I have heard it said that straight pipes will reduce horsepower because of loss of backpressure. Why is this the case...it would seem like straight pipes would increase power because there is nothing stopping the flow...?

 

Good question.
I dont' know why you need it, you just do.
Seems like straight thru would increase power, but it doesn't.

When I put muffler elims on my LT1 I lost some noticable torque and changed back to stock mufflers.

Go figure..

 

Yeah, I know a lot of people with 4 bangers that put huge exhaust pipes on there cars and high flow mufflers and lost tons of power I told them before they did it that their motor doesn't put out enough to have an exhaust like like. One of them has a 3" exhaust system on a 4 banger chevy s10 and it hurt the power big time.

 

not HP so much as some feel that you lose a pretty good deal of SOTP Torque.

Ive got a plugged Cat on my car right now...(not after tomorrow ) and I got to tell you, Ive got tons of backpressure and it feels much less powerful. I think some BP is good, and allows for good low end torque. But less BP allows for faster revving and much more usable torque i nthe upper RPMS.

Ive got straight pipe mufflers, and a gutted cat after tomorrow...so Ill give you a first hand description of any difference. l

 

You don't need backpressure if the motor is tuned for it. How much backpressure does a top fuel car have? None? Loss of torque? I don't think so.

 

Backpressure is resistance to airflow out the exhaust. If you put smaller pipes on a car, you increase backpressure because the air is being forced into a smaller volume and must increase its velocity as a result and it resists that acceleration which causes backpressure to result.

You dont need backpressure at all, it wont hurt an engine. But what you do need is the right exhaust velocity for OPTIMIZED power numbers. Larger pipes give lower velocity on low-end, and lower velocity means less torque output (less low end torque, more top end HP), too large a pipe means that the gas is moving slowly at any rpm, thus killing power altogether. (The peak exhaust velocity roughly coincides with the torque peak on a dyno.) Too small a pipe and the exhaust velocity peak is at a lower rpm, giving you more low-end but hurting the top-end horsepower. (on an engine that can make top-end HP) Whether your engine can make power at high rpm for you to even notice that is another separate question.

Pipe sizing moves the exhaust velocity to higher or lower RPM, and power peaks go along with it.

Now, if you put a huge pipe on an engine that cant push out the air and doesnt need it, you are basically moving the peak velocity (that corresponds to that pipes' size) out of the engines power range. A honda doesnt need a 1 3/4" header pipe with stock cams, for example, it needs pipe sizes tuned to correspond with the power range of the motor.

Vice versa, if you put a 1" header pipe on a 350, youre choking it, and although youll have high exhaust velocity at low rpm, the engine is pushing air out so hard at high rpm that it cant get out and the backpressure is killing your top end horsepower.

Straight pipes wont hurt you unless you go outrageously big.

 

I did a side by side compairison of mufflers on Crane Cams chassis dyno with my 86 VW GTI. Had about seven different mufflers that bolted up to the stock pipe. The straight pipe and glass pack were the worst HP/TQ producers with no back pressure. The stock muff and some other aftermarket products made about the same as factory HP/TQ numbers. A new guy on the block at that time was Flowmaster and their numbers were the highest, yet the curves were steeper than the rest. The best all around was a Walker "turbo-muff" with a 10 HP above stock and a generous curve that promoted both mid range and top end numbers.

Bottom line. Back pressure for normally aspirated engines is a requirement if you want a balance of low/mid/high end power.

My .02 worth

 

Very well stated Vader and JrRifle. I'm going to be trying an experiment in back pressure soon. 1 5/8" headers w/ 3" collectors, 3" connector pipes to 3" high flow cats located just beyond the auto trans. and then into LT4 pipes (2 3/4" w/ crossover) and finally into 2 1/2" Corsas. Just waiting on Stainless Works to get the fitment right for A4s. Any thoughts anyone? I don't mean to hijack the thread but I hope you all will think it a worthwhile addition to this most interesting discussion on back pressure. Food for thought, as it were. I'll add my sig so the essentials of my combination are known. Mostly street with low and mid-range torque and driveability my goals. Don't plan on changing from a 3.07 gear but would like a little more converter eventually.

 

With both of these guys. I believe when you open up exhausts, you just need to re-tune for it. Exhaust systems are developed for EPA and sound pollution control only. If not, there would be cats and long pipes with mufflers on dragsters.

 

Smaller pipes do work better in some cases. It is not because they provide backpressure, it is because they are sized correctly for scavenging! Scavenging simply means that the pressure pulses in the headers are timed correctly so that a pressure pulse from one cylinder arrives at the correct time to create a small negative pressure and assist the adjacent cylinders in extracting burnt gasses..

Any backpressure is bad.

 

Sorry, but you can't equate a T/F engine, which runs strictly at WOT and moves air like a wind tunnel, to street and even many race engines which must operate in a variety of RPM ranges. Also they already have way more HP and torque than they can ever hope to hook up.

 

I agree with all of that except....if any back pressure is bad, then how could smaller pipes EVER be beneficial? You would seem to contradict yourself. But yes, back pressure is one component of scavenging.

 

backpressure is the only reason I left the cat on my car

 

look at what vader said........you want the perfect mix of volume and velocity.............for example in a certain engine, maybe putting on 1/4" smaller pipes wont choke this certain engine...........they will give the exhaust gasses a faster exit though



think of water flowing through a hose.......what happens when you sqeeze the hose?.........................

 

The best exhaust system is: zoomies of the
correct lenght. Unfortunately, the best lenght at
5000 rpm is about sixty inches. Anything else
is a compromise for fitment, or other issues.

 

I was going to open up the exhaust of my stock 1990 back in the spring, but I got too many conflicting posts on the subject.

I was considering eliminaters only [ with 3 stock CATs ] then next year eliminating the precats [or new catless front Y ]

Any idea on if AND how much this will help or hurt me [stock engine with few free / cheap mods]

thanks
Paul

 

The ideal would be NO back pressure AND high velocity, no?
Unfortunately, physics doesn't allow it and you have to accept some back pressure to achieve desired velocity.
It's all a tuning trade-off/compromise: you accept some losses due to BP to get the larger, offsetting gains of V.
Back pressure is bad, but you have to live with some.

Keep in mind, back pressure from clogged cats, restrictive mufflers, etc. is NEVER good. Only BP from properly-sized pipes is acceptable because that's where the velocity comes from.

Larry

 

Uh, top fuel cars are supercharged.

Anyway, When your exhaust valve begins to open and the piston begins force the spent gasses out of the cylinder the gasses from the the last combustion cycle are still traveling out the pipe and behind that bunch of gas is a vacuum and, optimally, that vacuum, or negative pressure area, is present in the pipe as the exhaust valve begins to open and sort of helps suck the next bunch of spent combustion gas into the exhaust. Backpressure helps to control how fast the spent gasses travel down the exhaust passages. Not enough, or too much backpressure and the exhaust timing will be off. Hence, if you don't have enough backpressure the engine is most likely trying to force the spent gasses out of the cylinder with only the piston and no help from vacuum.

My $.02.

 

Ok - from what I've read (all good stuff), bottom line is if you're going to use your car as daily driver, not at the drags or racing, you need some backpressure - so no open pipes. AND take what the muffler/tailpipe companies say with a salt rock.

 

Here's a real time example....and as a bracket racer i know what my car will run in most weather conditions.

My car with a bone stock LT1 exhaust in 1500ft DA will run 11.30s at 118 with a mid 1.5 short time. Last Sunday I dropped the exhaust and ran open headers with short turndowns. Weather was 1500ft. Car went 11.20s at 121 with 1.6 shorts. The exhaust weighs about 60 lbs and the car picked up a little over a tenth...so about 1/2 the improvement was do to weight loss. The car made more HP (3 mph faster), but lost low end torque (almost a tenth slower on the short times).

I think people confuse backpressure and velocity in this case. Like Vader said, velocity will promote low end torque (SOTP), while flow will improve top end HP. Backpressure is not good in either case, but some back pressure may be required to get the velocity you need.


btw, this also says the LT1 exhaust aint too bad.