Posted By: MoparMarz
Rod Ratio - 05/03/13 05:55 PM
What are the premium bore to stroke ratio and the rod to stroke ratio when building a stroker 340, stroke is 4.125 with a 6.2 rod and a 4.125 bore.
Rod Ratio
Posted By: Cab_Burge
Re: Rod Ratio - 05/03/13 07:08 PM
Quote:Sounds like you already have the crank and rods so the option of optimizing the stroke/rod length ratio is not possible, unless you want to buy another set of rods
What are the premium bore to stroke ratio and the rod to stroke ratio when building a stroker 340, stroke is 4.125 with a 6.2 rod and a 4.125 bore.
I've heard from 1.60 to 1.80 ratio is best with 1.75 as the best, I understand the NHRA Pro Stock motors are near 1.82 to 1 ratio or higher 
Your current parts = 4.125 divided into 6.20 = 1.503 ratio I built a 4.00 inch stroke 340 block yeas ago for pump gas street use with NOS at the track, it is still runiing today, lots of street miles and a lot of hard passes on it. it has a set of stock length and spec. Eagle H beam rods in it 4.0 divided into 6.123 = 1.530 ratio 
Run it if you want, change it if you want The longer rod will help make the piston lighter also
Posted By: Dodgenu
Re: Rod Ratio - 05/03/13 10:18 PM
NHRA prostock runs a 1.7 rod ratio. Any more ratio and the piston speed is too slow. It is hard to achieve the 1.7 ratio unless you want to build a 360.
Posted By: dogdays
Re: Rod Ratio - 05/03/13 10:33 PM
I was a big proponent of the long rod theory made popular by the late Smokey Yunick. You can probably still find some of my posts on that view. But two funny things happened: Dynos became much more available and the EngineMasters competition started.
Dyno results for street engines seem to indicate that the long rod theory doesn't work that well. Mountain motor maven John Kaase said in an article that most of his engines are running in the 1.38:1 R/S ratio. He told of building an engine with a more "favorable" rod ratio and it didn't make any more power. He has also won EngineMasters a couple of times and I believe the last one had a R/S ratio of about 1.50.
Later, Hughes Engines built a motor with a 4.185" stroke and a rod about stock length which would put the R/S at under 1.50 and it made big power.
One of the keys to short rod engines is the ports need to flow big numbers. The piston moves away from TDC faster and can get a larger air column moving, is my way of explaining that.
So, run what you've got.
As for Bore/Stroke ratio, there's no magic in it. Basically you make the bore as big as the block can stand because you can fit bigger valves into it and the heads flow better. Then you pick a stroke that gives you the cubic inches you need or are limited to.
Sounds to me that your engine is going to be a monster.
R.
Dyno results for street engines seem to indicate that the long rod theory doesn't work that well. Mountain motor maven John Kaase said in an article that most of his engines are running in the 1.38:1 R/S ratio. He told of building an engine with a more "favorable" rod ratio and it didn't make any more power. He has also won EngineMasters a couple of times and I believe the last one had a R/S ratio of about 1.50.
Later, Hughes Engines built a motor with a 4.185" stroke and a rod about stock length which would put the R/S at under 1.50 and it made big power.
One of the keys to short rod engines is the ports need to flow big numbers. The piston moves away from TDC faster and can get a larger air column moving, is my way of explaining that.
So, run what you've got.
As for Bore/Stroke ratio, there's no magic in it. Basically you make the bore as big as the block can stand because you can fit bigger valves into it and the heads flow better. Then you pick a stroke that gives you the cubic inches you need or are limited to.
Sounds to me that your engine is going to be a monster.
R.
Posted By: MoparMarz
Re: Rod Ratio - 05/04/13 04:46 PM
thanks for the info guys actually just kicking around some ideas and really never knew to much about the ratio game. I'm kind of going towards that set up just really not sure yet. looking to spank a few chevy's down here in South Carolina. 
Posted By: Keith Black®
Re: Rod Ratio - 05/04/13 11:34 PM
The largest benefit of long rods is lighter pistons, provided the rods & crank weights are also in line 