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Ford Explorer Community - Maintenance - Modifications - Performance Upgrades - Problem Solving - Off-Road - Street
Explorer Forum Covers the Explorer ST, Explorer Sport, Explorer Sport Trac, Lincoln Aviator, Mercury Mountaineer, Mazda Navajo, Ford Ranger, Mazda Pickups, and the Ford Aerostar
If the 2.5" step gains you say 35hp, and the 3" step only gains 10hp more, is the extra 10hp worth the extra work and cost? I don't know what the actual hp figures would be, I just picked a couple of random possibilities. It might be 50hp versus 10, or 25 versus 5 etc.
In my opinion exhaust sizing involves compromises:
1. street, strip or street/strip
2. Cost vs performance
3. Space & weight limitations
4. Sound
5. Function: primaries, collectors, downpipes, post cat, post muffler
According to what you've posted:
1. You're willing to sacrifice street performance to reduce your track times
2. You'll pay more for better performance within reason
3. You've found room for a single 3 inch tailpipe and potential room for dual 2.5 inch tailpipes but it will be difficult to fit dual 3 inch tailpipes. You hope the extra weight of a dual muffler and tailpipe system will be offset by additional performance.
4. Loud is OK as long as it's not obnoxious and provides increased performance.
5. You've already sized the pre and post-cat system but the high back pressure indicates you're not getting full advantage of that.
Discussion:
The 75 horsepower (and associated airflow) to drive the supercharger must be included in the exhaust size computation. I think the law of thermodynamics requires the mass (air & fuel) going into the engine to equal the mass leaving the engine. Using your estimating rule your dual 2.5 inch dia. downpipes are good to 460 bhp and you estimate your engine is producing a 455 bhp. max. The exhaust temperature in the downpipes is hotter than in the tailpipe. If your downpipes are good for 460 bhp, dual 2.5 inch tailpipes should be good for more. Another consideration is that the larger the diameter of an exhaust pipe the slower the velocity of the gases. This matters near the exhaust valves but probably not after the cats. The honeycombs in the cats probably greatly decrease any tuning capability.
I suspect that the general pipe sizing rule is based on NA engines and not FI engines. I also suspect that the Ford rule of less than 8 psi at WOT under load is based on NA instead of FI engines. During boost, air is being pushed (instead of sucked) into the combustion chamber when the intake valve is open. Chamber pressures are higher under boost and it seems reasonable that exhaust downpipe pressures will be higher. With boost I wonder if lowering exhaust pressure significantly increases the airmass forced into the chamber. Even with an exhaust pressure of 10 psi, with a boost max of 15 psi there's still a differential pressure of 5 psi for chamber charge during open intake/exhaust valve overlap. You may be chasing a minimal performance increase in attempting to reduce your exhaust pressure. It would be helpful to find any testing of boosted engine's performance increase due to exhaust changes.
So you agree that the 75 HP should be added when computing for exhaust size. I keep second guessing myself because I don't see much discussion on the topic on internet forums. lol. Maybe I am just not reading the right threads.
I need to find more boosted threads.
You brought up weight, and hit the nail on the head with that one. I am thinking at the back of my mind that there will be a weight increase. Hopefully not too much of one. I plan to ask for the old muffler back so I can dissect it, and weigh it. Shifting the weight from middle to rear may be a benefit.
Most of the exhaust sizing info seems to state the 2.2 cfm per horsepower and 115 cfm per square inch straight pipe. So at least those may be common practice. The 8 lbs max back pressure is a general specification, boosted vehicles may differ. You are right in saying It would be helpful to find results of a boosted engine's performance increase due to exhaust changes.
I was looking for some back pressure testing results for supercharged vehicles but didn't find anything quickly and got side tracked. I will have to search again on that. I also plan to test my 4 door explorer, to see what stock back pressure with 250,000 miles will be. Maybe I can test before and after the cats too.
With a 5 or 6 psi differential from intake to exhaust I would agree that there may be no real problem on getting the clean A/F in. My other thought is that the restriction is making it harder to get the exhaust out and that may somehow rob power.
I will be on my way to the exhaust shop again, got to be there by 8.
Thanks for the input!
I have to post here. The link above is from where I live, and Tim is about 15 minutes away.
I guess we have more of a street scene here than I thought.
HA! that was done at motions old building on route 90 don. i also SWEAR one of my buddy's owned that car at one time, and took me for a ride in it. that was around 5 years ago
I went with 2.5 inch pipes and mufflers.
The mufflers are Flowmaster Super 44 Part # 942546
They are loud when I get on it, but they fit well! lol. They should flow well too.
It took 4 hours but I had a blast talking to everyone in the waiting room. There were some real characters in there today, made time fly by. Lots of laughs.
I can not wait to test back pressure! The stinking rain is preventing me from doing it now. I can not WOT this thing when it is wet out, all 4 spin and the truck goes sideways. Hoping for a dry spell tomorrow but the forecast calls for rain for the next 7 days. I hope they are wrong, I want to get to the track too.
Here are the pictures.
I have the back pressure tester installed behind the DS cat in the second O2 sensor hole, just waiting on dry weather to test it. I installed a tune to shut off the rear O2's just like before. Cant wait to see the results!
I will probably have some tips added in the future, I like the beveled look I have seen on the Saleen S7.
thats badass. i wish they would have thought of that for the mufflers when i got my exhaust done last year. then again, your pipes are out the back vs mine are off to the sides.
All I know is that it is a little louder, I only drove it home from the exhaust shop and then it started to rain so I put it in the garage and hooked up the back pressure tester.
If the roads stay dry enough while I am getting functional, I will go beat on it and test that back pressure at WOT.
The roads finally dried, got to test the back pressure on the new exhaust.
This is behind the cat, if I have time I will check before the cat to see total backpressure (I would assume 5-6).
The results show 3-4 lbs of back pressure at WOT. Huge drop from the 8-9 I had before with the single muffler.
Thanks again for everyones input!
