dust6928

anybody had any experience with using these rods in an engine. i have done strokers and know the gain you get with extra cubes but am planning just a 355 buildup and want to know if it is worth the money to go with longer rods. i have heard that with the better rod/stroke ratio you get a few extra ponies because of less friction and more longevity because of less sidewall pressure. anyways, if anyone has experienced or heard of anyone who has, fill me in.

treyZ28

Im not sure what you meant, but let me just clarify that rod length has no bearing on displacement.

with that said, in an NA/Nitrous motor, I would personally go with 6'' rods. It allows for a longer dwell at TDC amongst other things you can read about in smokey yunics "power secrets." Its a very well written, easy to read/understand book.

This is a grey area and everyone has their opinoins. I personnaly suggest a search in LT1 tech and advanced tech.

96z

Im going with 6" rods in my stroker with its biggest benefit being that there is more dwell at TDC...Most people go with a 6" rod with an NA motor and a 5.85" or 5.7" with an N20 or boost motor.

DjArcadian

What is "dwell"?

treyZ28

it is kind of hard to explain without moving my hands or showing you a picture...
wait hold on
i can get a picture for ya

http://auto.howstuffworks.com/engine.htm
look at the pic on the right

when the piston reaches TDC, it "dwells there" for a bit and does not instantiously come down again (dispite what you see). The connecting rod wont actually be at 90* when the piston reaches TDC

no imagine
with a longer connecting rod and a higher pin. The piston will dwell there for a bit, and longer with a longer connecting rod due to the increased angle.

to make numbers easy, the connecting rod makes a 10* angle "to the left" when it reaches the top. the time for it get to 10* "on the right" is the dwell.

It is REALLY hard for me to explain this by just typing. I hope you can see what i'm getting at.

anyway, it will have increased dwell with a longer connecting rod: stroke ratio.

I prob just confused you more sorry

anyway, while is dwelling, the combustion process is taking place. more dwell will allow a number of good things such as a more even distribution of the combustion on the piston, a better combustion overall and a number of other things i'm probobly forgetting. Hopefully injuneer or mindgame or someone else (so loung as its not rich krause )will come here and tell me to STFU and explain it better.


ps-
with heads and flat pistons like those, i think they modeled that after the LS1

rskrause

Trey: Not dead yet

You're right though, it is kind of hard to explain. But the piston motion is as you describe. The effects of rod length on piston motion are of a fairly small magnitude, though real. The shorter dwell near TDC of the piston with a short rod is where part of the advantage lies for a SC setup. By moving away from TDC faster during the first portion of the combustion process the short rod setup developes less peak cylinder pressure. This reduces the tendency toward detonation and both have the effect of being less likely to break a piston.

There are a number of other effects of rod length, which are even more subtle. But probably the biggest is that long rod combos have a lower piston+rod weight, which is a very good thing for a high revving setup. Typically, NA motors are set to rev high enough for this to be an issue while SC motors are not. With last years setup (SC+N2O) I shifted at 5,800-6,000rpm. As a comparison, Jason Short revs his solid roller NA car to 7,000rpm. So I have 5.7" rods and he has 6".

I know the weight thing is counterintuitive, but take a look at a piston catalog and a rod catalog. A6" rod is little, if any, heavier than a 5.7", but a comparable piston for a 5.7" rod is quite a bit heavier than for a 6". The piston for the short rod setup will have a higher compression height, that's where the extra weight comes from. But this is an advantage for a SC combo, because there is more room for a thick, dished piston crown and for thick (strong) ring lands. The dish allows for the desired lower compression ratio.

Anyway, this gets discussed frequently here, so you may want to do a search to find out more. Bottom line, Matt is right - NA, go long rod, supercharged go shorter. But keep in mind these are subtle differences at out level. Nothing to loose sleep over, but interesting to learn about. Now, if we were talking max effort race motors, it's another story altogether and these differences might be crucial.

Rich Krause

treyZ28

LOL!
That honestly made me laugh out loud!

as soon as i saw
I actually said "****! he saw it!"

ohh well, guess the old fart's still dancing around the bucket, just waitin to kick it ehh

I think I might head over this one of these weekends-
moral of the story?
dont die next week, trey might be coming over

Mindgame

I dunno.... but I'll play the devil's advocate here just to stir the pot a bit.

Maybe we could concede to a few points.

One, the closer you get to a 90* condition (crank centerline in relation to rod centerline) the better the mechanical advantage. This according to Heywood's book on IC engines, not Mindgame's. Therefore, the closer you get to 90* the more efficient the mechanical use of cylinder pressure. At least that's how I read it.

Now, we have a high efficiency combustion chamber. Burn rate is good and fast and generally, peak cylinder pressures occur at 22-25* ATDC.... or should I say, most professional engine builders who really know their stuff, try to arrive at a peak in this range. So, which rod is closer to the best mechanical efficiency point at the point of peak cylinder pressure?

Hmm...

From a cylinder pressure standpoint I'd have to disagree just based on this and what I've seen (torque-wise) from identical but differing r/s ratio motors.
Smokey was an advocate of using the biggest r/s ratio you can get (at least, that's the little snippet we get from his thoughts on the subject) I think there's probably more to it than that and if you listen to alot of the guys who've been around for a while they pretty much say the same thing.... read Larry Wydmer's thought's on it some time. You don't want peak cylinder pressure at TDC, it'd kill the crank so you want it closer to the best lever-efficiency point (90*). At least that's my oversimplified way of looking at it.

Just some thoughts to the contrary of what seems to be the norm.
The truth of the matter.... it probably doesn't even matter that much with the length changes we can feasibly make. Now, that might be all together different if we were talking 5.7 vs 6.25 but unless you have a tall deck block, it aint happening.

On weight, side loading..... agreed and no arguement..... not that that's what we're doing, just passing along other ideas.

-Mindgame