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 16th, 2009, 12:04 pm

Muad'Dib,
I fill this is a grate example of how a long rod make less side wall pressure.
Yes we can pick the details a part but ...........................it's a good simple explanation.

the other advantage of the long rod is to keep the piston up in the bore, that the longer stroke it trying to pull out the bottom :D

mendelmax wrote:
Muad'Dib wrote:How long did your last stroker run (or still running)? Miles?
It was built for my buddy, I don't know how many miles he has now, probably around 2000-3000. It's pretty new engine.

And I don't want to replace it, this time I'll be building mine ;)

I spent some time doing a simple drawing. I regret I used so small scale, since it's not as accurate as I would like it to be, but it shows my point. To make things clearly visible I used 60mm stroke and two extremely different rods: 70mm and 140 mm. I made drawings for 45, 90 and 135*, if someone is more ambitious I encourage to draw it larger with more angles. ;) It clearly shows why longer rod leaves the bottom part of the cylinder earlier and stays at the top longer. Sorry for poor scan, the parts drawn with pencil are not too well visible, but everything is clear.
Image

This drawing also shows why cylinder walls are wearing less in long-rod stroker- the angle between vertical axle and actual rod position is smaller, thus piston is supported more on the rod, less on cylinder wall. This wall pressure is a wasted energy, which also robs power from our engine. Not much, but little differences added together may make a real power.

To me the biggest drawback of using long rods seems to be the high ring position on the piston. They get close to the top, which in the long run may lead to seizing from overheating. Not a big deal on low duty applications, but for someone who wants to floor it often, use NOS or turbo this gets important. It can be somewhat solved by using ceramic piston coat. We did it on our last stroker and we hope it will help somewhat with excessive heat caused by propane (burns hotter than gasoline).
P.s. if you click on the pic, it will enlarge enough to get the idea ;)
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."

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 16th, 2009, 1:11 pm

I'm still of the belief that, in the applications that we are talking about: 4.0L jeeps installed with a 4.6L stroker... the change in rod length (and rod/stroke ratio) is going to be inconsequential to the target application (still, a jeep).
I don't have a TON of experience wheeling (well, practically none), but everything that I *have* done, and everyone I have talked to, is always looking for a "decently torquey motor" so they "don't have to go WOT on the trail"

Cyl wear issues, ring wear issues, TDC and BDC dwell, etc is almost always talked about in terms of WOT. at part throttle, the effects are DRAMATICALLY less. I personally am concentrating on cyl quench and cam selection to emphasize torque in the meat of my daily driver RPM areas. The 4.2 crank and rods work great for that, (with a slight offset grind on the crank)

User avatar
gradon
Donator
Donator
Posts: 1333
Joined: February 13th, 2008, 5:33 pm
Stroker Displacement: 4.6/280ci
Vehicle Year: 1996
Vehicle Make: Jeep
Vehicle Model: Cherokee
Location: DC

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

Post by gradon » June 16th, 2009, 2:58 pm

If(I know it should be when) I ever build another stroker, it will be with the 6.150" eagles and custom pistons. It's tough to grasp the concept, but as stated the longer rod will be straighter when the crank is at the 90* and 270* mark(for the particular cylinder), putting less of a load on the cylinder walls. It will also have less of the piston skirt sitting below the cylinder bore at BDC and will have more dwell at TDC(better penetrating punch, have you).

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 16th, 2009, 3:16 pm

The hardest time i am having with it all is cost. Going with a 4.0 rod and custom pistons (or kb944's) is $$$$. Those pistons cost a crap load and you still have to have them dished a bit after you buy them. I dont want to even think about adding $500 eagle rods to that :(
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!

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 16th, 2009, 3:22 pm

Why do you want to dish them? 22cc is plenty of space to have the proper compression. Then you can release some pressure with proper cam choice, and by polishing the combustion chamber which you can do yourself easily.

Also, remember that these KB's are forged pistons, so you can't compare their price to cheap cast ones. It's totally different story. Now if you compare the forged pistons for both short rods and long rods, and add the short-rod price, the difference gets smaller. It's still more, but isn't it worth to add these 200 or so bucks if you already spend a ton of money on whole project? If I would do it for myself, I wouldn't want to save these few bucks and then regret.

As a little example- my friend said he can't afford new cam. So we used the standard 93 one which we already had, to save these $300 or so. The result- huge disappointment and saving money for a new cam :doh:

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 16th, 2009, 6:07 pm

gradon wrote:If(I know it should be when) I ever build another stroker, it will be with the 6.150" eagles and custom pistons. It's tough to grasp the concept, but as stated the longer rod will be straighter when the crank is at the 90* and 270* mark(for the particular cylinder), putting less of a load on the cylinder walls. It will also have less of the piston skirt sitting below the cylinder bore at BDC and will have more dwell at TDC(better penetrating punch, have you).

For the same stroke, the TDC and BDC will be at the same points in the cyl, NO MATTER HOW LONG the rod.

User avatar
gradon
Donator
Donator
Posts: 1333
Joined: February 13th, 2008, 5:33 pm
Stroker Displacement: 4.6/280ci
Vehicle Year: 1996
Vehicle Make: Jeep
Vehicle Model: Cherokee
Location: DC

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

Post by gradon » June 16th, 2009, 8:23 pm

So say the kb944s were a floating piston and you used the 6.150" eagles and didn't have to deck the block at all and the piston stuck about .002-3" above the deck. You think that the piston would still be in the same place at BDC compared to say a 5.875" 4.2 rod and 802cp? Don't think so.
Last edited by gradon on June 16th, 2009, 10:09 pm, edited 1 time in total.

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 16th, 2009, 9:42 pm

gradon wrote:So say the kb944s were a floating piston and you used the 6.150" eagles and didn't have to deck the block at all and the piston stuck about .002-3" above the deck that the piston would still be in the same place at BDC with a 5.875" 4.2 rod and 802cp? Don't think so.
whoops, forgot to suggest that "For any Stroke that has the same TDC height measurement, BDC would be the same".

of course it will be different if you have different rods and compression heights that don't match.

The stroke of the motor is 100% determined by the crankshaft. if 2 combinations have the same piston to deck relationship at TDC< they will put the piston face the same distance down in the hole.

User avatar
RAPTORFAN85
Donator
Donator
Posts: 248
Joined: June 12th, 2008, 12:45 pm
Stroker Displacement: 4.6L
Vehicle Year: 2001
Vehicle Make: Jeep
Vehicle Model: TJ
Location: Mass

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

Post by RAPTORFAN85 » June 17th, 2009, 4:39 am

This seems to have turned into a pissing match about who knows the most about engine geometry and rod to stroke ratios etc. etc.
I don't claim to be an expert, but The facts are:

1. Long rod or short rod, the stroke is exactly the same for both.
2. Long rods will give a longer dwell time at both TDC and BDC allowing more charge to enter at BDC and better combustion and exhaust scavenging at TDC. (more power)
3. Long rods will have less side load on the pistons. Less friction, less wear, more power, (but it isn't a whole lot)
4. Long rods will give a better quench height with less block work.
5. Long rods will pull the piston skirt out of the bore less then a short rod.
6. long rods are marginally stronger.

With taking all this into consideration the long rods are the clear better choice witch is why I used them in my build.
"Strrrrroke me, stoke me...."

Billy Squire

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, 5:56 am

Muad'Dib wrote:The hardest time i am having with it all is cost. Going with a 4.0 rod and custom pistons (or kb944's) is $$$$. Those pistons cost a crap load and you still have to have them dished a bit after you buy them. I dont want to even think about adding $500 eagle rods to that :(

It is a tough Choose for sure.

If you want it to last, it going to cost you!

If you go the poor man route. there's always a .....?

But to be fair on the poor man stroker...........Percentage wise, there real have not been that many failers due to cast pistons.........but i happens :huh:

When i do mine it will be KBs or customs pistons(long rod)

RAPTORFAN85 the dwell thing, i don't thing is quite correct, as i thing the dwell is different between long and short rods top to bottom,(one has more on top and one has more on bottom) but would have to read up on that subject(that i have a hard time with any ways)
all the rest i agree completely :D
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 17th, 2009, 1:46 pm

It would make since to me that if your using the same stroke crank a long rod is going to spend more time higher in the bore compared to a shorter rod. As far as dwell time, i cant see one or the other (in my tiny little brain) dwell longer at TDC compared to BDC or vice versa. I would think that they would spend the same amount of time at the top and at the bottom... unless they were not going up and down in a straight bore. I could see a longer rod having more dwell time at both the top and the bottom compared to the shorter rod though... Thats how i see it anyway.. but i would love for there to be some factual evidence one way or the other ;).
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!

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 17th, 2009, 2:36 pm

Has anybody looked at my picture? It really shows the thing, and it is a practical evidence that explains the problem. It's simple as a nail construction, but it's hard to describe verbally ;)

The CR calculator also explains it wonderfully. Just enter few different IVC angles and observe how DCR changes. The changes are marginal for minimal angles, then they tend to increase. Compare the DCR at 52-58*ABDC, where most of our cams have their IVC angle. The differences are visible, though small. This way you can easily calculate, how much higher the long-roded piston will be.

For most of us, long rods are very beneficial. Longer dwell at TDC means better burn, and that is the most important factor to me. It will help both with power and fuel efficiency, which together with better durability are everything that anybody would like in a daily driver.

Just for example, take a look at marine engines- these huge diesels use very, very long rods for fuel efficiency and durability reasons. The situation is similar at steam machines. They also tend to use long rods.
The usage of short rods in automotive industry comes from few factors- they usually weight less (not the case in our rods, but in general), they help with engine packaging (less engine height) and they help at high revs, which is now the way of engine development. They are also cheaper to make, and allow better piston cooling by putting rings lower.

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 17th, 2009, 3:32 pm

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...?
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!

User avatar
gradon
Donator
Donator
Posts: 1333
Joined: February 13th, 2008, 5:33 pm
Stroker Displacement: 4.6/280ci
Vehicle Year: 1996
Vehicle Make: Jeep
Vehicle Model: Cherokee
Location: DC

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

Post by gradon » June 17th, 2009, 4:07 pm

There's more of a length difference between the 4.2 and 4.0 rods(5.875" vs 6.123", a .248" difference) than the 4.0 rods and eagle rods(6.123" vs 6.150, a .027" difference). Now the strength difference b/t the forged h-beam vs cast I beam. . .

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 17th, 2009, 5:53 pm

Your right .. for some reason i was thinking the eagle rods were like another 1/4 inch longer...
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest