If I try and run a smaller bore and longer stroke (to improve the R/S ratio) does this create bigger problems than the R/S ratio?
I barely know what I'm talking about, so take all of this with a grain of salt.
First let's make sure we are talking about the same thing. The ratio of crank stroke to rod length is usually called the rod ratio if that is what you are also talking about read on.
With any set crank stroke you can have a wide varity of rod lengths. So increasing the stroke and reducing the size of the bore is not the way to go to get a better rod ratio. You want to reduce the stroke and increase the bore and the rod length. A "shorter" stroke will let you have a longer rod in the same block. By reducing the stroke, enlarging the bore and making the rods longer you benifit from a number of things. A larger bore means larger valves, less shrouding of the valves and more air flow and air flow is what is going to make hp and torque. Now there are limitations to the maximum length of the rod for any application. As the rod gets longer it has to move up into the piston further, which moves the pin further up into the ring pack.
By lengthening the rods you have change the geometry of the rod angles in relation to the crank during the rotation of the crank. The longer rod is going to mean that the piston hangs around TDC longer making the combustion more effecient. This will mean more force on the rod/crank and more torque. Also since the combustion is more efficient it reduces detonation, which means you can run a higher compression ratio, which adds yet more hp/torque.
Take a look at
THIS ARTICLE from Hot Rod that I have posted on my site. It explains it a lot better. By going from the stock sbc 350 with a 4.00 bore and 3.48 stroke with 5.7 rods and a rod ratio of 1.64 to a 400 block with a 4.155 bore and a 327 crank with a 3.25 stroke and Ford 6 cylinder 6.350 rods they ended up with a motor of still 350 cu. in.,but now had a rod ratio of about 1.9, up from the 1.64.
What is impressive is this motor runs 11.0 compression ratio yet runs on 87 octane gas and puts out over 400 lb./ft. of torque from 2800 rpm to 5200 rpm with a max. hp of 413 and the torque maxed out at 440 lb/ft. This is a street motor 350 and not an all out race motor.
There are a lot of people who will debate rod ratios and if they will help, so this is just one opinion. They also can help from less friction on the cylinder walls as the rods are not pushing the pistons into the walls as much as a short rod does. At low rpm levels I doubt you would know the difference, but as the rpm goes up it sure can't hurt to work towards a higher rod ratio. Almost all builders seem to agree that if possible run the longest rod you can.
The one exception that I've read to all of this is a turbo motor where it might be better to "not" have the piston dwell at TDC. Also blower motors make power way further down the piston stroke than a non-blown motor will as there is a lot more air/fuel burning and providing power.
Don't forget also that stroking a motor gives you more power, but also increases the displacement. Nothing wrong with that as long as you stay in your class.
Hope this helps and remember this is a controversial subject and you have to do what you think is best. Me I want to build a long rod motor for my truck when finances permit.
c ya, Sum
<small>[ February 04, 2005, 02:37 PM: Message edited by: 1FATGMC ]</small>
