final drive swap

[3Phase]

... how come the Special and Standard basically have the same numbers in terms of performance from period tests?

It's not just the 16" vs 17" wheels. The base ignition timing is dialed back on the '79 Special and it has a different vacuum advance curve. It might have a different mechanical advance curve and/or total advance numbers but I'd have to look it up. There have also been confirmed stories of larger carburetor air and fuel jets from the factory in the '78 than the '79.

So, yeah, if the only difference was the wheel diameter then the '79 Special should blow the side covers off the '78 Standard but it doesn't because they reined it in a little. The slight additional mechanical advantage from the 16" wheel allowed the slightly detuned '79 engine to roughly equal the performance of the higher tuned '78 engine with its 17" wheel without exceeding it so the owners and the EPA were both happy.

[crazy steve]

Look at it this way... The Standard in 78 was the first 11 second bike. The Special when it came out, albeit with slightly different tune but with basically the same power as the original '78 based on dyno tests, was as quick, maybe quicker, but by a very small margin even with the shorter gearing from the 16" rear tire, insignificant difference is the term that comes to mind.

If the shorter gearing of a stock FD compared to the swapped FD is THAT much better in the acceleration department as has been highly touted 'round here, how come the Special and Standard basically have the same numbers in terms of performance from period tests? Most of what I've heard says the 16" vs 17" tire difference is about the same as the gearing difference in the stock FD vs. 750/850 FD. If that is accurate, I think we have a lot of armchair theorists out there blowin' smoke about how much of a "dog" the FD swapped bike is compared to a stock FD bike. Otherwise, a Stock Special should blow a Stock Standard into the weeds each and every time based on this same purported difference that some how comes to bear with an FD swapped bike, since the gearing is somehow so tall...

Help me here, am I smokin' something or missing something?[/QUOTE]

The '78 was a .1 of a second quicker in the quarter than the '79 Special, and .2 quicker than a '79 standard, due to the 'retune' for emissions on the later models. And Scott is correct, the '78 had a more aggressive ignition curve than the later bikes. Now, comparing apples to apples, the reasons given for the better performance of the '79 Special over it's same-year standard brother is 13 lbs less weight, a better tire, and the slight 'change' in final gear ratio due to the smaller tire. That last item isn't as much as you think; it's less than 5%, or an 'effective' FD ratio of about 3.4:1 vs 3.3:1 with the 4.50H-17. Figuring each .1 ratio change as being roughly worth about .1 second of quarter-mile time, dropping a 750/850 FD in means a .4 difference in ratio and you've just added .4 to your quarter mile times. So your 11.9 second Special is now .3 slower than an unmodified standard. That will show up the most in your 'short times' (0-60 ft, 330 ft, 660 ft) as once you reach 4th or 5th gear, it's more about horsepower. So will a FD-swap bike be a 'dog'? No, of course not; you'll still have a low-12-capable bike. But given two bikes in equal condition and two riders with equal skills, the bike with the OEM FD will outrun the 'swap bike' every time...

These are rough numbers, but I'd be willing to bet that if you ran a A/B comparison with a single rider/bike, only changing the FD, the results would be close to the above...

[jetmechmarty]

If we could just put the bikes on a dyno, we'd learn that the bike with the lower gear will develope peak torque and horsepower more quickly. I think that is where the real advantage lies.

Is it too hot to ride? I'm in the Netherlands wearing a jacket

[DAVINCI]

Hey, Marty, when you comin' back to JFK? I'm pretty sure I owe you beer. Or you owe me beer. What's the difference, let's have some beers. You'd better hurry, dude, I've got a 750FD w/ 49K on it that could explode any minute

[xsroadrunner]

Hey, Marty, when you comin' back to JFK? I'm pretty sure I owe you beer. Or you owe me beer. What's the difference, let's have some beers. You'd better hurry, dude, I've got a 750FD w/ 49K on it that could explode any minute

Its okay Randy as long as you're trying to make it fail...

[BA80]

its okay randy as long as you're trying to make it fail...

Ha Ha Ha Ha Ha Ha

[cywelchjr]

These are rough numbers, but I'd be willing to bet that if you ran a A/B comparison with a single rider/bike, only changing the FD, the results would be close to the above...

I'll go along with you there. And this is where Marty could get himself stomped with his "I'll take on any FD swapped bike in a drag with my non swapped bike and win". Greg's bike IIRC has a 78 engine in it, so a 78 engine in a special with the 750 FD against a stock 80 special changes the whole picture. A 78 standard will stomp ANY stock 80 any time any day. A special with a 78 engine, albeit with 750 FD may well overcome the drawback of the FD and put the bike onto even footing with the 80 special (I don't think an 80 standard would have a chance), making it a riders race. I will say, I'd like to be there and see the race though . And I'd like to be able to buy them both a beer (even though I don't drink myself) and pat them both on the back and congratulate the winner when it was over as well.

The point to take away from here is that each of us works on improving the aspects of the bike that appeal to us, making it a better fit to what we consider our perfect ride.

[DAVINCI]

The point to take away from here is that each of us works on improving the aspects of the bike that appeal to us, making it a better fit to what we consider our perfect ride.

Well put, Cy.

And that, ladies and germs, concludes the 750FD debate....in my dreams.

[Bonz]

The final drive ratio has nothing to do with a bike developing power more quickly. The shorter geared bike will gain rpm faster, therefore get into the power band more quickly. Just a point of clarification. The shorter geared bike will also go through the power band more quickly, necessitating more frequent up shifts while the taller geared bike will spend more time gaining speed in each gear before shifting...

Yeah, it all comes down to liking what you have, but the discussion about it all is so enjoyable and entertaining.

[cywelchjr]

This is the thing. We have been having a nice spirited debate about the MODIFICATION without anything getting personal and without AFAIK anybody getting bent out of shape, and have had a good natured rousing discussion of is issue. And I think we ALL have learned something, on BOTH sides of the isle. We have also learned now that they are not two subtypes of drive as was previously thought, but at least 3 of them, and it's unknown yet if the 3rd type is more suited to the mod than the other 2 (at a glance I would guess that it would be) since it seems to have been created for the more powerful 850. I'm guessing here, but that might have been the drive that with a larger bearing added and the pinning on the pinion that the 1100 has added to it might have been the 1100 FD had they stuck with that ratio for the 1100.

[skipper]

Ahhh, guys, when I posted this I wasn't so concerned about performance as durability when flailed on. I admit, I'm getten old and senile. Don't always make my thoughts clear (lots of time, even to myself).

[crazy steve]

One positive aspect of this swap that hasn't been mentioned is it will make the bike much less sensitive to throttle input at lower speeds. One thing I've noticed is the stock XS needs constant attention to throttle position to maintain speed, much more so than my Sportster with it's better low-speed torque and taller gearing. So from that standpoint, the swap will 'improve' driveability. The stock XS gearing gives quite a bit more engine braking...

But you 'swappers' are STILL going to DIE when Grandma runs over you in her '73 Caprice 'cause you CAN'T accelerate fast enough to get out of her WAY...

[cywelchjr]

One positive aspect of this swap that hasn't been mentioned is it will make the bike much less sensitive to throttle input at lower speeds. One thing I've noticed is the stock XS needs constant attention to throttle position to maintain speed, much more so than my Sportster with it's better low-speed torque and taller gearing. So from that standpoint, the swap will 'improve' driveability. The stock XS gearing gives quite a bit more engine braking...

But you 'swappers' are STILL going to DIE when Grandma runs over you in her '73 Caprice 'cause you CAN'T accelerate fast enough to get out of her WAY...

Ain't found a car I can't walk away from without breaking a sweat yet . And that's without bothering to downshift. The problem with the Caprice is gonna be when I can't stop and run right up her trunk .

[BA80]

The problem with the Caprice is gonna be when I can't stop and run right up her trunk .

CY! Up granny's trunk?

[BA80]

Greg's bike IIRC has a 78 engine in it,

According to the numbers on the cases ,2H7-021137, it's actually out of a 79F.

However, ever since I have had it in there I thought it ran rather smooth compared to other early engines with the big cams which tend to idle a little radical. The other day I had the valve cover off to fix a leaky cam plug and while I was there I checked valve clearances and they ar set very close to the 80 and up cams so I believe this thing has a later head on it.

I took this engine directly out of a parts bike that Bob Jones had stashed in the attic at his garage at home. I don't know what else may have been done to it. Hell, it might have an 1196 kit in it for all I know.

I know for certain it runs a LOT better than the 3J6 engine that was in there.