Exhaust Dump

[olds8894]

This is continued from "Cheap Power Increase" I just thought this might grab more attention.

Heres the deal. I'm thinking of putting an exhaust dump on my 88. I want it to be controlled from in the car for ease. Here are some sites I found that sell them, I also have a few questions.

http://www.atpturbo.com/root/releases/release091004.htm
http://www.quicktimeperformance.com/...austcutout.htm
http://www.dmhperformance.com/ecutout.htm

Questions:
Where should the dump be put on the exhaust? Before or after Cat?
Which one of the above might be best?
Will this throw any sensors and cause the chk. eng. light to come on?
Rough estimate of HP increase? I'm guessing about 10-15hp

Thanks.

[razr_88]

hmm.. i think the quicktimeperformance one would be the best. as for the location... well, that's up to you... if you put it after the cat, it'll be cleaner, but if before, the cat isn't in the way, resulting in more power.... however, you could asfixiate yourself off the fumes.

the only sensor i can think of off the top of my head that might be affected would be the o2 sensor... but i'm not sure

as for your estimates, i believe that they would be accurate...

[olds8894]

This just occured to me, I dont have a cat anymore. DuH! Well, I figure if I put it after the O2 sensors I should be fine since it will still have the same exhaust going through it all the time. I do need some ideas on how to make the dump pipe. I would like to run it out the back and make it look like dual exhause, kind of lame I know, since it would only run one or the other. So ideas will be welcomed. Thank you.

[razr_88]

maybe if you run a the gate from before the muffler, and then have it connect just on the other side of the muffler..idno.. just an idea.

[DrJay]

Questions:
--Where should the dump be put on the exhaust? Before or after Cat?
Before.

--Which one of the above might be best?
http://www.quicktimeperformance.com/QTEC/

--Will this throw any sensors and cause the chk. eng. light to come on?
No.

--Rough estimate of HP increase? I'm guessing about 10-15hp
4-5hp peak I'd guess, might be 0.

Something to keep in mind, an engine is an entire system and not just separate components in the same area. Any change you make to one area should compliment the other for maximum gain.

What you're likely to run into with using this is that your low-end is going to be total junk. I see it all the time and have driven more than a few. It feels like you're towing something off the line then it picks up. The reason is related to exhaust velocity which causes a scavenging effect in the cylinders. While the vast majority of it happens at the manifold, it carries on through most of the exhaust system. Enlarge the exhaust (or shorten it even) and it's going to take more airflow to create the same scavenging, thus you may see gains at 5,000rpm when the engine is pumping more air but might hurt you at the lower end.

Some of you may have even played with the idea in grade school. Try it again: Put a straw in water then cut it so that you can bend the top 1" of it at a 90* angle from the rest of the straw, so it makes an upside down L shape. Now blow really hard through the straw and watch the water get sucked up...scavenging in effect.

What you're doing with huge exhaust systems is lowering the exhaust velocity and thus lowering scavenging. This is like instead of using a small straw you "upgrade" to a 3" hose. Still think you can suck the water out? It has to do with Bernoulli's Principle. For those of you without the high-tech equipment needed to carry out the experiment, http://home.earthlink.net/~mmc1919/venturi.html has a neat interactive graphic to illustrate the point. Try making it an even size all the way, then open the "exhaust" a bit further.

Going back to my initial statement and complimenting components. A car that makes its power in the upper end of the RPM scale should always make modifications that help the upper end. The other way around for us low-end 3.8 owners. That's not always the case, but it's good practice. Turbocharged cars, for example, notoriously make their power in the higher end. With the words "Turbo" and "High end" in mind, go review the cars they used in the dyno comparison on quicktimeperformance. Now that you're giggling a little, have a look at the one graph they make readily available. A *stock* Trans Am does not make 324 *wheel* horsepower. There's a very good chance he was flowing too much for whatever system the car came with and..dare I say...needed something like this. That is not the case in most stock applications although that's not to say all exhaust mods are pointless for a stock car.

I've even seen extensive testing where replacing a decent aftermarket muffler with straight pipes actually lost a little power on the dyno. Heck to a certain extent quicktimeperformance even admits it:

"Q:What kind of gains are expected on my car?

A: Each car differs on the horsepower/torque gain. It all depends on your car and what modifications you have. You can gain as little as 5 horsepower on a car that already has a high flow exhaust system to a car that needs no back pressure and gain as much as 20-30hp. It all depends on your car. But it is a guarantee that you will get all the volume you want!"

Replace the erroneous use of "back pressure" with "exhaust velocity" and you'll get the point.

Point of the story: I wouldn't do it. Plus these engines sound like junk with open exhaust.

[razr_88]

what he is saying is correct... look up the exhaus pipe size on google or sumting... the smaller the pipe, the faster the pulses of gas must travel. the momentum of the gas creates a negative pressure behind it, which pulls out more exhaust out of the cylinders before the valve closes.

another thing, the longer the pipe to a certain extent the, more lower-end power you'll get. the shorter the pipe, the more higher-end power you'll get.
using a string or sumthing, measure the length of the stock pipe with the muffler, and make your straight flow pipe the same length, or longer, but with fewer bends.