| Re: [GMCnet] disk brake upgrade [message #234927] |
Fri, 03 January 2014 10:44  |
KB
Messages: 1262
Registered: September 2009
Karma: 0
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Senior Member |
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> Manny told me he also had a talk with you about DFCO. Now that REALLY
> seems extreme!
We talked about it a bit too. I think Manny's assumption was that in DFCO
the engine is basically at idle (~700rpm), which would result in abnormally
low pressures in the tranny. So I started watching what it really does in DFCO.
The RPM's never get anywhere near that low because the drive train is still
engaged even with the fuel cut off. I *think* it's a non-issue.
Karen
1973 23'
1975 26'
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Karen
1975 26'
San Jose, CA
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| Re: [GMCnet] disk brake upgrade [message #234930 is a reply to message #234927] |
Fri, 03 January 2014 10:58  |
Larry
Messages: 2875
Registered: January 2004
Location: Menomonie, WI
Karma: 10
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Senior Member |
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The thing I have found about DFCO is when going down a long grade, foot off the pedal, EFI goes into DFCO, meaning not only are we not burning any fuel, but as a result of not burning fuel, we are also not making any heat. I have seen the engine water temp drop off the bottom of the scale by the end of a long descent, so not heat was coming out of the dash heater, making for a cool ride in cold weather. Just what I noticed...
Larry 
78 Royale w/500 Caddy
Menomonie, WI.
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| Re: [GMCnet] disk brake upgrade [message #234981 is a reply to message #234927] |
Fri, 03 January 2014 19:57 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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My interpretation of Manny's position was that the closed throttle imposes
too much load on the transmission -- he prefers a higher idle speed than we
usually set with EFI. It's that closed throttle, with causing DFCO even
more drag since there's no combustion, that he opposes -- as I understood
him.
Ken H.
On Fri, Jan 3, 2014 at 11:44 AM, KB <kab7@sonic.net> wrote:
> > Manny told me he also had a talk with you about DFCO. Now that REALLY
> > seems extreme!
>
> We talked about it a bit too. I think Manny's assumption was that in DFCO
> the engine is basically at idle (~700rpm), which would result in abnormally
> low pressures in the tranny. So I started watching what it really does in
> DFCO.
> The RPM's never get anywhere near that low because the drive train is still
> engaged even with the fuel cut off. I *think* it's a non-issue.
>
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] disk brake upgrade [message #235008 is a reply to message #234981] |
Fri, 03 January 2014 22:47 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""My interpretation of Manny's position was that the closed throttle imposes
too much load on the transmission -- he prefers a higher idle speed than we
usually set with EFI. It's that closed throttle, with causing DFCO even
more drag since there's no combustion, that he opposes -- as I understood
him.
Ken H.
I hate to second guess Manny since he sure has to be the transmission expert here. My simplistic view is that the transmission is coupled to the engine through the torque converter that will back drive the engine within the slip ratio that will be around 80% of the drivetrain RPM. In other words if the engine quits or is shut down, the transmission will back drive the engine and maintain pump pressure way down to about 2000 rpm at which point the engine and trans will decouple and become independent. In otherwords, fuel cut off will slow the entire driveline until you hit around 2000rpm.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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