Hi Guys,
Sorry in advance if this question has already been asked a zillion times.
Question is: how much and what kind of performance difference can I expect if I swapped out the 32/10 final drive on my 1978 BMW R80/7/Dnepr chair rig with a lower 37/11 rear?
Currently the rig has plenty of grunt empty and will cruise about 75 in 5th (but that's with the throttle at the stop).
With a passenger or some other heavy cargo it really labors off the line and hills become mountains. Throttle pegged only gets me to indicated 65-70.
Will the lower rear give me more acceleration AND a little more at the top? Is there that much difference in gearing, or would I hardly notice? I have a line on a 37/11, but it's pretty pricey for me, or else I would just try it for myself.
Thanks in advance for any advice!
MotoJ, I have no experience with airheads, but I can relate my experience with my 883cc Sportster. I had a smaller front pulley installed to reduce the gearing ~4% with very good results. Take-off from stop now requires minimal clutch slippage, and my top speed improved 4mph according to the handheld GPS, and made the rig overall more satisfying to drive.
Your proposed change is ~5%, but your results may vary as your R80 airhead engine is more "flexible." Hopefully someone has more direct experience.
Lee
MB5+TW200+CRF250L+GTV300+INT650
XL883R w/Texas Ranger Sidecar
Zuma 50F + Burgman w/Texas Sidecar<Mrs. SwampFox
It's not a huge difference - whatever speed you are doing at 6000 rpm, you'll be doing it at 6300 rpm with the new gears. It'll probably help the acceleration and hill pulling some. Top speed increase is iffy. If you get an extra 5mph you'll be doing great. Those engines are rated for 50HP - only so much speed there. How much are they asking for the rear drive? You may get as much or more results from an aftermarket exhaust and carb jetting. Going to 1000cc cylinders/would be great - but not cheap.
What size and type of tire do you have on the rear? You might be able to get a 5% gearing drop by going to a lower ratio tire.
Thanks for the replies!
I'm already running dunstall style stainless mufflers and drilled jets.
The seller wants $450 for the rear. I don't know if that is a good price, honestly.
Is maybe the money better saved and put towards a 1000 conversion, you think?
IMO $450 is too high. I've seen a few on CL and ebay and they seem to run between about $150-250. You might consider holding out for a 31/9 (3.44:1) or 32/9 (3.56:1) for a little more oomph.
I'm trying to remember if the stock 800cc cylinders can be overbored for the R90 pistons but I'm getting kinda old.
http://flatracer.com/#/bmw-performance/4538136890 This place has 800cc big bore kits. They're in the UK and the price is 800 pounds - not cheap.
The problem with these here InterWebs is I keep seeing all these tasty tidbits for sale here and there! I need to lay off and be content with what I'm running. I do like to do my part to keep the economy going, though!
You'll get substantially more smiles from going with the higher displacement. The downside is a little less in the mpg dept, but it shouldn't be a drastic difference. I agree the price seems high on the rear drive.
I've got a 32/9 that I will let go for $250 if your interested. It's in good shape with good splines.
Hmmm... I'll mull this over. It may be TOO low...
This table might help you. Take a look. I did use final drive with a 1978 R-100/7 with a Ural hack attached and it worked quite well. YMMV with the R/80.
http://www.airheads.org/content/view/222/49/
I stand corrected, I don't think I ever did put this final drive on the 78 R/100, but only considered it. I think I was running a 34/11 on the R/100. It was many years ago and I think I have lost a lot of brain power since then.
If you are using 5th available at 75 mph and you are not increasing you are packing the engine. Try 4th products and see what happens.An old time BMW expert auto technician from the old region once informed me that when you throw the reduce on it should increase. If it does not you are packing the engine.
