Briggs 3hp build questions

[G.W]

Here's another in pretty orange: https://www.tsracing.com/30-Dipper-Plastic-w-Steel-Inserts-P3565.aspx

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Tha ks for the links. I think I might order both, they are pretty cheap. If they won't fit I'll replicate the design making it smaller to fit the rod.

I really hope to bring enough knowledge from trial and error in this build process to help others out who have the same mission as me. I've read several threads where people ask about building more HP with a smaller engine and their ideas are immediately shut down, not because it's impossible just because no one has tried.

 

[G.W]

Ok, did some research and it looks like the new target is 6000 rpm. The cast aluminum rod won't handle 8000. I was thinking of a forged steel rod, not cast aluminum lol

 

[panchothedog]

For higher rpm, I don't think it's so much the metal that the rod is made from. It's the fact that there is no bearing. To just have the bare metal of the inside of the rod and the rod cap, it's going to be pretty hard to mimic the tolerance of a rod bearing.

 

[G.W]

For higher rpm, I don't think it's so much the metal that the rod is made from. It's the fact that there is no bearing. To just have the bare metal of the inside of the rod and the rod cap, it's going to be pretty hard to mimic the tolerance of a rod bearing.

Interesting. I didn't know it had no bearing. I still am going to build this and see just how long it will last. Maybe I'll learn something useful in building my 79cc predator.

 

[panchothedog]

I'm just going by the picture you show in post #4. There's no bearing in that rod.
It looks similar to the rods on the stock predator engines I build. No bearing, just the inside of the rod and cap machined reasonably close. When you install the
ARC billet rod, it has a bearing insert, just like any automobile engine. The stock predator rod is good to about 5000 rpm, ( opinion varies ) where the billet rod is good to as much rpm as anyone has made one of these small engines to turn.
Better than 9000 rpm, I think.

 

[panchothedog]

See if the diameter of your crank is any where near the diameter of something that ARC manufacture's a rod for. I know that they make over sized bearings for undersized cranks of most common parts that they make.

 

[jetskijerry]

what else are you going to do to this 3.5hp engine to get all this power ? polishing the rod and a diff. dipper is not going to do it.

 

[G.W]

what else are you going to do to this 3.5hp engine to get all this power ? polishing the rod and a diff. dipper is not going to do it.

Some porting and polishing, increasing compression, modifing the cam lobes to have more lift, advancing timing, and a round slide carb. That's all I plan to do for now.

 

[jetskijerry]

sounds like a plan.

 

[G.W]

Oh, and shaving the eyebrows for better flow

 

[G.W]

Here's the one I'm probably gonna use. A freshly rebuilt (from what I was told) Briggs 3.5 HP. The serial number comes back as being made in 2001, I thought they quit making these long before that. Is has some good fresh oil in it, and just needs an ignition coil slapped on it. It's overall in great shape too.

 

[G.W]

I am definitely gonna use this engine
Not only does it have more displacement, but the exhaust also has bolt.holes for a flanged exhaust. That makes fitting a custom pipe much easier. Sure it might be a bit taller than the 3hp, but not by much.

 

[Hellion]

6,000 revs seems more reasonable but you won't get that much out of it on stock valve springs because it will "flutter" or "float" the valves at a certain RPM, meaning the stock springs just cannot open and close fast enough for the desired revs. It's a built-in rev limiter. Your highest revs might be 4-4.5K on stock springs.

Be neat to see where you get ungoverned though, before it grenades.

It's the fact that there is no bearing.

Nonsense, the engine is full of bearings. It has a bearing, just not separate rod bearing insert "shells" with two halves or what-have-you. Would be neat if it did for utter serviceability and longevity. Conceivably they could be added, custom though.

 

[G.W]

6,000 revs seems more reasonable but you won't get that much out of it on stock valve springs because it will "flutter" or "float" the valves at a certain RPM, meaning the stock springs just cannot open and close fast enough for the desired revs. It's a built-in rev limiter. Your highest revs might be 4-4.5K on stock springs.

Be neat to see where you get ungoverned though, before it grenades.



Nonsense, the engine is full of bearings. It has a bearing, just not separate rod bearing insert "shells" with two halves or what-have-you. Would be neat if it did for utter serviceability and longevity. Conceivably they could be added, custom though.

Yeah, I was gonna see what it would float at after modifying the dipper and replacing the flywheel. I've talked to my buddy a bit and he said although it isn't a great idea, I could try to grinding the cam with a file. I'm probably gonna snag 2 more cams for it off eBay so I can try one stock cam, one cam with the foot ground down with a file, and one cam with the cam lobes welded. That's probably gonna be as far as I go with DIY modifying the engine. It should however be a huge improvement on stock for the price of a billet flywheel.

 

[Whitetrashrocker]

I also have a 3hp sitting around. Im following to see what you get out of yours.
I think your on the right track though. As with any small industrial engine, adding some compression, better air flow and adding some timing will really wake it up.
Let the valves float and be the Rev limiter. I've had good luck with that combo so far.
Drill a hole in the top of the crank side on the rod. Chamfer it so some oil will make it's way into it. That should help prolong it's life.

 

[pearl111]

Alright, I'm looking into building a 3hp up to maybe 6 or 8 hp. Yes I know that's stupid and is gonna be expensive, but I don't care. I've set my mind to it, I'm going to do it. This build is however another year down the road, I'm just getting ideas stirred up. I can find an aluminum flywheel for the 3hp, but no rod. I don't wa t to pay to have one custom made either. I was talking to one of my buddies that used to work at a speed shop in the 70s and 8/s and he thinks I could polish the stock rod and it would survive 6k to 8k rpms. The only issue I can see is the steel oil dipper likes to snap off at about 5k rpm. I was thinking if I could polish the con rod and maybe make a stronger oil dipper I would be able to run the factory rod.

I got to thinking about what material to use, and I don't want to use aluminum, it's too unpredictable sometimes, especially on a smaller part like this. Maybe I could use some thicker steel on the new dipper, or what I was thinking was some Nylon. The only issue I could see with nylon is it flexes a lot, and might become brittle over time from the several heat cycles. Any Ideas you guys have on this would be great. Thanks!

Where did you get the info that the steel dipper likes to snap of at about 5k rpms?
Or have you experienced this issue yourself.

Does this only happen with the 3hp. motors? Or can this happen with the 5.5hp also?

Just wondering because I have some B&S, 5.5 motors that I removed the gov.

 

[G.W]

Where did you get the info that the steel dipper likes to snap of at about 5k rpms?
Or have you experienced this issue yourself.

Does this only happen with the 3hp. motors? Or can this happen with the 5.5hp also?

Just wondering because I have some B&S, 5.5 motors that I removed the gov.

I read it on a thread on some forum a while back, I believe it was a 5HP engine, but the same likely applies to all of them.

 

[G.W]

I also have a 3hp sitting around. Im following to see what you get out of yours.
I think your on the right track though. As with any small industrial engine, adding some compression, better air flow and adding some timing will really wake it up.
Let the valves float and be the Rev limiter. I've had good luck with that combo so far.
Drill a hole in the top of the crank side on the rod. Chamfer it so some oil will make it's way into it. That should help prolong it's life.

Which part in the rod? Could you maybe post a picture of what your talking about? Thanks!

 

[pearl111]

I also have a 3hp sitting around. Im following to see what you get out of yours.
I think your on the right track though. As with any small industrial engine, adding some compression, better air flow and adding some timing will really wake it up.
Let the valves float and be the Rev limiter. I've had good luck with that combo so far.
Drill a hole in the top of the crank side on the rod. Chamfer it so some oil will make it's way into it. That should help prolong it's life.

Like G.W. said...... but if you don't have any pictures..... could you try and draw a picture/diagram of the part and the area to drill the hole?

Thanks.

 

[Whitetrashrocker]