Rod length? Piston speed?
 

A question about rod length, piston speed.
I'm running a 6.160 rod. the stock length is 5.94. The stroke is 3.98.
Will the longer rod.
A= make more lower end torque?
B= make more power in the higher rpm ranges?
C= not make a hill of beans difference?
This is for a 4 cylinder car with an automatic (2600 pounds), I need all the torque I can get.

Re: Rod length? Piston speed?
 

Hi Art,
I think that most people will tell you that the answer is C, but if I were you I would run the shorter rod. For two simple reasons: One, a shorter rod is a lighter rod. Two, a shorter rod is more forgiving of detontation. This is just my 2 cents based on my experiences with large cubic inch engines. I have seen 700 plus cubic inch engines regularly turn 8000 rpm with rod to stroke ratios between 1.37 - 1.41 and when dealing with high compression ratios and or nitrous oxide, you want the piston to accelerate away from top dead as quickly as possible. IHRA pro stock engines run relatively short rods for this very reason. Again, just my 2 cents. Take care!

Chris Cogan
Kaizen Motorsports

Re: Rod length? Piston speed?
 

My gut answer is C, not a hill of beans.

Your changing from a stock rod ratio of 1.49 to 1.55. You generally need to make a major change of say 0.25 or greater to see any difference, but my reasoning in only therotical and not tried on the track or dyno. Any gain on airflow will probally be offset by additional rotating weight for probally a net zero change. I think engine friction should go down with longer rod, but this pretty small compared to increase in rotating mass.

But with a rod/stroke ratio of that low, your going to sucking REAL hard on the port the first 70 degrees of crank angle during the intake stroke.

Re: Rod length? Piston speed?
 

You'll benefit across the board if you optimize your cam profile to take advantage of the ratio, but it's going to help your torque more than anything.

Higher pin location is good, I'll assume you haven't offset the pins.
Shorter skirt, lighter piston....
Better cylinder filling and evacuation, IF the cam is profiled for it.....
Should be able to run less ignition lead, adding to cutting down on the load during the work cycle.

It's all good......the rod weight at the beam will make the least difference.

Re: Rod length? Piston speed?
 

http://www.gofastnews.com/board/tech...ill-going.html You want to see piston speed, check out this article

Re: Rod length? Piston speed?
 

Hey guys,
I got a little bored this morning and decided to be a geek for awhile and investigate this topic further. Keep in mind that ((stroke x rpm) / 6) is the equation for mean "average" piston speed in feet per minute. So rod length does not affect average piston speed. However, piston speed is sinusoidal and when you look at what is happening to instantaneous piston speed at every degree of crank rotation, you will see that piston speed is affected by rod length. Using Mr. Leong's cranktrain parameters at 8000 rpm, a 5.94" rod will have a peak piston speed of 8753 fpm @ 74 degrees leaving and approaching TDC. a 6.16" rod will have a peak piston speed of 8726 fpm also @ 74 degrees. Average piston speed for both rod lengths is the same at 5306 fpm. So if you picture a sinusoidal curve, the piston speed of the shorter rod is greater than the piston speed of the longer rod from 0 to 90 degrees, slower from 91 to 179, equal at 180, slower from 181 to 269, equal at 270, faster again from 271 back to 0. So with a shorter rod, the piston accelerates toward and away from TDC faster. this is a very slight change, but if you are trying to pluck frog hairs as most of us racers are, it should not be overlooked.

Again using Mr. Leong's parameters, consider this. At 8000 rpm with a 5.94 rod, the piston speed is greater than 6000 fpm for 170 degrees of every revolution.(47.2% of the time). At 9000 rpm with a 5.94 rod, the piston speed is greater than 6000 fpm for 192 degrees of every revolution. (53.3% of the time) And at 9000 rpm, the peak piston speed is 9847 fpm for the 5.94" rod and 9816 fpm for the 6.16" rod. So as rpm increases, the percentage of time that the engine sees piston speeds greater than the average also increases. Sorry if I bored the hell out of anybody, but I was curious to understand what's going on. Take care!

Chris Cogan
Kaizen Motorsports

Re: Rod length? Piston speed?
 

Sorry, in my last paragraph I meant to say, "So as rpm increases, the percentage of time that the engine sees piston speeds greater than 6000 fpm also increases."


Chris Cogan
Kaizen Motorsports

Re: Rod length? Piston speed?
 

Do you use a computer program to do your calculations, or do you just use a calculator? It takes me forever to do it on a TI-86, and I can't print out the results.

Re: Rod length? Piston speed?
 

Yes, I use Excel to do the math. Excel is great because it allows you to analyze the data in many different ways.(graphically, statistically, etc) So all I did in this case was create four columns, Crank Angle, Piston Position, Piston Speed and Piston Acceleration. Then I created cells for stroke, rod length, rpm and rod/stroke ratio (this saves a few parenthesis in the equations). Crank Angle goes from 0 to 719 degrees. Then copy in your equations for piston position, piston speed and piston acceleration at crank angle = 0 and then copy those cells down to the 719 cell and there you have it. Hope this helps. Take care.

Chris Cogan
Kaizen Motorsports
www.kaizen-ms.com

Re: Rod length? Piston speed?
 

Just for fun, what happens when you offset the piston pin?

Re: Rod length? Piston speed?
 

As a fellow geek, I had to play with this for a while. I derived the velocity as a function of angle and rotational speed and verified your answer for 6.16 and 8000. (Actually, I got 8763, but I won't quibble.) I then calculated the location of the instant center and, using velocity vectors, got the same answer. I then used ViaCAD for a graphical solution and, again, got the same answer.

I wanted to take the relationship for velocity and differentiate with respect to angle, set equal to zero, and find the maximum velocity angle. This turns into an algebraic nightmare, however, so, at that point, I resorted to the spreadsheet to find it.

Fun to do this sort of thing once in a while. Keeps my septuagenarian brain active.

Re: Rod length? Piston speed?
 

Art, to answer your question, all a long rod will do is broaden the power band. It has nothing to do with average piston speed.

Re: Rod length? Piston speed?
 

Its been a few years now but through some connections we got someone at nasa in houston to run a simulation on a cray supercomputer as to the effects of rod length the computer came up with results that said for any realistic changes in rod length the change in piston speed was marginal at best when the velocity graphs were overlayed to the naked eye you could not see a diference.

Re: Rod length? Piston speed?
 

The longer the rod, doesn't the piston stay at TDC longer, therefore slowing piston speed down? Why do a lot engine builders say that long rods won't work for a short stroke engine? I've been curious about this for awhile. Thanks


Scott Tilley
IHRA
SS/PDA2

Re: Rod length? Piston speed?
 

I think that a lot of the short stroke engines run a low deck block thus negating some some of the benefits of a long rod since you can only run so short of a compression height. But I did freshen an engine not to long ago that had a three inch stroke and 6.2 in rods in a standard block that was at the top of its class, made for some real light pistons.But there is just as many opinions as there are engine builders.

Re: Rod length? Piston speed?
 

You are both correct: a longer rod makes for a slightly lower piston speed.

Let's look at a 454BBC at 6500rpm. With the stock 6.135" rod, the peak piston speed is 7162.2 ft/min. If you go to a 6.385" rod, the piston slows slightly to 7135.4 ft/min. If you cram a 6.535" rod in there, you'll get the peak piston speed down to 7140.5 ft/min. That's less than a 1% difference.

Re: Rod length? Piston speed?
 

For a near 4" stroke either choice seems like a pretty short rod. I think longer is better in that instance unless you don't have the deck height for it.

Sinusoidal? Does that cause the pistons to self destruct?

Re: Rod length? Piston speed?
 

Well, a stock-length (6.135") rod gives a rod to stroke ratio of 1.53:1, which is not generally considered the best. But if you use a 6.385" rod, that brings the R/S ratio to 1.6:1, which many people consider a pretty good compromise. I think if you go any longer than 6.385", the pin is up in the rings with a standard deck block (9.8").

Re: Rod length? Piston speed?
 

Here's some experienced input.

http://www.pontiacpower.com/Long%20Rod%20Fallacy.html

Re: Rod length? Piston speed?
 

I run a four cylinder drag car myself. I have a 3.126 stroke and I changed from a 5.205 rod length to a 5.7 rod length. The 5.7 rod is 105 grams lighter and uses a small rod bearing diameter. The piston is 225 grams lighter and uses thinner rings. The longer rod engine makes more power up high- above 5000 rpm. the short rod motor makes more torque at 2700 rpm to about 3700rpm then the long rod engine makes more. The long rod engine is more particular about the cam timing. Overall I'm happy with the long rod engine. In the car the long rod engine runs a quicker et and more mph. My combination maybe different than yours the car is 2400 lbs. with me in it at race weight. Just talking from experience here.

Re: Rod length? Piston speed?
 

Look at all the used NASCAR stuff for sale on eBay...... Big bore, short stroke, long rod. Piston weight seems to be the priority. They spent a little money on research and development. So you know they've figured something out. Mind you, all of the secrets won't make it to ebay until it's common. So there could be a secret short rod combo... But I doubt it.
I could be comparing apples to oranges. But you can't ignore decades of homework.

Re: Rod length? Piston speed?
 

Well, you sure took the rod length at face value. There are "other" parameters in cup engines that have an affect on the "total" engine package. Just like in F-1. Yes, they are very well designed and tested for the purpose they are built for. If long rods are the answer then why did most of the 358 cu in PST engines (making well over 2.5 HP per cu in N/A) use very , very short rods and BB pins?

Re: Rod length? Piston speed?
 

That's the question I'd like to know the answer to. What is the advantage to a short deck block and short rods in a modified or comp engine? It has to do with the 9,000 to 11,000 rpm range...

Re: Rod length? Piston speed?
 

Intake manifold design

Re: Rod length? Piston speed?
 

I believe the thinking behind high rpm short rod motors is that the piston accelerates away from TDC faster which helps initiate intake flow plus you have the benefit of the lower piston / rod (rotating) weight. It appears to me this works well if you have adequate or more than adequate air flow but it’s my guess is that you need to keep these motors buzzing in a narrow RPM band.

Re: Rod length? Piston speed?
 

Adger, you mentioned intake design....Are you referring to mainly having a sheetmetal intake? Can you elaborate a little more? And at what RPM ranges do you think responds better to the short rod/short deck configuration?

Re: Rod length? Piston speed?
 

When you build an intake for a short deck engine you can stand the ports up.
You can also design the area under the carb better.. like tighten the runners up closer to the carb c/l. Then you can also build a port that enhances the short rod. The benefits go on and on......

Re: Rod length? Piston speed?
 

Let me ask this, If you do not have a short deck block, would there still be any advantage to having a shorter rod than a 5.7 for a stroke in the the 3.00 to 3.25 range running in the 8000-10000 rpm band?

Re: Rod length? Piston speed?
 

Maybe.. Depends on the head port/design.

Re: Rod length? Piston speed?
 

I thought the prevailing thought behind the long rod is to keep the piston at top dead center longer giving the fuel time to burn completely and create maximum cylinder pressure. The burn time for the fuel remains relatively constant even though the RPM increases, that is why you have to advance the timing as well.

Re: Rod length? Piston speed?
 

At TDC every piston has some dwell time where the crank is turning, but the piston is stationary.

In theory we want the piston in motion 100% of the time with no dwell at TDC because 1,2,3 deg of dead dwell time is 1,2,3 deg of lost power.

The longer the stroke, the longer the dwell time, the faster the piston speed and greater the piston velocity. With that understood, we find longer stroke engines have greater cylinder wear at top ring turn around.

Instead of a gentle transition at TDC, the piston rushes to the top, sits motionless for a few degrees, then rushes back down. Depending on ring design, some funky stuff can happen in that few degree time frame where the ring is flexing and eating away at the piston / cyl wall.

This is the same reason the effects of fine dust particles entering the combustion chamber show up sooner on longer stroke engines vs shorter stroke engines. Hope this helps.

Re: Rod length? Piston speed?
 

Just to clarify - "longer stroke" implies an increase in crankshaft stroke, an increase in rod length and a decrease in piston pin height (move the pin closer to the rings).

Dwell time, piston speed and piston velocity is increased with this combination of a true "long stroke"engine. Simply increasing the stroke of the crankshaft alone and leaving everything else the same will not change much of anything, including piston dwell at TDC. Hope this helps.