Welcome to www.JeepStrokers.com!!
Not yet a registered user? You are required to Register before you can post within this topic.

Short Rod VS Long Rod FAQ

All FAQ's Go here... Most basic questions can be answered if you look here!
User avatar
Flash
I love JeepStrokers.com!!
I love JeepStrokers.com!!
Posts: 693
Joined: February 17th, 2008, 10:45 pm

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by Flash » June 17th, 2009, 9:10 pm

Muad'Dib wrote:Also found this...

http://www.stahlheaders.com/Lit_Rod%20Length.htm



Im still not entirely convinced that the .25" difference between the 4.2 and 4.0 rods are worth that much... Now i do think thats a different story with the eagle rods..

However thats just my take on it all vs cost. Others definetly have a different opinion.

mendelmax .. maybe you should make a drawing using the 4.2 and 4.0 rod specifications...?
I new it was one way or the other :)
Short Rod is slower at BDC range and faster at TDC range.

Long Rod is faster at BDC range and slower at TDC range.
89 XJ with 300,000 on the original eng

"I've also never completed a motor, yet. My mouth (fingers) is also writing checks my ass can't cash."

User avatar
mendelmax
I made it to triple digits!
I made it to triple digits!
Posts: 110
Joined: March 16th, 2009, 4:25 pm
Stroker Displacement: 4.6
Location: Europe, Poland, Katowice

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by mendelmax » June 18th, 2009, 12:52 am

Muad'Dib wrote:mendelmax .. maybe you should make a drawing using the 4.2 and 4.0 rod specifications...?
I'll try, but I will have to make it pretty huge to make the difference easily visible, since the differences in lengths are so small.

As for the "worth/not worth" dilemma all I can say is it depends on your needs and expectations. Most strokers are short-roded and people are very happy with them. I don't think this is a bad sollution, but clearly the long ones are better. Now the thing is whether you want to pay few hundred bucks for "a bit better" or not. :cheers: I would, but I also fully understand people that wouldn't.

lafrad
Movin on up ^
Movin on up ^
Posts: 357
Joined: February 25th, 2009, 10:40 am

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by lafrad » June 18th, 2009, 6:45 am

http://ftlracing.com/tech/engine/rsratio.html

there are some graphs on there.

Some "boil it down" comments from the site:
Engine wear and failure issues:
* Rod angularity affects piston side loading. Reducing the maximum rod angle (a) will reduce side loading.
* Reducing piston acceleration from and toward TCD (point of maximum acceleration) reduces tensile loading of the rod.

Engine ignition and breathing issues:
* A piston that dwells at TDC longer allows the air/fuel charge a longer time to burn. This allows less ignition timing for peak power. Less ignition timing is useful because it reduces detonation allowing (slightly) higher compression ratios.
* A piston that dwells at TDC shorter increases the speed of the exhaust gasses during the overlap period. This increases the scavenging effect at low rpm and the engine makes more torque at low RPM.
* Reducing and delaying peak piston velocity allows the intake valves more time to open more to fill the cylinder. This allows a smaller intake running volume (and plenum) and better high RPM breathing.

Effects of a longer Rod
* Less rod angularity reduces wear.
* Lower piston velocity and acceleration reduces tensile loading of the rods.
* Less ignition timing is required which resist detonation.
* Compression can be increased slightly before detonation is a problem.
* Less intake runner volume is required and high rpm breathing is improved.
* Reduces scavenging at low rpm (weaker low RPM power).
* Longer TDC dwell time. (high RPM efficency).
Effects of a shorter Rod
* Increased rod angularity increases wear.
* Increased piston velocity and acceleration increases tensile loading of the rods.
* Increases scavenging at low rpm (increased low RPM power).
* Reduced TDC dwell time. (Reduced high RPM efficiency).
Now, the comparisons they have graphed on the page are comparing a 3.5" stroke and a 5" and 7" rod.

There is also information on going the "stroker motor" route.... its a "double whammy"

I'm still of the feeling: the 4.2L stroke and rod combo has been proven to be a reliable combo in AMC and Jeeps for a LONG TIME. many with 100's of thousands of miles with very little cyl wear (almost all I have seen have died first via Cam or bearings).
The ONLY DIFFERENCE when going to the 4.6/4.7L cheap strokers is piston diameter. The bigger diameter has more skirt surface area (less wear), and most cheap pistons can be found coated (less wear).

I'm going to stand by my view that the "cheap stroker" will perfectly fulfill the needs of any mild to hardcore Jeep enthusiast, and money is better spent dumping it into head porting, intake work, and exhaust manifolds. The short rod stroker will have a better bias towards low RPM torque (at the expense of high RPM torque... which, with a 4500-5500 rpm redline, is almost never seen). I would only chase after the long rodded stroker for a max effort, non-winter driver that will regularly see high RPM, high torque, and/or blower applications.

(I have a forged setup in another motor. forged pistons and cold winters are definitely annoying!)

User avatar
seanyb505
Donator
Donator
Posts: 447
Joined: February 16th, 2008, 9:34 am
Stroker Displacement: 280ci
Vehicle Year: 1997
Vehicle Make: Jeep
Vehicle Model: Cherokee
Location: West Palm Beach Florida
Contact:

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by seanyb505 » June 18th, 2009, 8:58 am

To people speaking of the Eagle forged 6.150 rod: do they even make those anymore?
Now I can be like all those other awesome people with more than one Jeep in their sig, but now I have to say one of them is sold:(
97 XJ 4.6
90 MJ 4.0 - sold

I want to have as many Jeeps as children. DD, offroader, drag MJ and another one. 4=4

User avatar
mendelmax
I made it to triple digits!
I made it to triple digits!
Posts: 110
Joined: March 16th, 2009, 4:25 pm
Stroker Displacement: 4.6
Location: Europe, Poland, Katowice

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by mendelmax » June 18th, 2009, 11:47 am

lafrad wrote:I'm still of the feeling: the 4.2L stroke and rod combo has been proven to be a reliable combo in AMC and Jeeps for a LONG TIME. many with 100's of thousands of miles with very little cyl wear (almost all I have seen have died first via Cam or bearings).
The ONLY DIFFERENCE when going to the 4.6/4.7L cheap strokers is piston diameter. The bigger diameter has more skirt surface area (less wear), and most cheap pistons can be found coated (less wear).
You have pointed some good thoughts, but I have some doubts about two of them. For example, I believe in our applications the long rods will make more torque. This is because our heads don't flow too well, and therefore letting it to stay at the top longer may help some with the exhaust. I mean it gives more time for chamber scavenging through this relatively small port. Then the pistons starts to travel down, which may suck some exhaust gases through still partially open exhaust valve. With longer rods you create less vacuum at first degrees ATDC, therefore engine will suck less gasses than short-roded one would. But that's only my understanding, I might obviously be wrong.

The other thing is that with 4.2 smaller bore we indeed get less surface to put the load on, but on the other hand the distance from piston rotation axis (pin) and cylinder wall is significantly smaller. This may have some impact on the surface pressure specifications. But here again, I'm not 100% positive whether it will increase or lower the load. And obviously, the average combustion pressure will be much, MUCH higher in the stroker than in 4.2. At least 10% more from the displacement itself, but you have to add higher CR etc. However you'd look at it, average stroker makes almost twice the 4.2 power. This is a very significant difference which we have to consider.

User avatar
1bolt
Global Moderator
Global Moderator
Posts: 545
Joined: January 18th, 2008, 4:06 pm
Location: Culpeper Virginia

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by 1bolt » June 18th, 2009, 9:48 pm

Not to hijack an interesting thread but the 4.0 HO Jeep heads do not breath poorly... For factory heads they actually breath quite well. There are large displacement V8 muscle car engines that had worse flowing heads than the 4.0 does...

Now back (sort of) to the rod length question. IMO quibbling about the rod length ignores the real meat of the matter, that's which rods allows the best build characteristics... Tight quench with a custom piston and performance oriented high compression, while allowing the cam to be chosen based on the performance goals and not on how much dynamic compression you need to throw away to avoid ping.
--
Simon
Looking for a 232 crankshaft see my want ad: http://www.jeepstrokers.com/forum/viewt ... =17&t=1292
http://www.jeepstrokers.com 94 XJ Stroked lifted locked. 89 MJ restored Work truck, 88 YJ going on third build up and second Stroker.

User avatar
Plechtan
Donator
Donator
Posts: 667
Joined: August 28th, 2008, 9:00 am
Stroker Displacement: 5.0L 4x4
Vehicle Year: 1988
Vehicle Make: Jeep
Vehicle Model: Comanche
Location: Woodstock, IL
Contact:

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by Plechtan » June 19th, 2009, 6:11 am

Just a little food for thought, most V8 small block engines have rod lenghts between 5 and 6" I believe tht the standard Chevy rod is 5.7" long and the ford small block is about 5.5" Our rods are more the length of a Big Block motor.

Indy car motors run very long rods maybe 2x the stroke.
Peter Lechtanski
The worlds Fastest Comanche Prroject

jsawduste
Strong Poster
Strong Poster
Posts: 969
Joined: February 28th, 2008, 3:13 am
Stroker Displacement: 4.9
Location: Michigan

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by jsawduste » June 19th, 2009, 11:08 am

Mr. Mendelmax,
first off I owe you an apology. While I must admit I had difficulty understanding the drawings you provided. I was convinced your analysis was based on hearsay.

After investigating the phenoma more closely and extending the question to others. Found out that I was in fact, wrong. A piston DOES have additional dwell time at BDC then at TDC. While the difference is small it is a fact.

Further reading can be found here.

http://speedtalk.com/forum/viewtopic.ph ... 814#180814

Yes I am the OPer on that forum thread.

User avatar
Flash
I love JeepStrokers.com!!
I love JeepStrokers.com!!
Posts: 693
Joined: February 17th, 2008, 10:45 pm

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by Flash » June 19th, 2009, 2:34 pm

I'm just as guilty as the next person but i thought the best statement was this!
Fwiw I think people leave way more hp behind with their tuneup that you will ever see from a 0.3" change in rod length.
hp difference with short and long rod is next to nothing with the way most 4.0 are stroked!

The importance between LONG and SHORT rod it how fare the skirt of the piston stick out the bottom of the block.

I don't even think side loads will be that much different between these two rod.
89 XJ with 300,000 on the original eng

"I've also never completed a motor, yet. My mouth (fingers) is also writing checks my ass can't cash."

User avatar
Muad'Dib
Site Admin / Owner
Site Admin / Owner
Posts: 1485
Joined: January 8th, 2008, 10:55 am
Stroker Displacement: 4.7L
Vehicle Year: 1990
Vehicle Make: Jeep
Vehicle Model: Cherokee
Location: Bend, Oregon
Contact:

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by Muad'Dib » June 19th, 2009, 8:21 pm

jsawduste wrote:Mr. Mendelmax,
first off I owe you an apology. While I must admit I had difficulty understanding the drawings you provided. I was convinced your analysis was based on hearsay.

After investigating the phenoma more closely and extending the question to others. Found out that I was in fact, wrong. A piston DOES have additional dwell time at BDC then at TDC. While the difference is small it is a fact.

Further reading can be found here.

http://speedtalk.com/forum/viewtopic.php?t=16910

Yes I am the OPer on that forum thread.
Good reading in that thread. Thanks!

More than anything i think it supports my theory that .25" difference in rod length is really not that significant to warrant the increased build cost.
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!

jsawduste
Strong Poster
Strong Poster
Posts: 969
Joined: February 28th, 2008, 3:13 am
Stroker Displacement: 4.9
Location: Michigan

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by jsawduste » June 19th, 2009, 9:07 pm

Muad'Dib wrote:
jsawduste wrote:Mr. Mendelmax,
first off I owe you an apology. While I must admit I had difficulty understanding the drawings you provided. I was convinced your analysis was based on hearsay.

After investigating the phenoma more closely and extending the question to others. Found out that I was in fact, wrong. A piston DOES have additional dwell time at BDC then at TDC. While the difference is small it is a fact.

Further reading can be found here.

http://speedtalk.com/forum/viewtopic.php?t=16910

Yes I am the OPer on that forum thread.
Good reading in that thread. Thanks!

More than anything i think it supports my theory that .25" difference in rod length is really not that significant to warrant the increased build cost.

Provided you can achieve a good quench number

User avatar
Muad'Dib
Site Admin / Owner
Site Admin / Owner
Posts: 1485
Joined: January 8th, 2008, 10:55 am
Stroker Displacement: 4.7L
Vehicle Year: 1990
Vehicle Make: Jeep
Vehicle Model: Cherokee
Location: Bend, Oregon
Contact:

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by Muad'Dib » June 19th, 2009, 9:36 pm

Hopefully by now most people will try to achieve a quench equal to that of thier head gasket thickness... Now if they didnt want to deck the block, then maybe going with long rods and kb944's will be the answer. Im pretty sure though that the cost difference between 944's and block decking is pretty significant.
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!

lafrad
Movin on up ^
Movin on up ^
Posts: 357
Joined: February 25th, 2009, 10:40 am

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by lafrad » June 20th, 2009, 12:42 am

Well, Decking a block and new pushrods.


Everything gets made up *somewhere*... unfortunately. The REAL answer was suggested by someone in another thread: get the hypereutectic piston guys to leave the current 4.0L piston with an extra 0.025" on the quench pads, and just machine down. sell "extended quench" pistons, and "stock replacement" pistons. same effort for them, but it would fix a TON of stuff for us. better dish volumes, better quench with no decking, etc etc etc etc

ah well, everyone will figure out their way to do it.

User avatar
SilverXJ
Global Moderator
Global Moderator
Posts: 5793
Joined: February 14th, 2008, 7:14 am
Stroker Displacement: 4.6L
Vehicle Year: 2000
Vehicle Make: Jeep
Vehicle Model: Cherokee
Location: Radford, Va
Contact:

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by SilverXJ » June 20th, 2009, 5:32 am

Muad'Dib wrote:Hopefully by now most people will try to achieve a quench equal to that of thier head gasket thickness... Now if they didnt want to deck the block, then maybe going with long rods and kb944's will be the answer. Im pretty sure though that the cost difference between 944's and block decking is pretty significant.
You still have to deck the block to get a 0 deck clearance with the kb944s. Which begs the question why they didn't put the pin lower on the piston, diving less deck clearance to begin with.

User avatar
1bolt
Global Moderator
Global Moderator
Posts: 545
Joined: January 18th, 2008, 4:06 pm
Location: Culpeper Virginia

Re: Short Rod VS Long Rod (Trying to wrap my head around it)

Post by 1bolt » June 20th, 2009, 7:43 am

Hmmm what is the LEAST decking needed to achieve zero deck clearance (gasket quench) with the "budget" stroker? Compared to custom/KB pistons/rods combo which need only a little shaved off?

Isn't it something very large? I'm not going on facts here, I'm going on what I seem to recall, but I recall that decking for "Gasket Quench" herein refered to by me as GQ with a short rod and off the shelf piston (because of the pin heights available) would end up with no deck left.

Again this isn't (or shouldn't be) about which rod gives "more benefits" on paper but which rods allows you to build the most well designed stroker, and in the end the most long term cost effective, and reliable as well as powerful stroker.

Getting your build to GQ and optimizing your compression ratio AFTER quench for the fuel you intend to use and the Cam you intend to use; is the proper way to build any performance engine... Otherwise as seen with all the so called "Budget" stroker recipes you end up having your fuel octane and your cam choice basically dictated to you by your choice of piston/rod/quench
--
Simon
Looking for a 232 crankshaft see my want ad: http://www.jeepstrokers.com/forum/viewt ... =17&t=1292
http://www.jeepstrokers.com 94 XJ Stroked lifted locked. 89 MJ restored Work truck, 88 YJ going on third build up and second Stroker.

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest