Backpressure Myth

[Seppo Kaitainen]

Ok, I'm tired of reading post where people say you'll lose power b/c of lack of back pressure.

1. This is not really true.
2. This shows a complete lack of understanding of what HP/TQ are.
3. Opinions are one thing; fact is another. If your not a 100% sure please state it as an opinion.

HP/TQ 101

FACT: Horsepower and Turque are curves and act as such. Much like women, not all curves are created equal. With the additions of mods these curves change in size and shape. To steal a line from Bill S "Horsepower is how fast you get to a brick wall. Torque is what moves the brick wall."

HP & TQ as compared to Backpressure
A High about of backpressure typically results in a medium and fat TQ curve and an HP curve with an early peak.

A medium about of BP typically results in a medium sized late peaking TQ curve. The HP curve being similar but peaking later than the TQ.

A low about of BP typically results in a TQ curve that peaks real late. Along with a HP curve that is also a late peaker.

This basically means this:
HIGH BP - quick start, low mid range and low high end umph
MEDIUM BP - solid start, good mid range and ok high end umph
LOW BP - pokey start, good mid range, and good high end umph

Shorthand: BP = backpressure HP = horsepower TQ = torque

[v6maro]

but here is something that is true: with out it, the O2 sensors do not get hot enough to read correctly, sending the PCM a wring signal, causing loss of hp/tq
On higherflowing engines, ie head & cam work, it feeds the air out enough so the o2's read correctly.
-Steve

[Magnus]

If you run open headers, you will have O2 contamination issues.

I run open y-pipe.. My o2's read just fine.

[TheBlackSwordsMan]

Magnus is your car a daily driver? I agree with seppo you don't actually lose your hp or torque the curves just change. Seppo what kind of backpressure do you reconmend??

[Magnus]

No my car is not.. When I do drive on the street however I run through the B&B.

[Arctc Wolf]

I agree w/ seppo w/ his description of backpressure and hp/tq. One thing people should consider when saying there's not enough backpressure in their exhaust is what the car is being built for. Some people don't want to put on heads/cam or a sc later on and just want some intake and exhaust mods. others will want to go all the way, and yet still others will go to a point in between the 2 extremes. lastly, for those who think they don't get enough backpressure if they run a 3" cat-back exhaust, you still have the headers and 2.25" cat converter causing backpressure. If you were to get everything done(headers, 3"cat, cat-back), make sure it allows you to take full advantage of any future mods you plan on doing, or and easier way to find out if you need a bigger exhaust is instead of building up(intake, exhaust, heads/cam, sc), build down by putting on an sc, heads/cam, intake, exhaust. This way you know how much air you're putting through the pipes, as well as being able to better dial in your chassis, suspension, and other components. Doing this will get you a very low torque peak and a fairly flat hp curve until you get exhaust. Then you can tweak the power curve by trying different size pipes and mufflers. Just an Idea.

[MustangEater8251]

hmm... here are some scenarios.

Car with full exhasut runs 16.2-16.3

Car with I- pipe disconnected, but clogged CAT runs high 15s

Car with no CAt, no I pipe, runs consistant 16.8s.

Car with no pipe, even extremely more clogged CAT, runs a 15.8s consistantly

Cat gets old fallls off car, I am running no CAt, back to the mid 16s with my car.

I buy a $15 muffler, a clamp and a 1/2 in. flat wrench, total cost $22, clamp sh|tty muffler to I-pipe, ran consistant 15.9s, with horrible track conditions. must have been mid to low 90s and very humid.


every tiem I made and adjsutment, the more freee flowing the exhaust, the less amount of torque I had but an increase on the top end, but eventually gets to a point, where there is a lack of power on top end and low end.

BTW, everytime I reset PCM.