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Onan JB Compression Ratio


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  #1  
Old 03-23-2011, 07:51:59 PM
Leon N. Leon N. is offline
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Default Onan JB Compression Ratio

Hello Folks:

Something that frequently puzzles me from time to time WRT compression ratio and performance.

I have a 705JB-3R/4703P , S/N 76903404 Looking at the PL for this particular machine, Onan calls out two cylinder heads. One is p/n 110A1439 designated for Gasoline Plants and the other is p/n 110B1440 designated for Gas & Gas-Gasoline Plants. The specified head gasket is the same for both heads.

Now short of measuring the compression ratio, incidentally I do not want to do that at this time, I am wondering what the designed compression ratio is for my plant? My 705JB came equipped with a tri-fueled carbureator.

I have never operated the plant on gasoline, however, in several places Onan indicated a 20% hit on output power only when using natural gas but not when using propane or gasoline. My plant can easily support full spec power, i.e, 32 amperes@240 volts, so I wonder if anyone knows what is the actual compression ratio of this tri-fueled plant?

My thinking is this plant has the lower compression ratio cylinder head and the reason my plant can deliver spec'd power is because Onan designed their plants very conservatively.

Does this make any sense?

Leon
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Old 03-24-2011, 08:50:02 AM
Max Thompson Max Thompson is offline
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Default Re: JB Compression Ratio

Looks like gasous fuel 9.2-1
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  #3  
Old 03-24-2011, 01:15:16 PM
Leon N. Leon N. is offline
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Default Re: JB Compression Ratio

Max:

Are you telling me that my tri-fuled JB comes with a 9.2:1 compression ratio? Does that mean that I could load my JB to 120% of its' rating if I run it on gasoline? Obviously I do not need to do that, but for brief periods that information, if correct, might some day come in handy. Onan did say I should expect a 20% reduction when using natural gas. Now I am confused.

What I can say is that--they don't build generator plants in that class like that today --.
The JB's are very rugged machines and it creates a nice feeling when it is called upon to perform.

Leon
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Old 03-24-2011, 01:32:10 PM
Max Thompson Max Thompson is offline
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Default Re: Onan JB Compression Ratio

I AM NOT SAYING ANYTHING. I am only posting info from Onan's JB JC manual. You would likely need the part # from your heads & pistons to know for sure what you have.
Just because you may have excess engine power does not mean you can over load the generator. Many Onanís have more engine capacity than the gen-end can use, since they used different gen-ends on the same engines.
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Old 03-24-2011, 02:11:49 PM
pegasuspinto pegasuspinto is offline
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Default Re: Onan JB Compression Ratio

Do a compression test. I would expect the low compression heads to give you a reading somewhere under 125 psi, and the high ones to go over 150. Make sure the plugs are all out and the throttle is held wide open.

As was said, the limits on output will be the generator end, in low overloads it's how much overheat it can stand, and in higher overload it's how big the wires are.

Robert
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Old 03-24-2011, 02:34:46 PM
Leon N. Leon N. is offline
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Default Re: Onan JB Compression Ratio

Sorry, I did not mean to upset you! Yes, I agree with you. Onan specs a cylinder head specifically for the gasoline version and a different cylinder head p/n for the gas & gas-gasoline plants irrespective of the production spec. My cylinder head p/n is 110B1440. From the Onan data I have, the piston p/n did not delineate the type of fuel until beginning spec S. My plant was built to spec P. So how do you interpret that?

Here's what I think:

Looking more closely at the JB cylinder head design and the parts list, I think the reason for the two different cylinder heads is because the gasoline-only plants did not accomodate the rocker arm oil line to lubricate the overhead valves because in those days, the gasoline fuel contained the lead additive. Where as, the gas & gas-gasoline plant design calls out a rocker cover lube oil line.

In short, folks using a gasoline-only version of the JB might want to consider adding some lubricant such as Mystery oil to the gasoline to minimize valve wear. Point of fact, if you study the JB pls you will find other subtle part number calls out for valves ,valve seats, etc for those plants which do not use the leaded gasoline.

In summary, I think my plant which was built to Spec P is of the low compression design.

Comments?

Leon
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Old 03-24-2011, 03:54:12 PM
Wayne 440 Wayne 440 is offline
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Default Re: Onan JB Compression Ratio

Since it is designed with the option to run on gasoline, it will be the low compression version.
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Old 03-24-2011, 07:04:58 PM
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EricWood EricWood is offline
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Default Re: Onan JB Compression Ratio

This high comp question pops up every so often. Look in the spark plug hole if you want to know what you have. Reference the attached picture, the item number 2A is the high comp/nat gas piston--you can see it has the dome edge--the other is flat top.

eric
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Old 03-24-2011, 08:19:15 PM
pegasuspinto pegasuspinto is offline
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Default Re: Onan JB Compression Ratio

The owners manual for your engine suggests using unleaded gasoline.

I can't ever find a reference that any engine maker was sorry to see lead go. It fouled up things and left deposits. Even aviation wants to do away with lead. Not done them any favors other then being the same fuel for a very, very long time.

Robert
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Old 03-24-2011, 09:40:10 PM
Leon N. Leon N. is offline
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Default Re: Onan JB Compression Ratio

Robert you are only partially correct. Show me where Onan says to use unleaded gasoline in the spec P JB engines.

My owners manual says the following for recommended fuel: Use clean, fresh regular automotive gasoline. Do not use highly leaded premium types.

I do recall back in the 50's, 60's and 70's gasoline engine designs took into account the benefical lubricating affects of the lead in gasoline to lubricate the intake valves, valve guides and valve seats. That, in my opinion is why Onan specified, for example, stellite material for the valves, valve seats in the early JB's when operating without the benefit of a leaded fuel, i.e., using natural gas or propane. That is why the gaseous (non lead fueled) engine designs included a lube oil line to the overhead valve chamber.

This is a subtle but important JB design feature which enhances the JB endurance. I got the best features of both , i.e., the harder material (stellite) and the rocker cover oil line when I purchased the tri-fueled version.

I'll bet many folks who converted a JB or even a CCK from gasoline to gaseous operation or who operate with modern day unleaded gasoline may see reduced endurance of their generator plants especially if run repeatedly near full load.

Leon

Eric

My JB pistons are flat topped. I looked a bit more and note that ALL Onan JB engines prior to spec S had a compression ratio of 6.5:1. The compression ratio for gas-only and LPG jumped to 9.2:1 beginning spec S.

The Penn State natural gas powered plants-only, had a compression ratio of 9.2:1 with no specification letter noted.

Leon
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