Denny2424 Posted February 20, 2015 Report Share Posted February 20, 2015 Hi I have a set of ported cylinders that were on a short rod 4 mill motor. The crank is no good. Can I run the cylinders on a stock stroke crank? I know the port timing is different on the long rod crank, but not sure if it is on the short rod crank. Thanks for any info. Quote Link to comment Share on other sites More sharing options...
blowit Posted February 20, 2015 Report Share Posted February 20, 2015 Yes, it will run but I would question why you would want to do that. Basically if the cylinders were ported right, they will have the port floors lowered 2mm so you will not have a nice transition at BDC. Not the end of the world though. The transfer duration will actually be pretty mild for a stock stroke so no worry there. The long rod has only a small effect on port timing. Remember that the longer rod is offset by the change in wrist pin location on the 795 series piston so the BDC and TDC heights are the same. The rod angle changes so that does affect the porting a small amount. Brandon Mull Engineering Quote Link to comment Share on other sites More sharing options...
sheerider11 Posted February 20, 2015 Report Share Posted February 20, 2015 Yes, it will run but I would question why you would want to do that. Basically if the cylinders were ported right, they will have the port floors lowered 2mm so you will not have a nice transition at BDC. Not the end of the world though. The transfer duration will actually be pretty mild for a stock stroke so no worry there. The long rod has only a small effect on port timing. Remember that the longer rod is offset by the change in wrist pin location on the 795 series piston so the BDC and TDC heights are the same. The rod angle changes so that does affect the porting a small amount. Brandon Mull Engineering what is the height difference from a long to a short rod? Quote Link to comment Share on other sites More sharing options...
blowit Posted February 21, 2015 Report Share Posted February 21, 2015 If you are referring to height as in BTC, TDC, there is no practical difference BUT the rod angle changes the game slightly. The higher the rod angle in degrees of rotation, the higher the delta angle. Glanced at the calcs and about .400*. I don't think I would get my panties in bunch about it. Quote Link to comment Share on other sites More sharing options...
Denny2424 Posted February 21, 2015 Author Report Share Posted February 21, 2015 So the cylinder ports are lowered the same for a short rod and long rod 4mm crank? I was just thinking of going with the stock stroke because I have one already. 1 Quote Link to comment Share on other sites More sharing options...
oldskool Posted February 21, 2015 Report Share Posted February 21, 2015 I would save my pennies and get a short rod 4mil vito's crank. They rip Quote Link to comment Share on other sites More sharing options...
blowit Posted February 21, 2015 Report Share Posted February 21, 2015 I think we should clarify a couple things. There is practically no timing differences between a LR and SR configuration! Way back in the day, the LR setup was actually a way to incorporate the Blaster pistons in a Banshee for a bigbore setup. They are nearly 5mm different in wrist pin location. So..... You have a 115mm rod with a 795 series piston which has the wrist pin relocated up 5mm to offset that. So, why the hell would I do that??? The laws of physics says that force applied in a line is directly applied but if we are not in a line, forces apply in separate vectors. What does this have to do with cranks? The rod angle! A perfect engine would NOT have any rod angle, the force applied to the piston from combustion would always push in a nice straight line but that does not happen. The LR helps to reduce this angle thus applying more force to the crank and less to the side of the piston and cylinder wall and also reduces max angle which is a function of rod speed and max rpm potential. So.... I would not base much of this build on selecting the short or long rod setup but I feel the LR setup is a better choice though not a radical difference. Lets just not get too hung up on LR and SR. This has NOTHING to do with the stroke of the engine at all! 1 Quote Link to comment Share on other sites More sharing options...
registered user Posted February 21, 2015 Report Share Posted February 21, 2015 dwell, acceleration and a few other things will be slightly different between the two rods but when comparing 110 vs 115 the differences would be pretty negligable. personally i dont see any reason to use the shorter rod if given the choice. the performance difference probly wouldnt even be noticable but youll have a bit less angle with the longer rod. over time im sure there would be a hair less fatigue on the piston, bearings, cylinders walls etc. hard to say if it would amount to much but it certainly wouldnt hurt Quote Link to comment Share on other sites More sharing options...
registered user Posted February 22, 2015 Report Share Posted February 22, 2015 not specificly comparing 110 to 115 but it shows a general idea what happens when rod lengths change Quote Link to comment Share on other sites More sharing options...
registered user Posted February 22, 2015 Report Share Posted February 22, 2015 Quote Link to comment Share on other sites More sharing options...
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