briggs and stratton 5hp engine : LUSENET : Junkyard Wars : One Thread

hey guys im really new to gokarts and i have just aquired a briggs and stratton 5hp engine and i was just woundering what mods i could do at a reasonable price. plus whats the best performance for the money. thaks

-- peter eddinger (, April 19, 2003


There is an article on the web that documents the build up of a 5hp B/S engine; they would perform a modification, and then run it on a dyno to see what worked best and what didn't. The biggest, cheapest, and easiest hp gain was when they removed the stock muffler and ran a 14" straight pipe.If you still need a muffler on it, a good idea is to go to a plumbing store and get a 3/4" female to 1" female adapter (about $1.50) and then run a larger muffler on the end if your 14" pipe. One interesting note is that B/S tunes their engines to put out their maximum hp at 3600 rpm.... so removing the governer and trying to spin the engine faster does no good without modifications. When they did the pipe modification, they gained about 1hp @ 4700 rpm. I have no idea of how safe it is to run a stock, non-race engine at this speed. I've heard that the oil-dipper is the weak link, shattering and causing bad things to happen. Check out the web for high performance dippers... about $10. They did more to the engine, such as porting, increasing the compression ratio, etc... but the best and cheapest gain over stock was yanking the muffler and putting on the 3/4" straight pipe. Hope this helps. I just found an old 1978 5hp B/S myself... can't wait to mess with it!

-- Gino (, May 05, 2003.

Hi- here's an excerpt from that article.... I lied to you about the 1 hp gain... it was about 3/4hp, but still the biggest gain they got out of it. 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. Futher 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.


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.

See how well it worked? You gain 1/2 hp @ stock rpm, and 3/4 hp at 4750 rpm... not bad, for a trip to the plumbing store! They say they're using a triple stage header, but for most purposes, a 3/4" x 14" pipe is the not-so-pretty way to accomplish close to the same thing. Also look at the torque readings.... on the stock engine the torque readings had already peaked and were dropping by 3600 rpm. With the pipe change, the torque increased all the way to 4750 rpm, meaning that this engine will keep pulling pretty hard at it's hp peak.

-- Gino (, May 05, 2003.

Here's the web address. My "reprint" looked horrible.

-- Gino (, May 05, 2003.

What about a cam shaft

-- jimmey Tirrey (, May 08, 2003.

Hello my name is Tommy i own SilentnightSpeed and i build R3,zilla,and ohv motors all day long so there isent much i cant answer,now about the cam stock lift on a 5 hp briggs is .233 very weak lift when u change the cam u change where your peak torque is so lets say u use a Dyno 98exp cam it is a jr dragster cam it starts to pull at 4500 rpm and keeps pulling up to and past 9000 rpm so u pick a cam to suit your driving conditions,,,and about the rod a normal cast alum 5hp rod is good till about 5000 rpm a R3 rod is ok till about 7300 rpm and a good billet rod is good for 10000+rpm for short burst...hope this helps e mail me any time if u have any Q's

-- Tommy Dodd (, September 15, 2003.

Tommy! Man, your like the 4th person I have seen of 4cycle on here!

Talk to ya later.

-- Jamie Miles (, May 12, 2004.

hey i am building an engine for fun...and i want the most power possible...i did the muffler deal and i also have all new piston and rings...block is bored +.020 alone with the head machined for more was a stock 5hp craftsman/briggs and much hp should i get now and is there anything else i can like some sites to buy parts from. please e-mail me at

-- Bobby Turgeon (, July 03, 2004.

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