Less can be More - tuning a banshee

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prossett
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Post by prossett » 27 Oct 2008, 9:19 am

After the engine re-build, and the breaking-in procedure, I've fine-tuned the banshee.

At first I automatically went up... OK, wait: the banshee is fueled by two carburettors, and getting the perfect mix of air/fuel is the objective to tune a carb engine. The amount of air and fuel can be regulated. The fuel amount is controlled by changing the jet (little screw with hole through it) by sizes... 230 is bigger than 200, etc.

So, I first automatically went up to 230 (200 is stock) to let more fuel in, but I wasn't too happy as it wasn't quite right... then I thought about it and asked myself why I immediately went bigger, without even thinking.

Well, we always want more power, right?

I thought about my goals with the banshee - covering distance, not racing up a dune over and over... and therefore reliability and fuel consumption are key to my intended use...

So I went down to 190 (10 less than stock) and you know what? Absolute perfection! 8)

It rises into the power band instantly with a flick of the throttle - I actually get MORE usable power, with less fuel consumption (to be verified) and better reliability over time (since the engine actually will produce less power - but more usable power.... if that makes any sense... more power at the wheels)

So, moral of the story, it is not always true that bigger, more power is better... perhaps perfect tuning is more a question of ideal balance instead of pushing for maximum power output.

Something to think about... 8)

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Neo
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Post by Neo » 27 Oct 2008, 9:54 am

The Power and the fuel efficiency of an engine is directly proportional. (Theoretically)
The perfect tune will get you the best power output.
At the same time will give a good fuel efficiency.
Evidently you are running lean with the current jet since its smaller than the stock.
As long as you have the right stoichiometric ratio, there arent any issues.
But your engine might run a bit hot and spark plugs might wear of faster if its too lean.

If you can somehow pump more air into the chamber you can increase the fuel and get more power... but that isnt the objective here.. nor the point.

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Post by Tamu » 27 Oct 2008, 10:37 am

hmmm, thats very interesting PR. Most (exaggerating here) stock turbo cars come tuned very lean throwing the AFR`s way out of the band it should be in. However, in your case you actually decreased the fuel ratio and didn`t really play around with the air...I am sure if you played with the air ratio and fuel together, you can get kick A$$ performance with reliability and the fuel effiecency just like neo mentioned....

I no for example in a stock evo if you get a basic flash or tune with no mods you get way better gas mileage and much much more power.......

But ya its more of trial and error thing until you really get the works around the hole aspect of tuning to perfection...maybe then you can use this skill on cars as well ;) .....

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Post by prossett » 28 Oct 2008, 8:04 am

The engine is not stock because I'm running a Powerhead, which basically is like shaving off the head. But I also threw out the racing pipes without mufflers, as I didn't want to go deaf on my rides, and I also took off the twin K&N exposed air filters and went back to the stock box.

So what I think was happening is the combustion chamber was pushing faster than what the exhaust could clear out, and in a sense it was clogging up.

I could have fixed it by allowing more air in, and also use the racing pipes for more air out - that would have been mean, but also aimed for short-distance racing.

Instead I reduced the air in by going to stock filter, reduced the air out by slapping on the stock pipes, but then the fuel was pouring in like a waterfall... so now it makes complete sense to also reduce the fuel supply.

And that brought it back into balance, so in a sense I de-tuned the engine to suit my requirements.

The less power I produce actually translates into better use of power compared to before for my intended use - but naturally it is less than what it could do with Powerhead + K&N + racing pipes... but that would be like driving a dragster on a long-distance trip!!!

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TNT
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Post by TNT » 28 Oct 2008, 3:48 pm

You keep mentioning long distance, what exactly do you use it for? You live in Al-Ain so does that mean you use it from your house to grocery store? lol :P

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Post by prossett » 28 Oct 2008, 5:45 pm

:lol: something like that - on the weekend of Nov 7 I'm riding Al Ain to Abu Dhabi fully off-road, camping there and back the next day! 8)

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Post by Tamu » 29 Oct 2008, 4:31 pm

Woow now that sounds like fun..something I would seriously want to do :D ....good luck with it, make sure you put up pics and a story to go along with your craziness :D ..lool

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Post by zeemmee » 29 Oct 2008, 9:41 pm

@Prossett looks like u have a lot of time in your hands, Keep up the good WORK :D (Y)

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Post by prossett » 29 Oct 2008, 9:45 pm

Thanks guys! 8)

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Post by Neo » 30 Oct 2008, 8:09 am

Prossett : I am jealous... So many reasons.. some i dont even want to tell...
:)

Back to the topic...
Even with fuel injected engines a fully open high flow exhaust results in decrease in fuel efficiency.
Because there is no back pressure due to restriction.


I know this will be more applicable in a carburator system since the amount of fuel in the cylinder depends on the air flowing thru the carb.
So due to non existance of backpressure or reduction of it, at high RPM a certain amount of fuel must be leaving the cylinder before its fully burnt...
Resulting in
1) Pollution
2) Less Gas mileage

In the current scenario,
When you went back to the stock system except for the head,
The Right amount of air entering to through the factory AIRBOX is leaving the Factory tuned exhaust... (Which is optimized my the manufacturer)
You could be getting a lean burn due to the improvement in the Head structure.

This is my theory... :)

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prossett
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Post by prossett » 31 Oct 2008, 9:21 am

Good points - let's work on them some more.

Let's add the fact that it's a 2-stroke engine, meaning lubricant is mixed into the petrol.

The normal ratio is 5 gallons of petrol to one quart. To help keep it on the rich side I've been going 4.5 gallons to a quart of synthetic.

Do you reckon that manipulating the oil/petrol mix is enough to balance out the lean head conditions?

The reasons I want to keep the powerhead are 1) it's already there and 2) it runs cooler than a stock head.

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Post by Neo » 01 Nov 2008, 10:55 pm

I had this feeling that u r running a 2 stroke... Scavenger!!!
In that case the whole intake exhaust has a huge impact...
Look ma no valves. :)
You run free flow, u could throw out a lot of your AF mixture...

Let the powerhead be on it if you feel its running cool.

Well i get confused with this gallon to liter conversion... Anyways whole country is going to go on Liter soon.. wait for it to happen..

The oil is there for a reason.. lubrication..
So your piston doesnt freeze at high RPM, lock up ur ride and throw u...


Lets look at this logically becoz I didnt concentrate well in the thermodyanamics class while doing my mech engg dip.

Oil doesnt vaporize like petrol, neither doest it burn
Adding more oil to same amount of petrol means...
Well as far as I can imagine...
Not good for the carbs,
Not good for combustion,
Not good for your exhaust coz the its going to get clogged.
Not good for the environment.

I think if you are doing fine now.. you keep as it is..

You know if your running too lean or rich when you rip your throttle abruptly while riding on load....
If the engine coughs or runs out of breath... voila...

But I would say keep the same jet..
And try with different combination...
1) Both intake and exhaust on
2) Only Intake and no exhaust
3) Only exhaust and no intake.

i remember you dont like the noise of the exhaust..
but seems like you got plenty of time to experiment.

By the way we didnt touch the spark or timing yet...
You might have to work on that too...
Boosting your spark always works..
Your could try advance and retarding.. Many options to play with.



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