Long rod vs. short rod

[Palehorse]

I'm not an engine guy so I would like some opinions. What are the advantages/disadvantages of each? I'm looking to have another limited motor built for mostly 3/8 mile tracks and wondering what guys are running these days and why.

[perfconn]

First thing to know is what classifys a long rod versus a short rod.Its all about rod length divided by stroke.A 4 inch stroke with a 6 inch rod is a short rod combination.A 3.50 stroke with a 5.70 rod is a short rod.A 3.28 stroke with a 6 inch rod is a long rod.
A long rod engine will have more topend but suffers off the corners.A short rod engine will have more torque off the corners but will have less topend.
Camshafts profiles become more critical when you stray away from middle of the road rod/stroke ratios.I like to keep the rod/stroke ratio of circle track engines around 1.75-1.80 rod/stroke ratio when possible but with longer strokes it becomes impossible because of block deck heights.

[andy16]

what ive been taught is a longer rod slows the piston movement down from like + or - 2 degrees from tdc. prolongs piston "dwell". supposedly this gives the fuel more time to burn evenly across the piston and at 4,000 rpm might not make much difference but when your turnin 9500 for 500 miles like a cup car it makes a differnce. thats just what i been taught and perfcons dead on w the rod to crank ratio theory. iv heard of a short rod theory that for a lower rpm engine a short rod may be better because the lower "dwell" time may pull a better vacuum on the intake valve and increase port velosity??? only way to be sure is to back to back dyno them on same motor on same day in same engine. for circle track i wouldn go lowwer than a 6.00 rod on any stroke that allowed it. imho

[dbr25]

Lots of different opinions on this. Terry summed it up pretty well. As Terry stated, the long rod seems to be more lazy off the corner, but has better topend, and I would like to add IMO, a flatter torque curve. The engine doesn't seem as "peaky". If you can build the engine to turn higher rpm, I feel like the added gear more than makes up for the lack of acceleration on corner exit, and your engine will pull hard the whole straightaway. However, its all about the engine combo,(can't just change rod and ignore cam and induction, bore/stroke combo etc.) as well as the track(s) your racing on. Makes a bigger difference the longer the straightaway. Here's some lengthy reads on the subject, with opinions on both sides: Don't know if I can post these?: http://speedtalk.com/forum/viewtopic.php?f=1&t=31001
http://speedtalk.com/forum/viewtopic.php?f=1&t=14079&hilit=rod+length

[Palehorse]

I was reading some of the stuff that you posted in the link, not sure if anybody picked up on this before but "raceman14" sounds a lot like a certain someone that posts in the crate forums on here a lot lol.

[dirty white boy]

I was reading some of the stuff that you posted in the link, not sure if anybody picked up on this before but "raceman14" sounds a lot like a certain someone that posts in the crate forums on here a lot lol.

its him...

[Palehorse]

I figured it was, guess I'm outta the loop these days on here

[dynoman14]

Well, I have a different opinion than most, what I can tell you is this. Look at some of the best performing engines in the world and then look at their rod/stroke ratios.

I just dyno'd an Road Racing 604 crate with 5.0" Ford rods ( over the counter piston ) in it and everything else exactly the way GM wants it in the Yellow Book...picked up 34ft# and 21HP over the exact set-up with 5.7" rods.

Wonder Why???

You would be well served to look at results from last years engine masters competition as rod and stroke are open to whatever you can fit in the engine. There are some pretty unique quotes in the winners summaries, about why they chose such short rods...

I have never seen much of a dis-advantage to them as most of my engines never see 9500rpm. In the ones that did I still had way shorter rods than most and made very adequate power with 830-850HP out of my better ARCA SB-2's, oh yea I was running flat tappets cams in them also just in case I needed to lease one to a Flat tappet customer. I used welded Stellite shafts from COMP and with proper break in and oil pressure around 80-100# they would live 1000-1500 laps until they were refreshed. My rod lengths in those engines varied from 5.4" , 5.7" and 5.850". Not too many 6" or above.

Do some research and find out why Charlie Fischer, and a ton of other fast sprint engine builders like 5.850's in their Sprint stuff so much...they run pretty good and make power at HighRPM's.

I am also pretty big on small bore big stroke stuff if your heads are great flowing...

My ideal combination is Small bore, big stroke, short rod and big head flow. I am working on a Ford right now and If I can talk Kaase into putting a small engine on his dyno again I will do it there for a good reference point as he has won a couple EngineMasters.

[Egoracing]

I just dyno'd an Road Racing 604 crate with 5.0" Ford rods ( over the counter piston ) in it and everything else exactly the way GM wants it in the Yellow Book...picked up 34ft# and 21HP over the exact set-up with 5.7" rods.
A 604 "Crate" with anything changed is a fast burn 350. NOT a "crate", as that monicker is the spec GM engine. It could not be sold or raced as a crate in ANY class.