A bunch of tech questions...

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OK, so I'm getting into cars now and I'd like to learn more. I've assembled a bunch of questions I'm hoping that some of you may be able to answer.

1.) Assuming your car can handle a dual exhaust, is it better to have two smaller ones than one large one?

2.) Is it possible to go too large on the exhaust? Some people have said that bigger is always better, but in my experiences with other thing, there always seems to be a sweet spot. If there is some "sweet spot" how do you go about calculating this? If bigger really is better, why not just pull the thing off (please ignore laws for this one, I'm speaking in the theoretical).

3.) How do you advance the timing in a car? This seems to be the "mod" that a lot of people go for right away (that's what chips do, right?). Is there a way to do it on a vehicle that doesn't depend on a computer? If so, how do you know how far to advance it?

4.) Why is DOHC better than single?

5.) I'm driving a Geo Metro right now, and I've seen a replacement crank pulley on ebay that promises a 6 to 10 percent hp increase. It's smaller in diameter than stock and 20% lighter, too. I know a bit about physics, so I know why this works (if it even works that well), but wouldn't that be bad? A smaller crank pulley means the water pump and alternator aren't being spun as fast as they were designed to.

posted by  Sonreir

1. That depends entirely upon the car. As long as the exhaust system allows for sufficient flow it really wouldn't matter. At that point, the determining factors would be space for routing and weight.

2. It is most certainly possible to go too large on the exhaust manifold or header. The larger the diameter the lower the velocity for any given engine and velocity is needed for proper cylinder scavenging. Header length and diameter can be changed to tune the power characteristics of the engine. After the collector you can prety much go as large as you want.

3. That again depends entirely upon the car. If the car has a distributor you simply move it... Unless timing is computer controlled then it will still TRY to retard itself to the proper setting. With a DIS or electronically conrolled distributor you have to get inside the PCM, so basically your looking at a chip.

4. DOHC is not better or worse than SOHC. They both have their advantages and disadvantages depending upon application.

5. You have the answer but don't even realize it. First off the pulley does not increase power. What it does is by turning the accesories slower, it uses less power to turn them. The obvious trade off is the fact that by turning them slower you will have lower output and that may decrease their efficiency.

posted by  vwhobo

What are the pros and cons of a DOHC vs SOHC comparison?

Also, would you consider using a smaller pulley? Would the trade off be worth it? I don't want to damage any parts of my engine, but it seems like a pretty sweet way to gain a few hp (to the wheels).

Thx again, hobo.

posted by  Sonreir

2. I don't think you'll have any problem with the charging system with the smaller pulley. The cooling system on your car is marginal at best so slowing down the water pump may cause overheating, but then again it may not. I have never, nor do I know anyone who has, installed un underdrive pulley on a Metro. It might come down to buying the pulley and experimenting with it yourself. That is all fact. My opinion? I wouldn't do it.

1. Without getting into a huge disertation on the pros and cons let me hit on what are probably accepted as the most important ones.

DOHC, pro: Generally speaking has lighter valvetrain parts and the potential for greater air flow. Both of those factors will likely (not absolutely) allow the engine to create more power.
SOHC, pro: In most case they create almost as much power as above with greater simplicity and fewer parts.

posted by  vwhobo

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