4

I know someone with an older Yamaha XS750. It's from the late 1970's. When I rev the engine in neutral it rev's up really slow. It comes down from the revving a slowly as well.

When I rev a 2005 GSXR 750 in neutral the rev's skyrocket very quickly and comes back down quickly too.

Why is this?

Why do old motorcycle engines from the past rev so slowly compared to newer motorcycle engines?

  • Not 100% sure but likely something to do with the bore and stroke of both engines. Engines with shorter strokes tend to respond more quickly to changes in throttle input. If you could get the dimensions of the bore and stroke for each engine that would be great – Zaid Dec 27 '15 at 20:54
  • Another (more obvious) reason is the material which the pistons and con rods are made of. Lighter materials = quicker acceleration – Zaid Dec 27 '15 at 20:59
5

Older motorcycle engines have more inertial weight than most newer motorcycle engines. I will say most because there are incredibly old design methodologies that are still in practice by some manufactures. These engines retain all of the characteristics from engines that were being produced from the 1920's to the 1970's. Ural and Harley-Davidson come to mind as stalwarts in technology.

In general there are characteristics that define your experience.

1. Over-square Bore

Many newer MC ICE's have a piston diameter that is bigger than the stroke of the piston. GSXR, Rotax for Aprilia, Ducati, Yamaha R1, all of the performance 4-stroke single cylinder dirtbikes have this in common. A short stroke and very big 4 valve combustion chambers. The short stroke requires less inertia to get the crank to spin back around and complete the combustion process. The oversquare piston bore allows this.

2. Less Inertial Mass

This maps to above and is a key reason newer MC engines are nice and snappy on the throttle response and rev high quickly. You can also hear this type of revving with Formula 1 engines pre-2013. Here is a Renault engine in the Lotus playing Happy Birthday. Many of the same principles apply to modern F1 engines. SHORT STROKE, big piston, big valves, low inertial mass.

Older Engines

The XS that you mentioned has a much longer stroke than say a GSXR, CBR or Ducati of the last 20 years. It's a 3 cylinder with a big flywheel and a counterbalancer system to smooth out the engine vibration due to the long stroke and much bigger crank that is required to accommodate for the longer piston stroke. Take into account the larger offset journals on the crank, bigger connecting rod, larger crank counterbalancer, the counterbalance system and you have a lot of weight. Getting that weight to spin up quickly requires a lot of energy and the motor doesn't produce enough to make it nice and snappy in terms of throttle response. Getting that weight moving is a lot of work.

In a nutshell. Newer MC engines have less inertial mass. That's the simplest form of an answer I can come up with.

Cheers!

  • Makes sense. Is this the same reason why new dirtbikes like the CRF450 rev up so quick compared to an old Yamaha single like an SRX? I'd upvote you but i'm out. I guess you can only vote a few times a day here. Happy New Year! – Ppoggio Dec 27 '15 at 22:20
  • Yes, essentially the same. Short stroke, big piston, huge valves, shim and bucket valve train = higher revving and usually less inertial mass overall. – DucatiKiller Dec 28 '15 at 4:36
  • 2
    +1 but would add not only do they have lighter internals and therefore less mass/inertia to accelerate but typically have more power also. Power to weight... not how you'd normally consider it but it's the same thing. – Sir Swears-a-lot Dec 28 '15 at 7:28
  • 1
    So I was right on the money then :) – Zaid Dec 28 '15 at 18:21
  • Great! TY, nice detail. – Ppoggio Dec 29 '15 at 5:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.