OK.... I was just wondering, Thanks for the reply.
Overheated Robin EH72-FI (28 hp) in PT-425 solved
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OK.... I was just wondering, Thanks for the reply.
joshlee
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So, do you know it the new PT's overheat?
joshlee
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So, do you like your PT?
joshlee
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Did you go to Virginia to pick up your PT ?
ponytug
Super Member
I agree with you about back pressure being misunderstood, but it isn't without issues.
One minor point with respect to the first link. Toward the end the author states that changing the back pressure won't have an effect on a computer (ECM, or engine control module) controlled (e.g. Fuel Injected) engine. This isn't quite true.
The feedback for the engine comes from the oxygen sensor in the exhaust, which is down stream of the valves, but ahead of the muffler. The engine control module has been programmed on the assumption (knowledge?) that the exhaust being sampled is x seconds downstream of the combustion chamber at a given rpm. Without the back pressure, the exhaust is actually "newer" or ahead of that. At constant rpm, it won't matter, but if you suddenly open the throttle or shut it down, the lag or hysteresis will be noticeable, perhaps to the point of stalling the engine. Factory ECMs are tuned for a bunch of factors, and this lag is one of them. When you add fuel injection to a carburetored engine, you end up having to tune many of these parameters.
You might ask why I know this- I had an older car that had an exhaust system that was a little resonant, so it was probably factory tweaked, and had some amount of back pressure. At one point the muffler blew out and boy did the performance on the engine change. Try to do 0-65 on an on-ramp was like telegraphing the ships engine room. You'd stomp on the accelerator, the engine would fumble, almost die, think about life for a few seconds, and then roar to life. (It happened to me about six hours after buying the car, just outside of MR's neck of the woods at about 10:00pm at night.) Let's just say I didn't drive as far as I was planning that night, because the nearest 24 hour muffler repair shop was more than few hours away. After the muffler was fixed, it was a great car for me. I put another 200,000 miles on it, with not much in the way of service besides the usual oil changes.
All the best,
Peter
One minor point with respect to the first link. Toward the end the author states that changing the back pressure won't have an effect on a computer (ECM, or engine control module) controlled (e.g. Fuel Injected) engine. This isn't quite true.
The feedback for the engine comes from the oxygen sensor in the exhaust, which is down stream of the valves, but ahead of the muffler. The engine control module has been programmed on the assumption (knowledge?) that the exhaust being sampled is x seconds downstream of the combustion chamber at a given rpm. Without the back pressure, the exhaust is actually "newer" or ahead of that. At constant rpm, it won't matter, but if you suddenly open the throttle or shut it down, the lag or hysteresis will be noticeable, perhaps to the point of stalling the engine. Factory ECMs are tuned for a bunch of factors, and this lag is one of them. When you add fuel injection to a carburetored engine, you end up having to tune many of these parameters.
You might ask why I know this- I had an older car that had an exhaust system that was a little resonant, so it was probably factory tweaked, and had some amount of back pressure. At one point the muffler blew out and boy did the performance on the engine change. Try to do 0-65 on an on-ramp was like telegraphing the ships engine room. You'd stomp on the accelerator, the engine would fumble, almost die, think about life for a few seconds, and then roar to life. (It happened to me about six hours after buying the car, just outside of MR's neck of the woods at about 10:00pm at night.) Let's just say I didn't drive as far as I was planning that night, because the nearest 24 hour muffler repair shop was more than few hours away. After the muffler was fixed, it was a great car for me. I put another 200,000 miles on it, with not much in the way of service besides the usual oil changes.
All the best,
Peter
Phils
Platinum Member
Nice job measuring temps!! We all knew the situation was bad, it's worse that I thought. I'm gonna be doing an exhaust change like that soon, thanks!
Phil
Phil
MossRoad
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I wouldn't think a muffler blowing out would affect the O2 sensor that much. Catalytic converter for sure, but muffler suprises me. I have had several fuel injected vehicles with straight pipes after the catalytic converter (I'm driving a '93 Suburban with a 3" exhaust and no muffler for 4-5 years currently) and they all ran fine. Loud, but fine.
Anyhow, I'm not disagreeing with you. And I can certainly understand you not wanting to stop in my neck of the woods at night.... :laughing:
Anyhow, I'm not disagreeing with you. And I can certainly understand you not wanting to stop in my neck of the woods at night.... :laughing:
ponytug
Super Member
Well, technically, the muffler unhooked itself from the exhaust pipe from the catalytic converter, so it was wide open. I'm not sure how it happened; an alligator or some piece of road debris that I didn't see at night, or just age (the car was used with 90,000 midwest miles on it.
Net, net, there was basically no muffler. I had to crawl under the car at night with no flashlight, and no tools and take a hanger and fasten it into something that would hold the muffler and exhaust pipe up off the pavement. Not my easiest automotive roadside repair, but not the hardest, either. Without the back pressure, it was fine at one throttle setting, but it really did not want to change rpms. Rather funny, as normally it was a very responsive engine. (Saab 2.0L)
The deeper story to the lag and ECMs was that I looked into retrofitting an old VW inline 5 cylinder engine with dual carbs to fuel injection, and discovered the complexities of creating an ECM tune that compensates for the various resonances and pressure shifts of an exhaust system. It sounds easy, and perhaps for some engines its, but others are rather... non-linear.
But back to the point at hand- GREAT retrofit!
All the best,
Peter
Net, net, there was basically no muffler. I had to crawl under the car at night with no flashlight, and no tools and take a hanger and fasten it into something that would hold the muffler and exhaust pipe up off the pavement. Not my easiest automotive roadside repair, but not the hardest, either. Without the back pressure, it was fine at one throttle setting, but it really did not want to change rpms. Rather funny, as normally it was a very responsive engine. (Saab 2.0L)
The deeper story to the lag and ECMs was that I looked into retrofitting an old VW inline 5 cylinder engine with dual carbs to fuel injection, and discovered the complexities of creating an ECM tune that compensates for the various resonances and pressure shifts of an exhaust system. It sounds easy, and perhaps for some engines its, but others are rather... non-linear.
But back to the point at hand- GREAT retrofit!
All the best,
Peter
MossRoad
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Yeah, no catalytic conterters in these little engines. Are there O2 sensors in the fuel injected exhaust system on these?
joshlee
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/Users/joshuaromig/Pictures/100_0164.JPG Would a bolt work better with a nylon lock nut work better here instead of the pivot pins? The pins are rolling and wearing the holes open.
