Isn’t is pretty wild how restricted stock Briggs engines are..

[Thepaetsguy]

So this is a carburetor off a stock 5hp Briggs at wide open throttle. So in a way even if you remove the governor it’s still only getting 75% throttle. and the eyebrows and the swirl piece in the intake. A 5hp is just a smaller really restricted 8hp in my opinion.

 

[sparkwizard]

As designed, installed in the intended application, they never see wide open throttle anyway, unless they are under a pretty heavy load, which limits RPM. They are made to do their work at about 2500-3000.

 

[Powerband83]

Bigger is not always better. The exhaust port on a 5hp is already too big, as one example. The threads inside the port work great for installing an exhaust reducer into it for added torque (Been proven on dyno's over the vast years).

 

[Thepaetsguy]

Bigger is not always better. The exhaust port on a 5hp is already too big, as one example. The threads inside the port work great for installing an exhaust reducer into it for added torque (Been proven on dyno's over the vast years).

I didn’t think the port is to big? I Ported a 5hp on the exhaust side and intake side to move the ports closer to each other. Then after getting it all together I set It up as 10t clutch 60t diff. and at just under 50mph was outrunning the big block 8hp. I did use a screw in exhaust so maybe I restricted it enough to not matter?

 

[Minimichael]

Of course, this whole discussion about exhaust assumes you're using a stock carburetor? I'm building a 5hp now, and I feel better after powerband83's point about threaded exhaust (because I'm going that route). But I suppose a mikuni on the intake changes everything?

 

[Thepaetsguy]

Of course, this whole discussion about exhaust assumes you're using a stock carburetor? I'm building a 5hp now, and I feel better after powerband83's point about threaded exhaust (because I'm going that route). But I suppose a mikuni on the intake changes everything?

Of course, this whole discussion about exhaust assumes you're using a stock carburetor? I'm building a 5hp now, and I feel better after powerband83's point about threaded exhaust (because I'm going that route). But I suppose a mikuni on the intake changes everything?

It’s the stock 5hp carburetor just with a lot done to it. I was not light on porting either side. And used a screw in exhaust. I was doing 47mph and know with bearing upgrades and hunching down in the seat I can break 55

 

[creia]

Bigger is not always better. The exhaust port on a 5hp is already too big, as one example. The threads inside the port work great for installing an exhaust reducer into it for added torque (Been proven on dyno's over the vast years).

Powerband is 100% correct. The (over) size exhaust port on the 5HP Briggs is a design error and actually REDUCES pertformance. (It is actually the same size as the Briggs 6,7,and 8HP engines.) If you are using a "screw-in" muffler it (somewhat) corrects this. If you are using a "bolt-on" flange for a header, years ago you could purchase a screw-in exhaust port reducer on eBay that increased performance by reducing the size of the exhaust port AND smoothed out (no exposed threads) the passage. (I have not seen them on eBay for years)They were illegal in Kart racing (no surprise) as they were considered an illegal modification that provided an advantage as they increased performance. About 12 years ago my son installed one of them on his Briggs Raptor 3 (they come with a higher performance carb than your "standard" Briggs 5HP- larger venturi, no "spiral" and no choke plate/slide) that powers a Bonanza BC-1200 and there was a noticeable difference. I would not advise enlarging the exhaust port AT ALL on a Briggs 5HP. If you want to do something to increase performance just (carefully) smooth out the intake and exhaust port/passages and remove any casting flaws or protrusions - do NOT enlarge it!
Michael

 

[Thepaetsguy]

Powerband is 100% correct. The (over) size exhaust port on the 5HP Briggs is a design error and actually REDUCES pertformance. (It is actually the same size as the Briggs 6,7,and 8HP engines.) If you are using a "screw-in" muffler it (somewhat) corrects this. If you are using a "bolt-on" flange for a header, years ago you could purchase a screw-in exhaust port reducer on eBay that increased performance by reducing the size of the exhaust port AND smoothed out (no exposed threads) the passage. (I have not seen them on eBay for years)They were illegal in Kart racing (no surprise) as they were considered an illegal modification that provided an advantage as they increased performance. About 12 years ago my son installed one of them on his Briggs Raptor 3 (they come with a higher performance carb than your "standard" Briggs 5HP- larger venturi, no "spiral" and no choke plate/slide) that powers a Bonanza BC-1200 and there was a noticeable difference. I would not advise enlarging the exhaust port AT ALL on a Briggs 5HP. If you want to do something to increase performance just (carefully) smooth out the intake and exhaust port/passages and remove any casting flaws or protrusions - do NOT enlarge it!
Michael

 

[Thepaetsguy]

Pocket change racers

 

[Bird Brain]

Ive heard it both ways about port/polishing, bowl blending, acid dipping after and the like. Mirror polishing exhaust ports makes sense for maxium out flow. Removing casting flash and hard turns and edges from an intake port the physics is easily understood also, like when your buddy stands in front of the fan it makes a difference in your comfort level right so you tell him to move. The science is in flow maximizing versus the surface finish left in the intake port. The micro roughness left further increases the air fuel blend on its route to the chamber for burning. That ratio of whats been worked versus whats been left alone is what makes some motors winners and some just get the partipation trophy. I know this isn't a winston cup car but the tech is the same. Get your dremels out and experiment some. Like the lottery, can't win if you don't play. Good luck all.