Compression.... - .: XT660.com

banksy · # 1 25-03-10, 22:56

Was reading today that the KTM Duke 690 has 65bhp!!! Its compression ratio is stated as 12.5:1... Now the stroke of the KTM and the XT660s is near as the same and the bore is omly 2mm wider on the Duke than the XTs..

Is the XTs compression alot lower than the KTMs? There is about 15/20hp difference between the motors. Has anyone upped their compression ratio alone to get more HP and if so how much HP did you get?

Kev · # 2 26-03-10, 01:17

The XT660 standard compression ratio is 10:1, I fitted a 11:1 with a 2mm bigger bore 2 years ago, the piston alone gave around a 1.5 to 2hp gain across the whole rev range.

To get HP like the KTM on the XT you need a combination of mods, high lift cam, bigger inlet valves, bigger throttle body, bigger bore with a higher compression ratio ETC.

The new 2010 KTM690R produces 70HP.

http://hellforleathermagazine.com/2009/11/2010-ktm-690-duke-r-690cc-of-s.html

Freez · # 3 26-03-10, 08:07

70HP is not that impressive. Also, not trying to put down KTM, just stating facts. KTM is a nice bike and the weight of the thing is brilliant. I also love the look of it.

Keep in mind that the 70HP is crank HP from a 690cc motor, running 12.5 compression. By the time it gets to the rear wheel, it has lost about 15%, giving you around 60HP at the wheel. Also, that 70HP is produced at around 7000 RPM, which is a 1000 RPM higher than the stock XT's power peak.

A XT with a 686cc motor, 11:1 compression, flowed head, bigger throttle body and a stage 2 cam, with a set of good pipes and DNA stg 2 filter can match that without any problems.

Also, getting 80HP plus with the Yamaha 686cc motor at the crank is also possible with a stg 3 cam, 12.5:1 compression and the bits above. Problem is, you need to change the ECU on the XT to do that as the motor has to produce peak power at 8000 RPM to do that. XT's stock ECU will limit the RPM at 7200 RPM.

banksy · # 4 26-03-10, 08:13

Thanks for the input guys.......... With a compression ratio of 10:1 i bet the motor on the XT will last longer than the KTMs , need less servicing and feel smoother on the road too..

Kev · # 5 26-03-10, 11:24

Are you sure the 70HP is at the crank as most of the KTM dyno sheets I have seen are stated at the rear wheel.

The dyno run bellow is from the old 690 motor with all the bikes run on the same dyno.

http://www.topspeed.com/motorcycles/...ure227331.html

mac-cos · # 6 26-03-10, 13:28

My old KTM 640 Duke made 50bhp on a dynojet RR at sea level. It was running the older Dellorto Slide carb not the power sucking mikuni CV with freeflow (KTM SX) can, open airbox lid and needle/jetting changes to suite.
RR chap was quite impressed with it.

Freez · # 7 30-03-10, 12:03

Hi Kev.

Most if not all manufacturers will claim crank HP in their documentation as rear wheel HP can be affected by tyre grip, pressures and so forth. It is by far more stable and repeatable than rear wheel HP. Crank HP also makes a better story on the marketing brochure.

Kev · # 8 30-03-10, 12:16

Quote:

Originally Posted by Freez

Hi Kev.

Most if not all manufacturers will claim crank HP in their documentation as rear wheel HP can be affected by tyre grip, pressures and so forth. It is by far more stable and repeatable than rear wheel HP. Crank HP also makes a better story on the marketing brochure.

I know what you mean, the Aprilia Dorsoduro claims 92HP, with most of the dyno runs that I have seen showing 75HP at the rear wheel.

Our XT's are not far off the mark from what Yamaha claim.

gianni1951 · # 9 30-03-10, 12:51

Quote:

Originally Posted by Kev

The XT660 standard compression ratio is 10:1, I fitted a 11:1 with a 2mm bigger bore 2 years ago, the piston alone gave around a 1.5 to 2hp gain across the whole rev range.

To get HP like the KTM on the XT you need a combination of mods, high lift cam, bigger inlet valves, bigger throttle body, bigger bore with a higher compression ratio ETC.

The new 2010 KTM690R produces 70HP.

http://hellforleathermagazine.com/20...90cc-of-s.html

Hi Kev,

in wich way you calculate the compression (from 10:1 to 11:1). I think that only a bigger bore don't change the compression. I think that compression is a ratio between the two volume (minimum for compression, maximum at the end of stroke). So, if you don't change the stroke of piston, according to me, you have the same compression independently by the piston diameter. Unless you have pull down the head (machining plane or changing gasket system).

Can you explain me ??? Tank you in advance.

Ciao.

Freez · # 10 30-03-10, 16:16

It is a bit complicated to explain, but normally if you increase bore size, even if you use a perfectly flat piston, you can increase compression a little, but normally thats not how it is done.

The easiest way is to install a higher compression piston and the dome on top of the piston is larger, resulting in it compression the air above the piston more than a piston with a lower dome.

You also can increase compression by skimming the head or barrel.

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