3 inch exhaust

[cah41]

So I have an 03 cobra with nothing on it except for a JLT cai. I was thinking about spending my next 700 bucks or so on a 2.80 pulley + sct dyno tune, but now I am thinking exhaust might be better to start.

1) is it definitely better to go with the exhaust before the 2.8 + tune and why

2) I was thinking about going for a 3 inch MAC catback. I have heard that this may not be advisable for my power level--if so, why, and given that my goal power level is eventually a stage 5+ port (so low, mid 500's I guess) sometime the next year and half, why would it still make sense to go with a 2.5" first?

Thoughts on anything?

 

[cah41]

anyone?

 

[rtusnake]

If your going to get a catback might as well get a 3" so you don't have to buy it twice.

 

[SlowSVT]

A 3" exhaust will lower the gas velocity reducing the scavenging effect and lower the resonance frequency of the exhaust also known as "drone". I would not consider a 3" system unless I'm making 700 hp and plan on keeping the tach north of 6000 rpm all day. I would stick with a 2 1/2" system. A 3" system is certainly overkill for a small 4.6 with a ported Eaton and will probably hinder your performance somewhat.

 

[gt347mustang]

A pulley and custom dyno tune will be 100x's better than a 3" exhaust!

 

[98 Saleen Cobra]

According to Bassani if your making over i think it was 450hp you should upgrade to a 3''. If I were you I would do a 3''.

 

[SlowSVT]

According to Bassani if your making over i think it was 450hp you should upgrade to a 3''. If I were you I would do a 3''.

And if your making 600 hp should you step up to a 3 1/2" system? I don't think the GT500 guys are going any bigger the 3" and they got 50 cubes on us.

 

[cah41]

A 3" exhaust will lower the gas velocity reducing the scavenging effect and lower the resonance frequency of the exhaust also known as "drone". I would not consider a 3" system unless I'm making 700 hp and plan on keeping the tach north of 6000 rpm all day. I would stick with a 2 1/2" system. A 3" system is certainly overkill for a small 4.6 with a ported Eaton and will probably hinder your performance somewhat.

Cool thanks for the info man. I had a feeling this might be the case.

So does it matter if I do the exhaust first or the pulley + dyno tune first? Will the stock exhaust really hinder the rest of the setup that much if I don't do it first? Because at this point I get the feeling that it's just preference.

Thanks in advance

 

[cah41]

According to Bassani if your making over i think it was 450hp you should upgrade to a 3''. If I were you I would do a 3''.

I think that quote you're referring to was in reference to big block chevy's, not a 4.6 modular motor. Thanks anyway

 

[SVTurbo 98]

A 3" exhaust will lower the gas velocity reducing the scavenging effect and lower the resonance frequency of the exhaust also known as "drone". I would not consider a 3" system unless I'm making 700 hp and plan on keeping the tach north of 6000 rpm all day. I would stick with a 2 1/2" system. A 3" system is certainly overkill for a small 4.6 with a ported Eaton and will probably hinder your performance somewhat.

Do you have any proof of this? Or are you just going by what someone told you?

 

[Stalker27]

There are more than enough guys on this forum & ModFords that run a 3" system & all comment that they have not felt any drop in power.

OP: I'm about to go with a 3" Mac myself so it gives me room to grow & i'm basically stock.

All this about 3" being to big is overkill. We are supercharged & make plenty of TQ down low. So what it you lose a lil bit down low, you're just moving the curve higher in the rpm which is where i want it anyway.
Any power you "MIGHT" lose you'll gain right back by adding the 2.80 pulley

 

[SlowSVT]

There are more than enough guys on this forum & ModFords that run a 3" system & all comment that they have not felt any drop in power.

OP: I'm about to go with a 3" Mac myself so it gives me room to grow & i'm basically stock.

All this about 3" being to big is overkill. We are supercharged & make plenty of TQ down low. So what it you lose a lil bit down low, you're just moving the curve higher in the rpm which is where i want it anyway.
Any power you "MIGHT" lose you'll gain right back by adding the 2.80 pulley

It is pretty well understood that an exhaust needs to be sized to promote optimal velocity thru the system to get the best performance. Too small a pipe diameter will be too restrictive and too big a diameter will slow the speed of the spent gasses and not provide enough back pressure. If a 3" system is better then a 2 1/2 wouldn't a 3 1/2" system be even better? See where I am going with this. Imagine all this is going thru the stock cast iron maniford which has a way smaller cross section then even the 2 1/2" pipe. Adding a set of headers only buys you about 10 hp over the stock logs. That crappy non-mandral bent 2 1/4" factory cat back is the worst offender in the entier system.

Yes there are guys who do run 3" systems but some of them actually when back to the 2 1/2" and stated the engine responds better with the smaller pipe if you wanna call a 2 1/2 pipe "small". Like I stated, if I was building a little 4.6 that was going to run high boost and revved to the moon then I might consider a 3" systems. But for the street 2 1/2" is a better choice which is what the vast majority of us are running.

 

[SlowSVT]

Cool thanks for the info man. I had a feeling this might be the case.

So does it matter if I do the exhaust first or the pulley + dyno tune first? Will the stock exhaust really hinder the rest of the setup that much if I don't do it first? Because at this point I get the feeling that it's just preference.

Thanks in advance

The best bang for the buck is a new cat back and a JLT intake. I have a 2 1/2" SS Magnaflow system which is very high quality, does not overly announce itself but gives you that classic angry Mustang howl and will "roar" when you lean on the gas. No tune needed with those items and you just added 20-30 hp to the wheels. Tune and pulley after you upgrade the intake and exhaust that way you won't have to retune later.

Don't forget cooling mods.

 

[firemanmike]

Any power you "MIGHT" lose you'll gain right back by adding the 2.80 pulley

Yeah, but the pullet is there to add HP, not offset losses due to other reasons.

 

[SVTurbo 98]

It is pretty well understood that an exhaust needs to be sized to promote optimal velocity thru the system to get the best performance. Too small a pipe diameter will be too restrictive and too big a diameter will slow the speed of the spent gasses and not provide enough back pressure. If a 3" system is better then a 2 1/2 wouldn't a 3 1/2" system be even better? See where I am going with this. Imagine all this is going thru the stock cast iron maniford which has a way smaller cross section then even the 2 1/2" pipe. Adding a set of headers only buys you about 10 hp over the stock logs. That crappy non-mandral bent 2 1/4" factory cat back is the worst offender in the entier system.

Yes there are guys who do run 3" systems but some of them actually when back to the 2 1/2" and stated the engine responds better with the smaller pipe if you wanna call a 2 1/2 pipe "small". Like I stated, if I was building a little 4.6 that was going to run high boost and revved to the moon then I might consider a 3" systems. But for the street 2 1/2" is a better choice which is what the vast majority of us are running.

So heresay is proof??? :rollseyes Why would backpressure be a good thing in a performance application?

 

[cobracide]

So heresay is proof??? :rollseyes Why would backpressure be a good thing in a performance application?

There are a couple of things that will slow exhaust gas down:

1) A sudden decrease in temperature. This is why auto makers could care less about cooling the exhaust.

2) Going from a small diameter to much larger diameter pipe. A larger pipe will slow the velocity of the exhaust down due to expansion.

Larger is not always better, the larger the pipe - the SLOWER the velocity. It's a balancing act and trade-off.

 

[Stalker27]

Larger is not always better, the larger the pipe - the SLOWER the velocity. It's a balancing act and trade-off.

Yeah ok, Then please explain why turbo's love a big exhaust like 4". Actually the bigger the better with them

Backpressure is only good off the line & will absolutly kill your top end cause i can't get rid of it fast enogh

 

[Stalker27]

Adding a set of headers only buys you about 10 hp over the stock logs.

You must be talking about peak power cause you gain more than that under the curve which is why everyone says there car runs like a raped ape & pulls harder to redline.

 

[SlowSVT]

Yeah ok, Then please explain why turbo's love a big exhaust like 4". Actually the bigger the better with them

Backpressure is only good off the line & will absolutly kill your top end cause i can't get rid of it fast enogh

Now I see where your philosophy is coming from.

2 completely different exhaust configurations. A Turbo will spool most efficiently when there is maximum pressure differential between the turbo inlet and the outlet. The ideal exhaust on a turbo is not having piping or any kind on the outlet of the exhaust scroll. That’s why you see 4” pipes after the exhaust scroll. A turbocharged engine has a substantial amount of backpressure from having multiple cylinders dumping into one or two turbos. A supercharged or NA engine will have more efficient exhaust because they can take advantages of the pressure waves created by each exhaust pulse (when you have a high pressure wave there is always a low pressure wave right behind it and you want to take advantage of that at the next exhaust cycle to help scavenge the cylinder). Turbo’s just pressurize the exhaust manifold to drive the impeller and diffuse the pulse pressure. While I am certainly no expert on exhaust system design I do have first and experience what the effect are if you install to large a system or minimize backpressure. THEY RUN LIKE CRAP! The engine stumbles at low speed and you can never seem to get the engine to run right. Backpressure and optimal exhaust velocity is what you are after something an overly large pipe won’t do.

 

[SlowSVT]

You must be talking about peak power cause you gain more than that under the curve which is why everyone says there car runs like a raped ape & pulls harder to redline.

These cars run like raped apes even without headers. Adding headers won't make a dramatic difference in these cars (the blower overshadows everything!). A high boost big hp benefit from headers way more then a mild engine will. On an NA engine they seem to make more of a difference. I have an issue of MM&FF with the Mod-for-Mods article showing dyno'd runs with both LT's and the stock logs and the lines in the graph pretty much overlapped one another all the way to redline and that was done with a straight pipe and no mufflers or cats. There have been quite a few people who were pissed as hell doing back-to-back dyno runs after installing headers. One guy recorded a 5 hp increase after spending quite a bit of dough installing headers. Its surprising how few people are running headers on their Terminators where on other cars it's one of the first mods you make. Go figure :shrug: