Tweet
Hello all,
I have had the opportunity to do some dyno testing on two different XS's in the last year or so and I thought I would post some results and maybe open up some discussion on tuning for performance. The first XS that I dynoed was a bike I built from two parts bikes. The frame and motor were from a 79SF and the carbs were from an 80G. I honed the cylinders, had a valve grind done at a local M/C machine shop, installed new rings, and ran it with a drilled out airbox, K&N, and a Mac 4-1 header. The 80G carbs came with 110 115 115 110 mains and I initially set it up with 115's all the way across. By the way, I left the pilots a stock 42.5. The bike ran great and idled very smooth but it didn't have the top end pull that I wanted. Logic might suggest that I go bigger mains to get more top end so I went to the Dyno to get a baseline. The results showed that above 5000, the bike was running way RICH! 8:1 air/fuel ratio exacatly. I was loosing power because it was too rich. I set the mains back to 110 all the way across and that set the ratio around 12:1 and dramatically increased the top end strength and provided about 38-42 miles per gallon. I eventually sold that bike and it is still running strong and reliable.
Back in May of 2002, I took my 79SF to the dyno for a baseline run. This bike was 100% stock right down to the air filter and exhaust. This bike gave 78 SAE HP and 60.4 SAE Torque and the mixture was about 13:1 on the idle circuit and dropped into rich territory at 10:1 on the mains above 5000. It ran great so I didn't see much sense in changing anything until my exhaust finally cracked for the last time and I decided to go with a Jardine 4-1, all black with the steel can. I took that down to the dyno and with just the 4-1 mod, it put out 87.1 SAE HP and 66.6 SAE torque. 9 more horse and 6 more ft-lbs! The surprising thing was that the mixture had gone richer across the board with the 4-1, averaging about 8-10:1. I had a spare airbox tray that I drilled out so I put that on to perhaps lean it more into a normal range and get more power. With a open airbox (drilled out), I gained 1.6 SAE HP and 1.1 ft-lbs Torque, getting better! Adding the open airbox made the greatest improvement in the mixture, bringing everything up into the 12:1-13:1 range, just about perfect for these motors. So, I was able to increase my SAE hp 10.7 and my torque 7.3 ft-lbs without changing a single jet!! I even have room to go 1 step smaller in the idle jet to clean out the bottom end but I might just be able to adjust the idle jet screw to get the proper mix down low. Now, I know that many of us like to assume that any mod to thses bikes has to require some bigger jets, BUT so far, I am finding that leaving well enough alone might be the solution for more power and economy. I think that Yamaha engineered these bike to flow at max potential straight from the factory and with 284 degrees of cam duration, flowing is their speciality!!!
My conclusive theory so far is this:
By installing the 4-1 header and increasing the scavenging power of the exhaust system, the motor was attempting to move more air through the stock intake system and therefore creating a stronger vacuum pulse against the carb diaphram. This, in turn, would move the diaphram up more at any given throttle setting, thereby giving a richer mixture through the main circuit. Opening the airbox provided more air on hand at any given time and decreased the vacuum pulse against the diaphram and brought the mixture more into a leaner balance. So, by equally opening the intake and exhaust flow patterns, the motor was able to flow better and provide more power and the jets didn't even have to change!! What a relief. Now, this is just a theory based on my experience and knowledge and I am fully open to contrasting theories and experiences. My only hope in sharing this info that we all learn from each other and if anyone else out there wants to hit up a dyno and post their results, we might even come up with a tuning baseline for these superb relics of brute power!!
I'll try now to post my dyno charts but if that doesn't work, email my off line and I'll send them to you for a reference.
On the first chart DYNORUN.004 is totally stock. DYNORUN.010 is the Jardine 4-1 only. DYNORUN.014 is 4-1 and open airbox.
On the second chart, I have the final results in standard hp and torque with the mixture in the bottom field.
I hope you enjoy reading all this and we'll see you in the twisties!
I have had the opportunity to do some dyno testing on two different XS's in the last year or so and I thought I would post some results and maybe open up some discussion on tuning for performance. The first XS that I dynoed was a bike I built from two parts bikes. The frame and motor were from a 79SF and the carbs were from an 80G. I honed the cylinders, had a valve grind done at a local M/C machine shop, installed new rings, and ran it with a drilled out airbox, K&N, and a Mac 4-1 header. The 80G carbs came with 110 115 115 110 mains and I initially set it up with 115's all the way across. By the way, I left the pilots a stock 42.5. The bike ran great and idled very smooth but it didn't have the top end pull that I wanted. Logic might suggest that I go bigger mains to get more top end so I went to the Dyno to get a baseline. The results showed that above 5000, the bike was running way RICH! 8:1 air/fuel ratio exacatly. I was loosing power because it was too rich. I set the mains back to 110 all the way across and that set the ratio around 12:1 and dramatically increased the top end strength and provided about 38-42 miles per gallon. I eventually sold that bike and it is still running strong and reliable.
Back in May of 2002, I took my 79SF to the dyno for a baseline run. This bike was 100% stock right down to the air filter and exhaust. This bike gave 78 SAE HP and 60.4 SAE Torque and the mixture was about 13:1 on the idle circuit and dropped into rich territory at 10:1 on the mains above 5000. It ran great so I didn't see much sense in changing anything until my exhaust finally cracked for the last time and I decided to go with a Jardine 4-1, all black with the steel can. I took that down to the dyno and with just the 4-1 mod, it put out 87.1 SAE HP and 66.6 SAE torque. 9 more horse and 6 more ft-lbs! The surprising thing was that the mixture had gone richer across the board with the 4-1, averaging about 8-10:1. I had a spare airbox tray that I drilled out so I put that on to perhaps lean it more into a normal range and get more power. With a open airbox (drilled out), I gained 1.6 SAE HP and 1.1 ft-lbs Torque, getting better! Adding the open airbox made the greatest improvement in the mixture, bringing everything up into the 12:1-13:1 range, just about perfect for these motors. So, I was able to increase my SAE hp 10.7 and my torque 7.3 ft-lbs without changing a single jet!! I even have room to go 1 step smaller in the idle jet to clean out the bottom end but I might just be able to adjust the idle jet screw to get the proper mix down low. Now, I know that many of us like to assume that any mod to thses bikes has to require some bigger jets, BUT so far, I am finding that leaving well enough alone might be the solution for more power and economy. I think that Yamaha engineered these bike to flow at max potential straight from the factory and with 284 degrees of cam duration, flowing is their speciality!!!
My conclusive theory so far is this:
By installing the 4-1 header and increasing the scavenging power of the exhaust system, the motor was attempting to move more air through the stock intake system and therefore creating a stronger vacuum pulse against the carb diaphram. This, in turn, would move the diaphram up more at any given throttle setting, thereby giving a richer mixture through the main circuit. Opening the airbox provided more air on hand at any given time and decreased the vacuum pulse against the diaphram and brought the mixture more into a leaner balance. So, by equally opening the intake and exhaust flow patterns, the motor was able to flow better and provide more power and the jets didn't even have to change!! What a relief. Now, this is just a theory based on my experience and knowledge and I am fully open to contrasting theories and experiences. My only hope in sharing this info that we all learn from each other and if anyone else out there wants to hit up a dyno and post their results, we might even come up with a tuning baseline for these superb relics of brute power!!
I'll try now to post my dyno charts but if that doesn't work, email my off line and I'll send them to you for a reference.
On the first chart DYNORUN.004 is totally stock. DYNORUN.010 is the Jardine 4-1 only. DYNORUN.014 is 4-1 and open airbox.
On the second chart, I have the final results in standard hp and torque with the mixture in the bottom field.
I hope you enjoy reading all this and we'll see you in the twisties!
If anyone out there is a specialist at this, maybe they could walk me through the process. Carbs are my speciality, not web links!! Anyway, take care all.
Comment