Cold Pipe tuning issue. What's your reading? Whats happening?

Dave Thacker /

Short history, finished a 70cc A35 setup with Estoril. Up-jetted to I forget what. The thing pulled like mad at 45 and would hit 50. After moving the jet around trying to find the sweet spot, it never goes past 44.

This evening it's cool out. I start it and leave the drive, withing a 100 yards or so, I'm at 45 and thinking I shouldn't be pushing a cold engine so hard. But there is power past 45. It really feels good even a touch rich.

So the difference to prior trials is I'm blasting right up to top speed on a cold pipe. When the pipe gets some heat, I'm held back to about 44. What is going on here?

Re: Cold Pipe tuning issue. What's your reading? Whats happening?

Things expand when heated and contract when cooled .

Your top end seems to be right at the point where that bit of change makes the difference you see . Maybe port timing ???

Re: Cold Pipe tuning issue. What's your reading? Whats happening?

Dave Thacker /

What I'm thinking about here is the speed of sound is faster in hot air than cold. Thus the pipe is effectively shorter when it's hot than when it's cold. So when I take off cold, the pipe is longer meaning the in -tune RPM band is slightly lower. Yet it wants to run faster.

I theorize the SHA mixture to grows richer at higher RPM. Not for the same reasons or by the same method as a chainsaw carb but it seems to be doing this none the less. On the cold pipe, I feel what seems like richness at the top that I don't feel on the second blast off because the pipe is now up to temp.

My understanding and theories are not lining up with results. I would expect to go faster with a hotter (read shorter by speed of sound) pipe and that is not happening in practice.

Certainly it's a complex system. It could be I have some sort of compression or ring issue that is changing at higher cly temps. I'm looking for some explanation that explains the results so I know what to chase next.

Re: Cold Pipe tuning issue. What's your reading? Whats happening?

> Dave Thacker Wrote:

> -------------------------------------------------------

> What I'm thinking about here is the speed of sound is faster in hot air

> than cold. Thus the pipe is effectively shorter when it's hot than when

> it's cold.

But in reality , physically , the pipe is shorter due to being cold and expands proportionately when at operating temp .

Re: Cold Pipe tuning issue. What's your reading? Whats happening?

Dirty30 Dillon /

The tuning issues you're experiencing definitely don't have to do with a cold pipe, it's probably more to do with a cold engine. The change in pipe temp is not going to brick you 44 mph.

I'd be more concerned with being overly rich, where a cold motor is now operating a good mix, but upon warming up is too rich. OR, it could be an old/malformed seal that after being heated expands to allow trace oil/air from the trans into the combustion chamber.

The speed of waves in the pipe DOES increases in hotter environments, but as you described, that would not inhibit the top speed as you've described.

Knowing your setup a little more might help someone posit a better hypothesis.

Re: Cold Pipe tuning issue. What's your reading? Whats happening?

Temperature of the exhause in the expansion chamber determines a lot about the reversion sound wave to over stuff the combustion chamber. Having the timing less advanced (closer to TDC) will help the combustion chamber heat faster and provide a more consistent and robust performance as well as warming up faster. Add some wrap but not on the header, start just before the beginning of the diffuser. Wrap all the way up to the cylinder will keep too much heat to the combustion chamber.

Re: Cold Pipe tuning issue. What's your reading? Whats happening?

Dave Thacker /

Thank you, these are some good answers. I have been suspicious it was rich. I think on my last day of endless jet changing, I had picked up speed as I moved up in jet but I will open it up and drop a jet size and see what happens. I am sensing it's rich on the top end after warm up.

Certainly the pipe does expand in length when hot. I have a machinists handbook and could calculate it but I doubt it's more than 100 thou.

Re: Cold Pipe tuning issue. What's your reading? Whats happening?

Before dropping a jet you could also remove some screens from the screen pack and see if that give you a touch more air to even out the jet.

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