750FD rpm question

[Bamper]

How much lower do the rpm's go with the 750FD vs stock.

[randy]

Enough to get 10%-15% better gas mileage. And, I hardly use 5th gear except when over 65 - 70 MPH.

I think XSchop posted the actual rpm changes?

[John]

You should see 3500 rpm at 60 mph. Over all, the engine speed will decrease by 500 rpm at most speeds.

[xschop]

rpm

Depends on your rear rim/tire heighth. John is about right at 3500 rpm in 5th gear...The final ratio is a 12.5% difference than stock, so deduct 12.5% rpm's off each gear... I rode my friend's XS11 a few weeks ago after I swapped his and that is right about where the tack was at 60 with his 16" rear tire from the special.....chop

[John]

I still have an XS with the stock FD, and every time I ride it, it seems like the engine is screaming at 65-70 mph.

[stoked]

Ye, from personal experience the drop, whatever it is, is dramatic. It makes crusing at 80 mph just feel likes it's where the bike is supposed to be at. I have to admit however I have not experienced great gains in mpg. But the move usuable rpm at higher speeds is well worth it to me.

[xschop]

mpg's

hey stoked, what exhaust are you running on that bike?

[dbeardslee]

Somebody double check my math here, but by my calculations this is what your overall gear ratios should be with the 750FD:

1st Gear - 10.62
2nd Gear - 7.72
3rd Gear - 6.1
4th Gear - 4.9
5th Gear - 4.19

Stock the overall ratios are:

1st Gear - 12.08
2nd Gear - 8.78
3rd Gear - 6.94
4th Gear - 5.58
5th Gear - 4.77

Numbers are for a '79 F. You can see by the numbers that by the time you hit 5th it's like a whole 'nother gear.

[trbig]

With my XJ and a 16" rear tire, my RPM at 70mph was right at 5k. After the FD swap, it runs right around 4100-4200.

I didn't get any noticeable fuel savings though, so don't do the swap EXPECTING it. Some do, some don't. BUT... I'm not constantly trying to find another gear like I used to. It just seems to be much easier on the bike.

Tod

[stoked]

Re: mpg's

Originally posted by xschop
hey stoked, what exhaust are you running on that bike?

Stock headers, PO cut cross over and old muffs off and installed non fiberglass packed but with baffle JC whitney trumpet styles. I have since added some fiberglass around the baffles.

So, stock looking JC whitneys with nice straight though baffle design, but no cross over.

[Bamper]

Has anyone done the 750FD swap and the 750 tire (18inch rim I think)? If so how much of a rpm drop did you get?

[xschop]

Flintstone Combo

You may wind up having to do a Fred Flintstone to get it going from first.

[dbeardslee]

Bamper - It's a 17"rim if you're going from a special to a standard size rim. If my math is correct that means the circumference of the 17" with a tire on it is about 4.104 inches bigger on the 17 (78.814") than on the 16 (74.71"). That means for each 18.204 revolutions of the 17 you have traveled far enought to equal 1 more full revolution on the 16. So at 5000 rpms you should be about 275 rpms higher on the 16, just from the rim size differential. Add that to the plus or minus 500 rpms from the FD mod and your rpms should be about 775 turns greater on the 16 at 5K. Or looking at it the other way, 5775 rpm's on your current setup should be 5k with the new stuff.

Or we could just wait for someone who has done it to chime in

[AlanB]

Hi Chop,

I always guessed that if the 750 could pull the 18" rim and the 750FD, the 1100 should manage?........

Or are the gearbox ratios different on the triples??

AlanB

[dbeardslee]

Bamper - I screwed up on the math. That would be 5000 rpms at the rear wheel, which would be like REALLY fast. Forgot to take into account the final gear ratio. Should be like 60rpms difference due to tire size. Also, I didn't look close enough at your question r.e. 750 wheel. Tire is a 4.0 X 18 which would be 79.128" in circumference, which is VERY close to the size of the 17" so 60-65 rpms should be close.