bajajoaquin
Senior Member
Probably had more to do with scavenging than velocity or back pressure.
I'm always ready to learn, if 2.5 isn't as good as 2.25 for the purpose, I want to hear why, so I can make an informed decision.
I'm a truck driver I see vehicles with oversize exhausts every day, a lot of it is about style rather than efficiency.
I haven't been able to on as much lately due to several cars needing general maintenance. I first questioned the 2.5" because for many people it is the default size picked most of the time. I knew that when I started taking in all of my cars to get new dual exhausts and one a single exhaust. The engines varied from a stock 360, to a stock 410, a stock 390 and a modified 302. My research lead me to calculating air flow through different sizes as I already had experience with the bigger is better when it comes to camshafts, valve sizing and intake runner size. Consequently my calculations showed that 2.25" was more than enough for my duals for all those engines. Had my 302 been stock, like my 289, then it would have gotten a 2" exhaust but it was no longer near stock. When I took my cars in and the shop guy asked what size, and I told him 2.25", he went smart choice as everyone else would have said 2.5" These guys have done my cars since 1990.
I actually did it the long way using math some years ago to make a quick table for myself at the 2.25 and 2.5 sizes.
Today there are similar tables now on the web that have done all the math for you without going into volume, mass, moles, % of O2 and so forth. All that stuff is for physic majors.
How To Calculate Muffler Size and Exhaust Pipe Diameter | Exhaust Videos
One for diesel trucks
http://www.asia.donaldson.com/en/exhaust/support/datalibrary/1053747.pdf
The smaller exhaust will have more back pressure which will help the engine, too little pressure works against the engine. Example, I had a 360 in a van went like hell with a Y pipe. Put same engine in my Dart with only headers and had valves floating over 3,000 rpm. It wanted the back pressure. Sold it and have to talk to the new owner and see how it is back in it's stock exhaust configuration.
I was just speaking from experience with the 360 I mentioned, and it didn't like the open exhaust, maybe the difference in load on the engine made a difference? Myth legend or whatever, it ran and made more power with a full length exhaust system. I'm not arguing anything, I don't have any other data than what I previously posted. Yep, it's an air pump. It's back in a van, and doing what it did before, running very well.Was looking for source for new exhaust system when I saw this thread. Thanks for the mention of the few aftermarket sources. So this is a reply to old these old posts as I am new on this forum but not new to performance cars, trucks, boats.
This is a pure urban street nonsense myths/opinion. No engine "likes" back pressure and all engines even my old AMC straight 6 benefits from as big as exhaust system as the engine bay layout and car will allow. You are speaking of a large air pump. The most air in and most air out makes most torque and power. If you do not believe my novice opinion check out all the you-tube videos on the subject by the guys at Hot Rod Mag, Road Kill or Engine Masters videos by past "Editor In Chief" David Freiburger and his team members. They have shown on engines and dynometer bigger is better.
put duals on my built gmc….lost low end torque...replaced duals with single 3 in..low ends torques back...duals aren't always the answer..real world ****....
never said you had to...like l said..my real world ****,,not yoursNot buying it. Nor are all the motor guys at Hot Rod. I go to race tracks NHRA, F1, NASCAR, Moto GP, never heard anyone in the Pits saying put back pressue into my exhaust system I want to go faster...
I'm not going to compare an LA 360 with an F1 or NASCAR engine, but even the NASCAR guy has more exhaust than I had.Not buying it. Nor are all the motor guys at Hot Rod. I go to race tracks NHRA, F1, NASCAR, Moto GP, never heard anyone in the Pits saying put back pressue into my exhaust system I want to go faster...
bigger is better.
My take away from listening to the engineer in the posted video is there is a science to this and no general rule of thumb applies to all. The objective as I understand it is to maintain exhaust speed which can be hindered by both too large and too small exhaust components. The key is to find the "Goldilocks" size for your particular motor.made more power with a full length exhaust system.