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Originally posted by RWD fords rule
Ok I read those posts, I have looked into the effects of rod ratio's quite extensively, here is what I found
Short rods = great with huge ports and a high cc bottom end such as 2.4 etc, but it is still a sticking plaster to cover up inefficient huge ports imho, also suits cams with huge lift at tdc which perform crap at low rpm
Long rods and well sized ports + a cam with a wider LSA is a winning combination, with longer rods the piston stays around tdc AND bdc for a little longer each revolution
The longer rods have much reduced piston to bore friction, the slower movement around tdc means the ignition flame has more time to spread out in the chamber burning more of the mixture before the piston moves well down the bore, the more mixture you can cleanly burn close to tdc the more power and torque the engine will make
Longer rods + shorter piston combinations are much lighter meaning the engine has a lot less mass to accelerate, the lighter the total weigh the better in terms of acceleration, this is why rotary engines have such fast acceleration even though they produce very little torque the rotating mass is very low like a motorbike engine
The longer rods put less pressure on the crank, high rpm breathing is improved, ALL 230bhp+ pinto engines I know of use at least 136mm rods, and most of them use 145mm rods, longer rods will make slightly less low end power but the reduced weight is going to make up for this
With the 5.7" rods we get a ratio of 1.88 to 1 which is not overly high, std ratio = 1.65 to 1
So in summary short rods make a little more low end power, suit huge ports and cams with a lot of lift on overlap, high cc bottom ends, this combination will work but it doesn't make this the best combination, there is a much better way to do it
Longer rods make more top end power, lower mass to accelerate, less pressure on crank, reduced piston to bore friction, more time for flame travel, also more time for cylinder filling as the piston moves away from tdc slower and also the piston raises up the bore slower after tdc, making more time for cylinder filling in the later half, suits ports with high velocity and good flow, sized just big enough to get the flow you need, suits cams with a wider LSA and short seat timing, (not an old profile with huge duration and overlap lift around tdc)
That is about it, have a look for high bhp 2.0 pinto's well over 200bhp and you will see ALL of them have much longer rods most of them having 5.7" or 145mm rods, the best of which being in Finland with billet alloy heads making 240bhp+ can't argue with that
I found a suitable set of rods for you: http://www.ebay.co.uk/itm/Eagle-H-Be...ht_3122wt_1110
They are the old eagle 5.7 rods a lot of people use in pinto's now being made again, all that needs be done is to surface grind about 0.20mm off each side of the rod, they are made too wide from the factory and big end bearings have been known to fail due to the gap between the side of the rod and crank closing up when they get hot, the oil not being able to escape the big ends, however when narrowed they will not fail up to 9k
Short rods = great with huge ports and a high cc bottom end such as 2.4 etc, but it is still a sticking plaster to cover up inefficient huge ports imho, also suits cams with huge lift at tdc which perform crap at low rpm
Long rods and well sized ports + a cam with a wider LSA is a winning combination, with longer rods the piston stays around tdc AND bdc for a little longer each revolution
The longer rods have much reduced piston to bore friction, the slower movement around tdc means the ignition flame has more time to spread out in the chamber burning more of the mixture before the piston moves well down the bore, the more mixture you can cleanly burn close to tdc the more power and torque the engine will make
Longer rods + shorter piston combinations are much lighter meaning the engine has a lot less mass to accelerate, the lighter the total weigh the better in terms of acceleration, this is why rotary engines have such fast acceleration even though they produce very little torque the rotating mass is very low like a motorbike engine
The longer rods put less pressure on the crank, high rpm breathing is improved, ALL 230bhp+ pinto engines I know of use at least 136mm rods, and most of them use 145mm rods, longer rods will make slightly less low end power but the reduced weight is going to make up for this
With the 5.7" rods we get a ratio of 1.88 to 1 which is not overly high, std ratio = 1.65 to 1
So in summary short rods make a little more low end power, suit huge ports and cams with a lot of lift on overlap, high cc bottom ends, this combination will work but it doesn't make this the best combination, there is a much better way to do it
Longer rods make more top end power, lower mass to accelerate, less pressure on crank, reduced piston to bore friction, more time for flame travel, also more time for cylinder filling as the piston moves away from tdc slower and also the piston raises up the bore slower after tdc, making more time for cylinder filling in the later half, suits ports with high velocity and good flow, sized just big enough to get the flow you need, suits cams with a wider LSA and short seat timing, (not an old profile with huge duration and overlap lift around tdc)
That is about it, have a look for high bhp 2.0 pinto's well over 200bhp and you will see ALL of them have much longer rods most of them having 5.7" or 145mm rods, the best of which being in Finland with billet alloy heads making 240bhp+ can't argue with that
I found a suitable set of rods for you: http://www.ebay.co.uk/itm/Eagle-H-Be...ht_3122wt_1110
They are the old eagle 5.7 rods a lot of people use in pinto's now being made again, all that needs be done is to surface grind about 0.20mm off each side of the rod, they are made too wide from the factory and big end bearings have been known to fail due to the gap between the side of the rod and crank closing up when they get hot, the oil not being able to escape the big ends, however when narrowed they will not fail up to 9k