3" vs 2.5" Exhaust?

[critter]

Sorry. I lost this thread for a while. My numbers were at the flywheel. That's what Jim was saying as well. Yes, his stuff is dated but the numbers still appear to hold up pretty well from what I've seen. The oval stuff makes a world of sense. Had it been out there when I did the exhaust the last time I seriously would have used it. I do have a friend who installed it with his car that makes 575hp at the flywheel and it's not acting the least bit restricted. His 3 inch oval has just as much clearance as my 2.5 inch round.

[wmhjr]

Wow! Thats a beautifull setup! It's so damn clean under there. My brother used to hav a 67 GTO. His wife totaled it though. Wasn't her fault she was rearended. I loved that car too. Where do you get the ceramic coating supplies? I just got a price for oval x-pipe and mufflers not including tail pipes. It's alot of money but im sure its worth it.

Thanks. The headers I bought from Thermal Tech in VA, who coated them. They buy raw headers and coat them as part of the sale. The rest of the exhaust I took to a local business who has done powder coating for me in the past. They did a pretty nice job, and they were very reasonable. The finish isn't quite as "polished" "chrome like" as the headers, but it turned out very nice.

[wmhjr]

Sorry. I lost this thread for a while. My numbers were at the flywheel. That's what Jim was saying as well. Yes, his stuff is dated but the numbers still appear to hold up pretty well from what I've seen. The oval stuff makes a world of sense. Had it been out there when I did the exhaust the last time I seriously would have used it. I do have a friend who installed it with his car that makes 575hp at the flywheel and it's not acting the least bit restricted. His 3 inch oval has just as much clearance as my 2.5 inch round.

Yep. I was going to do something else, but after researching it and seeing how much oval is used in NASCAR and other high end competition, I was sold. It's funny how when you're building a car, no matter what there will be stuff that comes out right AFTER you're about done with the build that you wish you could have gotten. In my case, it's the T56 Magnum.

[mike w.]

another thing to remember when talking horsepower is that a 500hp motor will only be making 500 at about 5-6500rpm! and , unless you are spending most time at the strip will you really notice any differences when running on the street as mostly the rpm's dont get much north of 3500 for any length of time as the speeds will be in the three digit range! a lot of "high powered" motors will only probably be kicking out 350 -400 hp at these rpm's so a 2.5" system will be just fine and easier to package than a 3 ", also, has anybody tried the spiral turbo baffles instead of mufflers? i have fitted several sets now and they sound awesome! no cackle or bark and no resonance either, the system looks good and clean without mufflers and helps out space and ground clearance wise too.

[wmhjr]

another thing to remember when talking horsepower is that a 500hp motor will only be making 500 at about 5-6500rpm! and , unless you are spending most time at the strip will you really notice any differences when running on the street as mostly the rpm's dont get much north of 3500 for any length of time as the speeds will be in the three digit range! a lot of "high powered" motors will only probably be kicking out 350 -400 hp at these rpm's so a 2.5" system will be just fine and easier to package than a 3 ", also, has anybody tried the spiral turbo baffles instead of mufflers? i have fitted several sets now and they sound awesome! no cackle or bark and no resonance either, the system looks good and clean without mufflers and helps out space and ground clearance wise too.

I think the point of Jim Hand, etc is not that you might be better served with 3" with a 500hp motor when it's actually making 500hp - but more that over the torque curve even from 3500 and up, a motor capable of making or exceeding 500hp may benefit (a little) from 3". In other words, he's not saying that the advantage STARTS when the power peak approaches 500hp. Otherwise, you could make the same argument about a 400hp motor only needing 2" exhaust. Or smaller. The motor is just a big air pump. Think about this. If you have 1000cfm going into the pump, then you heat that 1000cfm significantly, you can seee where both velocity and volume could be impacted at lower than 500hp.

This is a debate that will never go away, however. The differences are pretty small - just like some of the measurements that are out there comparing different X-Pipe designs between themselves, to H-pipes, and to non-crossoever systems. So many variables. If space and packing were a real concern to me, in a 500hp motor I'd go with oval 3" to round 2 1/2" at the tail pipes. There is really no reason not to go that route. That being said, on the street you'll likely see very little difference at all. Of course, on the street you also won't see many of the differences in suspension, etc that we also build into our cars.

[mike w.]

i see what you are saying, i was just trying to give a little info to some that might think it has to be a certain way to work, and in tight spaces a 2.5" will work fine, but a 3" is better if the space is there, it will also be a little noisier usually too, and as you also state, on the street in normal conditions/use most of what we build is not being used to its potential anyway,

[Skip Fix]

With a 500 HP motor a 3" H system with 2.5 tails was no where near the performance of a 3" X and 3" tails. Ultraflows are pretty quiet. Torque Tech's 3" tailpipes fit fine on my car, a little close to the 29.5 slicks.

[The WidowMaker]

a little off topic now, but someone mentioned the pypes stuff earlier. i personally would stay away from their tubing if doing a custom exhaust. the price is right and its already polished, but the tubing was very inconsistent in dia throughout the bend. if you took a 90 and made two 45's, then tried to weld the recently cut end of the 45 to a straight piece of tubing you were screwed. columbia river was much better.

just a heads up for those looking for materials.

[wmhjr]

On the same topic, I will say that the Ram Air stuff was extremely consistent and of excellent quality. In fact, we did modify the tail pipes by cutting, rotating and TIG welding. It was very consistent and resulted in an extremely smooth joint.

[wmhjr]

With a 500 HP motor a 3" H system with 2.5 tails was no where near the performance of a 3" X and 3" tails. Ultraflows are pretty quiet. Torque Tech's 3" tailpipes fit fine on my car, a little close to the 29.5 slicks.

How did you measure this? Everything I've seen in terms of numbers showed a very minor difference between 2.5 and 3" tailpipes. I'm honestly curious. I actually had planned to get one guy to help me with a completely custom exhaust, but he kept telling me 3" was stupid and I got tired of listening to it. I'm glad I went 3" but I personally don't think I'd even notice the difference if I swapped for 2.5" tailpipes. The exhaust gasses have cooled dramatically by then, significantly reducing volume. Stick your hand behind open headers on a warmed up motor, and then stick it behind a tail pipe. It's night and day.

[Lex]

a little off topic now, but someone mentioned the pypes stuff earlier. i personally would stay away from their tubing if doing a custom exhaust. the price is right and its already polished, but the tubing was very inconsistent in dia throughout the bend. if you took a 90 and made two 45's, then tried to weld the recently cut end of the 45 to a straight piece of tubing you were screwed. columbia river was much better.

just a heads up for those looking for materials.

Yes. I'm taking tha advice from WMHJR about using the exhaust kit from Ram Air Restorations. They do a 3" oval setup to the mufflers and 3" round tailpipes that exit behind the rear wheels.
At first I wanted to use the classic TA split tips and exit the back corners but RAR doesn't offer that. Torque Tech does have a kit to exit the back corners but I would have to modify the pipe to accomodate the tips. I'll just use different style tips for the RAR kit.

[Mbwicz]

Stainless Works does beautiful work, they do the t/a tips. I remember talking to them, and they would do the t/a tips in 3" on a custom basis. They are not cheap (street rod type stuff), but the parts are great.

http://www.stainlessworks.net/cart/p...oducts_id=1195

Mike

[Lex]

Thanks Mike. I will check this out.

Stainless Works does beautiful work, they do the t/a tips. I remember talking to them, and they would do the t/a tips in 3" on a custom basis. They are not cheap (street rod type stuff), but the parts are great.

http://www.stainlessworks.net/cart/p...oducts_id=1195

Mike

[parsonsj]

500 hp is the demarcation point for the 2.5 vs 3 exhaust. I'll do the calculations again:

Facts:
1. For each horsepower, you need 2.2 CFM from your exhaust setup.
2. Exhaust gas flows at about 115 CFM per square inch.

Calculation:
500 hp * 2.2 CFM / hp = flow requirement of 1100 CFM.
Since you are likely using dual exhaust, divide by 2: 1100 CFM / 2 = 550 CFM per exhaust pipe.

We need 550 CFM / 115 CFM / psi square inches of tube area = 4.8 square inches.

The area of a pipe is pi * r squared. We have 3.1 * 1.25 squared = 4.9 square inches.

So 500 hp uses all the available area of a 2.5" dual exhaust, with a big assumption: that the mufflers can flow 550 CFM.

For completeness, a 3" exhaust is good to 800 hp.



jp