Honda GX390 (Lacks Torque)

U

Ultimate Karting

New member
Hi Guys

We run almost standard 390cc Honda engines with 2mm skimmed heads and advancing the timing key by 2mm. Engines according to our dyno are not allowed to produce more than 24hsp and 49nm of torque.

Majority of the engines are sealed between 23.5hsp and 23.9hsp with torque of between 47 and 48nm. Now my race engine was producing 23.8hsp but was down in torque 46.7nm. (Spare engine 23.95hsp but 45.5nm)

How can I keep the hsp but increase the torque?

Does advancing the timing increase hsp only or does it increase both hsp and torque?
If I went to a standard timing key would that possibly drop hsp and allow me to take additional head gasket out which was added because the engine was coming in over on hsp?

We aren't allowed to modify carbs and are running the standard jetting, we did try one jet size higher (91 jet to 92 jet) but it didn't make much difference on the dyno.

Last question is by running a spacer behind the valve springs which obviously makes them slightly harder will that improve HSP or NM or both?

Any assistance will be appreciated
 
So are experiencing a lack of on track performance? Other than what the dyno has said, are there any issues?
First off, a dyno measures the amount of force or "torque" produced. Horsepower is simply a mathematical calculation using the torque numbers measured. Are all of the engines tested on the same dyno? On the same day? What type of dyno? If the dyno operator doesn't put a correction factor into the calculations to take ambient conditions into account, any numbers will be suspect and conditions change every day and throughout the day.
Do you use aftermarket open air filter and open header? If so, the jet sizes mentioned seem way low. I run a pure stock 390 in one class(with open filter and header) and typically have a .039 or .040 jet which would be a 102 to 105 (Keihin) or 100 to 102 (Dynojet) size. If you have stock air box and muffler then your jets may be OK but if not, your way lean I fear.
What rpms are you turning? The valve spring shim you spoke of will just allow you to possibly turn more rpm without floating the valves. You should be able to turn 5000 rpm with stock unshimed springs no problem. Simply increasing the valve spring pressure will hurt performance if its not needed to prevent valve float.
These are very interesting rules. Where do you run? There are others who may post who can better describe the relationship between torque and horsepower and how rpm is relavant, in an understandable fashion. (rpm X T) divided by 5252 = HP
 
Torque X RPM / 5252.1 = Horsepower. The only way to increase HP (a calculation) is to increase the torque, or produce the same amount of torque at a high RPM. At what RPM are you taking your readings?
What fuel are you running? What's the stock engines timing, in degrees? That seems to be a lot of advance, but not knowing what the factory setting is, I can't say for sure! What's "nm"? Is it metric, is there a conversion to decimal? What is "hsp"?
 
NM is newton Meter, the formula does not change

One metric horsepower is needed to lift 75 kilograms (avg. body weight of a person) by 1 meter (3.28 feet) in 1 second. Units called "horsepower" have differing definitions: The mechanical horsepower, also known as imperial horsepower, of exactly 550 foot-pounds per second is approximately equivalent to 745.7 watts

In order to use these numbers in your formula you need to convert NM to Lb/Ft 1 Lb-Ft = 1.35582 Nm.
 
Kart43 said:
NM is newton Meter, the formula does not change

In order to use these numbers in your formula you need to convert NM to Lb/Ft 1 Lb-Ft = 1.35582 Nm.
Click to expand...

one newton meter. = 0.737562 pounds force.
So 45nM = 33.19029 pounds of force.
So, converting his 45 newton meters to pounds.
Using my spreadsheet I get these numbers.
RPM------nM torque----HP
3,000------33.19------18.96
4,000------33.19------25.28
5,000------33.19------31.60
6,000------33.19------37.92
7,000------33.19------44.24
Do these engines make that much HP?
At what RPM do they take the readings?
There has been no follow up from the OP, so I guess it doesn't make any difference anyway. It was his first and only post.
 
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My point is your calculating his power with engine rpm and torque at the output shaft after the 2:1. The engine is revving to 14,000 rpm in your calculations above. The original poster can confirm if those torque figures are with a reduction, but considering a gx390 usually makes about 25 Nm I would say it is.
 
He never mentioned any RPM numbers. The ones I used were just for demonstration, to show how much HP the engine would be making if his torque number was at those RPM points.

Where did you get that 25nM number from.
 
Hey jsstump#70

The 1st engine which should 46.7nm and 23.7hsp was way down in qualifying (Qualified 5th where I normally qualify 1st or 2nd). I couldn't run with the top teams and on our track which has a steady uphill section I was being drilled on acceleration which I attribute to NM.
Top speed was ok but even in the tow I could only sit with the faster karts. Normally in the toe you land up pushing about halfway down the straight.

My 2nd engine which only had 45nm on the dyno was better on the racetrack but worse on the dyno. The dyno is a straight engine dyno and automatically adjusts pressure. The engines we supplied come standard with a 91 jet, should I try 100 jet because we are racing at altitude. Johannesburg is 1700 meters above sea level. We run a standard airrbox with World Formula Green Filter.
The Header we use is almost a 5 shape with silencer (Basically a car performance silencer) This is all restricted and the same for everyone.


We run a series in South Africa called F400, the idea is to keep the performance as equal as possible and not cheque book racing.
 
Guys the dyno readings might not be 100% correct as I know that with certain dyno's you need to have the correct weighted flywheel to give accurate figures.
However the dyno is the same for everyone so regardless of how far it maybe out (On our previous dyno our Hsp was around 17.3hsp and 43nm of torque.

The dyno procedure is to run the engine to 5000rpm over 5 runs to get an average reading, I'm not sure what the standard timing is but we take that half-moon key and cut 2mm off the key which allows us to advance the timing. As far as I'm aware 2mm is the maximum advancement you can make on these engines. What I don't know and since I don't own the dyno and have never been able to test std keyway versus advanced keyway.
 
5,500 feet, plus some maybe, is getting up there. Maybe someone from Denver CO can jump in? Denver is above 5,000 feet. Still, without the RPM numbers, your information, as good as it is, isn't of much value.
Not knowing the RPM where you got your reading is hindering my calculations. Do you know the RPM where your engine hits peak Nm? How about where it hits peak HP? It would be nice if I had all the torque & RPM numbers. I have fun doing this stuff.
I guess it's late there right now, so I assume I'll have to wait until Tues.

With a 1" crank, 2mm is a 9* advance.
With a .750" crank 2mm is a 12* advance.

Does the dyno software have correction factors? Are the numbers you gave us corrected?

Going from a 91mm jet to a 100mm jet is a 24.2% increase in area. That's a ton!! What does the plug look like?
 
I would try take timing out and add compression like you suggested, the theory here is that without timing the engine will fall over a bit at higher rpm and keep your HP number down but the extra compression might help the midrange? If you want to get a bit deeper into the engine you could look into moving the camshaft timing around a bit, maybe advance it a couple of degrees? Most typical "racing" mods will shift the torque up in the rpm increasing your HP, your aim is to produce a torque curve that makes a flat 24 hp without going over your torque limit, ie extending your usable range of power. Where on track are the other motors faster than you? Can you get the plot from the dyno so you can get an idea what the torque curve looks like and where you can work on increases without going over the limit?

edit- I missed you said where they were faster than you. Was the motor getting outrun dynoed before or after it had bad performance? I would be checking you dont have a mechanical problem with that motor first, bad sealing valve, bad rings etc etc.
 
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.
Click to expand...

I would not jump straight up to a 100 jet. With anything, I would test in increments and see if there is a gain or not. The dyno is a great place to do so with jets. Lots of variables with on track testing.
 
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