And remember the win-win part of a free flowing exhaust is not only horsepower gain but usually fuel economy improves as well.

 

Not exact no. Also, 2.5^2 + 2.5^2 = 3.5^2

The exhaust pressure doesn't increase as you go back. The pressure always goes down, otherwise it would flow backwards.

The velocity also goes down, because the exhaust gas shrinks more the further back it goes. This is why mufflers don't really get you much power on a vette... because not as much air is moving through em. More CFMs go through the headers than through the mufflers.

 

I said 3" was too small as a collector for two 2.5" pipes (and you show it would need 3.5"). I agree and that was MY point. Even two 2.25" is slightly forced thru 3"! What I have learned (from Vader) is that the exhaust is 2.25" vs. 2.5". (My muffler guy said he thought the pipes were bigger (2.5") in front of the main cat. I guess he was wrong). Anyway, if you're trying to make a point with the math (to me), I'm not getting it.

I assume you're pointing out that pressure goes down as the temp goes down (and temp goes down as exhaust travels thru the exhaust system). Vader stated that smaller pipes toward the rear helped keep velocity up.

Vader's point and your point (above) elude me. They aren't explained thoroughly enough to make sense.

It sounds like Vader recommends reducing pipe size toward the rear of exhaust (to keep velocity -- or pressure -- up). I did not see an explanation for this in here or on his website.

I'm way lost when you talk about getting power from mufflers???? I've heard some discussion about back pressure providing torque -- but even that escapes me!!! I'm guessing you're both trying to say something about backpressure, I just don't know what it is. More importantly how to consider it in a design.

 

It will be custom fab'd.

I'm sure there are generic "H" pipes in 2.25" but I don't think there's one designed for the L98. I would think it would need to be "pieced" in.

Does an "H" pipe (or "X" pipe) help with noise, smoothness, or performance?

Gregg

 

I agree with Jake, exhaust can be expensive and gains are limmited. that said I spent the $750 on TPIS headers and $200 on Flowmasters and gained maybe a .10 to .15 at the strip. I picked up .10 just gutting the rear cat (it was broke and rattling) and that was free. The headers were worthwhile to me because my heads have the wrong spark plug angle and they give great clearance. I havent burned any plug wires since I got them.
The Insider hints book is good, but take what you read with a grain of salt. Remember TPIS uses it as a selling tool for products they sell so its very biased.

 

Back pressure in the exhaust, from restrictions in airflow, be they from reducing pipe diameter or converters, reduce the velocity of the gases as they exit the engine. Increasing back pressure will inhibit the top-end ability to evacuate the gases and will move the peak gas velocity point lower in the rpm band, thus improving your low-end feel (torque basically). The gist is high gas velocity=torque, and that applies anywhere in the rpm range, but at high rpm it shows up in your HP number because your high rpm torque is not dropping.

The gas cools and slows as it travels and a larger pipe diameter will let it spread out into the pipe and slow down. You 'ideally' want the diameter to taper down slightly along the exhaust to counteract the effect, mainly so that you dont feel a loss of your low-end.

So, a big pipe will improve top-end slightly by moving that peak point higher, and a small pipe will limit the top-end because the gases cant get out fast enough and will effectively increase the back pressure in the system.



Youre not missing the point, I just dont think you are quite reading each other's posts right.

 

something you should keep in mind about long tube headers is, that you will have to adjust your fuel pressure up a couple notches, and for that you have to have the adjustable fuel pressure regulator, i do think running true duals is a good idea, no cats!

 

One correction to that...

Smaller pipes increase velocity. Increased velocity increases backpressure. Restrictions also increase backpressure. So... there's good backpressure and bad backpressure, just like cholesterol. The key really is finding the size that gives you a compromise between enough velocity to help with scavenging at lower rpms (increasing VE), but not too much velocity so that you have a ton of backpressure with the engine wound up to 5500rpm, so much that it will start reducing VE.


VE = volumetric efficiency and is a close indicator of horsepower.

 

Let me see if I can repeat this in my terms in order to confirm a layman's understanding....

Some backpressure is necessary. The reason I'm guessing it's necessary is that is gives the engine something to push against. Since piston engines are similar to giant pumps, fans, or blowers, they can create more power with more through-put and/or rpms. However, every action has an opposite reaction where some backward force is going to be applied. Since torque is a measurement of force vs. speed (HP?) pressure is being applied and that pressure needs something to press against. (Consider how much easier it is to push furniture across the room if your back is against a wall).

Practically applied to L98's, I'm guessing that some pretty big extremes would need to be introduced before a significant negative effect would be created. For example really short pipes larger than 4"-5"?

I my case, I own a 6-spd. I don't race, but I drive fairly vigorously. Specifically, I enjoy 1/2-3/4 throttle accelerations shifting before 4,500 rpm. With so many gears, wrapping the motor up higher seems so unnecessary. (Again, I'm not racing -- though I can't resist the occasional WOT!) This probably means I'm more of a torque lover than raw HP. That's also why I'm not sure that headers/intake work would benefit my "goals".

Though this thread contains a lot of (great) theory, I have stated my primary goal -- to update my exhaust and add side exhaust. If you have never considered this option, some thought will help you realize that our C4's represent HUGE challenges to this option. Since I'd like to keep sound levels fairly low, minimize loss of ground clearance, complete parts installation before the rear wheels, improve power (if possible), and keep or improve the emissions quality at or above where it is, I have a difficult task. Think about it!

I will also be installing a high-rise hood (current one is damaged). So, in the future, I could go crazy -- if I wanted to. But gas ain't cheap, parts aren't cheap, rebuilding motors ain't cheap, and I enjoy the car at it's current power levels. While improvements in power wouldn't go unused, I'm more focused on gaining the unique (yet utterly "Corvette") character of side exhaust.

I'm glad I'm learning about general theory while I'm also looking to apply that knowledge to immediate plans. (Keep in mind that the right comments COULD gear my goals more toward power vs. style if I thought it were reasonable to attain. So I'm also being open-minded!)



Gregg

 

After sleeping on this topic, I am warming up to the idea of putting long-tube headers on my car. (I note there is a special from GP stainless going until the end of January -- and I'm considering it).

IF I put headers on my vette, I'd be interested in keeping the sound levels down. I'm already putting a heftier sound system (than the Bose) in my car because the Bose didn't cut it. So, I wouldn't want exhuast that would overshadow the tunes.

It would be O.K. it "spirited" or WOT driving created sound levels above the music but I wouldn't want SIGNIFICANT resonance present all the time.

In my quest to install side pipes, cats and mufflers would have to be installed in front of the rear wheels. Can this be done while meeting the sound level requirements I've outlined? EVERY car I've heard with non-factory headers has been pretty loud -- louder than I'd want. However, since the goal of headers is to gain HP, I can understand that.

More importantly, this raises the issue of reducing backpressure (to get more HP) vs raising sound levels. My instinct is that there must be a direct trade-off between "opening" up exhaust and increasing sound levels. It would seem intuitive that less back pressure would mean less sound muffling. My conclusion would be that headers and dual exhaust with small high-flow cats and high-flow mufflers would be louder than I'd want.

What do you think?

 

I took out my pre-cats and the cat when they went bad. Replaced with a high flow cat and I still passed the sniffer in Ohio without pre-cats.

We just got good news. As of Jan 1 2006 no more e-check in Ohio. WOOHOO!!

 

Cars with side pipes are in general not as loud and dont have quite the bass of a rear exiting exhaust. So, it may not resonate badly at all. In particular, the Dodge Viper is this way. (no its not just because the V10 doesnt make as much bass as the V8) You may have a trade off to pay with extreme cabin heat around the doorsill (also like the Viper) depending on exactly how you do yours.

My car with Power Effects in the closed position will resonate at 2000rpm, with them open its at a different pitch but still there. I can hear the Bose just fine over the exhaust at other rpms, but on the hwy its bothersome.

A reason why coupes resonate is because of the big sound chamber you have behind the driver, and the exhaust underneath exiting directly below it. Thats why you can put extensions on the pipes, and add crossovers to reduce resonance on coupes. I have no idea what will work for a C4 with side pipes though. I have only heard one once before.

 

hi im new need help on horsepower 86 vette L-T1 engine flowmaster exaust K&N filter tornado in intake a air foil and smooth intake hose........need help on more mods got a race oagainst a 03 cobra mustang in one week all pointers much appreaciated

 

No. Backpressure isn't necessary. Properly sized pipes benefit you with scavenging and increased torque without hurting top end too much. Backpressure is just a byproduct of the pipes not being huge. Cats and mufflers also make backpressure, but do nothing to help with scavenging.

Exhaust is one case where bigger isn't always better. I could show you some fittings that flow better when reducing the pipe size and worse when increasing it. The best lesson in air flow, is that it can be very counterintuitive. Lots of self-proclaimed air flow experts in here, including me, so be careful with what advice you take to the bank.

 

Sell the tornado and save money for nos or drag radials. The tornado doesn't have any effect on the TPI. The 03 cobra can beat you by a full second unless he doesn't know how to drive a stick. I treed an 04 cobra in my 85, but he couldn't drive at all.

 

THATS NOT HELPFUL AT ALL who the hell would buy that tornado were street racing buddy.....lol ima seveteen yr hehe second vette in a year totaled the first one racing lol but anywayz i need somethinig fast i cant use nos im on my second engine and it might blow it i got 6 days now till the racce adn I CANT LOOSE drag readials rnt good either lol my cars got new tires kumos and it slides on dry asphalt and i cant even take it in the rain its so bad well more help needed aprreacate the thoughts

 

lol coaster what yr ur vette it looks almost identical to mine sweet ride

 

You're on the wrong forum then, and Mojo will lock down your threads on stuff like that. Go here: Corvettekillstories.com

Try and stay alive.

btw, Mine's an 85 with 6 speed swap, stock motor with small stuff done to it, I figure it's got about 250hp, best ET is 8.91@82 in the 1/8 mile. Have you been too the local dragstrip yet? You should definitely try it out, you might enjoy it more than street racing. And crashes are rare. Tickets and impounding are nonexistent.

 

whats a mojo locking thread if u dont mind me asking
and whtats the link for ???

 

that wuz some of the best *** rapings a mustangs ever seen puts a big smile on my face but thething u said abouta stick being faster my friends got a 98 stang with flows on it...its a 5 speed but i whoop him really bad......but he can drive a stick really well been doing it since he wuz 12(4 yrs) but its a six banger so idk if thats y or not