Honda GX390 (Lacks Torque)

Ultimate Karting said:
Hey jsstump#70

The engines we supplied come standard with a 91 jet, should I try 100 jet because we are racing at altitude.
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.091mm = .0358 inch
.1 = .0394
.089 = .0350
.086 = .034
That's an increase in the area of the hole of almost 17.4%. Wouldn't you want to use a smaller jet? You would if the engines were originally jetted for sea level.

What you need is an air density gauge!
 
I would use whichever engine performed best on track instead of chasing dyno numbers honestly...let your stopwatch be the guide. Even if your other engine made less hp and less Nm, it was faster on the track, so maybe that engine has a broader torque curve than the engine that shows better hp and Nm on the dyno....a broader torque curve is what you want in a race engne
 
I run the Dyno in question. The max torque is normally around 3500 RPM and Peak power at around 4500 rpm. The dyno has a dynamic correction system and is an inertia dyno. We get 16 degrees spark timing advance over the stock timing from cutting the key by 2mm. We measure the output between 2400 rpm and 5000 rpm. One variable that is not accounted for is variation in clutches as we test with the competitors own clutch. A worn clutch will yield a lower result because of slippage as will a worn sprocket because of extra drag. Our rules allow for a maximum of 16 degrees timing advance and a maximum of 3mm of skim on the head. We do not allow any modification to the carburetor including running alternative jetting. The plug shows the motor is not running lean or rich. Rules do not allow any changed to the stock motor except for the removal of the governor, free flow exhaust, better air filter and the 2 mods already mentioned.
 
FYI in our current configuration we have tested larger and smaller jetting and the optimum jetting is the stock one. Same goes for the ignition timing advance.
 
W5R said:
I would use whichever engine performed best on track instead of chasing dyno numbers honestly...let your stopwatch be the guide. Even if your other engine made less hp and less Nm, it was faster on the track, so maybe that engine has a broader torque curve than the engine that shows better hp and Nm on the dyno....a broader torque curve is what you want in a race engne
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My horsepower spreadsheet and graph have a calculation which shows the "total power under the curve". I think that's what you're talking about.
 
MaxSpeed said:
One variable that is not accounted for is variation in clutches as we test with the competitors own clutch. A worn clutch will yield a lower result because of slippage as will a worn sprocket because of extra drag. Our rules allow for a maximum of 16 degrees timing advance. We do not allow any modification to the carburetor including running alternative jetting. The plug shows the motor is not running lean or rich. Rules do not allow any changed to the stock motor except for the removal of the governor, free flow exhaust, better air filter and the 2 mods already mentioned.
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MaxSpeed said:
FYI in our current configuration we have tested larger and smaller jetting and the optimum jetting is the stock one. Same goes for the ignition timing advance.
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I cant help the original poster of the question in this case. It has been my experience on in most EVERY case, just putting on an open exhaust and open air filter will require a much larger jet to get optimal power. As mentioned, I run one class that is a stock 390 with governor removed and open exhaust and open filter. I cant imagine trying to run a stock sized jet. I run a .038 to .042 depending on conditions. Anything smaller shows lean lean lean on the plug and that's with stock timing. But I really don't know what is considered "stock" jet over there. I have seen fresh out of the box Honda carbs with a variety of jet sizes. The clutch deal is also a variable I would eliminate. Any numbers gathered would be extremely unreliable if in fact the clutch may be slipping excessively the whole time compared to one locked up solid. I assume ALL run the same tooth driver also? A rule dictating the size of the main jet opens up a pile of creative options to be performed in the low jet circuitry. Good luck
 
We use the 92 jet (0.92mm) on the GCAFH1 series engines out of China. One thing that was perhaps not made clear is that although we are allowed a better air filter we have to use the std plastic box meaning that you don't really get much advantage from the air filter. Once you are allowed to swop out the plastic box you most definitely need bigger jetting. In experimentation I found that with a simple pipe between the air filter and carb you need to go up from a 92 jet to a 105 jet (1.05mm). which is in line with your experience and what one generally expects with increased air flow. No modification is allowed to the idle/low circuit jetting either. The carb must be as supplied by Honda with the unmodified motor.
 
MaxSpeed said:
We use the 92 jet (0.92mm) on the GCAFH1 series engines out of China. One thing that was perhaps not made clear is that although we are allowed a better air filter we have to use the std plastic box meaning that you don't really get much advantage from the air filter. Once you are allowed to swop out the plastic box you most definitely need bigger jetting. In experimentation I found that with a simple pipe between the air filter and carb you need to go up from a 92 jet to a 105 jet (1.05mm). which is in line with your experience and what one generally expects with increased air flow. No modification is allowed to the idle/low circuit jetting either. The carb must be as supplied by Honda with the unmodified motor.
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92mm - 1.05mm is a 29.7% increase in jet area.

I don't know the dia. of the crank, but if it's 19.05 (.75") 2mm (.079") cut on the key is 11.92* advance. If the crank is 25.4mm (1.0") 2mm is 8.94* advance.

Tell me the bore and stroke and the stock CC's in the combustion chamber and I can tell you how much you changed the compression ratio. Real close anyway. As close as I can tell, 2mm is a 5 point increase (close) in the compression ratio. Do you run high octane fuel?
 
We run a 1 inch shaft. The 16 degree timing advance was however measured on the flywheel magnet, so confident that the info is correct. We run 93 and 05 octane fuel which is pump fuel in South Africa. The 93 runs with out pre-ignition issues although most teams run 95 octane. Stock bore and stroke.
 
I am also very confident in my calculations on advance. I have never heard of running 16* of timing advance. I have the bore and stroke numbers, but you need the combustion chamber CC's to determine compression ratio.

"pre-ignition?" don't you mean detonation? They are distinctly different things.

You said something about a 2-1 reduction, could that have something to do with your advance number?
 
youngengines I think you put your finger on the discrepancy. I am talking about 16 degrees in total, not 16 degrees on factory setting. Pre-ignition/pre-detonation, is this not perhaps subtle language differences rather than talking about different things. I am talking about what happens when you use fuel with an inadequate octane rating and get a diesel type of effect.
 
Alvin I understand how to calculate the compression ratio. I do not have the head in question so I am unable to measure the volume. My point was that 93 octane petrol (gas in US speak) works just fine.
 
16 total? According to the factory manuals (gx390K1) I have they come factory with 25 degrees fixed, unless you guys run a different variant over there.
 
We run the H1 series made in China .. or the T2 series made in Thailand. Most teams run the H1 as the T2 series has the CDI coil with the rev limiter so you need to toss the flywheel and coil and replace with clone parts to get it to rev.
 
MaxSpeed said:
youngengines I think you put your finger on the discrepancy. I am talking about 16 degrees in total, not 16 degrees on factory setting. Pre-ignition/pre-detonation, is this not perhaps subtle language differences rather than talking about different things. I am talking about what happens when you use fuel with an inadequate octane rating and get a diesel type of effect.
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Preignition occurs when deposits in the combustion chamber get red hot and ignite the fuel before the spark plug fires.

Detonation occurs when, after the spark plug fires, the pressure in the cylinder exceeds the octane rating of the fuel and causes some fuel to self detonate. Sometimes called dieseling. It sometimes sounds like a rock rattling around in the engine or in the exhaust. One thing; detonation can be occurring in your engine long before you hear it. It almost always shows itself by a clean area around the edge of the piston or head. Almost as if something had sandblasted the carbon off the piston in those areas. The piston, normally shiny, will have a dull appearance in those areas.

Any detonation at all will cause a serious drop in performance.

Have you ever seen somebody set fast time, and be leading the race, and about halfway through the race, the second-place guy catches and passes him? My guess is that he had the fuel mixture just about right for peak performance, for a few laps, but as the engine got hotter, it leaned out, (it could be getting some detonation) severely hurting his performance.
 
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