| Author | Message | ||
     Carbonbigfoot |
I've been poking around for an answer on the interwebz, and it's funny how I can't seem to find any specific answers to this. Here's the question: On an otherwise stock street vehicle, approximately how much power increase is there to be had with an "off road only" exhaust? I.e., gains as a percentage. Of course, I would never consider actually doing such a modification, but <ahem> purely out of curiosity. The results of my research thus far seem to indicate about 10-20%. Thoughts, anyone? Extra points for personal experience with dyno results... Thanks in advance! R | ||
| Sifo |
Too many variables to really say. How restrictive is the OEM exhaust? How free flowing is the replacement? How do the headers compliment what you are attempting to do? Are you tuning to match or just running the stock maps? Personally I would say that 10-20% is wildly optimistic. For my Sprint ST the manufacturer of the exhaust claims 6 HP can be gained with their slip on and a tune. That's about 5%. I think that's more realistic expectations based on their dyno testing. For me personally, I would ask what will it do to the entire torque curve? Peak HP means little for street riding. A broad, flat torque curve (such as my XB12 or Sprint ST) make for great street riding and even good track days.  | ||
| Carbonbigfoot |
Slip on's generally are the back half of an exhaust. What I'm specifically asking about is the restriction that comes with the catalytic converter. R | ||
| Benm2 |
That depends on the vehicle, and the location and type of catalytic converter. It also depends on the configuration of the remainder of the exhaust system. The lousy performance of OEM exhausts can be attributed to (1) durability (2) noise and (3) pollution control. Depending on configuration - most OEM exhaust manifolds are built to be durable. The "header tubes" are short, and the system is usually designed to keep the exhaust gases hot for cold-start emissions. If the vehicle has a close-coupled catalyst (attached to the exhaust manifold) then that is the area where big gains can be found. Replacing that with a tuned header can really open things up; near to the 10-20% you suggest. For vehicles that DON'T have a close coupled catalyst, the underfloor setup (older cars) really doesn't restrict that much. You'll get more gains from a better muffler and (as above) replacing the stock exhaust manifold with a properly tuned header. If you replace the header and cat-back exhaust, then replacing the catalytic converter itself might only get you an addition 2%. Finally - there are high-flow catalytic converters available that offer about the same flow as an open pipe. So if the other changes are made, then you could put in a high-flow cat and still be emissions legal. Again though - this will mostly apply with OLDER vehicles. Once the cars started using close-coupled catalysts, it is difficult to get a big performance upgrade without effectively deleting the catalyst. A quick note about aftermarket catalysts - they will not last as long as OEM. They don't use the same amount of catalytic material. OEM's are more conservative with the specifications and they have to meet tighter regulations (they are required to last longer by law). Does that help? | ||
| Carbonbigfoot |
Yep. The hypothetical application would be going from the stock exhaust on a 6.2L Ford Raptor to a Stainless Works off-road full system. Vs the same system with the high flow cats. Hypothetically. R | ||
| Etennuly |
Well.....In my day.....oh so long ago....converters were thought to be the enemy at the death of the pony cars in the early 70's. 1973 emissions controls on cars became more important than it's ability to move. Gasoline consumption was up because it was a secondary thought to pollution controls. Having said such a thing, it was a learning curve for the carmakers and the gobmt. They had not learned to work together on fixing what was then a runaway pollution problem in our cities at that time. Those cars of the mid 70's are not what you often find in automobile collections. As anemic as they were to drive they did not inspire memories like the cars of the 50's, 60's, and 70's. The early emissions cars I owned and drove had huge gas tanks that required filling often. Ten to twelve mpg was not unusual for a family car(not an SUV). If you had a mid size car and drove as if there was an egg under the gas pedal, you might get fifteen mpg. Of course there were small gas sippers around, like the 33 mpg VW, the Fiat sedans and Spyders, Toyotas, Datsuns, but they were small and as far as a family goes, not comfortable or crash worthy. Then there were the mechanics with a "cure" for a sluggish Camaro, Mustang, or Charger. The first thing they would do was cut off the converter and install headers. But then every new car came with a Federal Warning decal as to the amount of jail time that could be had for messing with emissions stuff. I recall changes in fuel mileage(if you could keep your foot out of a neat sounding V8)on the order of three to four mpg in a '73 Chevelle Laguna with a 350 2bbl automatic. That brought it up to about 16 mpg. Hp.....well dynos were not to be found in any local garages, but the "feel" in power was noticeable. It would get you to the gas station quicker. I think that car was rated at 140 hp. The change in pipes had to bring it up to 155 or so(but mostly it sounded great). It sure did not get anywhere faster.  To actually get the power that the gobmnt took away you had to back up a bit. Start with pistons that had some compression, then heads that could breathe into your new exhaust system, and a cam, intake and carburetor that could feed it. Then of course those 2.03 rear axle gears had to go. To your original question, dynos were for factories and big time race professionals. The cars I was involved in usually went with an older rebuilt engine or at least with an intake and carb change out. That Laguna was the only one that I messed with without any other changes than the exhaust that had rusted through in two winters. I would guess it helped power about 10%.....but 10% of near nothing is not much. There were companies making "test pipes" for the converters. Where they made a manufacturer replacement fit pipe that you could remove the converter to install the pipe for test purposes to see if a converter was the cause of a problem you might have been having.  Try looking up J.C.Whitney for "Test Pipe". I remember some of the adds claimed 15 to 20% power gain. Converters, electronic controls, and fuel injection, I believe are the reason most cities have air to breathe now. With any of the newer engines with electronic mapping everything is tuned to the pipe. Change one thing, you will have to change many to make it run as well as the original tune. | ||
| Xl1200r |
Pretty much all modern OEM cats are high flow and you gain little aside from noise by removing them. High flow means better MPG, that's why the manufacturers use them. Putting an aftermarket high flow cat in an otherwise off road exhaust is mostly moot. They are generally very small and, because of the headers which are much longer than the OEM headers or manifolds, puts them too far away from the hot gasses to heat up the cat enough to make a significant dent in emissions and in many cases you'll get the same CEL and code as if you didn't have a cat to start with. If you get cats, the only reason would be that you lived in a "visual emissions inspection" state (such as mine). They don't sniff the tailpipe, so as long as they can look at the exhaust and see the housing for a cat, you pass. | ||
| Natexlh1000 |
I'll wager that you would not detect too much in the way of gains in that new vehicle. You would probably get the most bang/buck with a cat-back system. Ever hear of the vacuum cleaner trick? I read about it in a British vintage bike mag. The author said the best way to see how much restriction is there in a muffler (carbon, soot, 2-stroke slime) is to put the exhaust on the exhaust of a vacuum. If the vacuum's motor changes pitch, you have a restriction. Perhaps you could apply this trick to your truck's components with a shop-vac to see if there is any noticeable restriction. | ||
| Thumper74 |
I know someone who may, or may not have welded the cat heatshields onto a performance X-pipe for a 5.0 Mustang since the emission testing station uses a mirror on a stick to check for the cats in Ohio... There was a HUGE increase in performance, but the anemic 5.0 (1982 model made 150 horsies) was swapped for a built 347 stroker. So, between the exhaust and the engine, there was around a 250 horsepower increase. | ||
| Crackhead |
Assuming Stainless Works did their homework, there should be very little difference because they would size the cat to the rest of the exhaust system. You should also look at the whole system flow to see if it is correct for your intended usage. dual 3" pipes are pretty big. | ||
| Carbonbigfoot |
TT the guys at SW, said if you are not doing a package with forced induction, there is maybe a 1 or 2% difference cat to open. Not enough to mess with. Thanks for the thoughts, guys. BTW, these guys really have their $hit together. Seems like a great operation. R | ||
| Azxb9r |
Too many variables to really say. That pretty much sums it up. Gains are all over the board, some vehicles will actually lose power with a "free flow" exhaust. | ||
| Midknyte |
I say skip it. Those things just cause accidents. Last time I replaced a catalytic converter, some damn kid hit my car and totaled it... |
