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Import Cars Technical Questions - Dear Dave

Geek-At-Large

Aug 1, 2008
Sccp_0808_01_z+dear_dave_technical+dave Photo 1/2   |   Import Cars Technical Questions - Dear Dave

Direct 'Dear Dave' tech letters to dave@eyesoreracing.com. Coleman will share mind-numbing details, earth-shattering revelations, and technical nerdisms in this space each month.

Q. Ask A Simple Question
I have a question on blow-off valves. I already know what they're used for and why, but I don't know how they work. How does the blow-off valve open when the throttle closes? Where does the vacuum line that is attached to the blow-off valve go? And what is it used for? I've noticed that some BOVs, like the GReddy Type-RS, are adjustable from soft to hard. What is that adjustment used for and what do the different settings do? Is there an advantage in setting it to 'hard' or 'soft' or is it better to try and stay in between?
Dexter Alleyne
Redmond, WA

A. When you're on boost with your foot pinned to the floor, there's a happy turbo pumping a huge volume of air through the intercooler, past the throttle body and into the engine. When you close the throttle in the midst of all this happiness, it's like when the cops come to shut down a party. The guy who answers the door gets the cold chills right away, but it takes a while for those in the backyard to get the message.

When the throttle closes, the air right in front of the turbo stops immediately, but the air behind it just keeps coming. Air piles into the throttle body until it builds up enough pressure to turn around and push back the other way. Eventually (in a party like this, eventually is a fraction of a second) this big wave of panicked air runs right back into the compressor outlet and tries to push its way out the back door.

Since the turbo is still spinning, this doesn't usually work. All the backpressure slows the turbo down quite a bit, but it's still pushing air into the system and the panicked mob of air turns around again and runs back for the throttle, where, predictably, it runs into a wall again and bounces back once more.

On a car with a big intercooler and long plumbing, this buzzkill makes a clearly audible choo...chooo...choooooo sound. With a smaller system with more compact plumbing, it will be a faster chi-chi-chi-chi. Either way, the sound is bad. At best, it tries to blow intercooler hoses off and slows the turbo, causing more lag when you finally get your foot off the clutch and get on with driving. At worst, it overloads the turbo's thrust bearings, leading to early failure (ball bearing turbos have strong enough thrust bearings to shrug off all this BS).

The blow-off valve is basically a side door. When the throttle slams shut, the side door opens and all the boost runs out there instead. This prevents the choo-choo train, and lets the turbo coast freely, so it's ready to rock as soon as the throttle opens again.

It's worth pointing out that if the engine uses a mass airflow meter (MAF), the blow-off valve should be plumbed so the air running out that side door runs back into the inlet of the turbo somewhere after the airflow meter. This prevents the MAF from telling the ECU that all the air running out the side door is going into the engine. Most people say that it's OK not to recirculate the blow-off valve, but most people have low standards for driveability, gas mileage, emission, and sooty rear bumpers. You just paid $12.95 a gallon for that gas, do you really want your ECU to go dumping it into your engine whenever the throttle is closed?

OK, so you claim you knew all that, so here's the part where I stop wasting time and answer the question:

There are three forces acting in a blow-off valve. First there's the air pressure acting on the bottom of the poppet valve itself. If there is 18psi in the intercooler pipe and the bottom of the valve is one square inch, that's 18 pounds trying to blow the valve open.

Sccp_0808_02_z+dear_dave_technical+blowoff_valve Photo 2/2   |   This GReddy Type-S BOV uses two springs to control how much the valve opens under part-throttle and full boost operation. The screw on top of the cap adjusts the pre-load of the springs pushing down on the black rubber diaphragm. When there is enough boost to overcome the spring and air pressure on the top side of the BOV, the piston on the bottom will push up and allow excess boost to dump out of the housing. The Type-S valve also comes with an additional nipple on the boost plumbing side of the diaphragm.

The second force is from a diaphragm at the other end of the valve. That vacuum hose provides air pressure from the intake manifold to this diaphragm. If the diaphragm is, say, two square inches, the same 18psi in the intake manifold will be pushing the valve closed with 32 pounds of force. 32 beats 18, so the valve stays shut. When the throttle is closed, though, there's still 18psi in the pipe, but the intake manifold is now pulling 10psi of vacuum. Suddenly, there are 18 pounds pushing up from the valve and 20 pounds pulling up from the diaphragm. The side door slams open and the party rushes outside.

This is all so elegantly simple, so why do we need a third force? The problem arises at part throttle, where the car might be cruising along with a slight vacuum in the intake manifold, but an eager turbo pumping boost into the system. In this situation, a dual-force blow-off valve would blow open. A blow-off valve that's open at cruise adds an annoyingly inconsistent delay when opening the throttle, so every blow-off valve also has a spring holding the valve shut.

This is where the adjustment is on adjustable blow-off valves; they're simply adjusting the pre-load on this spring.

There's no hard rule on how to adjust these valves. On a street car, it needs to open quickly when needed, so it should be on the softest setting that will still let it stay closed at cruise. On a recirculated system, it's even OK to let it leak under some cruise conditions to get this quick response. This setup usually gives the best boost response after the shift, but I've seen cars built by people who know a thing or two-like Prodrive-with valves set so tight (and deliberately undersized) that some boost remains in the intercooler plumbing during the shift.

Personally, I set them loose.

Q. Diesel Rotary
I have a Mazda RX-8 with all the basic bolt-ons and I absolutely love it. I fell in love with the rotary engine the day I understood cars. I also love diesels and I was wondering if there was any way of making a diesel rotary? I think that that would get the gas mileage up in a rotary and would sound really cool. I think it's possible, but I'm sure you will make me feel like an ass when you tell me why it can't work.
Will Aronson
Coolsville, ID

A. If you're an ass, so were the blokes at Rolls-Royce who tried to do just that in the '70s. There are two fundamental problems with making diesel rotaries. First, diesels require very high compression ratios and rotaries can't mechanically reach that compression without enlarging the rotor to ridiculous dimensions. Rolls-Royce solved this problem by using two rotors to do the compression. A larger rotor was used to pre-compress the intake charge and feed it into a second rotor, which then finished the compression, went bang, made some power, then fed the still-expanding exhaust back into the big rotor where it could expand some more and make even more power.

By stacking two of these two-stage, double-rotor contraptions into one engine, R-R managed to make one 929-pound monster that made 350bhp. Shockingly, this was about half the weight of a comparable piston diesel for the same application (tanks) and would have been perfect for the job at hand, which was to make tanks light enough to drop from airplanes. Before the design was refined enough for production, though, the British military realized that instead of blowing things up by dropping tanks nearby, it was much easier to just blow them up directly from the plane, eliminating the middle man and making the 900-pound diesel rotary nothing but an odd historical footnote.

The other problem, as anyone who has ever cranked the boost on an RX-7 knows, is that rotary apex seals really don't like the high pressure and violent combustion that pinging gas engines and perfectly healthy diesel engines see. This may be manageable in a tank engine with bigger, heavier seals and bunker oil pumped into the combustion chamber to lubricate them, but there aren't any emissions standards for tanks.

Q. Pick-A-Part University
I can't, for the life of me, visualize how a dual overhead cam engine with rocker arms works. I can see how an SOHC works with the cam in the middle, activating the rocker arms for both intake and exhaust. I've even seen how a dohc directly operates the valves by way of the bucket tappet. But I cannot get my head around how a dohc works with rocker arms, like in the 4G63. I can't find any movie clips on YouTube or anything. I've checked books and magazines, and can't find a visual layout for this. I guess the easiest thing to do is grab an engine and see for myself, right? Problem is, I'm broke, so I can't. And no shop will let me see. Please help me.
Lawrence Chaw
Grand Island, NE

A.Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime...

Turn off your computer, scrounge $2.00 for the entry fee, drive to your local self-service junkyard and educate yourself. You will learn more about how stuff works in one day at the junkyard than you will in a week of sitting on the toilet reading my answers.

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