Installing a GReddy Type-S Blow Off Valve in a DSM car with a stock BOV flange

This project relates to a Mitsubishi 4G63 turbo powered Eclipse/Talon/Laser (89-94)  but even greater improvement can be realized from replacing the BOV on the 95-99 cars, as the later design leaks at even a lower pressure than the early DSMs.

The stock BOV removed from a modified 1st generation DSM would be a good upgrade to a stock or lightly modified 2nd generation DSM.

Note: Some aftermarket DSM vendors can supply hard upper IC pipes with the Type-S flange instead of the Mitsubishi BOV flange. This would be preferable if you haven't purchased a hard IC pipe yet. This project converts an existing IC pipe flange to the Type-S style.

Purpose: the stock BOV will not hold higher boost pressures without modification. The stock BOV may leak even with a lightly modified engine.  A fast-acting aftermarket BOV protects expensive turbochargers from compressor surge.

Difficulty: This project is easy and suitable for anyone that can operate a hand drill.

Equipment needed: socket set, electric drill, RTV sealant, hand file (maybe) and an installed aftermarket boost gauge to check operation when completed.

Text and photographs copyright 2001 by Mark Miller (see PhillyDSM for email contact.)  Allow up to 60 seconds to load photos.
Remove your upper intercooler pipe containing the BOV by removing the two clamps  then remove the BOV by removing the hose,  two mounting bolts, and air return tube. 

(Do not allow any foreign objects or dirt to enter the air return tube or IC--stuff a clean rag into the openings)

Remove the large air return tube that connects to the turbo intake.

The new BOV (left) is a Gready Type-S. Compare to the  factory '89-94 style BOV in this flange view. 


When you unbolt the stock BOV from the upper intercooler pipe, you will notice that the flanges are different. 

It is possible to drill two mounting holes in the existing IC pipe flange to accept the Type-S BOV. Use the new gasket as a drilling template by centering it around the opening in the center of the flange. Mark the two outer holes for drilling. The two marks should be inline with the two existing mounting holes.

(Note: This modification is difficult to reverse, as the two holes you are about to drill would have to be filled if you go back to the stock BOV.)

Stock IC pipe is pictured here. 

Drill two holes at the locations you marked. Select a drill bit just large enough to accommodate the new mounting bolts. 

Remove all the scrap metal fillings from the IC pipe. Wash it out with a garden hose to make sure there is nothing to get into the engine. 

Buchur Racing stainless IC pipe is pictured here. 

Apply the new BOV gasket to the flange. Since there is only a thin mating border, I added some RTV silicone sealant on the inside step for some extra sealing capacity. 

Most people don't use RTV sealant here, but I didn't want to worry about leaks later, in case I really turn up the boost pressure. 


Using the supplied (stainless) mounting bolts and nuts, assemble the new BOV and gasket  to the IC pipe flange. I didn't use a torque wrench but did pull it very tight. You can also tap the flange instead of using nuts.

You can see how only half the gasket mates to the old flange. Hopefully it will be enough.

If you used RTV sealant, allow several hours for it to harden before operating the engine. 

Reinstall the upper IC pipe and tighten the clamps. 

(remove any protection you might have added to the IC / air return openings)

The small hose you removed from the stock BOV must be connected to the larger nipple on the anodized cap (closest to the adjustment screw.)

(Note: This connects to the intake after the throttle butterfly and will pull vacuum when the throttle is closed)

The smaller nipple on the cast aluminum body of the BOV may be spliced into the turbo outlet pipe or into the line between the turbo and wastgate actuator. Use the T-connector supplied. This connects to pressure even when the other line has vacuum. This will activate the BOV faster by applying pressure under the valve seat. 

The smaller nipple may also be left unconnected. The BOV will operate slower and more quietly. I did not test this mode. 

Replace the air return tube so metered air may be restored to the turbo intake. I had some trouble with fitment so I added some tape to the connection between BOV/tube. I also pulled up the neoprene hose cover that was found lower on that hose, so it covers and reinforces the return air connection. 

Some may allow the BOV to dump into the engine bay to increase the sound but there are reports of air/fuel ratio problems with that approach.

I find the sound is certainly loud enough with the return hose connected. 

The adjustment screw may be tuned for a harder or softer operation if both hoses are employed.  There is nothing to adjust if you do not use the second turbo pressure line. 

In the photo above you will notice the primary 
vacuum/boost hose (far left) is very close to the
fuse box cover. I used a rat-tail file to remove 
the edge of the box so the hose would not be
kinked. The green hose is silicone vacuum hose. 

The hose in the center is just the primary hose 
(on left) that loops under the BOV


Results: Without adjusting anything, I took the car out for a drive. The new BOV vents exactly as it should between shifts when under boost (or when the throttle is suddenly released when under boost.)  There is a very interesting "whoosh" sound that can even be heard by adjacent drivers. I also noticed that my max. boost seems to be about 1 psi higher. My old BOV (or gasket) may have been leaking. I may experiment with the adjustment screw in the future.

This article is based on my experience and my installation. You may decide that this information is useful to your installation, but use at your discretion.

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