xr650r on low octane fuel

10 replies to this topic
  • simon@vic

Posted October 13, 2006 - 08:52 AM


does any one know what the min octane you can get away with in an xr650r? and or know of a way to lower the compression a bit.

heading to places where fuel is sold out of the back of a truck...... or in a pop bottle.............................

thx for any help

  • SniperTeamBravo

Posted October 13, 2006 - 08:58 AM


If it is stock, I think it'll run on 86, but thats about as low as I'd go. Take a few bottles of octaine booster. I rember seeing on Hondas website that all the big XRs 600~650, this is XR-Rs now, were built with a modest stock compression to run on pump gas and gas in Mexico since they were designed as desert sleds.

  • simon@vic

Posted October 13, 2006 - 09:23 AM


what is stock compression on a xr650r? 9.5:1?

  • rockn6gs

Posted October 13, 2006 - 09:54 AM


Model: XR650R
Engine Type: 649cc liquid-cooled dry-sump single-cylinder four-stroke
Bore and Stroke: 100mm x 82.6mm
Compression Ratio: 10.0:1
Valve Train: SOHC; four-valve
Carburetion: Keihin 40mm piston-valve
Ignition: CD with electronic advance
Transmission: Five-speed
Final Drive: #520 O-ring-sealed endless chain; 14T/48T
Suspension : Front: 46mm leading-axle Kayaba cartridge fork with compression and rebound damping adjustability; 11.2 inches travel
Rear: Pro-Link Kayaba single shock with spring preload, compression and rebound damping adjustability; 12.1 inches travel
Brakes Front: Single 240mm disc with twin-piston caliper
Rear: Single 240mm disc with single-piston caliper
Tires Front: 80/100-21
Rear: 110/100-18
Wheelbase: 58.3 inches
Rake (Caster Angle): 27.8¡
Trail : 111mm (4.3 inches)
Seat Height: 36.8 inches
Ground Clearance: 12.0 inches
Dry Weight: 277 pounds
Fuel Capacity: 2.6 gallons, including 0.5-gallon reserve
Color: Red
Meets current CARB standards.
California version differs slightly due to emissions equipment.

  • HawkGT

Posted October 13, 2006 - 01:53 PM


A more aggressive cam will lower dynamic compression. Remember that the compression ratio assigned to a piston is a calculated figure for the piston (static compression). It's not really the final word on the dynamic compression of an engine--and that's the compression measurement that really matters.

Intake valves don't close until after the piston has already begun the compression stroke. The stock XR650R cam closes the intake valves at 45 deg after bottom dead center. That means the stock 10.0:1 static compression piston actually yields a little less than 9.0:1 of dynamic compression with the stock cam (at 1000ft elevation). Dynamic compression is always less than static compression. Take off about a 1.0 of dynamic comp for every 5000ft increase in elevation.

A Hotcams stage 1 closes the intakes at 54 deg ABDC, a stage 2 at 59.5 deg ABDC. That would reduce dynamic compression by about 0.5 and 0.7 points respectively.

This is why high compression pistons and aggressive cams so often go hand in hand. In some applications an aggressive aftermarket cam will actually lower overall performance unless a high compression piston is added as well. If you loose too much compression by leaving the valves open for a long time then you're going backwards (unless you make up for it with a higher comp piston). I'm not saying this is the case with the XR650R, but I've seen it happen on other engines....

Don't expect much help from auto parts store type octane boosters. Unless you're adding TEL or significant amounts of toluene (like 10-20%) to your fuel, there's not much you can do to improve the octane rating of low quality fuel.

FWIW, the manual recommends 92 (R+M)/2. 91 R+M/2 is all I can get at the pump. And it's crappy reformulated fuel. I've run it in the sand w/ a paddle (i.e. extremely high load) at 200ft in high temps with no audible knock.

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  • rebelventurer

Posted October 13, 2006 - 02:03 PM


Hawkgt is absolutely right! bad gas is bad gas nomatter how much octane boost you put in. fortunatly the big thumpers seem to run ok on bad gas just dont get in a race using the stuff.

  • simon@vic

Posted October 13, 2006 - 03:20 PM


cool. thx for the info guys.

im just going to run the 87 oct and hope for the best.

  • HawkGT

Posted October 13, 2006 - 03:45 PM


cool. thx for the info guys.

im just going to run the 87 oct and hope for the best.

I'd do the same. If you hear it knocking/pinging under load then you'll probably be able to ride around it. In other words, if it's detonating, back out of the throttle until it stops. Most engines can withstand mild deto for short periods without destroying themselves.

  • cleonard

Posted October 13, 2006 - 03:47 PM


cool. thx for the info guys.

im just going to run the 87 oct and hope for the best.

Just listen for the pinging. A little is actually OK. If it gets heavy, back off on the throttle some. Jetting a little richer can help minimize it as well. If you have it, try a one size larger main. I've run my 600 on 87 several times and it pings a lot when it gets hot or at wide open throttle.

  • simon@vic

Posted October 13, 2006 - 03:52 PM


isnt the 600 9:1 comp?

  • cleonard

Posted October 13, 2006 - 03:57 PM


isnt the 600 9:1 comp?

Yes it is. Air cooled engines must have lower compression to make up for the increased heat. When I have the bike jetted for max preformance it pings with 91 pump octane gas in high temperature and open throttle conditions. If I jet one step richer on the main and on the clip, it looses some power, but will run on 87.

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