short stroke L28 mods?

[Heavy Z]

Hello, I've got a '73 240z - triple weber 40's, E31 head, race cam, etc., but am looking for a different block.

 

I was considering an '81-'83 L28 when the idea came to me about possibly building a short-stroke L28.

 

My first question is, can it be done(240 rods?,240 crank?)? If so, is there any info on how these engines run? If 240 crank/rods(?) were used, I would assume this would be a big bore/short stroke quick-revving 260.

 

Any thoughts? Also, any ideas regarding a standard-stroke '81-'83 L28 block with my head/carb setup would be appreciated.

 

Thanks,

 

Brett

[johnc]

86mm bore with 74mm stroke would come out to 2.6L. My guess is that you'll have to use custom pistons. As an engineering exercise it would be interesting but I'm not sure how it would benefit street driving or track driving.

 

I think Bryan Little did this with his motor. I don't remember the link so you'll have to search the web for the Datsun Garage. Or maybe someone with a better memory will post the link.

[Dan Baldwin]

This comes up all the time. It assumes that all that added crank throw from a longer-throw crank is gonna reduce the engine's acceleration. It is bullshit. For one thing,

the added polar moment of the crank is minimal, and for another, the real load is not

the crank, but the mass of the car. The increased leverage at the crank will only

INCREASE engine acceleration under load. The only quick-revving contest a short-stroke engine will win is the one where the engines are turned over by hand!

 

Stick with the L28 crank. Or get an LD28 crank!

[240Z Turbo]

It is actually not a bad idea and I looked into it before doing my stroker motor. It is not that it will rev any faster, but it gives you the potential to rev higher. I had several conversations with Don Potter regarding this issue. It is kind of funny, the RB26DETT skyline motor has the exact stroke of the 240crank(73.7mm) and the bore of the 280z(86mm) to give a 2.6L motor. It would be intersting to see the results. I would think an 8K motor would be easily doable. Turbo of course!

[Dan Baldwin]

It is true that a longer stroke will reduce redline rpm. However, the loads in an engine vary with the square of rpm, and only linearly with stroke, so the increased rpm of the short-stroke motor is (pretty much) never enough to compensate for the additional torque of the longer-stroke motor. A 73.7mm-stroke L28 will have the same piston acceleration at 7530 rpm as an 83mm-stroke L28 at 7000, giving the short-stroke a ~7.6% higher redline (these numbers also account for the better rod/stroke ratio of the short-stroke motor). But the stroker has 12.6% greater torque from idle to 7000, and has about 4.6% greater peak power potential.

 

If you're in a displacement-limited class, it does make sense to go with the shorter-stroke/larger bore combo. If not, big bore AND stroke is best:)

 

And of course the stroker WILL rev up more quickly under load (say, when accelerating out of the tightest corner on your favorite road course, or from a stop through 1/4 mile).

[Dan Baldwin]

The previous post compared an L24 stroke w/ an LD28. L28 crank at same piston acceleration would be redlined at ~7215rpm. 7.2% greater torque and ~2.7% greater peak power potential than the L24 crank motor. Whoops, with 130.35mm rods that'd be 7200 rpm, and only ~2.5% greater peak power potential. Anyway, I know which crank I'd rather have in my engine. Displacement being unlimited, of course.

[DAW]

So why did Nissan change the configuration of their 3 Liter V6 from 87mm bore x 83mm stroke, to 93mm bore x 73.7mm stroke?...Hey, wait a minute, those stroke specs look familiar..83mm = LD28, 73.7mm = L24.

DAW

[Dan Baldwin]

For a number of reasons, I'm sure, none of which has anything to do with what will work best for more power in an L-series engine. If max power and torque are what you're after, an L28 or LD28 are the way to go. If you want more revs with less power, the L24 crank is certainly the correct choice.

 

I already said that for a FIXED displacement, most engines would benefit from having shorter stroke and larger bore for greater revs, oversquare designs (to a point) are inherently racier. But if anyone thinks a bored/destroked 2750cc L28 can compete with an equally prepared bored/stroked 3098cc L28 because of the increased rev potential, well I think they're wrong.

[DAW]

Of course a significant difference in displacement makes more power, but that's apples & oranges like "my 454 chevy will beat your 350 chevy". The question is will your 302 chevy beat my 305 chevy? I think it will. As to the comparison between the configuration of the old Nissan 3L V6 to the new, it doesn't matter if it's a V6 or an L6, I think it may allow for more tuning and power using a multivalve head and variable cam timing re the characteristics of a long-rod engine vs short (new r/s = 2.00, old = 1.86).

The important factor is the intended use of the engine, road race vs drag race vs autocross vs street, the budget of the builder ($/hp), and the fun factor for the effort/ingenuity/uniqueness in building the engine. Different strokes for different folks.

DAW

[Dan Baldwin]

Dude, read the original post and you'll see why I'm comparing an L28 w/ an L24 crank vs. L28 crank vs. LD28 crank. The question was NOT comparing engines of fixed displacement. Also, the primary benefit of oversquare designs is that you can rev faster, nothing to do with "tunability". r/s is a factor, but bore vs. stroke is more important for redline rpm. Speaking of r/s, it's been a while, but I remember reading about a SBC buildup in which the builder

discovered he could make more torque and power with 5.7" rods vs. 6"ers, so maximized r/s ain't necessarily the best thing. Think of the angle of the rod with the crank throw at max

cylinder pressure (maybe 18-20 deg ATDC?). If the rod is too long, it's not as nearly at a right angle w/ the crank throw, so less leverage.

 

If the intended use is road race, drag race, autoX, or street, the longer-stroke cranks are a much better choice. If cost is an issue, an L28-cranked L28 has gotta be cheaper and easier

to build than a custom L24-cranked L28.

 

To me, building an engine that revs to no purpose (less peak power) sounds like a big waste of effort/ingenuity/uniqueness.

 

But, as you said, diff'rent strokes.

 

Dan "whutchoo talkin' 'bout, Willis?" Baldwin