3" exhaust

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stlpsycho138

stlpsycho138

Junior
i was looking at a new exhaust for my car since mine was stolen.... found a kit that has 3" stainless from the cat back, and i have a choice of muffler that i want. would this be a good idea on a na motor? i heard too big is bad, that it actually makes you lose power.this is not a problem for me right now as long as it is all it will do, and it'll help me in the future. i plan on supercharging over the summer, so would it be better to just to go with the 3" now, or stick with the 2.5"?
 
Turbo and super charging both end up at one point or another requiring the same exhaust evacuation efficiency as each other. There's more air being shoved in either way, we know one needs a bigger exhaust, so the other will too. The method by which the procure more air does not dictate a change in evacuation needs. It's still internal combustion both ways.

Think about the cost of the exhaust either way you go. Let's say a 2.5" exhaust is $500, and the 3" is $750. If you plan on boosting it later, you'll want an exhaust that supports it. If you buy the 2.5" now because you aren't boosted, but don't want to lose any power until you're supercharged, you'll have invested a total of $1250 in your exhaust altogether. If you take the loss in performance now, you'll save money, time and effort. Let's say you don't get the forced induction down the road, you've not lost enough power that you were making more on your stock exhaust, so you'll still at a plus there, and the 3" carries more value, allowing you to sell it and procure the right size, possibly walking away with some money after you exchange them.
 
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thanks alot guys.... you just said everything i needed to hear to convince me. the site i'm getting it from has awesome prices on all sizes, so if any of you are interested i can get you the site. the 3" for my prelude is about $260 for the piping, plus muffler (i already have mine) and the 2.5" was about $245, so i figured the extra $15 couldn't hurt too much, lol. pm me if interested in the site.
 
ive seen many NA setups benefit from a larger exhaust. even going to a 3" when everyone said 2.5" is the largest to go on NA. think about this for a minute. rarely if ever do you see a car LOSE power by running open header. and im not talking about something with a megaphone collector or anything that is designed from the get-go to be open header, im talking normal headers that have just had the exhaust unhooked. most if not all of thetime you see a small increase in power. now if you can gain power like that, why couldnt you do the same by going from a 2.5" to 3"? 3" is fine for NA IMO and ill stick to that opinion until someone comes in here with a dyno chart showing a clear drop in power by doing so.

i actually plan on picking up a 3" thermal at some point (or have something made up) for my NA setup. since no one besides thermal makes a bolt on exhaust larger then 2.25" for the EF/CRX chassis. all the 2.5", 60mm, and 70mm kits are for EG's DC's etc.
 
I've had and seen several bolt-on 2.5" kits (non-custom built) for CRXs and the hatchbacks, maybe living in different areas has the most to do with that.

Personally, unless someone corrects my logic, I would think the proper exhaust diameter is found not only by porting the head and opening up how it breaths, but also a lot by the stroke and the bore. I imagine it's pulling and pushing a significant amount more of air at 84mms than at 81.5. Maybe not an actual large amount in reality, but I imagine the amount would be comparable to the change from 2.5" to 3.0" in exhaust diameter.

I'm just thinking that you don't have to be FI to make use of that size, I think if you are moving enough air to begin with NA, than the exhaust should match that need for larger piping.

Although, I have seen several dyno graphs of cars picking up power by restricting their exhaust size after being hyped into buying 3" exhausts but only supplying their car with an intake and some bling as far as the rest of the powerplant goes. The drop facilitated the backpressure needed to evacuate the exhaust more efficiently whereas the larger exhaust dropped the pressure and the exhaust moved more slowly. Think of a garden hose. When you kink it, it will stop, but if you relieve the kink to where it can flow, but constrained, and the stop at the point where there is the highest exiting pressure, then you've found the "ideal" sizing of the exit tube. That's the point of exhaust diameter selection. Just gotta figure out how much you are flowing, how the "kink" (head) is setup, and then configure you exit tube to match.

At least I think that's how it works I could be wrong, for serious
 
your analogy makes no sense. if you knew anything about fluid dynamics youd see the flaw. a very small opening would create a high pressure. and high velocity, but high velocity wont necessarily mean high flow. and in your analogy, the obvious ideal spot would be all the way open.

try to think about it this way. take a garden hose and put your thumb over the opening, and fill up a bucket. this way you increase "backpressure". exit velocity increases. now take your finger off and fill the bucket. the unobstructed hose would fill faster, even though it had a lower velocity, and its because it had a higher flow rate. this backpressure theory is BS.

like i said, if you can benefit from open header (essentially and infinitely lage pipe) then you can benefit from a larger exhaust where theres no chance of choking your setup. since every setup would theoretically need a different size pipe, but because of standardization, you cant buy say a 2.55" or 2.67" or 2.75" etc. so you go to the next one up.

and no one makes a 2.5" for the crx, ive looked. if they do it has a bottleneck somewhere in the system thats less then 2.5" (can only flow as much as the smallest section), and/or its size is misrepresented, ebay for example. and many people think the regular thermal is 2.5", but i have one and its not, its 2.25". also remember that all pipe is measured by OD, not ID, so the actual ID (the one that matters) will be smaller.
 
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GSRCRXsi said:
your analogy makes no sense. if you knew anything about fluid dynamics youd see the flaw. a very small opening would create a high pressure. and high velocity, but high velocity wont necessarily mean high flow. and in your analogy, the obvious ideal spot would be all the way open.

try to think about it this way. take a garden hose and put your thumb over the opening, and fill up a bucket. this way you increase "backpressure". exit velocity increases. now take your finger off and fill the bucket. the unobstructed hose would fill faster, even though it had a lower velocity, and its because it had a higher flow rate. this backpressure theory is BS.

like i said, if you can benefit from open header (essentially and infinitely lage pipe) then you can benefit from a larger exhaust where theres no chance of choking your setup. since every setup would theoretically need a different size pipe, but because of standardization, you cant buy say a 2.55" or 2.67" or 2.75" etc. so you go to the next one up.

and no one makes a 2.5" for the crx, ive looked. if they do it has a bottleneck somewhere in the system thats less then 2.5" (can only flow as much as the smallest section), and/or its size is misrepresented, ebay for example. and many people think the regular thermal is 2.5", but i have one and its not, its 2.25". also remember that all pipe is measured by OD, not ID, so the actual ID (the one that matters) will be smaller.
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the place i'm getting mine has 2 2.5" exhaust kits and 2 3" kits also, if your interested.
 
whats the brand? i can almost garantee its probably 2.25" and not a true 2.5", unles its a custom piece together kteller type thing. and not a bolt on kit.
 
If you are stock say good bye to your low end power.

3" seems real big for a n/a smaller engine.
 
yea, you just have to weld it all together yourself (or someone else that can weld), thats why its cheap. im talking bolt on kits.

i want to learn to weld, then ill make my own 3" setup. maybe even do it in alluminum :) ive seen an alluminum setup that john at ICON made up with v-band clamps and a burns muffler, looked and sounded sweet.

and yea jeef, 3" may be a bit much for a smaller engine(at least from an economical stance), like 1.6 and less, but on 1.8 and up 3" wont hurt anything.
 
well., that's okay with me tthat you have to weld it, i weld every day, so it won't take any time at all to put it together. weldings not hard, it just takes practice. i would suggest getting someone else to weld aluminum piping for you though. aluminum is so thin and the melting temp is so low it's extremely hard to work with, even to a seasoned professional.
 
one thing to watch out for though when you buy a 3" exhaust, there is little to no backpressure and sitting at idle it will jump up and down because the car will try to die and the ecu will rais it back up, we just took a 3" exhaust off of my brothers car for that reason, we put a 2.5 greddy back on it and it purs like a kitten sounds better cause its not ratty like the open 3", if your going supercharged though it will work out, just a precaution
 
is that a good price for a 3" exhaust? a friend of mine took his stock exhaust from the cat back to a local shop and they bent him up a 3" and welded the muffler on for around $75.
 
GSRCRXsi said:
backpressure is a myth. you dont need or want it.
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Pipe diameter and "backpressure" affect drivability, trust me. I ran open header from my house to the muffler shop (OEM usdm Type R mani) and trust me, it was LOUD and my idle was all over the place. Not to mention, every modulation of the throttle created loud burpy discontinuity in the exhaust flow. Backpressure is a good thing to the extent that you are not bottlenecking the system. If you are speaking of gas dynamics, every pipe diameter has a critical flow rate, at which point it will stop flowing. The larger the pipe, the higher the critical flow rate, the more air and therefore more fuel you can flow, leading to more power. However, exhaust scavenging is also important, and with a larger diameter pipe, you lower exh. gas velocity everywhere below the critcal flow rate, lowering the amount of available kinetic energy that effectively "pulls" on the following exhaust pulse. The larger the pipe, or in the case of the open header, the decreased mass of air flowing through the pipes, decreases the effectiveness of the scavenging, making the exhaust very ineffective in the midrange because of the lack of energy in the gas that would normally be pulling on the gas coming out behind it. Backpressure is a poor term, but as a general rule, the more continuous the gas flow, the better the exhaust will perform.
 
first off, your to worried about flow and the way the exhaust is coming out... do any of you know anything about tuning an exhaust? with no backpressure your not burning all your fuel thus loosing valuable hp, run it and it will burn black because there is unburnt fuel being thrown out, we all understand the theory of what goes in has to come out and water and whatnot but your missing the important details, yes you do need some backpressure like i and mike burgy stated, you do need it and if you dont have it your using unnecesary amouts of fuel with less combustion
 
94_gs-r said:
first off, your to worried about flow and the way the exhaust is coming out... do any of you know anything about tuning an exhaust? with no backpressure your not burning all your fuel thus loosing valuable hp, run it and it will burn black because there is unburnt fuel being thrown out, we all understand the theory of what goes in has to come out and water and whatnot but your missing the important details, yes you do need some backpressure like i and mike burgy stated, you do need it and if you dont have it your using unnecesary amouts of fuel with less combustion
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that is absolutely rediculous. You are talking about burning unburnt hydrocarbons in the exhaust tract? that is only going to help with anti-lag systems, and that is a completely different beast. The only fuel that contributes to energy (and thus power) production is that which is actually burned in the combustion chamber before the exhaust cam is at a "high lift point", allowing the exhaust gasses to exit the chamber. The unburned fuel that is left beyond the stoic point is there as a heat absorbtion agent, in order to reduce combustion temps. I only used backpressure as a loose term used to describe the continuity of the gas flow. If your exhaust is coming out black, you are running too rich or you are having blowby issues.
 
MikeBergy said:
Pipe diameter and "backpressure" affect drivability, trust me. I ran open header from my house to the muffler shop (OEM usdm Type R mani) and trust me, it was LOUD and my idle was all over the place. Not to mention, every modulation of the throttle created loud burpy discontinuity in the exhaust flow. Backpressure is a good thing to the extent that you are not bottlenecking the system. If you are speaking of gas dynamics, every pipe diameter has a critical flow rate, at which point it will stop flowing. The larger the pipe, the higher the critical flow rate, the more air and therefore more fuel you can flow, leading to more power. However, exhaust scavenging is also important, and with a larger diameter pipe, you lower exh. gas velocity everywhere below the critcal flow rate, lowering the amount of available kinetic energy that effectively "pulls" on the following exhaust pulse. The larger the pipe, or in the case of the open header, the decreased mass of air flowing through the pipes, decreases the effectiveness of the scavenging, making the exhaust very ineffective in the midrange because of the lack of energy in the gas that would normally be pulling on the gas coming out behind it. Backpressure is a poor term, but as a general rule, the more continuous the gas flow, the better the exhaust will perform.
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you can tune out any change in idle, it being loud doesnt really matter. obviously it will be loud. flow rate of the exhaust pipe doesnt really matter in terms of power (as long as you arent chocking it) because you are restricted on the intake side anyway. which is why i said going to a larger pipe wont hurt. ive still yet to see any car loose power from open header. its ALWAYS a small increase. so if you can benefit from open header, you can benefit from a larger exhaust. IMO.

are you talking about scavenging in the header? or scavenging at the tail pipe due to the lower pressure when you are moving? if you are talking about in the header, its really not something you can change, its all a variable of the header design. scavenging at the tailpipe will give marginal gains IMO. and also not exactly something you can change as the position of the tailpipe is set by the factory. unless you get all crazy like Bisi and break out the manometers to find the spot where the lowest pressure under the car is.
 
94_gs-r said:
first off, your to worried about flow and the way the exhaust is coming out... do any of you know anything about tuning an exhaust? with no backpressure your not burning all your fuel thus loosing valuable hp, run it and it will burn black because there is unburnt fuel being thrown out, we all understand the theory of what goes in has to come out and water and whatnot but your missing the important details, yes you do need some backpressure like i and mike burgy stated, you do need it and if you dont have it your using unnecesary amouts of fuel with less combustion
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you have no idea what you are talking about, no need to go into details bergy covered it. and unless you are "tuning" (changing) the length and path of the exhaust i dont know what you think tuning it means. the only thing under your control is the diameter. you can tune the ecu after changing an exhaust however.

somebody better tell all the racers about this backpressure thing. cause everyone at the track running open header and/or side exits must not know they are losing valuable HP by not running a full exhaust...:huh:, th exhaust is for sound and emmisions.
 
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