63 replies to this topic

[TrialsRfun]

Have followed this whole four stroke debate with interest but one thing puzzles me, why do 4 stroke engines run at a higher temperature than a 2 stroke when they are supposedly better lubricated and have only 1/2 the number of power strokes.
Also why are they noisier than a 2t.

[JTT]

Many more moving bits, all adding additional friction.

[Pete]

I think the noise is a bit of an illusion. A two stroke is quite noisy, but because of the higher frequency range the sound dissipates very quickly and doesn't carry, making it seem quieter. A four-stroke, however has a low frequency exhuast note which carries much further than a two stroke.
An example, when someone is playing loud music accross the road, you can hear the thumping bass beat an guitar, but not the high stuff like guitars and vocals.
No idea about the heat though!
Pete.

[TrialsRfun]

Should have included this with the topic starter about engine temp. but I forgot.

If the Sherco 4t has not got an oil pump then how does the oil get upstairs to the camshaft, valve stems, rockers etc etc plus all the other wearing bits & bobs ?

[bigfoot]

It only gets oil to the top when you fall off. No probs for me then, should run for ever.

Actually I don't know but would be interested in a sound explaination.

[g4321]

Quote

Actually I don't know but would be interested in a sound explaination.

The movement of the piston in the bore causes a change in pressure in the crankcase and this is used to 'force' the oil up to the top end'.

Not 100% sure but this was how I was lead to believe the original Huskys worked (same system).
Sounds like a plausable explanation to me?

[TrialsRfun]

Just looked on these pages saw Bigfoots avatar &

sorry what were we on about

[downunder]

Hi Guys n others

4t's run hotter 'cause all the running gear upstairs holds heat and slows disipation, so higher mass (weight and density) takes longer to cool. Just the fact that the camchain puts a gap between the outside world and the cylinder decreases disapation. Provision of space for a camchain alone makes it more difficult to get coolant around the cylinder in a small compact engine. Because of this there is a greater need to provide a cooling system that can cope with the increased retention of heat.

But for those of us that rode bikes pre water cooling just think what they had. The 2t's had massive great fins on the cylinder and head while the 4t's like old SL/XL/XR Honda's had next to nothing but ran all day. And believe it or not they had no oil pump, the camchain splashed through the sludge in the bottom end and dragged it up to the top end. Hell, the early Honda's didn't even have bearings for the cam, it just ran in the alloy head. My suspision is that the compession by-pass through rings combined with a breather out of the cam cover carried enough oil vapor to do the job.

In those days you could hound a motor for many hours but the eventual wear in the head on the early Honda's tended to be the biggest cause of worry not the heat.

More than willing to be corrected.............

BJDownunder

[g4321]

Quote

The 2t's had massive great fins on the cylinder and head while the 4t's like old SL/XL/XR Honda's had next to nothing but ran all day. And believe it or not they had no oil pump, the camchain splashed through the sludge in the bottom end and dragged it up to the top end. Hell, the early Honda's didn't even have bearings for the cam, it just ran in the alloy head. My suspision is that the compession by-pass through rings combined with a breather out of the cam cover carried enough oil vapor to do the job.

In those days you could hound a motor for many hours but the eventual wear in the head on the early Honda's tended to be the biggest cause of worry not the heat.

More than willing to be corrected.............

Sorry to correct you but the XL/XR 250/500 and the XL/SL125 based engines (ie 125's through to TLR250) all have oil pumps.

I agree with the wear on the cam wear in the head though although I found regular oil changes keeps this at bay!

Gordon

[AtomAnt]

One other key factor in heat generation is the cylinder compression ratio. !

If its high then a significant amount of heat is generated just in compressing the gases. Try taking a cycle pump and pumping it a few times in free air. then close of the end and pump it. The heat that you feel is mostly the friction of the gases being compressed (With some piston friction also of course)

Also, the extra force also places a greater load on the bearings which in turn, increases the level of friction again generating more heat.

Obviously then, if you minimise friction, you have a cool engine

[TrialsRfun]

2ts also have a high compression ratio as well as crankcase compression which is needed to force the incoming charge through to the cylinder ready to fire again.

[downunder]

G4321

I stand corrected although I can't recall a pump on my 78 XL100 but I think there was possibly an oil gallery that ran up beside one of the cylinder bolts so if it had a pump that was were the oil found its way up to the top end. If you really want plenty of oil up the top just crack a piston from gudgeon to gudgeon at high rev's. That tends to force it up and out just be ready for the walk home

Come to think of it........do I really want a 4t again?????

BJDownunder

[TrialsRfun]

Absolutely HRS the absence of a oil pump must surely compromise the engines durability, but does anyone know how the oil gets spread around the engines internals without one it is really puzzling ?

[Trials Lad]

I would have thought that all the unburnt fuel that goes goops up the exhust would take some of the heat away.

[downunder]

HondaRS

Like I said, crankcase pressure does get the oil up the top end, so did the Husaberg use grooved rings

I must find out how the Sherco does it, at least now I know the old Honda didn't just rely on me falling off often enough to get fresh oil up there. Mind you with the amout of time I spent picking it up, it didn't need an oil pump

BJDownunder