Will an engine spin faster with the spark plugs removed?

[bluechipx]

I had a drag race two stroke snowmobile that was over ported to a point that low end power was so poor that the engine was very hard starting. I put a drive socket on the end of a V-8 car starter and used 24 volts (two car batteries) for power. Being a bit nervous of the torque kickback, I tried it several times first with the spark plugs removed. 4200 RPM on a mechanical tach consistantly. Next I tried it with the plugs in, but with no spark. Any guesses what the results were? For verification, I went back and forth, plugs in and plugs out and got very consistent results. The answer was not what I was expecting. Guesses?

 

[AlephZero]

If the RPM is the same with and without the plugs, then you have no compression.

That would certainly explain why starting is difficult!

 

[bluechipx]

The compression is close to 175 PSI in each cylinder.

 

[jack action]

If it is not what you expected, I guess the rpm was faster with spark plugs than without.

That would be because, with spark plugs, the (higher) energy required to compress the air is entirely recovered on the down stroke. Without spark plugs, The air is still partially compressed, but it escapes through the plug holes. Hence, the energy cannot be recovered and more torque is required from the starter.

That would be my guess.

 

[Averagesupernova]

I think normally a starter that is properly sized to an engine will spin faster with the plugs out. But, in your case I would say that the starter was oversized so the results might not be the same.

 

[bluechipx]

If it is not what you expected, I guess the rpm was faster with spark plugs than without.

That would be because, with spark plugs, the (higher) energy required to compress the air is entirely recovered on the down stroke. Without spark plugs, The air is still partially compressed, but it escapes through the plug holes. Hence, the energy cannot be recovered and more torque is required from the starter.

That would be my guess.

Jack, bingo! The engine went from 4200 without plugs to 4800 with plugs for exactly the reason you stated. I didn't see this one coming and wouldn't have figured it unless I witnessed it personally and mentally analysed the reason or was asked the question and thought it must be a trick question! I've been told the same thing happens with a camshaft being installed in a V-8, the more rockers/valve springs the easier the cam turns.

 

[jack action]

Jack, bingo! The engine went from 4200 without plugs to 4800 with plugs for exactly the reason you stated. I didn't see this one coming and wouldn't have figured it unless I witnessed it personally and mentally analysed the reason or was asked the question and thought it must be a trick question! I've been told the same thing happens with a camshaft being installed in a V-8, the more rockers/valve springs the easier the cam turns.

To be honest, if I had answered the question in the title, alone, without thinking, I might have thought the same thing as you.

I'm glad you did - and share - the experiment; I'll be less stupid when I'll go to bed tonight!

 

[Averagesupernova]

4800 RPM without plugs in there will definitely cause drag by air rushing in and out.

 

[brewnog]

Depends on valve (or port!) timing.

 

[bluechipx]

Depends on valve (or port!) timing.

I don't see where port timing is a factor. Simply, without plugs you are pumping air out the open plug ang working to draw air back in on the down stroke. With plugs in, a large percentage of effort used to compress the air will be used to push the piston down, resulting is an easier rotating engine. This isn't my theory, I've done it and it definitely spins faster with the plugs in. I would be surprised if the reason I've given wasn't the right explanation.