Connecting Rod Length?
Discussion
Hey Phil I'd be keen for that info as well...
Correct me if I'm wrong, but if you keep the rod ratio fairly high, say over 2:1, you get the piston dwelling up around the top of the cylinder longer, due to simple geometry, and since most of the power (pressure) is generated early in the Otto cycle, this gives a better mechanical advantage over the crank, than a shorter rod ratio would, during the peak power part of the cycle...
Cheers,
Pete
Correct me if I'm wrong, but if you keep the rod ratio fairly high, say over 2:1, you get the piston dwelling up around the top of the cylinder longer, due to simple geometry, and since most of the power (pressure) is generated early in the Otto cycle, this gives a better mechanical advantage over the crank, than a shorter rod ratio would, during the peak power part of the cycle...
Cheers,
Pete
Did we clear up whether this is just someone wanting their homework done for them or not?
www.pistonheads.com/gassing/topic.asp?t=154003&f=66&h=0
www.pistonheads.com/gassing/topic.asp?t=154003&f=66&h=0
Simple rule make as long as practicaly possible, you are better of runing a smaller compresion height on the piston and a longer rod than the other way around, i have just had some pistons made that need oil support rings fitting just so i can run longer rods, basicaly on a rover v8 engine same capacity just going up .200 longer rod is worth around 15ftlb of torque
v8 racing said:
Simple rule make as long as practicaly possible, you are better of runing a smaller compresion height on the piston and a longer rod than the other way around, i have just had some pistons made that need oil support rings fitting just so i can run longer rods, basicaly on a rover v8 engine same capacity just going up .200 longer rod is worth around 15ftlb of torque
Do you get any clearance problems with the longer rods?
Or is that you just have to fiddle with the piston heights...
What sort of advantage would you get say, in a SBC, when you could use a block (Dart alloy race) that has an extra .5 inch of deck height...
the LSX guys use 6.125inch rods, and i think in taller deck aplications, so are the SBC guys. the problem with the SBC is there are soooooooooo many option that it is hard to point to 1 definative answer!
have a look on ls1tech.com on the SBC section and ask around. also on corvetteforum.com on the engine section (3/4 of the way down). these guys should beable to help more.
thanks Chris.
have a look on ls1tech.com on the SBC section and ask around. also on corvetteforum.com on the engine section (3/4 of the way down). these guys should beable to help more.
thanks Chris.
pittersuk said:
Does anyone know how to work outr or design a connecting rod length?
If you have the following
Bore
Stroke
rpm limit
cylinder displacment
combustion pressure
That sounds more like a question of crank throw to me.
Like the man said I guess longer is better, but you'll have to think about how much headroom you need for the cylinder head and manifolds, plenums and or carburation.
I guess you'll need to know the greatest predicted energy in the combustion chamber, and the torque/power required at the flywheel.
The energy is likely to be pretty much fixed for a normally aspirated engine, but you can choose what you like if the inlet manifold is above atmospheric.
The power output is going to be dependant on the drivetrain, and if it's a car the weight and performance of the vehicle. Given that you don't yet know the size of the engine, and that's going to be a major factor in the weight of the vehicle, I guess you'll be needing some sort of symultaneous equation!!!!!
Adam if you dont me asking is this for your project building a single cylinder 500cc? Have got an expected bhp to acheive, dotn forget once you have your short engine you are going need some pretty exotic valve gear to cope with the rpm you are talking about, back on to rod length you should be aiming for a ratio of 2.5-1 this will allow the piston to dwell at tdc for quite a while building more pressure on the piston and allowing the crank to swing over more before the actual power stroke happens,
In addition to V8's postings it may help you to look around at other engines which have been somewhat spectacular. 500cc cylinders were also found on motorbikes with long stroke combo's. There must be an old bugger on here who had a panther LOL Not me though, I just heard about them.
Boosted.
Not me though, I just heard about them. Boosted.
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