5hp FlatHead B/S Build-up

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Jerryburger

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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
 

Jerryburger

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PART TWO of TWO.....

4. Deck Block and Mill Head

Now the machine work begins. Taping off all the openings, the engine was left assembled except for removing the head. The head is milled to the specs listed in the WKA tech manual. The block was decked so that the piston protruded out of the block .0035". The engine was cleaned off and the head reinstalled with a stock gasket and the bolts torqued.


RPM Torque HP Change %Change

3600 - 6.69 0.120 1.83
4750 1.140 7.16 0.500 7.51
5000 1.320 7.06 0.580 8.95
5250 1.220 6.85 0.830 15.45
5500 1.080 6.35 0.620 10.82
5750 0.950 5.84 0.770 15.19
6000 0.820 5.26 1.050 24.94
Average 1.130 6.42 0.720 12.63


The engine liked more compression and the re-chambering of the combustion chamber. A 12% average increase is great for just doing some minor machine work. Importantly, this increased the engine's ability to work better at the higher rpms and produced the most significant increases above 5000 rpm.


5. Porting and Valve Job

The ports were de-burred and enlarged, using information from the flow bench as previously developed by Checkered Flag. It's very easy to get the exhaust port too big, so just a general deburring and smoothing of the port was done here. Most of the port work was concentrated on the intake port.


RPM Torque HP Change %Change

3600 - 7.02 0.330 4.93
4750 1.520 7.73 0.570 7.96
5000 1.420 7.60 0.540 7.65
5250 1.340 7.53 0.680 9.93
5500 1.190 7.01 0.660 10.39
5750 1.100 6.77 0.930 15.92
6000 0.920 5.91 0.650 12.36
Average 1.200 7.09 0.670 10.44


Joey felt he should have just de-burred the ports and made a dyno run so we could have compared exactly what a good port job would mean as compared to a karter just de-burring his ports with a hand grinder at home. Again, this porting work is just moving the power band up the rpm scale and allowing the earlier changes on the carb and fuel to take full effect.


6. Honing Cylinder and Clearance Rings

The engine was disassembled and the block was honed out to a 0.003" oversize. The ring end gaps before honing were:

Before Hone:
Top 0.016"
2nd ring 0.009"
Oil ring 0.016"

After Hone:
Top 0.002" (using a 0.010" in a std. bore)
2nd ring 0.018" (same ring w/0.003 more gap due to hone)
Oil ring 0.090" (ring thickness ground down from 0.130 to 0.095)


Cylinder had 0.0035" clearance on the piston with .002 out of round and 0.001" taper.
After Hone: 0.0065" piston clearance, 0.000" out of round and 0.000" taper.


RPM Torque HP Change %Change

3600 - 7.08 0.060 0.85
4750 1.550 7.92 0.190 2.20
5000 1.480 7.96 0.360 4.74
5250 1.380 7.79 0.260 3.45
5500 1.290 7.63 0.620 8.85
5750 1.180 7.30 0.530 7.83
6000 1.060 6.84 0.930 15.74
Average 1.320 7.57 0.480 6.77


The engine picked up at the high rpm due to the decrease in ring drag and reduced internal friction. The engine runs the longer races, these lower temps will result in reducing the internal friction and engine wear.


7. Synthetic Oil


Up until now, all runs had been made using 21 oz. of 30W Petroleum oil as Briggs specifies. On this run, 14 oz. of a light synthetic oil generally used by most karts was substituted.


RPM Torque HP Change %Change

3600 - 7.10 0.020 0.28
4750 1.585 8.03 0.110 1.38
5000 1.505 8.03 0.070 0.88
5250 1.395 7.81 0.020 0.26
5500 1.320 7.75 0.120 1.57
5750 1.230 7.55 0.250 3.42
6000 1.120 7.17 0.330 4.82
Average 1.359 7.72 0.150 1.98


Not a huge gain here - maybe if we'd used some of the "trick" (i.e. illegal) oils here, we might have seen more gain. In fact, Checkered Flag sells some of the base substances to mix with synthetic oils to make it "hot." Their own dyno testing shows little if any real gain on the dyno.


8. Ignition Timing

The stock timing from Briggs for all previous dyno runs had been 21(. Using a #5 offset timing key, it was advanced to 28(.


RPM Torque HP Change %Change

3600 - 7.25 0.150 2.11
4750 1.600 8.16 0.130 1.62
5000 1.500 8.05 0.020 0.25
5250 1.430 8.06 0.150 1.90
5000 1.340 7.91 0.160 2.06
5750 1.280 7.90 0.350 4.64
6000 1.170 7.54 0.370 5.16
Average 1.387 7.94 0.220 2.85


Again, just small gains with the timing advanced, all under a 5% gain. A Limited Modified engine with its more radical cam timing will show more results from the advanced ignition timing.


9. Camshaft Change

The stock Briggs cam was replaced with the ZX3 cam from Checkered Flag. This specifically profiled cam works the best in all their stock class engine blueprints.


RPM Torque HP Change %Change

3600 - 6.88 0.370 -5.10
4750 1.670 8.52 0.360 4.41
5000 1.620 8.70 0.650 8.07
5250 1.560 8.79 0.730 9.06
5500 1.460 8.62 0.710 8.98
5750 1.380 8.52 0.620 7.85
6000 1.310 8.44 0.900 11.19
Average 1.500 8.60 0.660 8.31


The engine initially lost power at 3600 rpm with the cam change because that rpm was out of the power curve for this cam profile. This cam provided a good power increase from 5000 to 5500 and another great power surge at the very top rpm of 6000.


Summary

The best horsepower number for this test engine came on the final run at 5250 rpm where it made 8.79 horsepower. Joey says the average engine out of their shop is 8.75 to 8.9 horsepower, so our test engine is right on target. The best WKA legal engine they have seen on their dyno is just over 9 hp!

Reviewing the step-by-step blueprinting process:


Modification Avg. HP Peak HP Gain

Exhaust Change 5.12 5.79 59%
Menthanol Fuel 5.38 6.27 5%
Blueprint Carb 5.70 6.66 6%
Deck Block, head 6.42 7.16 13%
Port Work 7.09 7.73 10%
Hone & Clearance 7.57 7.96 7%
Synthetic Oil 7.72 8.03 2%
Reset Timing 7.94 8.16 3%
Change Camshaft 8.60 8.79 8%

Total Blueprint 170%


Your results may vary according to how your dyno is set up and operated. It is very evident that the biggest gain was in the exhaust pipe and in the machine work and porting. The WKA has sought to restrict what can be gained from other things like camshafts and carbs, trying to even the playing field for all racers.

A blueprinted engine from Checkered Flag just like the one we have here would cost about $600. Just the machine work, decking the block ($20), porting the cylinder ($50) and milling the head ($10) would cost $80 and are the items the average karter would not have the equipment to do in his garage at home. But then, the experience and expertise of shops like Checkered Flag are sometimes worth the price of buying the total engine from them.

END OF ARTICLE.

...pretty interesting stuff, huh?
 

Jerryburger

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What's a real eye-opener is how the stock exhaust chokes the engine of 1/2hp, even at 3600 rpm... where the engine is supposed to run!

If I ever find such a well-documented article on the Honda GX200/Harbor Freight clone, I'll post it. I get the feeling it's going to be the next "Flathead Briggs" among karters.
 

gocartkid

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Thanks for posting that Jerry....I'm building up a Briggs 5hp right now and this will really help with my decisions on which mods I should do to it. :thumbsup:
 
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ironman

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What's a real eye-opener is how the stock exhaust chokes the engine of 1/2hp, even at 3600 rpm... where the engine is supposed to run!

If I ever find such a well-documented article on the Honda GX200/Harbor Freight clone, I'll post it. I get the feeling it's going to be the next "Flathead Briggs" among karters.

thats what i like to see info like that .jerry thats great info there...hum.......:toetap05: i remenber a old man on here{russ} that said you would not get any gain on the exhaust change.
;) this is really great info thanks jerry.
 

Russ2251

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thats what i like to see info like that .jerry thats great info there...hum.......:toetap05: i remenber a old man on here{russ} that said you would not get any gain on the exhaust change.
;) this is really great info thanks jerry.
Yeah well if your gonna do all that other stuff it will make a difference. If your running a governor these mods mean nothing.
As far as the 'old man' crack goes...you will never know what I know.
 

ironman

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i hear you russ if i knew have the stuff you knew i be in good shape.
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

3600 see the different governor or not.
 

kibble

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That's some good stuff posted by Jerry!

Yeah well if your gonna do all that other stuff it will make a difference. If your running a governor these mods mean nothing.
As far as the 'old man' crack goes...you will never know what I know.

Hey, how about those Brooklyn Dodgers, huh? :arf: :arf: :arf:
 

Jerryburger

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I think why the gain was so much on the exhaust side was the fact that those watchful eyes & ears at the EPA have pretty much mandated these things to be a bit too quiet. I'm sure the LawnBoys are seriously choked below their potential, even without messing with the intake, etc. This kinda gives the 4-stroke guys something to aim for, so that they can try to keep up with the 2 strokes.
 

tenlizard

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a dyno can measure hp? how much does a cheap dyno cost? i think they cost a lot don't they?
 

Kenny_McCormic

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Nope, dont worry about numbers, dynos are always off a few HP and only useful on small engines for comparing 2 with the same dyno.

Like I said dont worry about numbers increase is all that counts.
 
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