Long rod=broader power band

[NOSNUSE]

Okay my friends and I argue this myth all the time I say that a longer rod builds a broader power band due to the fact that it keeps the piston at BDC longer for more complete cylinder filling and at TDC for better combustion and as a side benefit you get a better rod to stroke ratio, they say that is wrong, so I am hoping you guys can bail me out here, who is right?

 

[kpt4321]

Do you mean longer stroke? A longer rod by itself won't do anything for the motor except put a big dent in the head.

 

[nutshot]

Do you mean longer stroke? A longer rod by itself won't do anything for the motor except put a big dent in the head.

Nope, he meant long rods. SBC fella's have been running 383 kits with the 5.7" 350 rod over the shorter 400 rod with special pistons. I have yet to see dyno sheets with proof of HP gain, but the current myth holds that you get more "Top Dead Center" (TDC, not BDC), which burns the fuel more efficiently.

 

[NOSNUSE]

If it stays at TDC longer won't it stay longer at BDC as well. This would work on any motor as far as I would think.

 

[kpt4321]

In this situation, I hve trouble seeing the difference over a regular setup.

You still have the same stroke, because the crank is the same. Assuming the setup places the top of the piston in the same place as it was stock, your cylinder volume at BDC is the same and your compression ratio is the same.

The pivot point of the rod/piston is a different distance from the center of rotation of the crank, but I am hard-pressed to think about how this will make a difference.

I'm trying to think about effects on the mechanical advantage on the top of the piston. I guess it might have more, due to the longer swingarm once it's past TDC...

 

[psychlow]

To me it just sounds like a way to snap a rod more easily. I'm with Kyle in that there's not a huge point to doing it.

 

[nutshot]

If it stays at TDC longer won't it stay longer at BDC as well. This would work on any motor as far as I would think.

Yeah, it will stay at both extreme's longer, I have a hard time remembering which (TDC or BDC) affects the fuel burn...

 

[sonicnofadz]

I'm a little confused here...if you are using the stock head, and stock block, and stock pistons and crank, a longer rod WILL NOT FIT. Am I wrong to assume this? Am I missing something here? If I am wrong someone please correct me. Maybe if you used a different crank in combination with a longer rod? Or are you guys talking about using different pistons with a longer rod? I Dunno Im confused...

 

[NOSNUSE]

Sonic this is using a longer rod but custom piston with the pin moved up in the piston to accomidate the longer rod to avoid it coming in contact with the head. TDC is the compression stroke and your cylinder volume increases the longer it stays at BDC. As far as the increase in power there may not be a great increase in over all horsepower but a broader power band meaning that there is more available power at any given RPM which makes for a better street driven car.

 

[kpt4321]

The time spent at the extremes really does not effect flame front speed to a noticable extent; the different pressures achieved because of the different times spent at TDC will a slight bit, but not much.

NOSNUSE, the extra time spent at BDC doesn't make the chamber any larger in volume. In fact, I am still debating with myself whether it would make any difference whatsoever in terms of cylinder filling.

Yes, the piston is down near BDC longer. However, since you still have the same time per stroke, that just means that the piston is moving faster in the middle of the stroke. A faster moving piston will generally decrease intake airflow.

On a NA car, this is 100% bad. Once the piston slows down and stops you don't have any more airflow, because you don't have a pressure differential. However, when it's going too fast the air doesn't flow well either.

On a turbo car, you can have intake air even if the piston is stopped because the intake manifold is pressurized. However, the exhaust manifold will generally be pressurized higher than the intake manifold, so....

I think the key here is in the mechanical advantage of the different rod size, but I have yet to decide exact what it's going to do.

 

[NOSNUSE]

the extra time spent at BDC doesn't make the chamber any larger in volume.

I am not saying that it would increase the chamber volume but rather that
if the piston is at BDC longer would this not allow more fuel mixture to enter the cylinder because the shorter rod would be moving up sooner there for pushing the mixture back up toward the chamber quicker then the longer rod, there for not allowing more fuel mixture to enter the cylinder. Like I states above I don't intend to increase the power output alot but rather make the engine more efficient by putting more availible power though out the power band, I am getting the feeling that I may lose this debate, but then again you learn something new everyday.

 

[Tnj4life]

Long rods will make more torque and a broader powerband. I had a longrod Yamaha Banshee ATV, it made a lot more torque and more usable power.

 

[DCJ98GST]

This is a good read on the subject. The consenses is that it helps

http://www.dsmtalk.com/forums/showthread.php?s=&threadid=86371&highlight=rod+ratio

Per Marco at Magnus:

"The Long Rod version which will only fit a 2.4L block, we extend the rod 6 more mm to get a 1.56 rod ratio. 156mm to 100mm crank.

The main feature to the whole long rod scenario was to not only get a better rod ratio but we were also able to get lighter parts. 329g vs 404g respectively.

The lighter parts and better rod ratio would work better in a race application (where this whole setup came from, our race slut) but for a full street application you would be splitting hairs to ask me to be able to tell the difference between the two.

Take this to the other extreme and put an 88mm crank in a 2.4 block and you have our 2.1L motor, which behaves very differently than a 2.0L or a 2.4L there is definetely a difference to be felt in this motor very different from the 2.3/2.4 and 2.0

This motor has a rod ratio of 1.84 and really likes the High RPM's, makes a bit more tq than the 2.0L but nowhere near as much as the 2.3/2.4L, one of the first things we noticed was how light it felt free revving.

There are alot of other things to consider when doing the last style as in you must run far less timing than a 2.0L because of the added time the piston dwells at TDC. "

 

[NOSNUSE]

....so basically stated the long rod (all else being equal) will build a broader power curve than a stock rod, and will create a more efficent motor over all, being able to burn the mixture better ,needing less timing therefore less chance for detonation and less stress on the over all internal componets. Of course I guess this wouldn't be noticeble so much on a street motor as it would on a endurance motor. I really want to thank you guys for all of your input about this subject.

 

[psychlow]

According to the quote above, you'd have to run "far less timing."

 

[NOSNUSE]

Sorry about that I fixed what I stated.