B21FT boost question

Okay finally got an intercooled overboost switch for the b21ft. My question is regarding the length of the hose going from the intake manifold to the turbo. I know when you shorten the hose you increase boost, but anyone with a factory IC car know how long the hose is? I don't want to cut it too short and screw anything up. Thanks

Seems like I remember it over on the side of the engine bay on the left there just above the kjet distributor. I'd ditch that and put a MBC a manual boost controller right on the turbo with super short hoses or put your stock overboost switch right at the turbo.

I could never get mine to work correctly. Just before I pulled the engine, and all the k-jet parts, I wired a switch to the overboost valve and was able to turn it on and off from the cabin. Not much difference. IIRC, the way it was supposed to work is that the RPM relay would energize the overboost solenoid for a few seconds when over 3700rpm.

Here's pics from when I first got my wagon. Click on thumb for full sized.


Oh, I just found the official answer in the B21FT-Intercooler_Boost_System_Installation_Instructions.pdf

Ohh, yes please.

Lovely, thanks for the help everyone.

Edit, looks like I'm still searching for an answer.

Overboost switch is the valve at the clutch pedal. Cuts the fuel pump ground when boost pressure is exceeded. The solenoid valve that you guys are talking about is used to delay the opening of the wastegate above 3700 rpm.

What Ethan is asking is how to increase boost pressure on a Mitsubishi TD04HL-15G turbo. With the TD05, you'd shorten the wastegate actuator hose to increase it.

ogamer777 said:

Okay finally got an intercooled overboost switch for the b21ft. My question is regarding the length of the hose going from the intake manifold to the turbo. I know when you shorten the hose you increase boost, but anyone with a factory IC car know how long the hose is? I don't want to cut it too short and screw anything up. Thanks

Click to expand...

John242Ti said:

What Ethan is asking is how to increase boost pressure on a Mitsubishi TD04HL-15G turbo. With the TD05, you'd shorten the wastegate actuator hose to increase it.

Click to expand...

Guess I completely misunderstood the question. What is your exact setup? Original K-Jet non-intercooled B21FT with upgrades? Swapped in Intercooled overboost switch and a 15G turbo? Are you adding a BOV / CBV or just using the one built into the 15G? Do you have what we've been calling the overpressure valve? What's your target boost level?

The hobbes overboost switch is purely a safety/limit switch. It cuts off the fuel pumps when it triggers. The only thing worse for your engine than cutting fuel at full boost WOT, is exceeding the max. boost pressure. An intercooled version of the boost switch triggers at ~14psi, so I'd guess it's OK with a target boost of 12ish psi.

Assuming a stock 15G and no solenoid bleed valve (what we've been calling the overpressure boost valve), I'd just leave the stock short little hose in place between the wastegate diaphragm and the outlet nipple, and adjust the wastegate rod to give ~12psi boost.

I don't know the tradeoffs when using a long hose to connect to the wastegate diaphragm to something other than reference pressure at the compressor outlet. Maybe Volvo decided to use reference pressure after the intercooler to account for the pressure drop across the intercooler? I'd guess that using a post-intercooler reference port would result in higher and better matching boost pressure at the intake manifold. I'd also guess that it would cause more initial overshoot in boost pressure.

From what I can guess at the op may be mixing up the bypass solenoid for the volvo IBS system. There is no performance benefit to using that setup. Just get a manual boost controller as suggested.

The bypass solenoid T'd off the wastegate hose to the solenoid and then the outlet hose goes to the turbo air inlet pipe. Bypassing that air is what gives you the extra couple psi above 3700rpm. No reason now to use this setup at all unless you are making a totally stock setup.

His car has been intercooled, but still had the original red overboost switch that cuts in around 10-11 psi. So, he bought the one that Jack was selling and planned to install it in place of the original unit, which triggers around 15.6 psi.

As far as I know, the solenoid bleed valve and relay setup hasn't been added to his car, partly because the parts I sold him came off my 245, and I'd removed those parts a few years back, then sold them/gave them away at iPd.

Looks like the solution might be to use the old Garrett T-3's actuator on the 15G, btw...

http://forums.turbobricks.com/showthread.php?t=314285

-J

dl242gt said:

From what I can guess at the op may be mixing up the bypass solenoid for the volvo IBS system. There is no performance benefit to using that setup. Just get a manual boost controller as suggested.

The bypass solenoid T'd off the wastegate hose to the solenoid and then the outlet hose goes to the turbo air inlet pipe. Bypassing that air is what gives you the extra couple psi above 3700rpm. No reason now to use this setup at all unless you are making a totally stock setup.

Click to expand...

The OP was talking about replacing the fuel cut-off overboost switch, then was trying to figure out how he'd increase the boost pressure, since he went from the original smoking T-3 to a 15G turbo.

In any case, it's the same as a Garrett T-3. Adjust the wastegate actuator rod. If necessary, use the T-3 actuator, since it has a stronger spring in it.

-J

Thanks for the correction.