Compact Car Questions - Dear Dave

Blowing Off MustangsI've got a Mustang SVO clone made out of a bunch of junkyard parts and I'm looking to mount a blow-off valve, since I run around with 15psi of boost. I want your opinion on the ideal location. It seems like it should go right after the compressor and before my front-mount intercooler, but there's plenty I don't know about this stuff. What's your opinion?Daniel GiesbrechtOutside a junkyard somewhere

First a little background on what the blow-off valve is supposed to do, in case there are any Schwinn drivers reading this letter. When Mr. Geisbrecht has the skinny pedal depressed, his junkyard turbo is spinning its heart out pumping air into the engine. If the good Mr. G. decides to close the throttle for some reason (shifting gears, avoiding puppies, a funny smell from the engine bay, etc.,) the turbo will find itself in a difficult situation. It's spinning along at something like 100,000rpm, moving heaps of air, when suddenly this throttle plate steps in the way and stops the party.

All that air has nowhere to go and the turbo, still spinning from its substantial inertia, is still pumping air into this increasingly crowded intercooler pipe. In this sad situation, the air eventually builds up enough pressure and has to just push itself backward through the turbo. This reverse flow brings the compressor to a screeching halt, puts huge loads on the turbo's bearings, and generally demoralizes what is already an emotionally sensitive engine part.

The blow-off valve opens up when the throttle closes, letting all that air shoot out of the pipe, where it can make noise and make you feel cooler than you really are. The turbo then gets to slow itself down naturally, easing the load on the bearings and, if Mr. G. is just letting off for a quick shift, keeps spinning for that fraction of a second, reducing turbo lag when the hammer goes back down.

Now, the blow-off valve can go anywhere between the turbo and the throttle body, but I like to put it as close to the throttle body as possible. The reasons are fairly simple. First, this keeps the air in the pipe and intercooler flowing in the same direction. If the valve is before the intercooler, all the air beyond the valve will have to reverse direction to get out of the pipe, then reverse again if the throttle is opened again.

Second, the cooler air on the cold side of the intercooler is easier on the blow-off valve itself. This only matters if you have a cheap blow-off valve, but that's just what I would expect in a junkyard SVO replica.

I am proud to say I have no idea what kind of engine management Ford used on the best Mustang ever, but if you have an airflow meter of any kind, you really should plumb the air from the blow-off valve back into the intake somewhere between the airflow meter and the turbo.

The reasoning here is also simple. The airflow meter has already measured this air, and will deliver fuel to match. If you dump the air, you'll have a dramatically over-rich mixture whenever the blow-off valve is open. If you plumb the air back into the intake, the airflow meter's reading will more accurately reflect the engine's actual fuel needs.

If you have to recirculate and you want the blow-off valve near the throttle body, you'll need a recirculation pipe that crosses back across the engine to connect the valve to the intake side. Big plumbing pain in the ass, if you ask me. In this case, it's perfectly acceptable to put the blow-off valve near the turbo so the recirculation plumbing can be shorter. In spite of my preference for cold-side blow-off valves, the drawbacks of blowing-off the hot side of the intercooler pipe are practically negligible, especially compared to the stumbles and hesitation caused by dumping metered airflow into the wind.