Hit & Miss Engine-Combustion Chamber

[neilaz]

New member. I've built a few Hit & Miss Engine kits and now I'm building my first bar stock engine (going backwards, I should have started here). I've never seen or read discussion concerning the Combustion Chamber. Am I correct in assuming it is not that important? With a fixed cylinder and fixed piston diameter what happens if the chamber depth is changed from 3/16' to 3/8" for example? How would this affect the starting? Again, because these engines are usually run with no load it probably makes no difference. I enjoy the building and running the completed engines but I'm no expert on solving problems of starting or other "why it doesn't run" questions. I would be pleased to see discussion on this. Thanks, Neil B.

 

[AussieJimG]

Hi Neil,

Check the effect on compression ration of changing the dimension you asked about.

Jim

 

[Cogsy]

If you increase the size of the combustion chamber you will be reducing the compression ratio. I believe most hit and miss engines have a reasonably low compression ratio already (I think it makes them easier to start and able to run at lower RPM) but reducing it too much will mean a drastic reduction in power and possibly won't even run.

You could always machine a new head for one of your engines, with some way of altering the compression ratio, and study the effects it has on the engine as you alter it.

 

[Mechanicboy]

Use syringe with fluid in ml or oz and fill up in the combustion camber via plug hole with closed valves and piston in TDC. Take a read of amount who is filled in combustion camber and calculate how much is compression ratio. To adjust compression ratio: Thick cylinder head gasket --> less compression ratio and vice versa.

 

[modelman1838]

I scratch built a hit and miss engine a few years ago and have shown it on this forum. I took a stab at the compression ratio and it started straight away but jumped about a bit, so I turned up a 1/2 inch spacer to fit between the head and the body and fitted this and it improved it no end. So I conclude in your case it will not be any problem. the bore on my engine was 1 1/4 inch dia.

 

[Brian Rupnow]

Compression ratio for a hit and miss engine should be in the range of 3.5:1 to 5:1----Brian

 

[neilaz]

Thanks fellows for the excellent response to my Combustion Chamber question. It turns out that what started my questioning was an error I made in calculating the Connecting rod length of my Philip Duclos bar stock engine. If Duclos had specified a combustion chamber length, which he didn’t, I would have found my mistake quickly. The combustion chamber length I’m referring to is the distance between piston at top dead center and the underside of the cylinder head. Your responses aided me in solving the problem myself. Having owned the Philip Duclos book “The Shop Wisdom of Philip Duclos” for many years I was naturally an advocate of his even though I had never built one of his engines. Trying to correct my error by going backwards proved unworkable without knowing that dimension. I wish he had shown the combustion chamber length. The engine I’m building is his “Odds ‘n Ends” engine. If you are interested in other engines I’ve built go to www.nbutterfield.com my “brag” personal web site. I was very impressed with the quick, informative responses to my question. Thanks, Neil B.