Free Flowing Exhaust
 

So I was wondering about an exhaust issue. I recently got a stock '88 5.0 5-Speed. No real modifications to motor or gears yet, but a bit of weight reduction (AC, rear seat, misc.) A friend has offered me a complete exhaust system with full-length/long tube headers, pre-cat dumps, cats and mufflers, but my question comes before all that. What kind of HP increase and or time drop in 1/4 mile would one see coming from this stock exhaust stock powered pony to the long tube headers with exhaust dumps directly after them. So basically a motor with long tubes on them and nothing else. Of course thats for the strip only, but I don't need every single pony possible while on road. Thanks for the patience with the simple questions!

Believe it or not but I think you would actually see a drop in power with that set-up. The engine needs some back pressure to operate properly. You would be making a lot of noise and would have nothing to show for it.

Alright, so I've laways heard the theory of backpresure .. how 3" exhasut drops over 2.5" etc, etc, but can anyone explain that for me? I understand the science of a free-flowing intake, and how torque is affected by making it flow too much air even if heads are up to it, but how does no pressure on release of exhasut affect it? All the top-fuel dragsters and funny cars I see all just have stright piped exhaust coming out with absolutely nothing. Is is maybe proportional to the HP produced? Thanks for any insight!

OK,.. I am not to keen on this but I will try to explain it and I am sure some of the other guys will either explain it better or tell me that i am wrong. So, here goes,
You need backpressure for sacvanging effects and torque for the street. If you were at the race track and race track only you supposed to make more horsepower to go faster but make alot less torque which is needed for the street. Plus sound ordinances.
Now there are exhust companies out there that claim you will make more power with there exhust over open headers. Generally companies that sell X-pipes.

I think a 2.5" exhaust setup is about right for a 5.0L up until around 500 horsepower. Then you would see benefits in 3".
Scavenging is just keeping the pipe small enough that exhaust velocity remains high. When the exhaust and intake valves are open at the same time, the higher exhaust velocity creates momentum and will vacuum in more intake charge, so you will make more power at lower RPM's with a smaller exhaust system. Granted, as RPM's climb, smaller tubing becomes a restriction as the velocity will increase due to the faster piston speeds. That's why high RPM drag motors don't run mufflers unless sound restrictions make them. They run just straight open headers...
You would have to get quite extreme with sizing for bigger exhaust to hurt PEAK horsepower...but it will definitely impact lower RPM ranges.
I personally believe longtubes and a 2.5" exhaust system should be the first thing you do on a stock 5.0L....the stock exhaust DEFINITELY restricts power output as has been proven time and time again with dyno tests.
Longtube headers will do better than shorties across the rev range as well.

When you are blowing 5000+ hp through the cylinders, you are making SO much pressure...that is why you see a TF dragster blow spark plugs like roman candles on one run.

Think of blowing through a straw compared to a hose. A kid will hardly be able to keep a steady flow of air through the straw...much less a hose. A guy my age can blow through a straw much harder and longer...because I have more power to blow and more to blow.....the more air you have in your lungs, the bigger the straw you can blow hard through...just like a car. The more power you produce, the bigger exhaust you can run.

That is kinda jacked, but I think you will get my jist.

Thanks so much.. especially JonnyK ... that makes alot more sense to me now, with the velocity speed aspect. I'm not quite at the 5,000HP range no less 300HP! So I'll prolly just look into a highflow cat and X-Pipe setup for my 300ish HP goal. Thanks again for the answers everyone!