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Removal of boost cutoff - 1988 740 Turbo

89_740Turbo

89_740Turbo

New member
Joined
Apr 10, 2018
Location
Central New Jersey
I recently got my hands on a super low milage ft motor and decided to go full motor swap. New seals, tranny and all. I'm running a 16T right now with a Yoshifab wastegate. The car purrs and wants to rip! Only problem is I'm on lh2.2

I am using Sbabbs chipped lh22 chip and hoping I can remove this boost cutoff on my car. Its cutting off at 15lbs and is like to run 18lbs.

I've researched the cutoff issue and have read nearly all the close to 2 decade old threads, which mostly have conflicting information. Some say there is something under the peddles, others say a hose near the ECU. Confusing. I'm running Jetronic ECU. Any help is appreciated
 
It's been a long time. But I seem to remember threads where people added a zener diode to limit the voltage value output on the air mass sensor so won't go to cutoff voltage.

You can also keep the turbo EZK117 ignition and change the fuel ecu to LH2.4 along with the appropriate sensors. You have to change a few wires around in the connectors for the computers. I made a thread about it and one of the members here did that setup. Will find it and post back.

Here is the LH2.4 fuel computer swap.https://forums.turbobricks.com/showthread.php?t=347716

How to put the zener diode in courtesy of thelostartof.https://forums.turbobricks.com/showthread.php?t=245837&highlight=Zener+diode
 
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dl242gt said:
It's been a long time. But I seem to remember threads where people added a zener diode to limit the voltage value output on the air mass sensor so won't go to cutoff voltage.
Click to expand...

I think that's only an LH2.4 thing? Could be wrong.

Unless they changed it up on different model year 740s I think your boost cut switch should be mounted like right on top of your ECU.
 
dl242gt said:
It's been a long time. But I seem to remember threads where people added a zener diode to limit the voltage value output on the air mass sensor so won't go to cutoff voltage.

You can also keep the turbo EZK117 ignition and change the fuel ecu to LH2.4 along with the appropriate sensors. You have to change a few wires around in the connectors for the computers. I made a thread about it and one of the members here did that setup. Will find it and post back.

Here is the LH2.4 fuel computer swap.https://forums.turbobricks.com/showthread.php?t=347716

How to put the zener diode in courtesy of thelostartof.https://forums.turbobricks.com/showthread.php?t=245837&highlight=Zener+diode
Click to expand...

I'll have to take a look at the lh2.4 chip one. Appreciate that!
 
Cwazywazy said:
I think that's only an LH2.4 thing? Could be wrong.

Unless they changed it up on different model year 740s I think your boost cut switch should be mounted like right on top of your ECU.
Click to expand...

I'll have to take a look. Its a jetronic ecu as opposed to matronic so you should be right but I'll have to take a look.
 
For a LH 2.2 car like you have there is a boost cut off switch under the dash IIRC. Check under the cluster for vac hose routing and you should find it. From there you can either just bypass it OR remove the glue from one end and adjust the screw.
 
thelostartof said:
For a LH 2.2 car like you have there is a boost cut off switch under the dash IIRC. Check under the cluster for vac hose routing and you should find it. From there you can either just bypass it OR remove the glue from one end and adjust the screw.
Click to expand...

I think this is what you're talking about?

Cldd54B.jpg


I'm just going to remove and put a BC on to control boost
 
The car wants to blow past 15lbs real quick.

What are people's AFR readings under boost. Reading mid 14s not under boost and 10.6 to 11.5 under boost. Idle rich
 
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89_740Turbo said:
I'm just going to remove and put a BC on to control boost
Click to expand...

This does not control boost, it just breaks the connection between the two wires when it hits a certain amount of boost pressure. That causes one of the grounds in the fuel pump relay to open, which then kills power to fuel system.

I ran my track car without that switch. You have to use a butt connector to connect the two wires together that go into the switch to bypass it. Then plug the vacuum line that ran to the switch.
 
rb92673 said:
This does not control boost, it just breaks the connection between the two wires when it hits a certain amount of boost pressure. That causes one of the grounds in the fuel pump relay to open, which then kills power to fuel system.

I ran my track car without that switch. You have to use a butt connector to connect the two wires together that go into the switch to bypass it. Then plug the vacuum line that ran to the switch.
Click to expand...

So hypothetically, if I connect those 2 wires together, plug the hose back into it and put a boost controller on setting it at the boost i want, for my setup like 16lbs, i would be able to breach the original boost cutoff of the car and allow myself to reach the desired goals?
 
89_740Turbo said:
So hypothetically, if I connect those 2 wires together, plug the hose back into it and put a boost controller on setting it at the boost i want, for my setup like 16lbs, i would be able to breach the original boost cutoff of the car and allow myself to reach the desired goals?
Click to expand...

If you connect the two wires together, the fuel will no longer be cut when you have hit the boost cutoff level. So you could go to however many lbs of boost you want. I have no idea if your system has the capability to add enough fuel to keep up for the additional boost you desire. Without the cutoff switch, your right foot becomes the cutoff.

YMMV on how long your head gasket will last, how long your rods remain trapped in your block, and how intact your pistons remain. I say go for all the boost for science. If it blows, document it with lots of pictures and just dial it down a bit with the next engine.
 
rb92673 said:
If you connect the two wires together, the fuel will no longer be cut when you have hit the boost cutoff level. So you could go to however many lbs of boost you want. I have no idea if your system has the capability to add enough fuel to keep up for the additional boost you desire. Without the cutoff switch, your right foot becomes the cutoff.

YMMV on how long your head gasket will last, how long your rods remain trapped in your block, and how intact your pistons remain. I say go for all the boost for science. If it blows, document it with lots of pictures and just dial it down a bit with the next engine.
Click to expand...

Not looking to go crazy. 15-16lbs of boost maximum on this setup. Internals should hold up well against that. That being it's a 94 squirter block from a 940.
 
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