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      09-18-2020, 02:19 PM   #1
BetweenTheWheels
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Performance after retrofitting an m235 exhaust on a 228

Hi all - if this needs to be in the engine/exhaust tuning thread, I can move it.

First, I have a 228 that I enjoy but I wanted more noise from the exhaust (natch) and retro'd an m235 axle back unit about a month ago. Transforms how this car feels to drive! Sounds great. Looks great. A nice balance between civilized and some extra snarl when pushing it hard.

Question: when I am giving full acceleration, the car sounds great and feels great 1st to 2nd, 2nd to 3rd and 3rd to 4th (I have a ZF). The car is plenty quick and even feels more lively with the new exhaust, which may be placebo. What I'm curious about is a bit of drag I feel in gear transitions as if the Hulk has hooked his finger under the rear of the car and gives me a little tug backward at each shift. It comes in time with the growls/burble from the exhaust and my thought is the higher level of back pressure that's being expelled via the dual pipes is pulling some flow from the turbo causing it to hang or lag a fraction of a second each shift. 1) Am I diagnosing that correctly ? And 2) While it's not a big deal, is my solution (should I choose to address it) a tune which increases my boost?
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      09-20-2020, 08:19 AM   #2
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OK, let's clarify things a bit here - "back pressure" is resistance to normal exhaust flow, caused by a more restrictive exhaust tract. It is NOT the pressure of the exhaust coming from the engine. If anything, the new muffler has increased the flow, but that would not cause any kind of lag and is not "pulling some flow" from the turbo. All the exhaust flow goes through the turbine side when the wastegate is fully closed, When the throttle is backed off (such as when shifting gears), built-up pressure in the compressor side is relieved by a valve that routes it back into the pre-turbo intake tract so as not to stall the compressor. If the function of this valve isn't working correctly or if the wastegate is opening (thereby reducing compressor speed), lag would be induced until the turbine can get the compressor back up to speed. Check those items, and also check the electronic connector to the valve on the muffler that opens/closes the passenger-side pipe and make sure that side is opening. So no, you're not diagnosing it correctly and no, increased boost isn't going to solve the problem.
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      09-20-2020, 09:20 AM   #3
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Quote:
Originally Posted by RoundelM3 View Post
OK, let's clarify things a bit here - "back pressure" is resistance to normal exhaust flow, caused by a more restrictive exhaust tract. It is NOT the pressure of the exhaust coming from the engine. If anything, the new muffler has increased the flow, but that would not cause any kind of lag and is not "pulling some flow" from the turbo. All the exhaust flow goes through the turbine side when the wastegate is fully closed, When the throttle is backed off (such as when shifting gears), built-up pressure in the compressor side is relieved by a valve that routes it back into the pre-turbo intake tract so as not to stall the compressor. If the function of this valve isn't working correctly or if the wastegate is opening (thereby reducing compressor speed), lag would be induced until the turbine can get the compressor back up to speed. Check those items, and also check the electronic connector to the valve on the muffler that opens/closes the passenger-side pipe and make sure that side is opening. So no, you're not diagnosing it correctly and no, increased boost isn't going to solve the problem.
Thanks for the info!
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      09-21-2020, 07:52 AM   #4
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Quote:
Originally Posted by BetweenTheWheels View Post
Thanks for the info!
You're welcome!
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