Carburetor efficiency question..
 

Well here's my first and probably only post. :D

Now, I don't have the most efficient vehicle ever made, but I bought it because it meets 2 conditions..:

1)It's 4x4
2)It gets decent mpg

The Suzuki Samurai :eek:

In the summer I've been getting about 30 mpg on the highway and almost as good in the city. It's has a 2 barrel carburetor, with a single primary and a single secondary.

***So my QUESTION is this:

Will an equal throttle position in different gears (4th or 5th) ALWAYS equal the same mpg?

I'm wondering this because I'm thinking the higher RPM can suck the fuel/air in faster and even though the valve is open the same amount, it gets sucked through it faster, resulting in more fuel burn.

I also have a vacuum gauge ready to install. With this, will I choose 4th or 5th depending on the condition (hills or headwind) based on which ever gear gives the highest vacuum?

Thanks.

...simple answer: No.

...because the engine RPM's are necessarily different in different gears...and, the faster the engine turns, the lower it's fuel economy will be.

..."Rule-of-thumb" -- best FE occurs in the highest gear at the slowest engine speed that doesn't 'lug' the engine.

...a vacuum gauge is an excellent indicator to have and use!

What Old Tele Man said.

Well it's hard to tell if I'm lugging a 1.3L that puts 40 hp to the ground! When I cruise at 65 mph in 5th gear, I'm doing 3600 RPM. It's not lugging, it's just suffering from sheer lack of power.

So a real life example..

65 mph, 5th gear, 3600 RPM, 3/4 throttle
65 mph, 4th gear, 4100 RPM, 3/4 throttle

Even though I'm going the SAME speed and have the SAME throttle position, 5th gear should yield higher mpg?

I use 5th gear on the highway 90% of the time, but once in a while due to the barrenness of ND, winds can get crazy high and the truck doesn't have the power to hold speed at 60 mph even when it's to the floor. So I drop it to 4th.. Usually it's not that bad, and to go 60 mph I keep it about 3/4 though.

After all this is said though, once I get the vacuum gauge installed, I should just go by which ever gear produces the most vacuum to hold the speed I want, correct?

...what does the vacuum gauge say at each setting?

...bet there's a measurable difference with the higher gear showing a lower vacuum number, indicating heavier "breathing"--an indication of better BSFC.

Dunno, it's not installed yet. :D

I have used a few samurais over the years, great little rigs but they do indeed suffer from a lack of horse power when runnin at highway speeds. With a webber carb nd 235/65s I was able to keep going on level ground at 70, on the first bbl in 5th any wind or uphill down shifting was pretty much required. My milage when in town was about 30ish till I moved to the bush then I was lucky to get 20mpgh. (4x4 engaged and slogging through dirt, snow, short trips and the like)

When cruising in 4th or 5th gear even with identical throttle positions you are sucking more air through at higher rpm which mean more fuel as well.

You can feel with the accelerator petal where the second bbl on the carb opens up ride wide open on the first barrel and let the truck do as best as she can, down shift when you have to and use the second barrel if that can't keep you moving fast enough.

Cal mini performance, petroworks, and rocky road outfitters have stuff that may help. a low rpm torque cam Will likely be high on my list for the next one. The help in fuel economy wont necessarily pay for a cam, but they do help in the off road drivablity department.

I am pretty sure that the 1.3 puts out about 65ish.

is the carb progressive secondary?
if so, is it vacuum valve, thermal controlled?

if use of one barrel, yes you want the whistle of atomizing...and that also leads to a very decent surge into the second barrel. The samuri is not that old, I assume progressive.

30 is doing good. samuri with larger wheels still has low gearing, but it helps. and also, do not remove engine driven fan, that is a helper not a killer. the downdraft loves it. it is an "anti-thumper", carbs like smooth.

Stock carb is progressive iirc

I'm not too sure what you mean there. Can you rephrase it?

daverbmxer,
i opened a thread with the same question. -- i already had my VAC gauge installed.
i did find conditions, like a highway on-ramp, where the lower gear yielded better vacuum.
i have an automatic transmission, so lugging is pretty unlikely to happen, but i come close.

the only reasonable response i received was old tele man's response.

my vehicle is too old for a scanguage to compare these gears, but i think he made great sense.

Luvit - You could install a marine fuel meter, such as a flowscan, to determine GPH usage of the engine.

what do i do with that GPH figure? ..kinda see GPH in either gear?

Those can't be cheap..

If you know GPH and MPH, you can compute MPG. They aren't overly cheap, about $150 for an analog gauge type, more for digital display. For a carb, however ,there isn't much other choice.

Here is the answer :)

A tall velocity stack on the carb perhaps?

When I first got my Scangauge, that was the first test I did - a 4th, 5th, and 6th gear mpg comparison drive at the same 35 mph speed on a slight uphill climb about a mile long. The mpg results showed 5th gear beat 4th, and 6th gear beat 5th. This was in my SE-R.

Here's another. I've run the same 10 mile loop in all three of my family's cars in top gear and the next gear down. That's 35 mph in top gear v. 30 mph in the next lower gear. The route is essentially level, driving at sea level with at most about a 50' elevation gain in a couple of places. Each car showed better mpg in top gear.

...the ecomodder jingle, repeat after me:

"the slowest RPM in the highest GEAR that doesn't 'lug' the engine."

"the slowest RPM in the highest GEAR that doesn't 'lug' the engine."

"the slowest RPM in the highest GEAR that doesn't 'lug' the engine."

Old Tele man is right. That's why low end torque is good. It allows you to run in a higher gear in a lot of situations.

Haha yeah, I fully agree with that but my 1.3L has very little torque to say the least, especially at a lower RPM.

I'll try to keep it in 5th gear as much as I can from now on. I'll be going up from 205/70R15s to 215/75R15 eventually, so it might make it a little harder.

I've still got to go through the rig and do some tune up stuff. Going to check the timing and maybe bump it up a bit, adjust valves, tweak carb, grease wheel bearings, etc.

BTW, are there any people that have noticed an increase in MPG or power with Iridium (or similar) spark plugs? I've heard numerous times that they're a joke and don't help, but on the Suzuki forum I'm on it was mentioned that they help.