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Message started by ZSteele on 10/16/19 at 15:32:58

Title: Shorter Piston Rod?
Post by ZSteele on 10/16/19 at 15:32:58

After some reading it seems like an endgame set back is the length of our piston rods. Has anyone toyed with the idea of milling the bottom of the cylinder down and running a shorter rod and camchain? The sides wouldn't quite line up but all of the dowel holes/camchain slot/sleeve should be aligned still as they're parallel and straight through.

Title: Re: Shorter Piston Rod?
Post by DragBikeMike on 10/17/19 at 13:13:42

Are you asking about the connecting rod? If so, it would be much better to leave the cylinder height alone and install a "longer" rod. We already have a negative deck height (-.162"). That's about 4mm. So a connecting rod that is 3.5mm longer than stock would reduce the negative deck to about .02" (about 0.5mm). That would increase compression and also provide about .04" quench. Both good things.

The downside is that you would have to find a suitable connecting rod, and then install it. I bet it would be very hard. It requires disassembling the three-piece crank assembly and then reassembling and truing. Very exacting work. The assembly is pressed together, and the right & left wheels are different diameter and also not full-circle. That makes truing even harder. The assembly would also have to be rebalanced. What a project. :P

But hey, I have thought long and hard about trying it. The longer rod would improve the rod vs stroke ratio. Longer rod engines make better torque and horsepower. The longer rod increases the piston dwell at TDC, which results in more time for pressure to build and improved mechanical advantage during the power stroke.

I looked at the DR rod bearing to see if it was the same as the LS. It's not. So even if the DR rod is longer (I have no idea if it is or isn't), it probably won't fit in the LS wheels. Hope rings eternal. A long-rod crank assembly would be sweeeeeet. It would increase compression, create proper quench, increase piston dwell, improve mechanical advantage, and allow use of the standard cam chain and tensioner assembly. It would be a real nice modification.

Title: Re: Shorter Piston Rod?
Post by ZSteele on 10/17/19 at 16:29:51

I'm talking about the rod the piston is attached to, guessing we're talking about the same thing, I just dont know proper terminology.

Title: Re: Shorter Piston Rod?
Post by Armen on 10/18/19 at 06:04:31

I'm with DBM. Might be a good idea.
Used to rebuild the cranks on our Bultaco racer. Not fun. Got ok at it, not great. But that was a 17HP putt-putt. By the time we were done it made over 30 HP, and the cranks like to walk apart with not a ton of hours on them.
I'm guessing the Savage crank would take a lot more press.
On our SR500 racer, we had the rod made and sent the crank out to be rebuilt. Serious press to get that apart.
Had Carillo make a rod for that, and for a BMW Airhead project. As I remember, they were north of $400 each.
Looked into doing similar illness on my buds little Italian racer. Actually found a Japanese 2 stroke rod that fit the bill without having to have a custom one made!
So, if it was me, I'd say start by pulling apart a stock crank, and make some measurements. See if anything out there would/could work. If you find something, send the whole mess off to someone who rebuilds roller cranks and have them do it.
I actually made up charts and graphed the difference in dwell time at TDC with the longer rod. It does make a positive change.
In Smoky Yunich's book on hot rodding Chevy's his simple advice was 'Put the longest darn rod you can fit in there!". I was aiming for 2.2-1 rod to stroke ratio, as that number was suggested by a few folks who know much more than me.
Let us know how it works out!
I had made a 20 ton hydraulic press where I worked. Not there anymore, so I don't have access to it. I'll leave this project to you.'

Title: Re: Shorter Piston Rod?
Post by ZSteele on 10/18/19 at 06:48:40

Pfffffff, I think you guys are grossly overestimating my knowledge of engines and engineering. Should I have gone to engineering school instead of music, probably. Am I good at reading and piecing together the years of work you guys have put together, ask me after I put the engine back together...

I'll stick the long arm/rebuilt crank on the bulletin with the Ti header for later :P

Title: Re: Shorter Piston Rod?
Post by Fast 650 on 10/18/19 at 12:02:21

Just curious, but what advantages are you thinking that you would gain with a shorter rod?

Title: Re: Shorter Piston Rod?
Post by ZSteele on 10/18/19 at 14:04:42

No clue! Thought I read somewhere that the piston rods length was a bottleneck towards the end of the tinkering stages. I doubt anyone would need to go that far but Lancer or someone with that many mods.

Title: Re: Shorter Piston Rod?
Post by Fast 650 on 10/18/19 at 19:12:07

Okay, I was just wondering since almost everything is a negative with a short rod. Short rods are generally cost cutting measures from manufacturers, like the small block Chevy. Deck height of a 350 and 400 are the same, but the 400 has a longer stroke so they opted for a shorter rod in the 400. Cheaper than making a new block specific to the 400, but at the expense of increased piston side loading from the more severe rod angle.