CAM SPEC QUESTION(S)

[sid pileski]

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

[Vixen]

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

[AVTUR]

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

[sid pileski]

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

[Jo]

Before you spend too much time on this: Have you priced up the Helical gears  Or do you intend on cutting your own 

Jo

[Jasonb]

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

[sid pileski]

Before you spend too much time on this: Have you priced up the Helical gears  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

[sid pileski]

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

[Jasonb]

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.

[Longboy]

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.