Jerryburger
Admiral Annoy
Okay.... this is a VERY informative article posted originally in a Briggs & Stratton racing forum. FULL CREDIT goes to the original Author/Poster. Although this article concerns the 5hp flathead, I'm sure the basic principles can also be applied to other engines that were designed as "utility" and being modified for karting/racing. Hope this helps....
PART ONE of TWO:
Blueprinting the Briggs Stocker - One Step at a Time
By Joey Padgett of Checkered Flag Fuels
World Karting Magazine, November, 1994
Nobody runs a box stock Briggs 5hp engine in WKA races. Everybody has "blueprinted" their engine, increasing the performance from the stock 5hp configuration to a considerably higher output. All these modifications and the increased performance hopefully falls inside the rules governing the Briggs Stock 5hp classes racing in WKA.
When it all began, Briggs racers bought a bone-stock 5hp engine, raced it for a while, then took it apart and rubbed on it, or had a buddy "who is real sharp with Briggs" rub on it till it ran faster and faster each race. The Briggs Box Stock engine rules, currently used by the WKA, mirror what evolved over time as modifications by some of the best "blueprinters" in WKA racing.
Today, a top Briggs engine shop will buy new engines, season them in their own way and then begin the blueprinting process. Usually, the engine is torn down to the bare block, all machine work done after carefully measuring the stock engine. A blue printed carb and modified cam are added, the engine is run in on the dyno. The cost for all this ranges from $500 to $700 depending on the engine bulder and the end result.
Joey Padgett and his crew at Checkered Flag Racing Fuels have always wondered how each stop of their engine blueprinting process affected the overall performance of the Briggs stock 5hp engine. Well, to satisfy their curiosity, and to educate and inform WKA members, the WKA donated a Box Stock 5hp to Padgett and crew to blueprint and dyno in a step-by-step process, carefully measuring the effect of each change along the way.
The Checkered Flag crew for this test consisted of Joey Padgett, chief engine builder, and Brandon Creedle, chief dyno operator and grunt labor. Checkered Flag has a fully equipped machine shop and their shop's talents at building Briggs 5hp engines are nationally recognized with several WKA National wins. They use a Stuska water brake dyno and have worked very hard at getting accurate and repeatabe results from their dyno.
1. Engine Break-In
The first step in preparing a race engine from the Stock 5hp Briggs out of the box (hence, box stock), was to run the engine in on the dyno for three hours at 5800 rpm. This was done, monitoring the temperature and putting the engine under a small load on the dyno. The goal was to season the block and get all the moving parts familiar with one another.
A dyno pull was made after engine break-in on the stock but now seasoned engine. A reading was made at 3600 rpm., since Briggs uses that rpm figure for establishing their 5hp rating. The engine made 5.09 horsepower at 3600 rpm, just a tad more than Briggs claims for this engine. Further runs were made to establish the power curve of the engine in the rpm range where we expect to run the engine on the track.
RPM Torque HP*
3600 - 5.09
4750 .880 4.48
5000 .750 4.02
5250 .670 3.77
5500 .560 3.30
5750 .430 2.65
6000 .330 2.12
Average .603 3.39
*All dyno runs in this article are corrected to standard temperature, humidity and atmospheric pressure to reduce any weather related influences. Torque figures are gauge readings, and not actual corrected torque.
Modifications
The step-by-step modifications to e performed on this stock engine were the same as done to each blueprint Checkered Flag engine. The sequence of the modifications was selected to resemble what a typical racer might try if he were blueprinting the engine himself. Also, the sequence was selected to minimize the amount of teardowns and was accomplished with tearing the motor down only twice.
Exhaust Change
The stock Briggs muffler was removed and a .930 triple stage header was installed on the stock Briggs 5hp test engine. Again, the engine was running on pump gas with 21 ounces of 30W petroleum oil in the sump as per Briggs standard recommendations.
RPM Torque HP Change %Change
3600 - 5.52 0.430 8.44
4750 1.145 5.79 1.310 29.94
5000 1.060 5.65 1.630 40.55
5250 0.980 5.48 1.710 45.36
5500 0.905 5.30 2.000 60.60
5750 0.720 4.41 1.760 66.41
6000 0.640 4.09 1.970 92.92
Average 0.908 5.12 1.730 59.41
Obviously, the stock Briggs muffler was very restrictive to the exhaust flow. The huge increase in horsepower from 5000 to 6000 rpm reflects this.
2. Change to Menthanol Fuel
RPM Torque HP Change %Change
3600 - 6.35 0.830 15.04
4750 1.240 6.27 0.480 8.29
5000 1.120 5.96 0.310 5.49
5250 1.040 5.81 0.330 6.02
5500 0.950 5.56 0.260 4.90
5750 0.740 4.53 0.120 2.72
6000 0.650 4.15 0.006 1.47
Average 0.957 5.38 0.260 5.08
The change to menthanol fuel produced the biggest gain on the bottom end of the power curve and diminished as the RPM increased. The temperature indicated that a smaller jet may have produced more power with the rest of the motor in the stock condition.
3. Blueprinted Carb
The stock carb we had been using for the test up to this point was flowed and blueprinted and flowed again. Before the blueprinting, the carb flowed 21.03 cfm, after the blueprint job, it flowed 24.36 cfm, an increase of 16%.
RPM Torque HP Change %Change
3600 - 6.57 0.220 3.46
4750 1.320 6.66 0.390 6.22
5000 1.220 6.48 0.520 8.72
5250 1.080 6.02 0.210 3.61
5500 0.980 5.73 0.170 3.06
5750 0.830 5.07 0.540 11.92
6000 0.660 4.21 0.060 1.45
Average 1.015 5.70 0.320 5.95
The blueprint carb was a good change for the bottom of the power curve, but the .052" jet may have put a hole in the fuel curve as indicated by the percentage change around 5500, 5750 and at the top rpm at 6000. Further modifying this engine should take advantage of the extra rich fuel curve on the top end of this motor.
....to be continued in next post in this thread. -JerryAssburger
PART ONE of TWO:
Blueprinting the Briggs Stocker - One Step at a Time
By Joey Padgett of Checkered Flag Fuels
World Karting Magazine, November, 1994
Nobody runs a box stock Briggs 5hp engine in WKA races. Everybody has "blueprinted" their engine, increasing the performance from the stock 5hp configuration to a considerably higher output. All these modifications and the increased performance hopefully falls inside the rules governing the Briggs Stock 5hp classes racing in WKA.
When it all began, Briggs racers bought a bone-stock 5hp engine, raced it for a while, then took it apart and rubbed on it, or had a buddy "who is real sharp with Briggs" rub on it till it ran faster and faster each race. The Briggs Box Stock engine rules, currently used by the WKA, mirror what evolved over time as modifications by some of the best "blueprinters" in WKA racing.
Today, a top Briggs engine shop will buy new engines, season them in their own way and then begin the blueprinting process. Usually, the engine is torn down to the bare block, all machine work done after carefully measuring the stock engine. A blue printed carb and modified cam are added, the engine is run in on the dyno. The cost for all this ranges from $500 to $700 depending on the engine bulder and the end result.
Joey Padgett and his crew at Checkered Flag Racing Fuels have always wondered how each stop of their engine blueprinting process affected the overall performance of the Briggs stock 5hp engine. Well, to satisfy their curiosity, and to educate and inform WKA members, the WKA donated a Box Stock 5hp to Padgett and crew to blueprint and dyno in a step-by-step process, carefully measuring the effect of each change along the way.
The Checkered Flag crew for this test consisted of Joey Padgett, chief engine builder, and Brandon Creedle, chief dyno operator and grunt labor. Checkered Flag has a fully equipped machine shop and their shop's talents at building Briggs 5hp engines are nationally recognized with several WKA National wins. They use a Stuska water brake dyno and have worked very hard at getting accurate and repeatabe results from their dyno.
1. Engine Break-In
The first step in preparing a race engine from the Stock 5hp Briggs out of the box (hence, box stock), was to run the engine in on the dyno for three hours at 5800 rpm. This was done, monitoring the temperature and putting the engine under a small load on the dyno. The goal was to season the block and get all the moving parts familiar with one another.
A dyno pull was made after engine break-in on the stock but now seasoned engine. A reading was made at 3600 rpm., since Briggs uses that rpm figure for establishing their 5hp rating. The engine made 5.09 horsepower at 3600 rpm, just a tad more than Briggs claims for this engine. Further runs were made to establish the power curve of the engine in the rpm range where we expect to run the engine on the track.
RPM Torque HP*
3600 - 5.09
4750 .880 4.48
5000 .750 4.02
5250 .670 3.77
5500 .560 3.30
5750 .430 2.65
6000 .330 2.12
Average .603 3.39
*All dyno runs in this article are corrected to standard temperature, humidity and atmospheric pressure to reduce any weather related influences. Torque figures are gauge readings, and not actual corrected torque.
Modifications
The step-by-step modifications to e performed on this stock engine were the same as done to each blueprint Checkered Flag engine. The sequence of the modifications was selected to resemble what a typical racer might try if he were blueprinting the engine himself. Also, the sequence was selected to minimize the amount of teardowns and was accomplished with tearing the motor down only twice.
Exhaust Change
The stock Briggs muffler was removed and a .930 triple stage header was installed on the stock Briggs 5hp test engine. Again, the engine was running on pump gas with 21 ounces of 30W petroleum oil in the sump as per Briggs standard recommendations.
RPM Torque HP Change %Change
3600 - 5.52 0.430 8.44
4750 1.145 5.79 1.310 29.94
5000 1.060 5.65 1.630 40.55
5250 0.980 5.48 1.710 45.36
5500 0.905 5.30 2.000 60.60
5750 0.720 4.41 1.760 66.41
6000 0.640 4.09 1.970 92.92
Average 0.908 5.12 1.730 59.41
Obviously, the stock Briggs muffler was very restrictive to the exhaust flow. The huge increase in horsepower from 5000 to 6000 rpm reflects this.
2. Change to Menthanol Fuel
RPM Torque HP Change %Change
3600 - 6.35 0.830 15.04
4750 1.240 6.27 0.480 8.29
5000 1.120 5.96 0.310 5.49
5250 1.040 5.81 0.330 6.02
5500 0.950 5.56 0.260 4.90
5750 0.740 4.53 0.120 2.72
6000 0.650 4.15 0.006 1.47
Average 0.957 5.38 0.260 5.08
The change to menthanol fuel produced the biggest gain on the bottom end of the power curve and diminished as the RPM increased. The temperature indicated that a smaller jet may have produced more power with the rest of the motor in the stock condition.
3. Blueprinted Carb
The stock carb we had been using for the test up to this point was flowed and blueprinted and flowed again. Before the blueprinting, the carb flowed 21.03 cfm, after the blueprint job, it flowed 24.36 cfm, an increase of 16%.
RPM Torque HP Change %Change
3600 - 6.57 0.220 3.46
4750 1.320 6.66 0.390 6.22
5000 1.220 6.48 0.520 8.72
5250 1.080 6.02 0.210 3.61
5500 0.980 5.73 0.170 3.06
5750 0.830 5.07 0.540 11.92
6000 0.660 4.21 0.060 1.45
Average 1.015 5.70 0.320 5.95
The blueprint carb was a good change for the bottom of the power curve, but the .052" jet may have put a hole in the fuel curve as indicated by the percentage change around 5500, 5750 and at the top rpm at 6000. Further modifying this engine should take advantage of the extra rich fuel curve on the top end of this motor.
....to be continued in next post in this thread. -JerryAssburger