exhaust diameter in question?

[stockbee]

Hi all, I'm trading my Sr straight up next month for a RB25 swap and spending about 1,200 on mckinney mounts, driveshaft, and greddy intake manifold from a friend.

As far as the exhaust goes, I would love to have an apex'i gt spec with 3.75 inch taper diameter, but I can get a stainless exhaust from a guy here in town, custom made, for cheaper. Only issue is that its 4-inch.

Is .25 of an inch much of a difference to consider at all, considering that its after the turbo and will not need much backpressure in the first place. Im trying to save money if at all possible, but will spend it if its necessary.

It will most likely have the stock turbo on it initially, but Ill be trying out a potentially good 3076R the same week I get it, if that makes much of a difference. Thanks for the help in advance, Look forward to getting this in and running smooth.

[spolitte]

First, Back pressure is bad. Don't believe anyone who tells you otherwise.

Second, depending on how good of an exhaust and if it has very little restriction or not, both 3.75 and 4" exhaust seem overkill on an otherwise stock rb25 other than the intake and maybe new turbo??? With that said, you shouldn't loose any top end HP with the 4" over 3.75 if it as well engineered as the apex'i is; however, you'd probably loose a tad bit of low end power.

Now, if it truly as well engineered (has as little restriction as) the apex'i, is cheaper, and you plan to eventually make 400+ hp then bigger is better, right? Plus, the rb25's are pretty torquey anyway from what I understand?

Also, have you checked into BRM exhaust's? I know they only go up to 3" on the website, but if you want bigger the guys there might be able to make something for you and they are comparitively cheap, and I hear nothing but good things.

[Joe]

First, Back pressure is bad. Don't believe anyone who tells you otherwise.

Second, depending on how good of an exhaust and if it has very little restriction or not, both 3.75 and 4" exhaust seem overkill on an otherwise stock rb25 other than the intake and maybe new turbo??? With that said, you shouldn't loose any top end HP with the 4" over 3.75 if it as well engineered as the apex'i is; however, you'd probably loose a tad bit of low end power.

Now, if it truly as well engineered (has as little restriction as) the apex'i, is cheaper, and you plan to eventually make 400+ hp then bigger is better, right? Plus, the rb25's are pretty torquey anyway from what I understand?

Also, have you checked into BRM exhaust's? I know they only go up to 3" on the website, but if you want bigger the guys there might be able to make something for you and they are comparitively cheap, and I hear nothing but good things.

lol backpressure it NOT bad. backpressure allows for proper cylinder scavenging. (go google it, ill wait). with that being said exhaust size is FAR more important on a NA car.

on a turbo car all the backpressure the engine needs is created by the turbo so it really makes no difference what you do AFTER the turbo provided you arent choking it. a 4" is pointless and overkill but whatever you want man. a 3" will support you to 500whp if you really want it to. arguing the difference between .25 is like arguing who is hotter, kate beckinsale or bar rafeli. both are hot. both will get the job done.

[Cjmartz2k]

OP- I don't think you have a good grasp on exhaust diameter vs. exhaust gas velocity vs. back pressure. Bottom line, back pressure is always, 100% of the time, with out exception a hindrance to performance. Exhaust gas velocity is good. On n/a cars, if your pipe is to big, you loose exhaust gas velocity and it reduces scavenging which hurts your low end. You don't loose low end because you don't have enough back pressure. You want the straightest, smoothest exhaust, that's as large as you can get with out killing your exhaust gas velocity, or conversly, you could say you want the smallest exhaust you can get that doesn't create a lot of back pressure.

All that being said, you have a turbo car which negates any of that. The bigger the better. For a stock turbo or anything under 400rwhp, 3" pipe is just fine. If you run 4", it's not needed, but it'll support more later on.

*Joe--I typed this when I only saw the original post. It wasn't directed at you, but I think you are also confusing back pressure vs. exhaust gas velocity. Think about it. How would a restriction in the exhaust help pull more gasses out of the cylinder? You are right about the turbo negating things though. It's not that it helps by creating back pressure, it's just that it stops the added effect of better exhaust gas scavenging from the rest of the gasses on there way out of the tail pipe helping pull what's behind it out. Yes, the turbine wheel hurts performance a bit, but the compressor side of things more than makes up for it
Modified by Cjmartz2k at 1:24 PM 11/2/2009

[stockbee]

Thank you all for your responses.

I was afraid I wasn't going to get much of a response as it does admittedly sound like a noobish question. I simply haven't owned a turbocharged vehicle before.

Cjmartz2k, thank you for explaining things further in relation to exhaust scavenging too, I really get the idea you want to help everyone on here learn more than they currently do if possible.

All in all, I don't think four inches is necessary. I will be getting the 3076R and keeping it if it is good. My friend who owns is had it on his series 1 recently tuned with a power FC and had it putting down 363 at the wheels. I simply want to make sure I'm doing all I can for the car as far as the easy bolt-ons are concerned and not creating any bottlenecks from the get-go.

Once again, thanks for the help. Look forward to posting pics of the car when the swap is done.

[Mistaken]

Someone correct me if im wrong.

I believe the apexi GT spec isnt full 3.75. It starts off 3 inch and steps up to 3.75.

[spolitte]

on a turbo car all the backpressure the engine needs is created by the turbo so it really makes no difference what you do AFTER the turbo provided you arent choking it. a 4" is pointless and overkill but whatever you want man. a 3" will support you to 500whp if you really want it to. arguing the difference between .25 is like arguing who is hotter, kate beckinsale or bar rafeli. both are hot. both will get the job done.

I guess I should have prefaced my statement with 'turbo back'. I shouldn't have assumed that was what we were talking about, but I thought the apexi' and the custom system will be from the cat back, maybe even from the downpipe back, but either way not before the turbo?

Either way, the exhaust header/manifold you use, as well as the turbo it's coupled with also play a big role in how your exhaust works, which is what I believe joe was getting at when he talked about the turbo creating back pressure? From my understanding, the purpose of a custom built exhaust header is twofold; one, it is designed to line up exhaust pulses coming from each different cylinder, and two, it is designed to 'speed up' the exhaust gas velocity so that it can push the hot end of the turbo faster, thus speeding up things on the cold side too.

Heres my best explanation of what I am talking about 'lining up' exhaust gas pulses, hope its comprehendable. In a I-4 and I-6 engine, you have 4 and 6 different exhaust pulses respectively in one full cycle of the engine, which happens to be two full revolutions. These pulses happen in a sequence, just as your cylinders fire in a sequence, the exhaust valves open in a sequence, etc. etc. One more thing you need to know about these exhaust pulses is that they are comprised of three parts, the head (a high pressure area leading the exhaust out), the body (a medium pressure area containing most the exhaust gasses, and a tail (you guessed it, a low pressure area that trails this pulse we are talking about and creates a vacuum in the pulses wake). Now, if you have ever looked at one of those super expensive race headers for sr20's, rb25's etc etc, you will notice that each cylinder has its own individual primary tube coming from the exhaust ports which bends around in smooth twists with the other primary tubes from the other cylinders until they come together to either one or two different turbo flanges depending on the application. Anyways, the point of bending these pipes together, and trying to make them all the same length is to ensure that these different exhaust pulses 'line up' when they finally join into one tube before the they hit the turbo. This is a form of exhaust scavenging, using the tail of the previous exhaust pulse to effectively pull the next one out faster by creating a larger pressure difference in the system.

When you get into V shaped engines the headers become even crazier because you have competing exhaust pulses, but i'll leave that for someone else to explain.

Also, as I mentioned above, some headers are designed to actually squeeze the exhaust pulses thru the primary tubes thus speeding up exhaust speed. The reason for this is the faster your exhaust gasses are traveling when they reach the turbo, the faster the turbo should spin. And yes, this 'squeeze' is technically a form of back pressure, and by the loose definition of the word a turbo charger in itself is technically a form of back pressure as well. Cj explains this pretty well as he explains a bit about the subject of exhaust gas velocity, which in itself is another factor dealing with how exhaust systems work. However, one thing I would say is that it is not fair to go around telling people back pressure is good because by loose definition of the word how a turbo works is technically a form of back pressure. Turbos are good, but the back pressure created by the turbo is not actually good for how the exhaust gasses travel thru your exhaust; therefore, back pressure is still ALWAYS bad.

Exhaust gas velocity has a direct relation to exhaust gas temperature, and for this reason you can go too big sometimes, especially in the case of N/A cars, as Cj as touched on this subject. If the exhaust system is too big, from headers-back on n/a and from turbo-back on boosted apps, then the exhaust gasses will cool down considerably as they are traveling out the system, slowing them down, and thus reducing the overall effectiveness of your super-big, super-straight system.

And stockbee, trust me when i say this is definately not a noobish question, and many people have many answers to give you concerning this subject. If you want to learn a little bit more about exhaust systems, and how they work go check out this thread...zerothread?id=456815

[stockbee]

Someone correct me if im wrong.

I believe the apexi GT spec isnt full 3.75. It starts off 3 inch and steps up to 3.75.

Yes, the exhaust is actually tapered in design. I should have clarified that earlier.

And stockbee, trust me when i say this is definately not a noobish question, and many people have many answers to give you concerning this subject. If you want to learn a little bit more about exhaust systems, and how they work go check out this thread...zerothread?id=456815

I appreciate the helpful insight. Will definitely be reading up on this more so, after I finish this dang research paper on the oil embargo of 1973....

[Cjmartz2k]

Damn spotlite, how long did that take to type LOL. Pretty much right on with my understanding. Kinda piggy backing on that with respect to what you were talking about with exhaust gas pulses (which are also negated after the turbo) and manifold design. I'm sure most people have heard of equal length headers vs. shorty headers. Equal length are supposed to be better at low end for exactly the reasons you mentioned, while shorty's come in handy when you are shooting for more top end power (and for fitment in a tight engine bay, but that is neither here nor there for this discussion. Joe's advice still applies the same on turbo cars as ours, just different reasoning behind it.

[spolitte]

Haha, yah it took a bit of time. But that was O.K. because I had about an hour free that morning while I was trying to catch a ride up to go watch my car on the dyno and finally pick it up! Needless to say, my mind was thinking nothing but nissan/cars that morning, so it was pretty easy to write that short little essay on exhausts.

Anyways, one thing I did want to mention to the OP was to make sure if you were thinking about going with the local custom fabber then make sure he uses mandrel-bent tube as opposed to crush-bent. Well, basically just make sure the quality of craftsmanship will be the same if not better than the Apex'i from bending method to overall straightness etc. etc.

[bryantkalagian]

on a turbo motor i wouldnt worry about it, but im not sure. i got a straight pipe 3" on mine.