i am building a 377 stroker and was wondering what the difference would be between a 5.7in rod and a 6.0in?


350 block
377ci
3.750'' stroke

 

The crank would be made to clear either just the 6.0in one or both, the pistons would be different and of course the rods.

The pistons have what is called a compression height, if you change the rod length the compression height needs to be changed. A longer rod requires a shorter compression height.

Bret

 

the diffrence is in the rod/stoke angles. each rod yields a diffrent RSA and will effect the powerband and the way the motor will act.

i.e. the 5.7 rod / by the 3.75" stroke on the crank nets you a 1.52 RSA

the 6.0 rod comes out to a 1.60.

the higher the r/s angle the faster the motor will wind. the only downside is that when you up the angle it deprives cyllinder filling a bit. for n/a applications it really is the way to go but if you're planning a full race nitrous/blower app, you'd want it lower like to get as much nitrous/extra air in the cyllinders as possible.

thats why the 396 ci cars really like blowers/nitrous. it's just one more factor to toss into the mix of where do you want to make you're power and what do you want to do with the combo. ;D happy rodding,

 

Thats rod/stroke ratio, and I wouldn't worry too much about it.

The difference between 6.0 and 5.7 is .3.

Don't agree with the faster acceleration from a longer rod theory. Shorter rod has a faster acceleration, a stronger pull on the intake and will do just fine in a drag race engine. Likes a bigger intake port, big cam, carb etcetera. The 6" is fine though, just buy a set and forget about all the rod/stroke ratio nonsense.

-Mindgame

 

potato / pu-tot-oh ...

diffrent person diffrent opinion.

where did that come from?

it's important. any serious engine builder will take this factor into consideration, as it seriously effects the powerband. read some of Smokey Yunick's stuff, he has a great writeup on it in his power secrets book.

 

I'm with Mindgame.

I like long rods, but I would rather has less mass in there total, so a lower deck height and shorter rods with a very light piston is even better. But when acceleration doesn't matter in the engine, then the longer the rod the better IMHO.

Read this.... http://victorylibrary.com/mopar/rod-tech-c.htm

Rod to Stroke Ratio is a over hyped topic. If you have a Small Block Chevy then go from a 6.0 to 6.25" rod in a NA motor and on a longer stroke Blown motor you might not be able to fit enough piston in there so go with a 5.85" one.

Bret

 

Alright, you made me break out the calculator...

Math, just calculate it and you'll see.

5.7" rod (l), 3.48" stroke (r), 7,000 rpm, @TDC:

Acc max = ((7,000² x 3.48)/2,189) x (1 + (r x .5/l))
Acc max = 101,658 ft/sec²

Now take a long rod, let's say its 6.25":

Acc max = ((7,000² x 3.48)/2,189) x (1 + (r x .5/l))
Acc max = 99,554 ft/sec²

Those formulas are from the Automotive Math Handbook by Forbes Aird.

I've read Smokey and Jenkins and alot of stuff over the past 30 years. I think you might have missed what they were saying.

It's important for an engine that's gonna be pushing the limits for a long period of time.... say an IRL engine. Because it's all about mileage on the parts in those engines... cycle time.
Not as big a deal for a daily driver though. Even a lot of drag racers choose to go with shorter rods because of the acceleration. Different people, different reasons.

-Mindgame

 

Bret,

i've read the victory writeup many times, it's one of my favorites. i agree with what you're saying. i think that the problem here is one of un specified information, and the language used. basically i think we're saying the same thing.

"It's important for an engine that's gonna be pushing the limits for a long period of time.... "

what do you think a daily driver does?

"Even a lot of drag racers choose to go with shorter rods because of the acceleration."

mindgame, please dont generalize here.. bring out one in specific. and by acceleration are you talking pistion speed or general car acceleration? their reasoning for the shorter rod may not be simply because of max pistion speed.

remember what we're looking for in specific is combination, combination, combination.. for any given application you're going to want to determine pistion speed as a part of this. taking the rod ratio into the formula is just one more part of the ordeal. im not saying that it's the allmighty key to unlocking hidden power, but it is important.

back to the topic, the only diffrence is in price of the rods, new pistions, and Rod-stroke Ratio

 

A daily driver doesn't push the limits all the time. It does so at stop lights, pasing situtations, drag strips, autoX etc..... A daily driver lives a mundane life for a engine, one of extremes, which is why it's different from a Winston Cup motor that goes from 6000-9500 for a few hours then it's done.

The price is the only difference really, so a 6.0" is about ideal since it costs the same as a 5.7" rod.

Bret

 

Read the article at Jere Stahl's website too.. good stuff on rod/stroke ratio.

"It's important for an engine that's gonna be pushing the limits for a long period of time.... "

Maybe we should include definitions.....
Long period to me means, an engine going *****-to-the-wall for 'long periods' of time, ie. IRL, NASCAR, LeMans, Trans-Am, etc, etc, etc..
I don't know about you but I don't drive my street car to work at 90-100% of my maximum engine speed.
If you ever indulge yourself to some of the technical write ups on IRL or endurance racing in general you'll see that maximum piston speed, mean piston speed and g-forces inside the engine are really important things for an engine that's gotta make it 500 miles before it quits. That's why those guys are so concerned with the rpm limits, minimum block heights and the like. But don't take my word for it.. there are quite a few articles across the net that discuss the new Ilmor, Infinity and the Aurora race engines... just for starters.

"Even a lot of drag racers choose to go with shorter rods because of the acceleration."

One specific... the use of the short deck big-block Chevrolet engine. If you're into BBC then you know how popular they are with the race crowd. Shorter pushrods (less weight in the valvetrain), shorter rods (less weight), etc.
In acceleration I'm talking piston. I didn't say that the only reason was max piston speed. Please go back and read what I said in my first response.
Most guys are running Powerglides (I raise my hand) in their race cars. One thing you'll find out about running a 2-speed tranny in a race car is that the engine has to be built a little diff then those running the 3-speeds. More rpm drop between the gears, more velocity is needed through the intake port, short rod helps in creating more velocity and car can run stronger and to a higher rpm because it can tolerate more cylinder head. Believe me, I've seen it first hand in my own cars.
Do you wanna debate this some more?

At least we've determined that it's the rod stroke ratio.
But please be more specific... what is the formula? How would you go about determining an "optimum" rod/stroke ratio? I'm all ears.

-Mindgame

 

i would think that if you ran a longer rod like a 6.o instead of a 5.7 your piston would stay at top dead center longer for more power?

 

Yeah or a 6.25" rod, it does stay there longer, no doubt. It should also require less ignition advance too, but I have never seen it.

I like a longer rod going into a engine, rather than a shorter one, the real differences are very had to come up.

A short deck engine like Mindgame says has it's advantages, you might need a long deck BBC of other things.

Bret

 

You can also look at it from the standpoint that if it stays at TDC longer, then it spends less time moving up and down in the cylinder, which gives you less scavanging to fill the cylinder. Supposedly, this starts becoming a significant problem in high-revving motors with rod ratios well above 2:1 (which isn't going to be a problem for most domestic V8 guys).

Rod ratio can be a significant factor; however, as many have pointed out, there's just not much difference between a 5.7" and a 6.0" rod in this application. Besides, it's interesting to note that many Honda car (B16/18, H22) and motorcycle (most of theiir high-perf line-up) engines run rod ratios in the 1.5:1 range and don't have problems with longevity.

 

Well, I'll throw my 2 cents in here. First, the main benefit I see in longer rods is reduced weight in the reciprocating assembly. Yes, Pro Stock, and even the now defunct Pro Stock Truck used VERY short deck blocks. Like mindgame stated, it has to do with shorter and lighter valvetrain (more stable at high rpms), and shorter and lighter reciprocating assembly. But, even at that, most Pro cars and trucks still used the shortest compression height they could get away with, usually less than 1.25". It all has to do with the ring package, valve notch depth, wrist pin diameter. Everything is done to increase stability and reduce weight, both are possible in a short deck block.

Now, one other VERY small benefit of longer rods that wasn't mentioned is they increase the piston velocity at max valve lift. This is when piston speed and thus piston "suction" is needed the most. Max valve lift on most cams occurs at roughly 102-106 degrees ATDC. At 104 degrees ATDC, the velocities are as follows for a 3.48" stroke:

5.700" - 7303 fpm
5.850" - 7321 fpm
6.000" - 7338 fpm
6.125" - 7351 fpm
6.250" - 7364 fpm

Not much difference, I know, but in reality the differences between a 5.70" rod and a 6.00" rod are not even discernable on a graph. Everyone likes to talk about dwell time at TDC. The actual figures show that the differences are MINUTE at best. The distance from TDC at 2 degrees ATDC for a 3.48" stroke is as follows:

5.700" - 0.001383"
6.000" - 0.001367"

That's only 0.000016". Not even measurable!!! Even at 5 degrees ATDC for a 3.48" stroke is as follows:

5.700" - 0.008639"
6.000" - 0.008538"

That's only 0.0001". That's one TEN-THOUSANDTH of an inch. Do you really think that makes a big difference????? Most mikes won't even measure that!!!

Anyone interested in seeing the real values for different rod lengths, including charts, etc. can e-mail me. I built an excel spreadsheet that calculates and graphs the distances, velocities, and accelerations of any rotating assembly, short stroke or long stroke, short rod or long rod. I think you will find all this talk about the advantages and disadvantages of longer or shorter rods is just HYPE. Totally blown out of proportion by the media and by most of us.

I still say it boils down to what works for you. For me, I wanted the lightest reciprocating assembly I could get, so I have 6.125" aluminum rods and custom pistons with 1.135" compression height. But, I'm running 043" rings, and my top ring is very close to the top of the piston (0.180"). Also, I'm naturally aspirated with no nitrous, so I can get away with it, and I'm planning on turning over 8,500 rpm, so weight was a key concern. It all just depends on what your trying to do.

Sorry for the length, but this whole debate over rod length has grown old. Too many myths, not enough facts.

I guess that was more like 25 cents!!!!

 

Awesome post.

Yeah I agree and all it really does come down to is weight.

Bret