What determines redline in these motors?

[Steve]

John Park, you just got me thinking of a frankenbuild of sorts - a 250 top end on the 300. The 250 had 10:1 CR compared to the 300's 8.9:1 CR. Hmmm....

Before you start some random "frankenbuilding", find out HOW the 250 got its 10:1 ratio.

I'll bet it wasn't just the head.

.

[Ric]

Oh absolutely, Steve. I tend to research like mad anyways - it helps keep me from asking yall off-the-wall questions, like "will a 6-speed trans fit in place of the 300's 5-speed?" amongst other things.

Not that I know whether the 6-speed would fit. There would just be so little if any benefit to it after looking at the gear ratios, that it ain't worth the time and effort.

I'll bet it wasn't just the head.

I have a sneaking suspicion that you might know a little sumpin bout that, eh?

[Steve]

Nope. I have absatively, posilutely NO experience with any GS smaller than a 450.

I am just guessing that there might be domed pistons or a lower deck height in use, so it's not going to be a simple matter of swapping over a head from another engine. However, you might get lucky and find at least some improvement in CR there.

.

[Guest]

I can't find any specs on the 300, but the lower compression would indicate that the cam is closing the valves sooner which is more conducive to a dirt bike type power band. Cam and compression are closely related.

I recall the 250 CR to have been 10.5:1 and the 8 valve 400 was 10:1, but that may have been Euro specs. As to redline, with a 44mm stroke I'd think breathing and valve float to be the only limits. The valve springs on a 400 8 valve seem pretty soft to me, but maybe that's just because the valves are so tiny. My ear says they start to float at 12,000??..

[Ric]

Per the owner's manual, redline on a 300 is 9500. Being a bike meant for the US, I doubt it wound up with any hp restricted cams - a truly fearsome beast, she is. B(

No, I would figure I need to tach it up to cart my heavy butt around. Kinda stoked to see how much I can squeeze out of it by fixing the the lean condition. I'll likely swap a smaller rear sprocket on it afterward to make the gears more useable. I seriously am hitting 2nd in less than fifteen feet from a stop... and that's when I'm actually winding it up.

[Guest]

Per the owner's manual, redline on a 300 is 9500. Being a bike meant for the US, I doubt it wound up with any hp restricted cams - a truly fearsome beast, she is. B(

No, I would figure I need to tach it up to cart my heavy butt around. Kinda stoked to see how much I can squeeze out of it by fixing the the lean condition. I'll likely swap a smaller rear sprocket on it afterward to make the gears more useable. I seriously am hitting 2nd in less than fifteen feet from a stop... and that's when I'm actually winding it up.

I'm not talking about restricting horsepower but making the power band more linear with less top end rush for the uninitiated. They already dropped the CR; if they wanted max hp it would be 10:1 like the 400 and probably similar or the same cam. Just because the line on the tach says stop at 9500 doesn't guarantee it's still making much worthwhile power there.

How hard would it be to see if the 250 cams and 300 cams are the same part number?

[Steve]

How hard would it be to see if the 250 cams and 300 cams are the same part number?

That part is easy.

'81 250T
EX cam 12721-11401
IN cam 12711-11401

'82 300L
EX cam 12721-11900
IN cam 12711-11900

What's different? No idea, I will leave that to someone else.

.

[Ric]

Steve, according to what I found on the AlGoreNet that part number you have for the 250 replaces the part number you have for the 300... ON the 300, of all things.

I typed this, and I have a hard time making sense of it!