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Let say if I have stock P150X engine and aim for better gas mileage but good performance for daily ride and fitted Polini Box exhaust, which carb setup gives better mileage?

SI 24/24, up jetted, and tight pad clearance

or

PWK/PE 24 with reed valve.

...

Also, is it possible to make a custom expansion chamber which optimized for low-mid rpm as what the stock engine normally run?
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UTC quote
xperimental21 wrote:
Let say if I have stock P150X engine and aim for better gas mileage but good performance for daily ride and fitted Polini Box exhaust, which carb setup gives better mileage?

SI 24/24, up jetted, and tight pad clearance

or

PWK/PE 24 with reed valve.

...

Also, is it possible to make a custom expansion chamber which optimized for low-mid rpm as what the stock engine normally run?
Unless you're set on an expansion chamber a polini box will give you all the bottom end grunt you need, I've got one on my vmc super g and it pulls ( a sidecar) from very low down all the way up to 7500rpm. comparable if not better than a px200.
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Your best option for better gas mileage is keeping the motor bone stock.

After that, your next best option would be a si 20/20. You're not going to be able to pump enough air to get the 24/24 working well, so it'll be tough to tune, especially in the 1/4-1/2 throttle range.
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chandlerman wrote:
Your best option for better gas mileage is keeping the motor bone stock.

After that, your next best option would be a si 20/20. You're not going to be able to pump enough air to get the 24/24 working well, so it'll be tough to tune, especially in the 1/4-1/2 throttle range.
Agreed, and the smaller carb will give you a better bottom end, and in theory if you've got enough grunt you could up gear to improve your fuel consumption.
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UTC quote
xperimental21 wrote:
Also, is it possible to make a custom expansion chamber which optimized for low-mid rpm as what the stock engine normally run?
Forgot to answer this...short answer: No.

Longer answer: one of the Big Box exhausts will get you some resonance on the top end while still being quieter than a full expansion chamber. It'll give you a wider, flatter, but also somewhat lower max HP power curve.

For most people, that's what they want for day-to-day riding. Less shifting, but not much more noise than stock.
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chandlerman wrote:
Forgot to answer this...short answer: No.

Longer answer: one of the Big Box exhausts will get you some resonance on the top end while still being quieter than a full expansion chamber. It'll give you a wider, flatter, but also somewhat lower max HP power curve.

For most people, that's what they want for day-to-day riding. Less shifting, but not much more noise than stock.
Would that resonance from big box also pulls the mixture back into the combustion chamber?
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UTC quote
Yes. That funky segmented expanding pipe leading to the box is there for that.
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76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 79 P200E, 66 Lammy S3
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UTC quote
stimuli wrote:
Yes. That funky segmented expanding pipe leading to the box is there for that.
Yep. If you cut one open, you'll see that a cone intrudes into the box, but doesn't narrow back down like a proper tuned pipe, so basically a semi-tuned pipe.
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