Indian PowerPlus Compression Ratio

[Shaky Jake]

Has anyone measured the Compression Ratio of a PowerPlus Indian? I ask because I recently did, and I was surprised at the result. I used a graduated burrette to fill the combustion chamber and measured its volume, then I used that information to calculate th compression ratio. I'm looking to compare notes.


Thanks!



Kevin

[gharper]

Kevin,

I have not done that measurement but would suspect it is in the 4.5/1 range. I'll be doing it soon on my early chief and would guess it to be very similar as the 22-26 Chief cylinders are basically larger Powerplus cylinders.

A few years back I did a complete rebuild of my 24 Chief. I fitted Bonneville pistons in an effort to get a bit more oomph out of the old girl. Even with the 1/8" taller pistons fitted I was still able to slip my sausage fingers in through the valve holes and between the top of the combustion chamber and the piston. So, I'll be measuring the stock and modified compression ratios and bet there isn't much difference.

I'm curious to hear what number you came up with, or are you holding out for comparisons?

Gene

[TechNoir]

Hello Kevin, I have not seen it measured exactly that way before, usually its an assembled engine and thicker oil but I cant fault the theory on the assumption that you have a flat topped piston?

John

[Shaky Jake]

Kevin,

I have not done that measurement but would suspect it is in the 4.5/1 range. I'll be doing it soon on my early chief and would guess it to be very similar as the 22-26 Chief cylinders are basically larger Powerplus cylinders.

A few years back I did a complete rebuild of my 24 Chief. I fitted Bonneville pistons in an effort to get a bit more oomph out of the old girl. Even with the 1/8" taller pistons fitted I was still able to slip my sausage fingers in through the valve holes and between the top of the combustion chamber and the piston. So, I'll be measuring the stock and modified compression ratios and bet there isn't much difference.

I'm curious to hear what number you came up with, or are you holding out for comparisons?

Gene

Sorry Gene, I posted the video on my other thread, but I forgot to put it here. I'll post a link below. The result is at the end of the video. I've also taken some measurements on the cylinder, and I believe I can stuff a 1917 piston, which is 1/4 inch taller, into the 1916 cylinder and still have about 0.200 inches clearance at the top. It would raise the CR by about 1/2 a point, from 3-1/2 to 4:1. Here is the video:

https://youtu.be/1ldR3Zi_-sw




Kevin


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[Shaky Jake]

Hello Kevin, I have not seen it measured exactly that way before, usually its an assembled engine and thicker oil but I cant fault the theory on the assumption that you have a flat topped piston?

John

Yes, you are correct, it is a flat top piston.

The problem I see with a thicker (more viscous) oil in this setup is that it would be harder to tell when it's right at the top of the pin, because it will have more of a "meniscus." A thicker oil also has more of a tendency to trap air bubbles in the nooks and crannies of the valve pocket.

I did think about taking the measurement with the engine assembled, but I wanted to avoid the challenge and potential mess of draining and disassembling the cylinder after it had been full of oil.

Anyway, thanks for responding. I always value your input.




Kevin


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[gharper]

Kevin,

Nice job, the video was great! I've done that same thing on Excelsior cylinders and get about 4:1 as I recall. As I said, I'll be doing that same thing on my early chief cylinders soon, checking the original compression ratio vs the higher ratio using the 1/8" taller bonneville pistons.

These are very low numbers, but appropriate for the conditions and fuels of the times. With modern fuels we can easily raise the compression ratio without "knocking" from pre-ignition. The million dollar question then becomes how much additional stress can the cylinder handle? Indian made 2 sizes of exhaust valve plugs for the early chief, which also fit the later big valve powerplus engines. One set was almost flat on the underside, the other hollowed out, about 3/4". So this would be a high and low compression set of plugs. Other than that, there were no factory accommodations to raise compression until 1927 when the removable cylinder head appeared on the chief. Compression was increased considerably which then required a different gear ratio on the kick starter starting in 28.

Gene

[Shaky Jake]

Kevin,

Nice job, the video was great! I've done that same thing on Excelsior cylinders and get about 4:1 as I recall. As I said, I'll be doing that same thing on my early chief cylinders soon, checking the original compression ratio vs the higher ratio using the 1/8" taller bonneville pistons.

These are very low numbers, but appropriate for the conditions and fuels of the times. With modern fuels we can easily raise the compression ratio without "knocking" from pre-ignition. The million dollar question then becomes how much additional stress can the cylinder handle? Indian made 2 sizes of exhaust valve plugs for the early chief, which also fit the later big valve powerplus engines. One set was almost flat on the underside, the other hollowed out, about 3/4". So this would be a high and low compression set of plugs. Other than that, there were no factory accommodations to raise compression until 1927 when the removable cylinder head appeared on the chief. Compression was increased considerably which then required a different gear ratio on the kick starter starting in 28.

Gene

Thanks Gene.

Yes, I was looking at making longer valve hole plugs that extended to the bottom of the threads. It would increase the CR by about 1/4 of a point.




Kevin


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[Tom Lovejoy]

The late Lee Standley told me, ideally try for about .060 clearance between head and piston. That is with remove able heads, but I would think it would apply here too ? I can't remember if he was talking about flat pistons, but I think not. As he liked Sport Scout racers.

[gharper]

Tom,

For max HP on a flathead I seem to recall.030" is the ideal "squish", again on removable heads. The early Chief and I assume powerplus cylinders, aren't the true Ricardo combustion chamber design. The area over the piston is domed and gently rolls over to the valve chamber, leaving lots of room. In my mind if I were tiny and could stand on top of the piston at TDC, it would look like the Sistine Chapel overhead!!! In comparison, the 27 and later Indian flatheads would be flat and right above my head with a ramp to the valve chamber. Not sure if that visual makes sense......

Bottom line is with the blind cylinders, there is a lot of room above the piston at TDC, hence a very low compression ratio.

Gene

[Shaky Jake]

In addition to the above considerations, I feel like it would also be advisable to not allow the top of the piston to extend above the top of the cylinder bore, when using a flat top piston. In the PowerPlus cylinder, the bore is shorter on the valve side than on the opposite side. This will limit how close the piston can come to the top of the combustion chamber.




Kevin


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[TechNoir]

In addition to the above considerations, I feel like it would also be advisable to not allow the top of the piston to extend above the top of the cylinder bore, when using a flat top piston. In the PowerPlus cylinder, the bore is shorter on the valve side than on the opposite side. This will limit how close the piston can come to the top of the combustion chamber.

Kevin
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Hi Kevin, I would have though it was important to ensure that the top of the top ring stays below the top of the valve side of the bore. Why should the top of the piston stay below i too?

John

[Shaky Jake]

Hi Kevin, I would have though it was important to ensure that the top of the top ring stays below the top of the valve side of the bore. Why should the top of the piston stay below i too?

John

During combustion in a side valve engine there is a flow of gases from the valve pocket to the cylinder. If a flat top piston extends above the top of the cylinder, I would be concerned about the affects of side loads on the piston that may result from that flow.




Kevin


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[TechNoir]

OK thanks Kevin, that makes sense. It would be interesting to know what sort of loading that would put on a piston although I doubt anyone has the answer.

John

[Shaky Jake]

OK thanks Kevin, that makes sense. It would be interesting to know what sort of loading that would put on a piston although I doubt anyone has the answer.

John

It's one of those things; you never know until you try, and then it's too late.





Kevin


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[Spacke2speed]

Kevin
On the later Indian Chief “74” standard motors, the pistons stop below the top of the cylinders. On the “74” Bonneville motors, the pistons stop even with the top of the cylinders, as the pistons are different. On the “80” motors, the pistons stop above the top of the cylinders, as the motor is stoked, using the standard pistons.
Burgie