Author Topic: CAM SPEC QUESTION(S)  (Read 1721 times)

Offline sid pileski

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CAM SPEC QUESTION(S)
« on: January 28, 2022, 04:49:06 PM »
Good day everyone.

I'm in the very early stages of designing an engine that will be loosely based on one that a friend of mine has a full size version of.
It's a single cylinder 4 stroke boat engine, probably from the 20's-early 30's.
I like it because it used a set of 90* helical open gears to run the cam, to a set of rockers, to push rods, to another set of rockers, to the valves.

The bore right now is 1.5". Stroke not yet determined.
I want this to run slow.

It will, per the prototype have a large solid flywheel. I'm thinking no more than 5:1 compression.

Now the cam spec question.
What will be the best specs for a good slow runner?
From some reading, no over lap.
But, what else makes for good idle performance?

Also, small carb

Thanks, Sid

Online Vixen

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Re: CAM SPEC QUESTION(S)
« Reply #1 on: January 28, 2022, 04:58:51 PM »
Hello Sid

There are some discussions about various  valve/ cam timing events over on the 'Bristol Hercules sleeve valve radial engine plans' topic. Around Reply #62 various valve/ cam timing events are discussed (not just sleeve valves). You may something useful there.

Mike
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline AVTUR

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Re: CAM SPEC QUESTION(S)
« Reply #2 on: January 28, 2022, 05:16:24 PM »
Sid

I think you have included the main features for good idle performance; large flywheel, mild valve timings with low lift and low compression ratio. A stroke greater than the bore could help.

All these would reduce the engine's acceleration but I doubt if that is of concern. There is something rather nice about soft engines.

AVTUR
There is no such thing as a stupid question.

Offline sid pileski

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Re: CAM SPEC QUESTION(S)
« Reply #3 on: January 28, 2022, 05:24:12 PM »
Hello Sid

There are some discussions about various  valve/ cam timing events over on the 'Bristol Hercules sleeve valve radial engine plans' topic. Around Reply #62 various valve/ cam timing events are discussed (not just sleeve valves). You may something useful there.

Mike

I'll check it out.

Any other insights?

Thanks, Sid

Online Jo

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Re: CAM SPEC QUESTION(S)
« Reply #4 on: January 28, 2022, 06:40:21 PM »
Before you spend too much time on this: Have you priced up the Helical gears  :paranoia: Or do you intend on cutting your own  :)

Jo
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Online Jasonb

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Re: CAM SPEC QUESTION(S)
« Reply #5 on: January 28, 2022, 06:49:18 PM »
You could take a look at some of the other designs from the period but would need to weed out the ones that were more designed with performance n mind.

If you can also arrange an easy way to advance and retard the ignition timing that will allow you to slow things down a lot.

This 1934 design that I scaled down also had the valve lift reduced quite a bit as I wanted it to run slow and sound good as it will never get used in a tether boat

<a href="https://www.youtube.com/watch?v=v1WTcKEt--s" target="_blank">http://www.youtube.com/watch?v=v1WTcKEt--s</a>

helical gears need not be that expensive if you can use standard helix angles, I've used these for an engine which I thought were very reasonably priced

https://ghw-modellbau.de/index.php?cPath=91_83_88_211_213

Offline sid pileski

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Re: CAM SPEC QUESTION(S)
« Reply #6 on: January 28, 2022, 08:29:01 PM »
Before you spend too much time on this: Have you priced up the Helical gears  :paranoia: Or do you intend on cutting your own  :)

Jo

Helical cam gears are already on order and in the mail hopefully from HPC gears.
They had just what I was looking for, and in stock at a very reasonable price.

Sid

Offline sid pileski

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Re: CAM SPEC QUESTION(S)
« Reply #7 on: January 28, 2022, 08:30:25 PM »
You could take a look at some of the other designs from the period but would need to weed out the ones that were more designed with performance n mind.

If you can also arrange an easy way to advance and retard the ignition timing that will allow you to slow things down a lot.

This 1934 design that I scaled down also had the valve lift reduced quite a bit as I wanted it to run slow and sound good as it will never get used in a tether boat

<a href="https://www.youtube.com/watch?v=v1WTcKEt--s" target="_blank">http://www.youtube.com/watch?v=v1WTcKEt--s</a>

helical gears need not be that expensive if you can use standard helix angles, I've used these for an engine which I thought were very reasonably priced

https://ghw-modellbau.de/index.php?cPath=91_83_88_211_213

Yes, ignition timing goes without saying.
Guess I should have mentioned that too.

But thanks for pointing it out.

Sid

Online Jasonb

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Re: CAM SPEC QUESTION(S)
« Reply #8 on: January 28, 2022, 08:49:31 PM »
Keep us posted on progress, I've fancied doing an old boat engine like a Calvert or similar for a while. I've also got an inkling for an early outboard.

Offline Longboy

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Re: CAM SPEC QUESTION(S)
« Reply #9 on: February 01, 2022, 10:09:46 PM »
Good day everyone.
I want this to run slow.
It will, per the prototype have a large solid flywheel. I'm thinking no more than 5:1 compression.
Now the cam spec question.
What will be the best specs for a good slow runner?
From some reading, no over lap.
But, what else makes for good idle performance?
Also, small carb.
Thanks, Sid

Slow stead idles kinda blends in a perception of easy running and a steadfast reliability in model engines. Your exhaust valves should close right near TDC. The compression should be lower than 5:1.  Ignition timing right around TDC. A .10/.15 model engine carburetor the right size. The greatest influence on engine speed will be the flywheel diameter. You can knock down over a hundred RPM increasing your diameter by one inch.
I like bevel gears too! Look over on EBAY for the China specials.   Luck to you, .....Dave.




« Last Edit: February 01, 2022, 10:21:08 PM by Longboy »
Wadda you mean, "It don't run"???

 

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