Long or short stroke??

[colorado-high]

I used to own a 01 KTM400 E/XC that was a nice bike but very under powered for where I ride,95% of the time I am over 9000',you could not even lift the front end to cross a small log or puddle. I had sooooo many problems with it that I sold it shortly after I bought it.I realize now that I must of got a lemon because no one now seems to have many problems with them. I would like to build a bike that would preform well at high altitude.I need some suggestions on what combo of engine mods and what bike to start with.The trails are all rocks and roots and log crossings, you are mostly in 1st ,2nd and 3rd not too much high speed stuff just some connector roads to get to the trails.Do I start with a SX, MXC or Exc,400,450 ,520,525.Big bore, long stroke,short stroke,which cam to use?Which combo would give me the desired results?The magic button does not matter to me. I plan on installing a auto clutch.Or is there some thing that I could do with a Yamaha 400,426 or 450? I am a big guy(245 with gear) I need big power but I still need it to be managable

 

[cujet]

Sounds like the 525EXC would be the ticket for you. They have plenty of power, even at 9000feet.

Chris

 

[luvtolean]

Colorado, while I haven't had a chance to try it yet at high altitudes, you may want to look into oxygenated race fuel as well. I've been told by a couple people now (one is SFO) that it runs noticably better at high altitudes with it.

 

[Robcolo]

THere's just no substitute for cubic inches and High compression up here. A few bikes back I had a YZ 490 with the head milled. Even though it was down on power at 12,000' it still had a whole lot left. It's a lot less expensive to up the compression on a 2 stroke than on a 4. A KX 500 will hold it's own with bikes twice the price.

 

[nephron]

The loss in hp @ that altitude should not be that great if properly jetted. It'd be more likely that your bike was not jetted properly than it being a "lemon".

 

[Lorin]

It is my understanding that even conservatively speaking, 15 to 20 % power loss is not uncommon between bikes at sea level as compared to an altitude of 9000 - 10000 feet. As to jetting, I have been to Colorado the last two years and even with re-jetting, my 500 had a Very noticeable loss of power. On another note, when riding in Colorado, there is rarely an ideal jetting setting, as you can change altitude by more than a few thousand feet within a few miles. I ended up going 5 full steps leaner on the main jet, and almost two turns in on the pilot to get my bike running well. I was able to ride 500 miles without fouling a plug, but there were still times that the bike wouldn't run very well. I second the idea of a big bore two-stroke for Colorado. My WR 500 performed well in the diverse terrain I experienced in Colorado, and would still do the higher speeds needed for the paved roads.

 

[wayneg]

What about buying or trying a bike with fuel injection?? Gas Gas and Cannondale both have four strokes available which use fuel injection. The fuel injection is meant to be able to automatically adjust itself for elevation, and I was wondering if it would work or not. Do you know of any riders that have tried either of these bikes at high elevations? It would be interesting to hear from anyone that has one of these bikes who has tried it somewhere like this.

 

[Robcolo]

Lorin, We're down 15% on power right here at 5200ft. Somewhere I have a table equating air density / HP but can't find it right now. As I remember at 10,000 the loss is closer to 35 - 40% ! -- and we have several trails within an hour of Denver that go over 12,500. You won't believe how anemic a [properly jetted] 250MXer is at that altitude.

 

[tightwoods]

Originally posted by Robcolo
As I remember at 10,000 the loss is closer to 35 - 40% ! -- and we have several trails within an hour of Denver that go over 12,500. You won't believe how anemic a [properly jetted] 250MXer is at that altitude. [/B]

Robcolo is right. Above 9,000 feet all bikes feel like they lost half their HP, especially the 2 strokes. Get a 525 EXC and install a high compression piston.

 

[cujet]

Actually the HP lapse rate is easily calculated. Any pilot can tell you the percent of power available at any given altitude. Rather than guess at the power loss, look it up. For instance, 18,000 feet equals 50% power output.

Chris

 

[Lorin]

Suprisingly enough, one of the Pitkin locals that showed us around was doing extremely well on his Jack Piner KTM 200. Although it was impressive, I needed a larger, more forgiving abundance of power to make up for my lack of skill. The majority of bikes we saw being used in Colorado were large, 4-strokes (DRZ's, KTM's, and YZF's). I would strongly consider any of the aforementioned bikes if I lived and rode in Colorado.

 

[cujet]

Oops, forgot to include the formula for HP loss at altitude. the following is a fairly accurate way to determine power loss at altitude.

sea level HP x 0.03 x altitude divided by 1000

Chris

5000 feet equals a 15% loss.
6000 feet equals a 18% loss.
7000 feet equals a 21% loss.
10,000 feet equals a 30% loss.

 

[colorado-high]

Everone seems to suggest a 525 to cure my problem. The problem is a 525 puts out appo.50 hp. if you compensate for high altitude you get 35hp. That is not enough.I need some thing that puts out a least 40-50 hp at high altitude. I would like more torque and to come sooner in the power band. I would like to know what combo of crank,rod ,piston and cam would give me the desired results.

 

[agitt73]

souds like you want a 1100 cc street bike

 

[marcusgunby]

How about a big bore KTM 525-you can go as large as 570.

 

[ziadel]

I seem to remember something about a turbo XR 600.....
do some searches, see what you come up with...

 

[cujet]

Some 2 strokes show a slight advantage as far as power loss at altitude vs. a typical 4 stroke. It is often said that 2 strokes have a supercharging effect due to crankcase presurization. This is not really true, as it is very difficult to get a 2 stroke better than 100% VE. However, some 2 stroke ultralight engines have shown a remarkable ability to perform at altitude. These are simple engines and certainly are no where the technology of current dirt bike engines. I suspect it is a matter of intake and exhaust tuning. to be more specific, I suspect that the exhaust is slightly restrictive in the stinger/silencer/muffler area and the intake is tuned for long duration. Some of these engines perform well up to 25,000 feet! It takes a turbo 4 stroke to match the same altitude.

Chris

 

[flying W]

I also ride high altitude in CO. While I think that big bore bikes are the best at high altitude I don't think that a big bore will address your concerns about the lack of snappy power. Most tend to judge dirtbikes not by overall power but by how much power can be instantly generated. In your case, the KTM400 probably still made enough power but because it was running so rich due to altitude gain that it lost alot of throttle response and no longer had snappy power. I have a KDX200 and had a CR500. At high altitude the KDX has better throttle response and wheelies easier then the CR. However, the CR does a much better job of chugging up steep, rocky hills. I would say to get the KTM525 and consider a smaller front sprocket. Figure out what the lowest elevation you will ride and then jet it a bit lean at that elevation. Even with all this, don't feel upset if the KTM won't ride wheelies at 12,000'.

 

[cujet]

Also consider using higher compression to help make up for altitude.

Chris

 

[shed]

I think you should bolt a turbocharger onto an XR650. Should be enough power to get you out of trouble, and minimal power loss at altitude! There is plenty of room to package a turbo beneath the carb where Honda should have put an electric boot. You should also be able to run an open exhaust, thus offsetting the extra weight by binning the standard silencer!

If you dont build it then I will. Just what I need to conquer a wet Welsh bog!

 

[KYHU]

If you absolutely want to wheelie your bike at 12000 feet, why not use the clutch ? You could wheelie a 80cc bike at 12000 by using the clutch.

If you have ridden on a 125 enduro bike, you know that using the clutch to wheelie in slow, technical parts is almost mandatory.

Otherwise, it is possible without too many problems to tune a 525 to make about 60 rwhp.



david