Help selecting a rod stroke ratio for a turbo build

[turbobluestreak]

Ok I'm looking into a future build up for an turbo L6. First off I'm thinking about just staying with the L6 heads and forget about the 24 valve head but enough of that on with my subject. I understand that you can go long or short with the rod ratio and the effects and reasons to pick each of these but all my literature seemed to geared toward Naturally Aspirated motors with multiple head valves and didn’t explain too much if any on how it would effect a turbo motor.. I understand that a rod ratio is only one of many variables in building up great engine.

 

Yes I've searched and found in the turbo section a long rod turbo L6 theory I fell that this subject should be discussed once again but how about some of you higher HP L6 guys show up and post (you know who you are).

 

Ok guys, which would be better for a build, long or Short rod ratio?

I'll get the ball rolling, I am now thinking of a long rod ratio something between 1.75:1 to 1.8:1 because of the increased dwell time that the piston stays at on TDC because of our older two valve heads. I don't know I could be wrong though so if I am please enlighten me.

 

tbs

[Mack]

yes, for our old crappy heads, you want a long rod. when smokey ynick and the gang were building long rod motors, they had to do this to "work the head" as they put it. I wouldn't get too hung up on a particular ratio tho. An L24 has an r/s ratio of about 1.8 stock. I would say get some custom trick pistons and use LD28 rods with an L28 crank. almost perfect 1.75.

 

on the other side of the coin, a short rod will also work the head at lower RPM. very tricky subject.

 

Now the question becomes, how big to go on your bore? If you shot for 90mm (cyl walls getting thin for a megaHP turbo) you;d have an almost exact copy of an FJ20. 80mm(L28= 79mm) stroke with a 90MM bore and 140MM rods. we know the FJ20 made good power and was durable. then again, it did have head you could put drain stoppers into the intakes on...

 

I am contemplating this for a high HP na build and already found some stock pistons from another make that has is almost perfect in regards to pin height and pin diameter, as well as bore!

[jeffp]

Well I set my engine up with a 1.75:1 ratio. I could have gone to 1.8:1 but I did not want to push the wrist pin into the oil ring. I am running an LD28 crank of 83mm stroke. The rods are 5.350" long. The engine torque ratings look a little weird when you look at the dyno graph. I made 413Hp @ 475 foot pounds of torque. Is that good enough?

Generally, the longer rod will increase the torque readings, the shorter rods are for higher prm applications.

So for me it was a longer rod, that enabled me to decrease the side loading on the piston and cylinder walls.

Now the bore, well lets just say to bore 88mm and 89mm, the differences between the two are nominal gains if you take the time to do the calculations, and for me I opted for the stronger cylinder walls. The increases for the 89mm bore were not worth any of the down sides of going bigger.

[JMortensen]

That's the first time I've ever heard low r/s ratio goes with high rpms. In fact I've always heard the opposite, basically that ~1.8 is what we should be shooting for in an L series engine. That's why your usual high rpm forged piston NA L28 uses L24 rods.

 

Also Mack, have you checked the weight of the LD28 rod? I don't know about the LD in particular, but most diesels have HEAVY rods and pistons. Really heavy. Like putting a heavy flywheel on heavy.

[Mack]

funny you should mention that, Jmortensen! I did weigh an LD28 rod a while back, and I dont recall the exact numbers, but it only wieghed 80 grams more than a stock L28 rod! I was very surprised, to say the least.... I thought for sure those deisel rods would weigh a ton more... I know 80 grams is a fair chunk, but no where near where I though it would be.

 

 

You could make up the difference with a set of pistons, as 1 fast Z has shown.

[JMortensen]

That's surprisingly light. I would have expected 150 to 200 grams heavier. Still, bummer there isn't really enough room to use the L20B rod.

[thehelix112]

Sorry to be a pest and thread hi-jack, but how long is the LD28 rod?

 

Dave

[JMortensen]

140mm

[thehelix112]

Ta.

[Mack]

I was talking to Tony D a while back and he said he knew of someone building an L28 using an L20B rod... might have been him, not sure.. now THATS a tight ring pack!

 

yeah, 1 fast Z and I weighed the LD28 against an L28 rod on his shadow-gram (?) machine, suppoes to be extremely accurate. Funny thing was, we got a 100g difference, then we looked at the LD28 rod and realized we forgot to take out the rod bearing!

 

JeffP's mentioning of short rods being better for HIgh RPM was also the first I had heard of it.

 

some food for thought tho...... A stock CA18DET has an 83.6mm stroke with a 133mm rod.. Sound familiar? stroker? ANYWAY, the CA18DET's are THE go to motor for high RPM's. you can tach a stock out to 8500 to 9000rpm every day. of course, they have an excellent DOHC head as well.

 

point being, rod/stroke ratio isnt as important as head flow. perhaps one would spend money more wisely on head work than custom rods/pistons?

[thehelix112]

point being, rod/stroke ratio isnt as important as head flow. perhaps one would spend money more wisely on head work than custom rods/pistons?

 

Agreed.

 

Dave

[thehelix112]

DAW,

 

To be fair, this discussion is focussing on rod ratio effects. Not strokers. That was a sidetrack.

 

We all appreciate your input into stroker L6s and what you've said makes perfect sense, its just not what we're talking about.

 

Dave

[Mack]

then comes the problem of how far out you can bore an LD28 block..... has anyone measured the cyl wall thickness?? If one could for sure get +3mm out of it (87.5mm bore) you could build one hell of a 3.0L motor.

 

I would like to do this, and this is the sole reason I got a complete LD28 shortblock, instead of just ripping out the crankshaft in the U pull it yard. damn it all to hell!! Jason510's web page with all the L/Z motor specs is NLA. I had a really good recipe cooked from all the specs on there, but now its all gone! oh well

 

Yes, you probably coyuld build an awesome torque monster using a long rod and an LD28 block, but not many people have access to that particular block. FOr the average person, they would be better off spending the cash on head work or a custom intake manifold.

 

it woudl be fun tho.

[thehelix112]

Now now, I wasn't meaning to be offensive. But I don't like being called deaf when you're talking about

strokes greater than L24

and we're talking about

which would be better for a [turbo'] build' date=' long or Short rod ratio[/quote']

 

Anyway, I seem to recall reading that a longer rod increases the effective compression at higher rpms as compared to a shorter rod. I don't know if this is what you want with a turbocharged engine as effective compression at rpm is not going to be an issue. I'll try to search to find the link.

 

However longer rods do reduce piston side loading which is always a good thing.

 

Dave

[(goldfish)]

I think this was the thread stating the long rod engine had better detonation resistance.

 

http://forums.hybridz.org/showthread.php?t=94959

[thehelix112]

Interesting.. the thread doesn't appear to explain why. Would someone like to clarify that? Is it because the pistons accelleration towards TDC is slower hence the temperature increase of the charge occurs more slowly making it less likely to detonate?

 

Dave

[Eric]

how well would this actualy work?

 

I'm tossing around the idea of building myself a new engine for my Z' date=' and trying something a little different. I have tons of L series parts available, basically everything from every model of engine.

 

So the rod/stroke ratio isn't terribly good in most L series, not since the L24 anyway. Since rod/stroke ratio greatly influences detonation resistance this should be a good area to design a turbo engine around.

 

What I have in mind is a L24 crank, L24 rods, F54 block, turbo pistons, N42 head, setup. This will put me right around 8.0:1 CR, or slightly higher with a surface plane done. (Let's say 8.25:1 for arguments sake) And the rod/stroke will be 1.80. Displacement however is reduced to 2569cc on a standard bore.

 

Let's use my current engine to compare with. Stock 82 turbo engine, slight head surface, around 7.5:1CR, 2753cc displacement, 1.65 rod/stroke.

 

So the question is: What engine is going to have more off boost response? The 7.5:1CR 2753cc or the 8.25:1CR 2569cc? My thoughts are that they would be quite similar in this respect. (all other things being equal)

 

I would expect the 1.80 engine to rev more smoothly, have more resistance to detonation, get better off boost fuel economy, and in the end be able to run more boost safely. Thoughts?

 

Generally I don't expect reducing displacement to be a performance enhancer... just feeling out your collective thougthts on the matter. Who knows, someone here may have allready tried this out![/quote']

 

is the F54 the L26 block? i'm not up on all the diff codes. if this is a good setup, i'd like to do it to my L28.

[TheNeedForZ]

F54 is commonly called the "turbo block" It is the engine block of 280ZX turbo. What makes it different is the extra webs cast between cylinders

(1-2,2-3 and 4-5,5-6) to make the block stronger.

 

Some experts actaully encourage using regular blocks without the webs because the webs make cylinder distortion uneven(out of round).

[Mack]

As I understand it the "webs" were cast in to get better coolant circulation around the cylinders, thus cutting down "hot spots", at least thats MY understanding of it all.

 

If you pop a freeze plug out of an F54, chances are you will see straight through to the other side.

 

The N42 block is the one that has solid metal inbetween the cylinders, making it the stronger of the 2 for over boring and what not.

 

 

Ive been there when 1 fast Z demonstrated this interesting factoid. He had an N42 and an F54 side by side. when you popped freeze plugs out of both sides, and shined a flashlight through, you could see light out the other side of the F54, but not the N42.

[crazy280]

I don't know about which block is stronger when overbored, but with a stock bore the turbo block is stronger.