I lost some power somewhere... anyone have a guess where?
 

Since I am paying close attention to speed, RPM and vacuum level, I noticed something disturbing in the last couple days.

I seem to have lost about 10 average MPH when maintaining the vac gauge at 10. I don't know if I am making less power in the engine, or if it is getting sapped away by one form of drag or another, but I can definitely see the change in speed at that throttle position as compared to a couple days earlier.

I can only identify 3 things that have changed about the time this started:

1. I got the new wheels and tires, same size as before.
2. It got significantly cooler out.
3. It has been somewhat more windy.

1 is possibility, I guess, if I have increased my rolling resistance -- but 10 MPH worth? That would be depressing. The old tires were not significantly worn, just dryrotted, so it is not like I was running tread-less before.

2 seems like it should create *more* power, with an accompanying drop in efficiency, all else being equal, but maybe I am thinking of that wrong.

3 is a good bet, but I am noticing the power loss pretty consistently regardless of the direction I am traveling.

Of course, there is a 4, and that is "something else happened to break/degrade/go wrong about the time of these other changes", and I'm not sure what to look for there. Engine runs fine and vacuum is still 19 or 20 at idle.

It is bugging me because when attempting to drive with load I am now cresting gentle rolling hills at 40mph or below on 55mph roads, which starts to get a little risky, imo.

Any thoughts?

New tires will add some resistance. The big killer is cold and wind. Ive been noticing that here too.

Cold air = more dense = more power needed to push through it = dropped speed while DWL

I've noticed it a bit in my VX as of late as well.

wagonman76 -

That's what I was thinking.

CarloSW2

Yep! Right, on all accounts! :thumbup:

1. Rock-hard 'bald' tires will give you the least resistance!
   
   If you want to go for the gold, you should be running 'Land Speed' tires:
   
   Goodyear Land Speed Tires (low resistance, lightweight, and rated to 300 mph)
   
   
2. Cold/wet air is VERY dense! When I took ground school (for flying planes) air density was drummed into us.
   
   With cars, sure air density affects FE, but with planes it can and will kill you!
   
   In the state of Alaska, they have their own set of flight tables because the air is so cold and dense that it doesn't take hardly any power and speed to get off the ground.
   
   In Arizona, when it got 122 degrees, the FAA grounded all commercial flights, because they were afraid that planes wouldn't be able to take off and land safely.

Anyway, yes, you're taking a triple hit! ;)

Okay, gotcha.

So to my as of yet fictional data-gathering rig, in addition to OBD II, injector pulse rate (and derived MPG), rate of climb/descent, ambient temp and manifold vacuum data, I need to add barometric pressure and humidity.

That's all do-able, eventually. The real pisser is a real-time way to measure wind speed and direction, subtracting out that generated by my own movement. Must be a way, I think they do this on sailboats...

(Don't look for most of this before next spring, though, at the earliest.)

Er... A final thought...

I don't know about the powerband in a Geo Metro - never drove one - but...

My Honda B16 motor is widely ridiculed as a "torqueless wonder". At 8000+ rpm it will propel my ride like a white hot comet, but "cresting a gentle hill" at even 3000 rpm will cause my engine to labor, and the speed to drop precipitously!

The workaround is to simply drop back into 4th gear and gently build some revs... ;)

Have you tried this?

It will allow you to maintain your vehicle speed without laboring the motor - shouldn't affect FE too adversely - and retain your vacuum readings, yes?

Peak torque in the geo is 3300, and it pulls real well from 3k to 4k in any gear. But my understanding was that high revs were the devil with regard to FE. 55 MPH in 5th is about 2800 or so. 60 is around 3k. The only time I have even touched anything above 3 grand is when trying to accelerate quickly in certain situations that require it for safety, and even then I am shifting right on 3300. Mostly I am shifting around 2500 and cruising wherever 10 on the vac leaves me, which has been at pretty low speeds overall. About 45 MPH average in the current weather, I'd say.

Now, when I am going up a hill and the revs are dropping to about 2k in 5th, I'll downshift to 4th, but keeping me vac up still leaves me going between 35 and 40 mph and not gaining any ground until I hit the crest. If I want to maintain speed up a hill I am going to need to give it some gas, one way or the other.

I'll be able to experiment more when I get some of the stuff I ordered yesterday. I just filled up a few minutes ago, 49.6 MPG. Little disappointing, actually, considering how radically conservative I have been driving (I was hoping to be in the low 50's), but given other comments here about the effects of temp and wind, maybe I didn't do too badly.

EDIT: I forgot to calc for a -.83% odometer error! I broke 50! Whoohoo! 50.05 MPG.

ive noticed the same thing laitly too, seems i need bit less vacuum to maintain old speeds. im usualy between 15-17 in-hg on flat roads, those 1L must be working it to cruse at 10 in-hg :P

The weather changed here in my region and I already can tell my MPG's are going down.

Next mod = Block heater