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oil pan 4 02-05-2017 02:38 PM

Primary tube length for 305cc engine
Exactly like this : but for a different generator.
Engine is a single cylinder 420cc Briggs and Stratton OHV engine, with 3.54 inch bore and 2.6 inch stroke that runs at 3,600rpm on my Home made solar inverter generator hybrid - EcoRenovator
The exhaust system on my 7kw generator is shaking it's self apart and in the thin air here it struggles to power a reasonable 20 amp load. It's also loud.
I would like to put on a longer exhaust pipe to optimize power, a bigger muffler to quiet it down a little since this generator's engine doesn't run unloaded much.
What would be the ideal length and diameter to maximize power?

One calculator is saying 30 inched for a 3/4 inch pipe.
Another is saying the ideal tube diameter is 1/2 inch. For practicality I'm think I am going to try to stick with 3/4 inch.

freebeard 02-09-2017 07:26 AM

Someone should respond to your post. :(

It's hard to get scavenging out of a single cylinder isn't it. I'd trust the calculations, wrap it with Lava Tape and bellmouth the end.

Why is it shaking to pieces? Maybe it needs better mounts and hangers. Bend the 30" into a spiral and it will reduce the lever arm.

oil pan 4 02-09-2017 11:53 AM

The muffler is big, made of thin metal and not well attached.
I also tried to power my 2.5hp air compressor that runs close to 5hp at shutoff with this generator and it couldn't get the compressor going up to speed.
It just couldn't handle the start up surge.

Stubby79 02-09-2017 12:24 PM


Originally Posted by oil pan 4 (Post 534061)
The muffler is big, made of thin metal and not well attached.
I also tried to power my 2.5hp air compressor that runs close to 5hp at shutoff with this generator and it couldn't get the compressor going up to speed.
It just couldn't handle the start up surge.

Engine slow down? If it was maintaining speed, then the weak link isn't the engine.

Hold on a sec...7kw? That's presumably surge rating. For a second or two while something is starting up.

That motor is rated at 8hp gross...which are about 15% now it's 6.8hp. You're trying to get 5hp out of it. Is that measured at the plug? I'll assume so. So, if your engine is putting out what it's supposed to, then you need your generator head to be at least 73.5% efficient. I think that's a bit on the high side (though maybe at peak they're noticeably more efficient).

Anyway, I think your machine is grossly overrated. And that you won't be able to get that much juice out of it. The generator heads I see online recommend ~13hp for that kind of power. That's not a small difference that you might be able to squeeze out of it with a tuned exhaust.

If you don't need as much airflow as it puts out, you could consider changing the pulley size so that the motor isn't loaded nearly so much. Assuming it's belt driven.

oil pan 4 02-09-2017 01:00 PM

It's direct drive, runs at 3,600rpm.
7kw is the run rating, the surge is something like 10,800.
I was going to do exhaust and air intake because it's got one of this air cleaners you see on 5hp lawn mowers. I'm at 4,400ft above sea level, which means I have 12% less air here.

I was going to do a 2 prong approach, increase engine power and add a soft start to the compressor.

Yeah when I hit the generator with the compressor startup load it slowed down to 47hz and never recovered.
When connected to power grid power the compressor surges to 45 amps.

Stubby79 02-09-2017 01:26 PM

this: says that your generator has a 420cc engine. Which is much more reasonable. Same model?

No wonder you expect it to keep up with your compressor.

Better be sure before someone tells you what size pipe to run. 420cc also happens to be almost half the size of your other(920cc) engine, which has 2 cylinders instead of 1, so the sizing is going to be close to the same. At a guess.

45 amps? at 120v? I sure hope you have a thick cord going to that thing!

Wonder if your governor is working properly...can you reach the throttle linkage to try and force it to open the throttle wider, just to see if it helps?

oil pan 4 02-09-2017 04:24 PM

Yes that it. Only difference mine has solar panels, charge controllers, 900w kiasae stand alone pure sine inverter.
My mistake for thinking it's 305cc.
The 922cc (461cc per cylinder) generator needs 52 inch long 3/4 inch primary tubes. For the 420cc maybe do 48 inch long 3/4 wide tube.

The air compressor is 240v. My amp meter says it draws 45 amps, I'm sure it could be more for a brief instant. It draws around 20 amps at shutoff.

freebeard 02-09-2017 08:27 PM

Here's a 1 1/4" Supertrapp muffler that can be had as clamp-on or NPT threaded.

oil pan 4 02-09-2017 10:48 PM

I looked around to find a bendable weldable pipe and found 1 inch outside diameter, 7/8 inch inside diameter. pipe.
3/4 inch water pipe would work but the inside of the pipe isn't that good, each joint there is step, plus the plumbing pipe don't have wide bends.
I'm going to take this pipe weld up one end, pack it full of sand weld a cap on the other end and take it to work and bend it on the big conduit bender.
For a muffler I have collected some motor cycle mufflers I would like to try. If those are too loud I would just stick a car muffler on there.

Stubby79 02-10-2017 05:42 AM

I'm interested to see the results. I have a 420cc engine sitting on a shelf for a future project.

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