Sunday, September 13, 2009

VUN Power Upgrade #1

The VUN's light weight means that it can accelerate quickly to 100km/h despite putting out only 54bhp in standard trim. But there are a few things about the standard VUN's power delivery that could be improved. I'll go through them...
  • It is definitely sluggish in the mid-range. The engine doesn't really pull until you get into the power band. In the mid-range you can feel it is gasping for air (you can hear this too).
  • The rev limit is about 7,500 RMP. Because of the first problem, if you want to ride hard, you will be always trying to keep the engine RPM high to stay in the power band. I found myself constantly hitting the rev limiter when pulling hard out of slow corners in 1st or 2nd gears. When you do this it feels like you just threw out an anchor, there is just not enough range.
  • There is a slightly annoying splutter at very low throttle openings at any RPM (except idle). Actually, I didn't really mind this too much. It's fairly easy to ride around the problem by selecting a higher gear or by slipping the clutch at the right moment.
Air Filter Upgrade

I asked VUN dealer for Bologna (Italy), Franco Cavina, what simple things he would suggest to improve the power. He said the first thing to do is exchange the standard paper air filters with free flowing metal gauze filters. When you do this, a change to the fuel-injection map is also needed to keep optimum the air-fuel mixture.

Even though my paper filters were still in good condition I decided that, when I was in Milano, I would ask the guys in CR&S to make this modification as the mid-range flat spot was really bothering me.

A CR&S technician fits high-flow air filters to my VUN. Roberto Crepaldi is lurking in the background.

The VUN has two ducts either side of the seat that channel air into a still air box in the ducktail. From there the air is fed through two rectangular cartridge filters to the injector body intake duct. Changing air filters is as simple as removing the seat (two screws), then swapping out the air filters.

Select high quality, high-flow metal gauze cartridges. These cost more than the paper cartridges but it is worth it and they last longer since they can be washed whenever they get clogged up.

Dump those Catalytic Converters

Two catalytic converters are mounted inside small bulges in the header pipes, one on each side. I don't know how you feel about it but I resent catalytic converters. If you live in a country where you can still get away with not having them, then I suggest getting rid of them.

I don't know if it is possible to take off the headers and hook them out. In my case, I swapped the original header pipes with some "track-only" pipes which don't contain catalytic converters.

Software Upgrade

Changing the filters and removing the catalytic converters requires a new fuel-map to ensure the correct air-fuel mixture is maintained. I had this done at CR&S. It was pretty simple. They just plug a laptop into the communication port on the right side under the fuel tank and select an appropriate fuel map for the modifications.

I asked if they would also raise the rev limiter by 500 RPM. I figured that would be enough to stop me from hitting the rev limiter in 1st and 2nd gears under hard acceleration. They said it would be OK, so my rev limit has gone up from 7,500 RPM to a more respectable 8,000 RPM.

Testing the Changes

I would get these changes done at your friendly local VUN dealer. They should have the software, fuel maps and expertise to do this properly. Without changing the fuel map for the injection system the air-filter change will probably make your VUN run worse.

This all took just a couple of hours, then it was time to take the VUN for a test ride... WOW! I expected an improvement, but this was much better than I ever expected!

Someone once told me that it is hard to notice any power increase unless it's more than about 10%. I don't know if that is true but I could feel a definite power boost across the entire RPM range and at every throttle opening. The mid-range flat spot was completely gone and the peak power was definitely boosted. Even the splutter at low throttle openings has disappeared! It feels like my VUN finally has the power delivery that it deserves.

I would recommend these changes to every VUN owner. It is simple and cheap and it makes your VUN much more fun to ride.

I was able to make one fairly credible measure of the peak power increase. On my ride down to Italy I measured my top speed at about 182 km/h with a big tank bag and backpack (without the luggage I normally get about 192 km/h). Top speed is determined by the point where wind resistance and turbulence overcomes the power delivered by your engine. On the way home, with the same tank bag and backpack, I could reach over 192 km/h. That's a simple measure but it represents a pretty significant improvement.

The real payoff can be felt on those twisty, switchback, mountain passes... the "mistostretto". Pulling out of corners is smoother and more responsive. The engine now gives you more flexibility and there is no slamming into that rev limit.

Appropriate Warnings

Raising the maximum RPM might void your waranty. This is probably up to the discretion of your dealer (or CR&S). Get this done by your dealer and make sure they are OK about it. Definitely don't do this until after your engine is properly run-in. I changed mine after about 5000 km.

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