3 cyl. vs 4 cyl.

[NewToy]

Well ok sort of kinda. The piston in the bike doesn't melt because of the speed of the piston. It melts because of the heat produced. If you design an engine that can't handle the heat it produces it will burn up. You can find some diesel truck owners that found this out the hard way. They installed a 150+hp chip and ran up a long grade with a heavy trailer for 45 minutes. The engine isn't designed to handle the extra fuel producing the 1000 degree exhaust temps, something has to give, piston, turbo, tranny, etc.

I don't think the wear and piston travel have a lot to do with each other either. Dirty oil, air, bad machining, poor assembly and other impurities cause the wear, mainly the dirty things.

Piston speed is a major factor limiting engine RPM's. Faster the piston, more friction (heat) generated. This is not related to the load or amount of fuel being consumed. Piston speed and valve float are the two major considerations in determining an engines redline.

John

 

[RobJ]

Piston speed is a major factor limiting engine RPM's. Faster the piston, more friction (heat) generated. This is not related to the load or amount of fuel being consumed. Piston speed and valve float are the two major considerations in determining an engines redline.

John

i see you have never worked with diesels on a dyno like i have. an engine running 2500rpm with no load puts out about 200-300 degree+/- exhaust temps, start adding the water brake and the temps rise to 900 in seconds. paint melts off the new muffler, etc. the whole engine is under more load, piston, rods, rings, crank.

valve float is not a major consideration in a diesel that tops out at 3000rpm. if it is then can tweak the springs. people worry more about that in a 10,000 rpm race car.

piston speed also has more to do with physics, more weight, more inertia (that thing). you don't see a 5000# ship engine rod turning more that a 100 rpm or so. but no problem with a 50cc 2 stroke or a chain saw engine turning 15000rpm.

 

[john_bud]

I go with the other guy, rods are sized based on the loads they are expected to carry. Not to be big just to be big.

No, not for the fun of making a bench racing bragging point. Ya got me there! But when you only have one per hole, you have more engineering degrees of freedom. You can make it bigger and forget about it. When you have 2 per hole, there is less room to make it thicker, so you have to make it stronger in more intelligent ways. (read --> more costly and time consuming)

People have run cummins engines up to 3x the factory hp with factory rods. That's just a fact, it may show that some people have too much time on there hands, but still a fact. I don't think that the good engineers at Cummins sat around and put the engineering design limits at 1000 hp. I do think that they made it big because they could make it big --> then it wasn't a worry in any way, shape or form. One less hot potato to juggle in the engineering equation. The trade off of slightly greater stress on the crank is minor as the max rpm's are limited to about 4500 by the nature of diesel flame front propagation. The electronics limit the engine to what, 3200? 3400? It would be a different story at 6-8000 rpm !

As far as high engine speed use, if that is what you want then perhaps a Viper is more up your alley as a truck engine the Cummins has its place making gobs of torque at low RPM and returning excellent fuel economy for the job done compared to other options,

I've driven a Viper quite a few times, they are a brutal fun ride. But high RPM? no. I don't think of rpm's as high unless the redline is above 8,000. Bike guys may put that at 12,000 or 14,000! It's all in your perspective. Then again, my Ford tractor has a redline of 2400...

And you are soooo right about fuel economy. My buddies dodge gets 2-4 mpg more than my duramax. Not a lot, but he has a long bed dually 1 ton and I have a short bed single 3/4. It's 8-12 more than his 4 door long bed dually 1 ton duramax. But the duramax's are automatics and the dodge is a stick.


jb

 

[NewToy]

i see you have never worked with diesels on a dyno like i have. an engine running 2500rpm with no load puts out about 200-300 degree+/- exhaust temps, start adding the water brake and the temps rise to 900 in seconds. paint melts off the new muffler, etc. the whole engine is under more load, piston, rods, rings, crank.

valve float is not a major consideration in a diesel that tops out at 3000rpm. if it is then can tweak the springs. people worry more about that in a 10,000 rpm race car.

piston speed also has more to do with physics, more weight, more inertia (that thing). you don't see a 5000# ship engine rod turning more that a 100 rpm or so. but no problem with a 50cc 2 stroke or a chain saw engine turning 15000rpm.

We are talking about two totally different things here. Even without significant dyno experience (like you have) I realize load on an engine will increase operating temps.
I was just trying to make the point that piston speed is a consideration in engine design regardless of any load factors.

John