Why do cars lose low end torque with open exhaust systems?

[crazypete]

I have an offroad bassani x-pipe imminent and I am worried about losing all my bottom end umph! I'm pretty sure I got gyped on my torque converter cause she runs at very conservative maybe 1300 - 2000 rpms and starts pulling right at 800-1000. Therefore, I need low end umph until I get a real 2400 rpm converter. Teaches me to use stupid ebay.

Anyhow, I hear that cars lose all their low end torque on hpipes. Why and what can I do to get this back?

It's funny cause with the stock 4-cats back on, she feels very..... 'restricted' and toned down. I'm planning on being all clever and sawing the heatshields off the rear cats and welding them to the x-pipe to make it look like I have cats, you know, just in case the roar of my exhaust attracts unwanted attention, I can bluff my way through!

[mustangII460]

Changing backpressure is like changing cams.

When the exhaust valve is about to close the intake opens. This is called valve overlap. With the exhaust valve open a new fuel mixture is sucked into the chamber.

Two much overlap can be caused by (No backpressure or low exhaust manifold pressure.) The air entering the cylinder flows too fast or too long, causing lost torque, less efficient.

At higher RPM pressure will increase, giving some power back.

[Ackbar00]

Lets see, before the turbo, I had MAC 1 5/8 Longtubes, a 3In H-Pipe, and 3In Dyno Max Race welds (Straight through design). I dynoed 324 Ft lbs to the wheels. I would not worry about it at all. Run your X-Pipe without fear:cool:

[tireburner163]

To tell you the true I can't notice the performance difference between open headers, open H-pipe, and full exhaust on my LX

*shrug*

[crazypete]

Actually, I had an offroad h-pipe with welded in aftermarket cats. It was rusted a bit and I could never get the crossover to line up (supposedly the bassani comes in 3 sections so everything will be easy to install). The cats were disintegrating inside so I switched back to the stock 4 cat to pass emissions.

MAN!!

My car feels choked!! It idles cold so much better but she feels just.......like........slow! Plus those stockers are like 3x as heavy as the aftermarket stuff. It does make a difference to have open exhaust!

[HISSIN50]

do an internet search on "exhaust scavenging."

the pulses in the exhaust create vacuum to help scavenge (or pull) exhaust out of the motor. when you lose that backpressure, you lose that 'suction' force.

that is why crotchrockets for awhile now have had valves in the exhaust to maintain backpressure at lower RPM, while allowing better flow at higher RPM.

[crazypete]

Interesting. Could this problem be "fixed" with a cam or some 1.7 roller rockers? I dont particularly want to change the power band cause spinning things up to even 5000 rpms invites breakage, especially with a 130,000 mile motor. I'm gonna put in some 4.56 gears and even then, it's 3000 rpms at 80 mph. If I make the valves open more, would that gain back some of the exhaust scavenging. I'll look up exhaust scavenging tonight.

I'm still running in and out of the house. I'm doing "spring cleaning", which consists of spraying the entire underside of the car with 1500 degree ceramic engine paint. Most of it falls off by the next year so spring is when I freshen it up. Once done, I blast the entire underside with tire silicone cause it makes a nice sludge that really protects against rust and makes the car smell like a showroom =) . I have this feeling that I get mad respect whenever I take her to a shop just because the underside is even cleaner than the topside.

[MiracleMax]

A word on back pressure.... It sucks in any form.

Back pressure should be called what it really is. Negative torque pressure.

ba... I mean negative torque pressure is force the engine has to work against in order to expell spent exhaust gasses.

Ideally you want an exhaust system that has no back pressure without losing velocity or over scavenging the cylinder. This is the best system for a typical street car that has an engine that isnt optimized to take advantage of the suction energy created by a tuned exhuast system.

Now depending on how you look at things, this probably something equally wierd (as saying all back pressure sucks), but very small tube headers with a nice small dia exhaust (upto the mufflers) will do a good job.

by small I mean 1-1/2 primaries and 2-1/4" x-pipe (if you can find it), flair it up to 2-1/2 bullet mufflers then do whatever you want after that (2-1/2 inch pipe I think would work best, but isn't as important as the ego factor of big exhaust tips), if you can get the entire system thermal barrier coated, even better. The longer you can maintain heat and velocity in the system, the better it will perform.

[crazypete]

I keep my exhaust painted headers to just before the tips with 1500 degree ceramic paint.

My bassani x-pipe just arrived!

<oink! oink! Pete rolls around in happiness, grunting loudly!>

I'm using 1 5/8 headers running through a 2 1/2 system all the way to the back. That's a close approximation!

My 4.56 gears should _utterly_..._totally_ cancel out any torque loss! Yeah!

[Nixon1]

Say goodbye to traction. First gear will become a granny gear. Get a rev limiter....you'll probably need it with how quickly that thing is going to bang through gears now.

I think you'll like the X pipe. Not as loud as H-pipes I hear, but it does a very good job and makes an interesting exhaust note.

[crazypete]

According to the calculators, with my .067 OD, I'll only be pulling 3000 at 75 mph. Not bad at all. Thats the same as my 3.73 equipped 25.5 inch height tire'ed lx 5.0 used to do.

I picked gears based on hwy rpms at 75 mph. I love being in the power band in OD.

I get (or used to get, I should say ! =P ) sursprisingly good traction for a 5.0. I'll just have to remember to be careful with how I punch the gas in the middle of turns and such stuff. I might find myself throwing sandbags in the back soon!

How big of a radial will fit in these stock tire wells? I wouldnt mind upping the tiresize a bit.

[MiracleMax]

A 275/50R15 fits back there nicely, a buddy of mine who is a chronic cheap spender on tires went with a set of 295/50R15's but rolled the inner fender lip and got rid of the kicker shocks.

[crazypete]

Aaaah, you are thinking of width, my friend, I was thinking more of diameter and radius! I think I've seen people referring to their 17 inch and I think I even saw someone post a while ago with 18 inch wheels.....

You guys know what I'm talking about! The old 70's cars with tails in the air and noses in the dirt! I wonder if you can mount a larger tire on a 17 inch rim? Tires = cheap(relatively compared to) rims = expensive.

[MiracleMax]

This is a nominal estimate but,

225 x .60 / 25.4 x 2 +15 = 25.63 inches tall

235 x .60 / 25.4 x 2 + 15 = 26.10 inches tall

275 x .50 / 25.4 x 2 + 15 = 25.83 inches tall.

295 x .50 / 25.4 x 2 + 15 = 26.61 inches tall

225 x .55 / 25.4 x 2 + 16 = 25.75 inches tall

245 x .45 / 25.4 x 2 +17 = 25.68 inches tall

See how it works, so your looking at a tire that probably can be around 27 inches without a problem. Interestingly most 275/40R17's and 245/45R17's measure in the manufacturers data sheets as specing out the same. Again though using the math gives you a nominal measurement, not the actual tire dia.

[~The Jester~]

GEARS-Hey Pete, 4.56 is alot of gear. I did the same "figuring" when I decided I wanted them too. Along came a set of 4.30 for the right price so I installed them, took out 4.10 by the way. Now here's the deal, yes with the OD I am still VERY capable of cruising on a highway at 65-70 (Or more, DOH!) But now the car spends NO time in first gear! Definitely get a rev limiter in that car if you don't have one already!!!!! And make sure your headrest is sturdy! Ha!

PIPES- A friend of mine just had Kromer Kraft custom build headers for his W8 powered Demon. There is a very small, but noticeable DECREASE in diameter in the 4" collector, according to the guy that built them, this is for "street torque". $1300 for a set of headers, I'm thinking he knows what he's talking about. I guess the point to my post here is I think Bassani has already thought of this too. It doesn't take much to affect flow, and I bet if you look, they already took care of you!

Good luck!

[crazypete]

Now, technically, I wouldnt need to worry about rev limiting because the auto will shift on it's own when the rpm's get high and I generally wouldnt drive the car fast enough to harm the motor with rpm's (car shakes violently around 110) in OD. I ordered a hardened input shaft and an A+ servo since I'll spend so much time in OD.

[Nixon1]

Technically, yes. But don't trust the transmission to do the work for you. It doesn't always. And tirespin always throws it for a loop....

[crazypete]

The msd-AL6 comes with these little plug in ....I guess they're resistors... that have different numbers stamped on top for different rev limitations. I think the stock one is 6K rpms. Thats pretty reasonable. Dont think I'll throw a rod at 6k rpms.

Something just occured to me.....

I'm gonna be out of 2nd gear by like 10 mph =) . Getting a 3000 stall lockup converter is relatively pointless other than launches cause I'll be locked up almost immediatley.

My goal in life.....really.....is to get a car that can put it's front wheels in the air when it launches.....thats all I ask. I dont need to win any races or anything...as long as those front wheels are in the air, I'll be struttin my stuff back in the pits.

I dont think I can do that on radials, though....can I?

[~The Jester~]

NMRA Drag Radial class does it all the time, and they have 8 second time slips too! hehehehehe