Here are some more opinions on the TBI setups.
We had an analog Holley Projection. It was ok, but didn't really have many
of the features that make fuel injection so appealing. It offered very
rough adjustments and had no way (other than the blinking light on the
closed-loop kit) to monitor its operation. The one we had (actually we
even bought a second one to troubleshoot a problem we were having with it)
really didn't have any more adjustment than a carb.
When we got our motor rebuilt (it did have burnt valves) during the
frame-off, we decided we didn't want to risk the "new" motor to the
Pro-Jection we had been using. We got an Accel TBI setup. It has been
very good so far (~16,000 miles). It offers all the idle control, timing
control, cold start, warm-up enrichment, etc that people think of when they
think "fuel injection". Everything can be easily adjusted and
monitored/logged with a laptop. It now starts at a turn of the key without
fiddling with the gas pedal. It idles at any RPM we want. It idles at the
same RPM in gear or in neutral/park. It goes from fast idle to normal idle
through an adjustable map based on water temp rather than an abrupt
kick-down. It controls the timing (we installed a GM computer controlled
HEI from an Olds car) and has a knock sensor to protect it from pinging.
It has really made driving/using our GMC much more pleasurable.
I've heard that the newer digital Holley ProJections also have similar
features to our Accel unit. They apparently now hook up to a computer for
monitoring and finer/repeatable adjustments. They also have idle control
and many other adjustments (cold start, etc).
The one thing I don't like about some of the TBI systems that use GM
computers is that the map is not adjustable at all by the user. A new chip
has to be burned every time to change any of the fuel
curves/idle/timing/etc. From what I have heard, this is where the real
difficulties with these systems come in. I have heard stories where people
have gone back to Howell several times to have new chips burned.
Apparently if your 455 is exactly like the one they used to originally
setup the map, this type of system works well. If it is a decent amount
different, on the other hand, it takes a much longer time to get it right
because a chip has to be burned and installed in the computer every time a
potential change is tried out.
If they used a MAF (Mass Air Flow) style system, this wouldn't be as
critical because that type of system actually measures the exact amount of
air coming into the engine and can make adjustments based on it. Just
about all the aftermarket systems (Accel, new digital Holley, Howell, Turbo
City) , however, use a MAP (Manifold Air Pressure) system which takes the
pressure in the intake manifold and uses a computer table to calculate out
how much air it thinks is coming into the engine. The computer table needs
to be tailored to the particular engine's efficiencies at different
loads/RPMs. The MAP setup is used in aftermarket systems because it is
much easier to install, the sensor is less expensive, and it doesn't
restrict the intake in any way.
Factory GM setups have switched back and forth between MAP and MAF. Since
there is little variation in production-line engines, one computer table
can be developed for a MAP system that will work well on all the engines of
that type. GMC Motorhome engines usually have many miles on them or have
been rebuilt, so there is quite a bit of variance from engine to engine.
This is where the problem with a one-map-fits all approach comes in. The
systems can make some adjustments to the map internally based on the oxygen
sensor feedback, but the range of these adjustments is limited. The
systems also can go "open-loop" under heavy throttle conditions where input
from the oxygen sensor is mostly ignored.
Zak
>Gene:
>
>While at Myrtle Beach, there was a lot of talk about the TBI system. I
>understand that a number of owners who installed the TBI having adjustment
>controls inside the coach have experienced some serious engine problems (e.
>g. burned pistons, etc.) due to lean mixture settings. I talked to a couple
>individuals who were either in the process of or have removed the TBI they
>had installed.
>
>On the other hand, a Turbo City dealer had a later generation TBI system
>there on display.
>
>My recommendation is to thoroughly check out the system you plan to install
>before installation and satisfy yourself that it is for you. Perhaps talk
>to some of those who have reservations/bad experiences with TBI.
>
> Paul Bartz
>
>From: JDDP32B [mailto:JDDP32B]
>Sent: Thursday, April 01, 1999 7:00 PM
>Subject: Re: GMC: Servant of Master?
>
>I am anxious to put on my TBI injector and upgrade the rear
>
We had an analog Holley Projection. It was ok, but didn't really have many
of the features that make fuel injection so appealing. It offered very
rough adjustments and had no way (other than the blinking light on the
closed-loop kit) to monitor its operation. The one we had (actually we
even bought a second one to troubleshoot a problem we were having with it)
really didn't have any more adjustment than a carb.
When we got our motor rebuilt (it did have burnt valves) during the
frame-off, we decided we didn't want to risk the "new" motor to the
Pro-Jection we had been using. We got an Accel TBI setup. It has been
very good so far (~16,000 miles). It offers all the idle control, timing
control, cold start, warm-up enrichment, etc that people think of when they
think "fuel injection". Everything can be easily adjusted and
monitored/logged with a laptop. It now starts at a turn of the key without
fiddling with the gas pedal. It idles at any RPM we want. It idles at the
same RPM in gear or in neutral/park. It goes from fast idle to normal idle
through an adjustable map based on water temp rather than an abrupt
kick-down. It controls the timing (we installed a GM computer controlled
HEI from an Olds car) and has a knock sensor to protect it from pinging.
It has really made driving/using our GMC much more pleasurable.
I've heard that the newer digital Holley ProJections also have similar
features to our Accel unit. They apparently now hook up to a computer for
monitoring and finer/repeatable adjustments. They also have idle control
and many other adjustments (cold start, etc).
The one thing I don't like about some of the TBI systems that use GM
computers is that the map is not adjustable at all by the user. A new chip
has to be burned every time to change any of the fuel
curves/idle/timing/etc. From what I have heard, this is where the real
difficulties with these systems come in. I have heard stories where people
have gone back to Howell several times to have new chips burned.
Apparently if your 455 is exactly like the one they used to originally
setup the map, this type of system works well. If it is a decent amount
different, on the other hand, it takes a much longer time to get it right
because a chip has to be burned and installed in the computer every time a
potential change is tried out.
If they used a MAF (Mass Air Flow) style system, this wouldn't be as
critical because that type of system actually measures the exact amount of
air coming into the engine and can make adjustments based on it. Just
about all the aftermarket systems (Accel, new digital Holley, Howell, Turbo
City) , however, use a MAP (Manifold Air Pressure) system which takes the
pressure in the intake manifold and uses a computer table to calculate out
how much air it thinks is coming into the engine. The computer table needs
to be tailored to the particular engine's efficiencies at different
loads/RPMs. The MAP setup is used in aftermarket systems because it is
much easier to install, the sensor is less expensive, and it doesn't
restrict the intake in any way.
Factory GM setups have switched back and forth between MAP and MAF. Since
there is little variation in production-line engines, one computer table
can be developed for a MAP system that will work well on all the engines of
that type. GMC Motorhome engines usually have many miles on them or have
been rebuilt, so there is quite a bit of variance from engine to engine.
This is where the problem with a one-map-fits all approach comes in. The
systems can make some adjustments to the map internally based on the oxygen
sensor feedback, but the range of these adjustments is limited. The
systems also can go "open-loop" under heavy throttle conditions where input
from the oxygen sensor is mostly ignored.
Zak
>Gene:
>
>While at Myrtle Beach, there was a lot of talk about the TBI system. I
>understand that a number of owners who installed the TBI having adjustment
>controls inside the coach have experienced some serious engine problems (e.
>g. burned pistons, etc.) due to lean mixture settings. I talked to a couple
>individuals who were either in the process of or have removed the TBI they
>had installed.
>
>On the other hand, a Turbo City dealer had a later generation TBI system
>there on display.
>
>My recommendation is to thoroughly check out the system you plan to install
>before installation and satisfy yourself that it is for you. Perhaps talk
>to some of those who have reservations/bad experiences with TBI.
>
> Paul Bartz
>
>From: JDDP32B [mailto:JDDP32B]
>Sent: Thursday, April 01, 1999 7:00 PM
>Subject: Re: GMC: Servant of Master?
>
>I am anxious to put on my TBI injector and upgrade the rear
>
