up top, but you will LOSE 5% torque. Basically, you can expect to lose power to the tune of ~2.5%.

 

And, again, word is the rpm is limited more by crank torsional harmonics than by peak piston acceleration anyway.[/quote']

 

 

I get it. Thanks.

[Mack]

sorry, I should have clarified on the "ideal r/s" statement. for a street motor, that is. I would hate to even think what F1 motors are running for a r/s.

 

and yes, in my original post I mentioned the L24 being rated at the crank.

 

I was just thinking this engine would be a good alternative to a stock rebuild, if someone wanted to try a little experiment and not be out that much cash if it sucked. plus, you could always re-hone the bores and build a traditional L31 with the block and rods (albeit slightly modified)!

 

It would have the same TQ as a regular old L28 (big bores make big torque as well as big strokes), but it would rev like an L24. Ive always found the L24 to be more "rev happy" than the L28s, carbed or injected for either motor.

 

 

oh and Im sure some crappy cast toyota pistons would give out long before you would find the rev limits on the L24 cranks!

 

of course, one could use these pistons for rebuilding an L28, get the rods honed out for the bigger pins and bore the block, skip the LD crank, use the stock rods and crank and end up with a 3.0L as well.

[TheNeedForZ]

L24

 

redline : 6500rpm

rod : 133mm

stroke : 73.7mm

 

maximum inertial load at redline : about 2220G

--------------------------------------------

Chewievette's setup

 

rod : 140mm

stroke : 69.7mm

 

if maximum inertial load (the load rod and bolt sees) is to be kept the same, the new redline is moved to 6750rpm. A 250rpm gain, not much.

 

If the LD28 or FJ rods are very strong(I am sure they are), and the pistons/pins are very light, then redline rpm can be extended higher. That only takes care of the con rods so they don't break. To get really high rpm, there's tons of work to be done. Oil alone is a lot of trouble.

 

To rev Chewievette's setup to 9000rpm, the rods must withstand 3900G of inertial load.

[JMortensen]

I thought stock L24 redline was 7000...

[Clifton]

I agree with Dan and jmortensen. Giving up discplacement isn't the way to go. L28's already have a short stroke compared to a 7M (91mm) and there's no way I would shorten it to rev a little more. Even if you can make the same HP with more revs it will not be as streetable as you'll loose torque and the HP will come on higher. Ask any one who has done a stroker L28 which they liked driving more and how it revs.