Changed Compression Ratio and Lost Gas Milage/RPM

[StevenRankin]

Howdy All,
I recently had my 84, R80RT cylinders bored out and sleeves put in place of the Nikasil linings. I installed 9.5:1 pistons instead of the 8.5:1 or "stock" pistons.
Before I averaged 40mpg, now I get 30mpg.
Before, at 4000RPM I was reading 70mph now it is 5000RPM to reach 70mph. At 5000RPM, I was running at 80mph. now I am running at 70mph. At 6000, it was 90mph, now 80mph.
The clutch is NOT slipping and the speedometer/odometer checks out with my GPS. Indicated speed on the speedometer is very close to GPS Speed. not enough error to be of concern. Also, my normal trip to visit family now means a gas stop in between where as I used to make it there and back on one tank of gas.

Ok, long winded, what do you all think might be going on? St.

[jdmetzger]

Howdy All,
I recently had my 84, R80RT cylinders bored out and sleeves put in place of the Nikasil linings. I installed 9.5:1 pistons instead of the 8.5:1 or "stock" pistons.
Before I averaged 40mpg, now I get 30mpg.
Before, at 4000RPM I was reading 70mph now it is 5000RPM to reach 70mph. At 5000RPM, I was running at 80mph. now I am running at 70mph. At 6000, it was 90mph, now 80mph.
The clutch is NOT slipping and the speedometer/odometer checks out with my GPS. Indicated speed on the speedometer is very close to GPS Speed. not enough error to be of concern. Also, my normal trip to visit family now means a gas stop in between where as I used to make it there and back on one tank of gas.

Ok, long winded, what do you all think might be going on? St.

First, if you wanted something to bore you, I would have taken those Nikasil cylinders off your hands and given you my non-nikasil 800cc cylinders. I've been looking for a good used set of Nikasils.

Anyhow, back on topic. Did you re-jet your carbs? That's a major change that will affect your fuel mileage if you haven't.

[kbasa]

How can the rpms at a given speed change without a change to the gearbox or final drive?

[StevenRankin]

Hello Josh,
Thanks for the answer.
You would not have wanted my old cylinders, the piston skirts had slapped the tar out of them. Ring gap was wide enough to drive a truck into it. The bike has 140K miles on it.

I will look into re-jetting the carbs and see what I come up with.
Dave,
I still feel funny with the concept of higher RPM to Make original MPH. There has been no gearing change at all. Plus I used the stock connecting rods. St.

[kbasa]

Dave,
I still feel funny with the concept of higher RPM to Make original MPH. There has been no gearing change at all. Plus I used the stock connecting rods. St.

Yeah, that doesn't make any sense at all to me. If nothing's changed in the driveline (final drive, trans), it should mean the same speed at the same rpms, right?

[Screamineagle]

How can the rpms at a given speed change without a change to the gearbox or final drive?

Absolutely correct. Engine work never changes the rpm of the engine.

Steven, what else was done to your machine? I can understand a lower fuel economy based upon engine rebuild, but not a difference in rpm.

[ccolwell]

How can the rpms at a given speed change without a change to the gearbox or final drive?

It couldn't. The possiblities are: 1) some change between the engine and the rear wheel. 2) a change in the speedo [did you check it against the GPS prior to all this?]; 3) a change in the tach. BTW, did the compression change require a change in the ignition timing? Not getting that right could explain a change in MPG, although not the change in the relation between RPM and speed.

[StevenRankin]

Hey Everyone,

Speedometer has not changed before or after rebuild.
I have not changed anything except the pistons and jugs, no drive train changes. I did change clutch disk and pressure plate. Clutch is clean and NOT slipping.

I haven't changed or tried re-jetting. Timing is according to the book and the advance is right on.

I can understand a change in fuel economy but I don't understand the higher RPM needed to achieve MPH.

[ccolwell]

I haven't changed or tried re-jetting. Timing is according to the book and the advance is right on.

OK, but it is possible that the stck timing is not the correct timing for high compression pistons. I don't know whether that is the case, but if you haven't investigated it yet, it is a question that needs answering. Hopefully someone here can tell you. Well, us, as now I'm curious.

[Friedle]

How can the rpms at a given speed change without a change to the gearbox or final drive?


Easily, if the clutch is slipping either due to incorrect adjustment or contamination. :bikes

Friedle

[28796]

Easily, if the clutch is slipping either due to incorrect adjustment or contamination. :bikes

Friedle


That or a funky guage are the only things that could account for a 1000 rpm change. As far as mpg it could be lots of things. When did you last do a valve job? Have you checked compression since the rebuild? How many miles have you put on the new pistons?

[cjack]

Howdy All,
I recently had my 84, R80RT cylinders bored out and sleeves put in place of the Nikasil linings. I installed 9.5:1 pistons instead of the 8.5:1 or "stock" pistons.
Before I averaged 40mpg, now I get 30mpg.
Before, at 4000RPM I was reading 70mph now it is 5000RPM to reach 70mph. At 5000RPM, I was running at 80mph. now I am running at 70mph. At 6000, it was 90mph, now 80mph.
The clutch is NOT slipping and the speedometer/odometer checks out with my GPS. Indicated speed on the speedometer is very close to GPS Speed. not enough error to be of concern. Also, my normal trip to visit family now means a gas stop in between where as I used to make it there and back on one tank of gas.

Ok, long winded, what do you all think might be going on? St.

The only things other than the clutch that could slip is the rear drive coupling in the swing arm and the two in the trans. Your drive shaft has a torsion damper which is splined to the shaft, so I would think if your damper were jumping 180 degrees continuously, you should be able to feel that. There is another one in the input shaft of the transmission as well...same deal though. The earlier driveshafts were able to slip without detection, however they quickly became uncoupled. There is a similar slippable coupling between the output shaft of the transmission and the four bolt flange. I've never heard of that slipping, but that would be the only one on your bike that could...I think.
It really sounds more like you are not in top gear...forgot how many there were?

[lkchris]

Speedo error could explain both your rpm variances--which are otherwise impossible-- as well as your mpg changes, as obviously you're not getting accurate distance readings.

And, I won't confuse you with Steve Rankin of West, TX, former Texas Airheads Airmarshall.

[Bob_M]

Speedo error could explain both your rpm variances--which are otherwise impossible-- as well as your mpg changes, as obviously you're not getting accurate distance readings.

And, I won't confuse you with Steve Rankin of West, TX, former Texas Airheads Airmarshall.

I had a similar problem with my 82RS and got the same reply from the same poster (thanks Kent). After I got a performance award I replace the speedo the problem was fixed.
Even if you have not done any work on the speedometer, they are old and sometimes unrelated failures occur at the same time.

I can not explain how the GPS validates the speedo. That throws the whole theory out the window. :dunno Perhaps you are riding near a breach in the time/space continuum, and a string of a parallel universe is resonating in a harmonic only your BMW can percieve. (they are timeless you know)

[StevenRankin]

Hello BMW Friends!

I am surprised at the number of replies I have gotten to my posting.

Just to clear a few things more. Valves and heads were done 10K before the major overhaul. Compression if I recall is 150 per side. I now have 5K on the new rings and pistons. Oil consumption is nearly zero.

So, it looks like I should check the basics, valves, timing and jetting. Plus double check the clutch is not contaminated and slipping. I had another rider ride it and He did not think it was.

The up side to this is there does seem to be more zip to the bike, more than just the stock overhaul would account for. Of course now I don't have a stock bike to run against.

Again, thanks for the information. St.

[20774]

Could there be something hinky in the electronic tach? Some kind of change in the pick up off the coils that might explain the tach variation... Sounds like the speedo is spot on based upon measurements against a GPS system.

Steven, is the physical noise and vibration level different at the old speeds or are you just basing this on RPM vs MPH readings?

Kurt in S.A.

[StevenRankin]

Kurt,
The noise is about the same as before. Except The higher RPM to get to the MPH.

I wonder: Could it be that I have set myself up to do more work to reach a certain MPH than I did in the old set up? I mean I have to run a higher RPM now to do the work I needed to do Before. The difference in speedometer and tachometer reading now is just a red herring. The real question is the change in fuel economy?
I don't know enough about racing to know that jetting and timing will most certainly change performance. After all, that is the fun of "tinkering" with engines, the challenge of making things go faster more efficiently.
That is one reason why I LOVE my Airheads. St.

[bikerfish1100]

IF your speedo/odo is accurate (measure against mile markers on the hiway?), 30 mpg from an R80 is pretty damn abyssmal, even with 9.5 pistons. Thrashing my '78 R100RS (with big carbs & pipes), i could drive it down into the low 40's, but that would be as bad as it would ever get. an R80 should be more efficient, not less.
check your basics- timing, jetting, valve adj, etc.

[rinty]

Bikerfish:

I don't think we know what the new displacement is; it might be as much as 1,000 cc. But the mileage is still way less than what it should be.

Rinty

[StevenRankin]

I got out the book and did some checking. Jetting for an older R80 bike was MUCH different than for my 84 R80RT. The jets are listed at 150 for the earlier bike versus 132 for my carbs now. The needles are different as well.
Given this tidbit, I think a jet change is in order.