Exhaust back pressure concern....

[Guest]

I added pod filters and rejetted the carbs. So naturally with more air and more fuel is more exhaust right? I've got the stock exhaust. I'm worried it will be a little restrictive for the changes.

I was thinking of open pipe, but that is going to get annoying to me. Then I thought about buying some less restricted mufflers, but that costs money. Then I thought about modifying my current mufflers baffles to accept more flow. I figured if I ruin them I can buy some less restrictive mufflers later?

Does anyone have any suggestions on what I should do? Or will it even be a problem?

[Steve]

Keep them stock. Not just because of the noise factor, you would be surprised how efficient the stock system is. Most of the restriction is in the stock air box and you have removed that. Good luck, hope you got the jetting close.

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[mmattockx]

I added pod filters and rejetted the carbs. So naturally with more air and more fuel is more exhaust right? I've got the stock exhaust. I'm worried it will be a little restrictive for the changes.

Using the stock exhaust isn't going to cause problems, it just won't let your engine make as much power as it could with a more open exhaust. If you like it stock, then leave it that way and just accept that you could have a bit more power if you went with a performance exhaust system instead.


Mark

[Steve]

That is pretty much true, but there is a larger power gain by switching to pods than there is by switching exhaust.

Suzuki did a remarkable job of silencing the exhaust without hurting flow very much.

It would be very helpful if we knew what bike was being discussed (hint, hint), but let's say he had a bike that was factory-rated at 75 hp. Adding pods might get him to 80. Switching exhaust might add another two, for a total of 82. Had he kept the stock airbox and switched exhaust, there might not even be an increase, simply because of the restrictive airbox.

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[TinkersCustom]

That is pretty much true, but there is a larger power gain by switching to pods than there is by switching exhaust.

Suzuki did a remarkable job of silencing the exhaust without hurting flow very much.

It would be very helpful if we knew what bike was being discussed (hint, hint), but let's say he had a bike that was factory-rated at 75 hp. Adding pods might get him to 80. Switching exhaust might add another two, for a total of 82. Had he kept the stock airbox and switched exhaust, there might not even be an increase, simply because of the restrictive airbox.

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1981 Suzuki GS450L is the bike in question.

[Nessism]

Many times adding pods and an exhaust causes power loss, not increase. That's because Suzuki optimized the intake and exhaust systems to work together with the engine's mechanical components and changes to one part of the system takes something away from the whole. For example, the rubber boots that connect from the airbox to the back side of the carbs are actually velocity stacks. Also, many of the so called "headers" that were sold on the market are not tuned for power, they are just bent pipes designed to look a certain way and save weight and make noise. That's the main reason to change to one; you can drop some significant weight and change the sound. Don't expect to make more power though. At best, a header in conjunction with reduced intake restriction will add a small increase at high rpms, but don't be surprised if you lose power in the mid and low ranges.

[gsrick]

Many times adding pods and an exhaust causes power loss, not increase. That's because Suzuki optimized the intake and exhaust systems to work together with the engine's mechanical components and changes to one part of the system takes something away from the whole. For example, the rubber boots that connect from the airbox to the back side of the carbs are actually velocity stacks. Also, many of the so called "headers" that were sold on the market are not tuned for power, they are just bent pipes designed to look a certain way and save weight and make noise. That's the main reason to change to one; you can drop some significant weight and change the sound. Don't expect to make more power though. At best, a header in conjunction with reduced intake restriction will add a small increase at high rpms, but don't be surprised if you lose power in the mid and low ranges.

Even if that happened, wouldn't a slight loss in power be gained back by the loss in weight?

[John Park]

Even if that happened, wouldn't a slight loss in power be gained back by the loss in weight?

Given an all up weight of 700 lbs., 2% would mean that you'd need to drop 15 lbs. off the exhaust. Wind drag would remain the same.

[Brendan W]

Just happened to have a chart on the desk about inlet and exhaust pressure.
This is for an 1172cc four cylinder at 2000 rpm.

At constant exhaust pressure, increasing inlet pressure by 1 psi increases airflow by 15%
At constant inlet pressure, increasing exhaust pressure by 1 psi reduced airflow by 2%

Airflow is much more sensitive to the inlet configuration.
Turbos exploit this.

[TinkersCustom]

Just happened to have a chart on the desk about inlet and exhaust pressure.
This is for an 1172cc four cylinder at 2000 rpm.

At constant exhaust pressure, increasing inlet pressure by 1 psi increases airflow by 15%
At constant inlet pressure, increasing exhaust pressure by 1 psi reduced airflow by 2%

Airflow is much more sensitive to the inlet configuration.
Turbos exploit this.

I should be OK then?

[Brendan W]

I think the main issue after pods is jetting. If that is sorted I wouldn't be worried a the few HP you might be losing in the mufflers.
Even if you did free off the exhaust it could well be at the expense of another jetting exercise and mid range torque which would probably slow you more than a few horses at the top.

[GS1150Pilot]

This was a good thread to find as I begin digging into the CB900f. I love the stock exhaust and have no desire to replace it. I will probably do a Dynojet kit and put a good aftermarket air filter in, but essentially, I want this bike as close to stock as possible. I've even decided to repaint it in the stock silver and blue after watching old race videos.

[posplayr]

Just happened to have a chart on the desk about inlet and exhaust pressure.
This is for an 1172cc four cylinder at 2000 rpm.

At constant exhaust pressure, increasing inlet pressure by 1 psi increases airflow by 15%
At constant inlet pressure, increasing exhaust pressure by 1 psi reduced airflow by 2%

Airflow is much more sensitive to the inlet configuration.
Turbos exploit this.

Very nice; thanks for sharing

[Steve]

This was a good thread to find as I begin digging into the CB900f. I love the stock exhaust and have no desire to replace it. I will probably do a Dynojet kit and put a good aftermarket air filter in, but essentially, I want this bike as close to stock as possible. I've even decided to repaint it in the stock silver and blue after watching old race videos.

Not sure how this would relate to your CB900, but I have a header of unknown origin on my GS850, and have a K&N filter insert in the stock airbox. I have not done official plug chops, but I have reduced the main jets from 125 to 120 (115 is stock). Needles have not been shimmed, pilot (mixture) screws are all in the 2.5 to 3.0 range.

Any weight reduction from the header installation has been GREATLY offset by installation of the fairing, saddlebags and trunk.
The bottom line is that I have a utilitarian classic GS that happens to sound pretty good, I don't care if it's a bit slower or faster.

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[mmattockx]

Just happened to have a chart on the desk about inlet and exhaust pressure.
This is for an 1172cc four cylinder at 2000 rpm.

At constant exhaust pressure, increasing inlet pressure by 1 psi increases airflow by 15%
At constant inlet pressure, increasing exhaust pressure by 1 psi reduced airflow by 2%

Airflow is much more sensitive to the inlet configuration.
Turbos exploit this.

One chart is not going to cover more than one specific motorcycle. What bike/engine is it for and what were the details/operating conditions? Those numbers aren't really a surprise, inlet pressure is maximum 14.7psi (in a perfect, theoretical world) so increasing it by 1psi is significant whereas exhaust pressure is much higher so increasing it by 1psi is much less significant.


Mark