Holley HP EFI
Thread Starter
Burning Brakes
Joined: Aug 2001
Posts: 1,038
Likes: 15
From: Brick- Next to the land of Mojo NJ
Holley HP EFI
Well, I pulled the trigger on a Holley HP EFI ECU.
I will be using this to control the engine in my 1991 L98/ ZF6.
I am planning on retaining the stock ECM in order to run the gauges/ interface with the ABS, etc.
After having reviewed the schematics for both my 1991 Vette and the Holley ECU, I am uncertain as to whether I should:
1) Purchase the Holley TPI/ MPFI harness(es) and have to cut into my stock harness anyway to remove what is not needed
2) Cut into my stock harness and repin what is needed (injectors, TPS, ECT, MAP, etc) for the Holley ECU
At a crossroads here and I do not feel like making an expensive mistake.
Thanks
I will be using this to control the engine in my 1991 L98/ ZF6.
I am planning on retaining the stock ECM in order to run the gauges/ interface with the ABS, etc.
After having reviewed the schematics for both my 1991 Vette and the Holley ECU, I am uncertain as to whether I should:
1) Purchase the Holley TPI/ MPFI harness(es) and have to cut into my stock harness anyway to remove what is not needed
2) Cut into my stock harness and repin what is needed (injectors, TPS, ECT, MAP, etc) for the Holley ECU
At a crossroads here and I do not feel like making an expensive mistake.
Thanks
Instructor
Joined: Sep 2007
Posts: 127
Likes: 13
From: Apollo Beach. Fla
Well, I pulled the trigger on a Holley HP EFI ECU.
I will be using this to control the engine in my 1991 L98/ ZF6.
I am planning on retaining the stock ECM in order to run the gauges/ interface with the ABS, etc.
After having reviewed the schematics for both my 1991 Vette and the Holley ECU, I am uncertain as to whether I should:
1) Purchase the Holley TPI/ MPFI harness(es) and have to cut into my stock harness anyway to remove what is not needed
2) Cut into my stock harness and repin what is needed (injectors, TPS, ECT, MAP, etc) for the Holley ECU
At a crossroads here and I do not feel like making an expensive mistake.
Thanks
I will be using this to control the engine in my 1991 L98/ ZF6.
I am planning on retaining the stock ECM in order to run the gauges/ interface with the ABS, etc.
After having reviewed the schematics for both my 1991 Vette and the Holley ECU, I am uncertain as to whether I should:
1) Purchase the Holley TPI/ MPFI harness(es) and have to cut into my stock harness anyway to remove what is not needed
2) Cut into my stock harness and repin what is needed (injectors, TPS, ECT, MAP, etc) for the Holley ECU
At a crossroads here and I do not feel like making an expensive mistake.
Thanks
Melting Slicks
Joined: Apr 2007
Posts: 2,616
Likes: 111
I run the Holley HP ECM along with the wiring harness. But I am running it in an 85 with the aftermarket Vett Aid dash cluster with Autometer gauges. Works wonderfully w/o the 870 ECM at all. Now I can hook up to a lap top. Your later model dash ....not sure....I would run if with your existing factory ECM to be safe. I am thinking your OEM ECM controls some dash functions.
Thread Starter
Burning Brakes
Joined: Aug 2001
Posts: 1,038
Likes: 15
From: Brick- Next to the land of Mojo NJ
Yes stock ECM is staying in place for certain.
I am just at a crossroads whether I want to shell out for the Holley harness and dissect what what is not needed any longer from the stock harness.......OR..........re-pin some parts of the stock harness for the Holley ECU.
I am just at a crossroads whether I want to shell out for the Holley harness and dissect what what is not needed any longer from the stock harness.......OR..........re-pin some parts of the stock harness for the Holley ECU.
Team Owner
Joined: Mar 2000
Posts: 83,383
Likes: 87
From: Boston, MA
I'm strongly considering doing the same conversion with my '87 and also hoped to retain as much of my stock digi-dash functionality as possible. I haven't researched it yet but was thinking of seeing how hard it would be to essentially create an adapter that would bridge the Holley and stock harnesses where necessary to feed pertinent data to the stock ECM necessary to feed the dash or control other items which require feedback from the engine systems.
A question for others already running this system; did you retain your large cap HEI and what did you use to interface with it. Most of the Holley TPI stuff seems to be for small cap HEI.
A question for others already running this system; did you retain your large cap HEI and what did you use to interface with it. Most of the Holley TPI stuff seems to be for small cap HEI.
Melting Slicks
Joined: Apr 2007
Posts: 2,616
Likes: 111
Kept the existing distributor/cap/rotar ...no smaller unit required. The Holley harness sensors worked fine along with fuel management sensors on the injectors.
You will be converting to MAP vs. MAF....but not sure if you already have that on your year. For me that was another benefit...no more one off MAF specific to the 85 or relays.... I run the ZZ9 roller cam with 0 issues as well.
You want to make sure you get the proper GM HEI /TPI wiring harness. They have the part numbers listed.
You will be converting to MAP vs. MAF....but not sure if you already have that on your year. For me that was another benefit...no more one off MAF specific to the 85 or relays.... I run the ZZ9 roller cam with 0 issues as well.
You want to make sure you get the proper GM HEI /TPI wiring harness. They have the part numbers listed.
Last edited by 856SPEED; Jul 18, 2018 at 12:40 PM.
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Team Owner
Joined: Mar 2000
Posts: 83,383
Likes: 87
From: Boston, MA
That's great to hear, thank you.
You will be converting to MAP vs. MAF....but not sure if you already have that on your year.
I run the ZZ9 roller cam with 0 issues as well.
Safety Car
Joined: May 2001
Posts: 4,746
Likes: 1,640
From: Fairfax Virginia
I would get the Holley harness and tie it into your existing wiring only where and when absolutely needed. Holley makes good quality stuff and I would trust their harness is probably, well made, well marked and easy to follow. I might be inclined to let the car's ECM control the timing since it has the ability to retard under knocking which most aftermarket ignitions are not able to do.
You really don't want to trust the original harness unless it is in "great" shape. The harness in my 1988 coupe is not in the best shape after 30 years, the wires are starting to turn brittle and cracks in the insulation are becoming more frequent. In my engine compartment the wire insulation is faded and brittle probably from the high heat and time. My 1988 has 103,000 miles on it so I would not expect the wiring to be in this sad of a shape. I also have a 1968 C3 and it's wiring is in better shape than the wires in the car made 20 years later. I might be wrong but I think GM tried saving a few bucks here and used cheaper wire in the 1980's.
On my 1968 Corvette I have added an MSD Ignition system, electric fuel pump, Methanol injection, dual electric fans and other accessories. I ran (fused) battery power to a secondary fuse block, this source is supplied by a #6 awg flexible marine grade wire and is located under the dash panel in front of the passenger. From here I supply power to everything I have added to the car, it makes it much easier than going in and out of the original harness. I also number the wires I have added to the car so I know where each wire goes. The little books of numbers are available in most Electrical sections of good hardware stores. This makes my car easy to upgrade while keeping it original for the future owners. I also like having several colors of wire so you don't make all the connections with red or black wire. It is fairly easy to buy the connectors like they use in modern cars that are waterproof and very high quality. I have kits that have everything from a single wire to multiples up to eight wires per connection, this makes for very professional and clean installations. The best part is that they are water-resistant and seem very secure. These are now available at Eastwood and go under the name of "Crimp-Right". These little connectors are a real bonus when adding to your car's electrical system and are a far sight better than what came in my 1968 Corvette. Of course if you have a "Bloomington Gold" car you might not want to use them.
I bought a 3 ring binder and I keep every "Installation Manual" from anything I put in the car in this book. This has saved my butt numerous times and is a good practice. If something happened to me it would help the new owner understand what I did and how to maintain it.
Running wires should be done very neatly and secured so they don't move around while driving. Just be very careful when adding your Holley fuel injection system that you keep "power wires" and "signal wires" apart from each other. Any power wires should be routed away in a different path so they don't touch or get close to the signal wires. Otherwise the signals might be adversely affected by picking up noise from the power wires. This is critical when working on cars with hi-tech electronics and computers in general.
I hope that your Holley Install goes smoothly! Please keep us posted on any problems or tricks you learned as it will benefit us all! I will be doing the same type of conversion in my 1968 BB Corvette when I get tired of rebuilding those big Holley carburetors I currently have.
You really don't want to trust the original harness unless it is in "great" shape. The harness in my 1988 coupe is not in the best shape after 30 years, the wires are starting to turn brittle and cracks in the insulation are becoming more frequent. In my engine compartment the wire insulation is faded and brittle probably from the high heat and time. My 1988 has 103,000 miles on it so I would not expect the wiring to be in this sad of a shape. I also have a 1968 C3 and it's wiring is in better shape than the wires in the car made 20 years later. I might be wrong but I think GM tried saving a few bucks here and used cheaper wire in the 1980's.
On my 1968 Corvette I have added an MSD Ignition system, electric fuel pump, Methanol injection, dual electric fans and other accessories. I ran (fused) battery power to a secondary fuse block, this source is supplied by a #6 awg flexible marine grade wire and is located under the dash panel in front of the passenger. From here I supply power to everything I have added to the car, it makes it much easier than going in and out of the original harness. I also number the wires I have added to the car so I know where each wire goes. The little books of numbers are available in most Electrical sections of good hardware stores. This makes my car easy to upgrade while keeping it original for the future owners. I also like having several colors of wire so you don't make all the connections with red or black wire. It is fairly easy to buy the connectors like they use in modern cars that are waterproof and very high quality. I have kits that have everything from a single wire to multiples up to eight wires per connection, this makes for very professional and clean installations. The best part is that they are water-resistant and seem very secure. These are now available at Eastwood and go under the name of "Crimp-Right". These little connectors are a real bonus when adding to your car's electrical system and are a far sight better than what came in my 1968 Corvette. Of course if you have a "Bloomington Gold" car you might not want to use them.
I bought a 3 ring binder and I keep every "Installation Manual" from anything I put in the car in this book. This has saved my butt numerous times and is a good practice. If something happened to me it would help the new owner understand what I did and how to maintain it.
Running wires should be done very neatly and secured so they don't move around while driving. Just be very careful when adding your Holley fuel injection system that you keep "power wires" and "signal wires" apart from each other. Any power wires should be routed away in a different path so they don't touch or get close to the signal wires. Otherwise the signals might be adversely affected by picking up noise from the power wires. This is critical when working on cars with hi-tech electronics and computers in general.
I hope that your Holley Install goes smoothly! Please keep us posted on any problems or tricks you learned as it will benefit us all! I will be doing the same type of conversion in my 1968 BB Corvette when I get tired of rebuilding those big Holley carburetors I currently have.
Thread Starter
Burning Brakes
Joined: Aug 2001
Posts: 1,038
Likes: 15
From: Brick- Next to the land of Mojo NJ
Racer
Joined: Sep 2011
Posts: 445
Likes: 11
From: Indiana
Thread Starter
Burning Brakes
Joined: Aug 2001
Posts: 1,038
Likes: 15
From: Brick- Next to the land of Mojo NJ
Yes there are only a few circuits on the stock ecm that I am going to need to keep (vss, serial data, and possibly the air conditioning inputs/ outputs)
Will most likely go with the Holley harness.
Will most likely go with the Holley harness.
Le Mans Master
Joined: Feb 2005
Posts: 6,477
Likes: 565
From: Clearwater Florida
..... I've been using Holley EFI stuff forever … started with the Commander 950 , which is a perfect set-up for early EFI cars like ours … am now using the Dominator … Buy the Holley harness and let the HP control what it needs to run the engine … the HP will control all ignition functions including DIS (distributor-less ignition systems) and CNP (coil near plug) and will control timing via knock sensors … fully lap top tuneable or use a touch screen for more basic set-ups … your only regret will be not having done it sooner …..
Thread Starter
Burning Brakes
Joined: Aug 2001
Posts: 1,038
Likes: 15
From: Brick- Next to the land of Mojo NJ
It seems to me that keeping the stock ecm is complicating things that don't need to be complicated. The biggest "item" you will need from the 91 ECM is the VSS output signal (4000 PPM) grn/wht wire. The HP can output this signal, no problem. The speedo is driven by the CCM, the CCM gets its speed input on the grn/wht wire. With the ecm removed the CCM will set a code 46 which results in a SYS error, but you'll have to live with it. And the fuel computations won't work. But I feel these little issues are tolerable if you can eliminate the ecm and the associated potential problems.
The A/C idle-kicker and clutch relay control (if used on 91, I don't remember) is available in the Dominator, it probably is in the HP as well, but I'm not certain. The A/C control head also receives VSS on the grn/wht wire. The A/C head receives CTS and RPM on the data line from the ECM. One possible work-around for the A/C control is to retro-fit C60 Manual Controls if your car has C68 electronic A/C controls.
I've done two Dominator installations in Syclones. I use the 10' main harness, and terminate the engine end with my own lay-out. The Holley plug-n-play harnesses make it easy, but the install ends up a bit cluttered because the harnesses are "fit-all" lengths. I like my harness and installs really clean, so I terminate the engine end myself.
There's some ideas. Please post updatse as your project progresses.
The A/C idle-kicker and clutch relay control (if used on 91, I don't remember) is available in the Dominator, it probably is in the HP as well, but I'm not certain. The A/C control head also receives VSS on the grn/wht wire. The A/C head receives CTS and RPM on the data line from the ECM. One possible work-around for the A/C control is to retro-fit C60 Manual Controls if your car has C68 electronic A/C controls.
I've done two Dominator installations in Syclones. I use the 10' main harness, and terminate the engine end with my own lay-out. The Holley plug-n-play harnesses make it easy, but the install ends up a bit cluttered because the harnesses are "fit-all" lengths. I like my harness and installs really clean, so I terminate the engine end myself.
There's some ideas. Please post updatse as your project progresses.
Correct me if I am wrong, but i thought the VSS signal is carried from the ECM to the CCM via serial data? How would I go about setting this up on the HP ecu?
I pretty much have everything else figured out from looking at the schematics, etc.
Thread Starter
Burning Brakes
Joined: Aug 2001
Posts: 1,038
Likes: 15
From: Brick- Next to the land of Mojo NJ
Woops! Just looked at the schematic again, the VSS signal from the ECM to the CCM is separate from the serial data.
Will taking the ECM out of the loop affect my ABS? It looks like that is fed by serial data.
The HP ecu arrived via the big brown truck yesterday, so no turning back now!
Will taking the ECM out of the loop affect my ABS? It looks like that is fed by serial data.
The HP ecu arrived via the big brown truck yesterday, so no turning back now!
Last edited by 19corvette91; Jul 20, 2018 at 09:33 AM.
Team Owner
Joined: Mar 2000
Posts: 83,383
Likes: 87
From: Boston, MA
For those of you that installed in a C4 where did you mount the main unit? If I go this route I'm hoping to tuck it under the dash but that sounds like finding a somewhat clear place to cut into the firewall and use a grommet to pass the harness through.
Advanced
Joined: Jun 2018
Posts: 50
Likes: 5
From: Springfield Mo
With the ecm removed the CCM will set a code 46 which results in a SYS error, but you'll have to live with it. And the fuel computations won't work. But I feel these little issues are tolerable if you can eliminate the ecm and the associated potential problems.
The dominator system has a bunch of inputs and output options and I am trying to research if it is possible to imulate, or mimmic the factory system with the dominator ECU to retain this factory gauge.
Team Owner
Joined: Mar 2000
Posts: 83,383
Likes: 87
From: Boston, MA
The 84-89 "Atari Dash" is self contained. The signal from the VSS sensor on the transmission VSS is fed directly to the Cluster. I believe there is a "VSS output" single wire, probably brown, that sends speed info to the ECM. The ECM uses speed for several computations.
- C15, BLK (452), Vehicle Speed Signal Input. USE AS GROUND
- D12, BRN/WHT (437), Vehicle Speed Signal Output to ECM, Switching from 12 to 0 volts.
- D11, YEL (500), Vehicle Speed Signal Input. USE C15 AS GROUND, 340-500 Ohms
There is also D10 ORN (461) Serial Data Output to ECM toggling 2-4 volts which piques my interest a little. I'd love to know how the fuel economy and range display stuff works within the dash as there doesn't seem to be anything crazy complicated being fed into it.