|
|
11-11-2007, 07:56 PM | #1 |
VIP Member
Join Date: Dec 2001
Location: Richmond Hill, Georgia
Posts: 2,003
Likes: 0
Liked 1 Time in 1 Post
|
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.
__________________
Art Leong 2095 SS |
11-11-2007, 11:00 PM | #2 |
Junior Member
Join Date: Apr 2007
Location: Defiance, OH
Posts: 47
Likes: 0
Liked 0 Times in 0 Posts
|
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 |
11-11-2007, 11:03 PM | #3 |
Senior Member
Join Date: Jan 2002
Posts: 653
Likes: 8
Liked 238 Times in 25 Posts
|
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. |
11-12-2007, 12:18 PM | #4 |
Member
Join Date: Dec 2006
Location: Indy
Posts: 422
Likes: 0
Liked 0 Times in 0 Posts
|
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. |
12-06-2007, 01:44 PM | #5 |
Member
Join Date: May 2003
Location: Billings Mt
Posts: 279
Likes: 122
Liked 54 Times in 18 Posts
|
Re: Rod length? Piston speed?
http://www.gofastnews.com/board/tech...ill-going.html You want to see piston speed, check out this article
|
12-07-2007, 02:13 PM | #6 |
Junior Member
Join Date: Apr 2007
Location: Defiance, OH
Posts: 47
Likes: 0
Liked 0 Times in 0 Posts
|
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 |
12-07-2007, 02:24 PM | #7 |
Junior Member
Join Date: Apr 2007
Location: Defiance, OH
Posts: 47
Likes: 0
Liked 0 Times in 0 Posts
|
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 |
04-17-2008, 02:19 PM | #8 |
Guest
Posts: n/a
|
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 |
04-17-2008, 04:21 PM | #9 |
Junior Member
Join Date: Mar 2008
Posts: 9
Likes: 0
Liked 0 Times in 0 Posts
|
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.
|
04-27-2008, 02:04 AM | #10 |
Member
Join Date: Sep 2004
Location: Kalamazoo, MI
Posts: 117
Likes: 0
Liked 0 Times in 0 Posts
|
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.
__________________
~Pete 1970 Nova |
|
|