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I noticed this when I got my exhaust put on back in April. If I'm driving on the highway, with OD off, and let's say I'm coming to an on ramp, so I'll just floor it all the way...when I let off the gas, the exhaust is really loud, it gives out a low, type of muscular rumbling sound..it sounds cool, but I was wondering why it happens when OD is off? The exhaust note goes completely away when I let off the gas with OD on.

Mike
 

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Your car is not shifting into overdrive. Therefore your engine just slows down producing the rumbling sound when you are in that lower gear. My 78 thunderbird had glass packs and would do the same thing If I manually shifted it.
 

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basically the engine will stay in the current gear....so when you slow down and the engine revs down...a deep resonation tone will be heard..sometimes followed by a backfire. :biggrin:

So basically what your hearing is an engine Decelerating. Neat eh?

With OD on..the car would normally just shift to the next gear
 

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The car is always in overdrive. Turning it off is the idea of the situation, so actually you would wonder about driving with it off all the time. I do not know about the long term effects of having it off all the time. Transmission might not like it, or gas mileage.
 

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I recently bought the turbo book by that corkey guy. I've read the first few chapters and it said in there that as rpm increases so do the forces on the pistons and such, for example if the RPMs were at 4K they would have twice the force acting on the pistons as there would be at 2K RPM; therefore if you go driving on the highway all day with OD off you would probably greatly reduce the lifespan of your engine.
 

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Originally posted by: arskiracer
I recently bought the turbo book by that corkey guy. I've read the first few chapters and it said in there that as rpm increases so do the forces on the pistons and such, for example if the RPMs were at 4K they would have twice the force acting on the pistons as there would be at 2K RPM; therefore if you go driving on the highway all day with OD off you would probably greatly reduce the lifespan of your engine.

Right, obviously the faster you're going, the more you are stressing the engine because the crankshaft's gotta revolve faster. OD is similar to driving a manual 6 speed, and putting it in 6th gear, you see the rpm's drop while the car just maintains speed; on the other hand if you were doing the same thing in say 3rd or 4th gear, you'd notice the rpm's up around 4-6k, and it may sound like the engine was straining....taking the car out of o/d is acceptable occasionally, but leaving it out of o/d all the time is just going to wear out your engine faster as it works unnecessarily.
 
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