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Onan JB/JC High Compression Pistons

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Old 05-17-2016, 02:36:25 PM
ArodaPowerCo ArodaPowerCo is online now
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Default Re: JB/JC high compression pistons

I'm with Motorhead. At 8700' elevation, turbocharging would be appropriate here. All you would be doing is bringing it's performance up to what it is at sea level. No real strain on the engine.

Raising compression will help some by making more efficient use of the existing fuel/air, but the majority of power loss is from lack of oxygen.
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Old 05-17-2016, 03:20:03 PM
len k len k is offline
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Default Re: JB/JC high compression pistons

Nor sure how much bang for the effort your going to get but an alternative to turbos and superchargers is to do what race cars drivers do. They reduce flow restrictions on the intake and exhaust to help engine breath better (get more mix into cylinders). Heard they open up the dia of the intake manifold and sand smooth the inside rough sand cast finish, this reduces drag of air flowing across it and allows mix to flow faster into the cylinders, net effect is more mix gets into cylinders. They also sand smooth rough finish of both valve ports and heads. $30 air die grinder with a round stone on end. They go as far as making flow test bench powered by a vac cleaner and measure flow backpressure. Might be a bit of black art skill in it too.

Can also try using larger exhaust pipe and muffler to reduce back pressure to increase hp. I noticed a propeller airplane I was in didn't have a muffler.

Supercharge/turbo shouldn't stress the engine, it'll just bring back cylinder pressure and hp to what engine was designed to make.

Maybe high compression will help a smidge, but might need high octance gas to not ping.
But without a supercharger or turbo your mostly limited by not being able to get enough fuel into the engine to make the power you want. Same thing happened in WW2 planes.

Technically because of lack of O2 you can't add enough fuel into cylinder and still have mix's air/fuel ratio in the flammable range, without a turbo or supercharger to make the incoming mix denser (higher pressure). Basically after the mix burns the gas temp is not high enough to make normal pressure to drive the the piston down, because there's not enough fuel in cylinder. High altitude has exactly the same effect as not opening the throttle all the way at sea level. (note at sea level and low throttle setting you have a vac in the intake manifold.)

Last edited by len k; 05-17-2016 at 04:46:41 PM.
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Old 05-17-2016, 04:51:26 PM
Gunny Gunny is offline
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Default Re: Onan JB/JC high compression pistons

Having a good amount of experience with dry fueled engines, Onan and others and turbo charged engines, I strongly recommend against turbo charging an air cooled engine.

You start running a dry fueled engine near their maximum design limit, a little can make a lot go wrong quick. If you run knock sensors and have a way to take load off of it, back the timing down, adjust the mixture or take off turbo boost you are flirting with disaster. If you are lucky you'll just burn exhaust valves, if unlucky, you'll drop an exhaust valve then then you have a lot of broken and melted parts. Need to also closely monitor exhaust temperatures and take appropriate steps when they start getting out of hand.
The air cooled Onans are great engines but I would not run and air cooled dry fueled engine too hard for too long. Too long can be just a couple of minutes.

Be happy to help or pass on experience and advice but would think that you would do better with a larger engine/generator and not try to squeeze every last bit of power out of one.
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Old 05-17-2016, 07:13:00 PM
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Default Re: Onan JB/JC high compression pistons

I have been looking for high compression pistons for my JB for a while now. There is a set of 2 on ebay for a while now. But in my opinion, for the price they want for those it doesn't justify the extra output. I would really like to find the design specs or blueprint of those high compression pistons and ask an aftermarket manufacturer about making some.
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Old 05-17-2016, 08:38:46 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan JB/JC high compression pistons

On line Sears Parts says they have them, I did not call however! Over a couple hundred dollars each piston though.
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Old 05-17-2016, 09:11:15 PM
JohnnyC JohnnyC is offline
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Default Re: Onan JB/JC high compression pistons

My .02 cents - In your case mill the heads, if possible. Easiest and least costly way. It is critical to know the clearances to ensure there are no possibilities of collision.

New Jersey
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Old 05-17-2016, 10:36:05 PM
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Default Re: Onan JB/JC high compression pistons

Depending on how much you mill off of the heads, you would want to shorten the push rods the same amount. This keeps the rocker arm angle the same.
The suggestion of turbocharging is only to bring the air charge to a close to sea level range, Not hot rodding the Onan.
If you look at some of the larger natural gas Onans, they use a 460 Ford engine and have a small turbocharger on it.
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Old 05-17-2016, 10:53:03 PM
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Default Re: Onan JB/JC high compression pistons

I want to say that the combustion chamber volume on the JC would be too large to be affected much by milling the heads. To make a real difference in the compression ratio you would have to take a quarter of an inch off the head which I don't think would work. It would screw up the valve train geometry among other things. With a nominal CR of 7-1 you would have to go up to 10 or 11-1 to make a significant difference. In a typical JC I believe the cranking compression is about 110-120 psi , on my Nat gas spec JC it is around 225-230 Psi so all other things being equal the squished volume would be half. Unless you actually make that much of a change it would be pointless. The amount of power lost according to Onans own chart for natural gas is close to 40% at 8700 ft or 30% for propane . No amount of raising compression is going to give a 40% increase in power, the only way to achieve that is turbo or supercharging which is commonly used in aircraft to give sea level power up to 10,000 Ft. Cheers Dan
It's always something simple.
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Old 05-17-2016, 11:14:03 PM
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Default Re: JB/JC high compression pistons

Originally Posted by Motorhead View Post
Someone tried it here on the site. Can't remember who.
I'd like to be proven wrong, but I don't think it's been posted here before.

I started a thread on the topic last summer but, since I'm fresh out of spare JB heads to tinker at, haven't tried it yet.

Anyway, the increased CR isn't going to make up for 9000 feet ASL. Darts just needs a bigger genset...

Last edited by Jim McIntyre; 05-18-2016 at 08:37:36 AM.
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Old 05-18-2016, 12:40:06 AM
Dennis19508 Dennis19508 is online now
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Default Re: JB/JC high compression pistons

Atmospheric pressure at sea level is about 14.5 psi, and at 10,000 ft it is only about 10.2 psi. (I know you are only at 8700, just rounded up for easy math and fudge factor) So 4.3 psi is the amount of boost needed to perform like at sea level. So a small turbo could easily do the job. Hardest part will be finding a turbo small enough.

Given that a JC is 120 C.I. (1.97 L) and 15 kw is only a little over 20 hp you might be able to get there with a larger carb and better intake. High compression will help as will a free flowing exhaust. But the carb is the primary restriction I think. Only way to know for sure would be to tap a vacuum port into the intake and measure the vacuum when running at max load when the carb is fully open. If more then two inches of vacuum then the carb could be made larger.

What ever way you decide to pursue, it sounds like a fun project.

Also cooling might be affected. Since the air is only 2/3 as dense as at sea level it will not remove heat as effectively. Should monitor the gen head as well as the engine. First full power test will show if it will be a problem.
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