After months of debating I took the advice from the gurus here on H-T and went with full 3" exhaust on my 94 DC with a stock low mileage B20. It had stock piping with just a WS2 axle back. Now its full 3" from the cat-back (cat is 2.5).

I immediately noticed a loss of low end torque, of course I havent fully tuned the car yet to take full advantage of the exhaust but I honestly can't see me getting the torque back. However it does seem to pull much harder up top now and all the way to redline where it used to be really weak. However, this being my daily driver (85 miles commute daily) I miss the low end grunt of the B20 on these mountain roads.

Anyone else notice this with 3" piping??

 

Some back pressure will give you low end power.
But its kills the top end as your seeing.
Gotta decide where u need the power..

 

I kinda figured the OEM cat and my small collector header would keep enough back pressure to prevent this but guess not, either way I think I'll get used to it, I just like torque/low end power and hate to rev something to 8K to make power but I've learned thats simply something a Honda wasn't designed to do! It definitely pulls much harder/smoother all the way to redline now though!

 

You're losing low end torque because it isn't tuned..

You want NO back pressure..back pressure isn't even a real word in the SAE books..its a myth thought up by old guys with their 325 chevy's back in the day..

Smaller piping can increase air flow to a point, but then that point is, how high in the rpms can it flow without hurting itself. It may have felt like you lost low end, but you didn't.. just get it tuned.

 

ALL MOTOR- Isn't it true that many OEM's design exhaust systems with restrictions in place to purposely create a certain amount of back pressure or restriction? I know this influences the power curve of a given engine substaintially.

I'm new to tuning Hondas, not new to tuning overall. Using Crome Pro what changes are normally needed when upgrading to large diameter exhaust like this? Do the Honda ECM's use adaptive strategy like fords/GM's to compensate for correcting Air/fuel with changes like exhaust? Is it likely that I just need to fatten up the fuel map to compensate for this much better flowing exhaust?

 

Now that you've changed say a cat-back system, the stock cat and stock header are going to need to be changed to fully see what you've gained.

B series dont really respond super awesome to bolt on mods, like the k motors do. Food for thought and i know this is just a "street" comparsion but my buddy's 2011 civic si bone stock would run neck and neck with a legacy GT 2012. This weekend we put on a Aem 3'' cai and a skunk2 mega power 70mm exhaust and he put almost a full car on him.

The b series ecu doesn't change, what it doesn't see..the k series is also a better designed motor. As far as i know, the ECU has a certain tolerance for adaptive. E.I IAT's, ECT and what not.. its not like say a AUDI or porsche which has sensors all over the place so the ecm can adjust ign timing and afr's for the added mod.

When using CromePRO you'll want to look @ your low end 1500-4000k range.
On your motor, use stock p30 ign maps, shoot for a AFR of around 13.3-13.1 AFR @ WOT. I've always used the p30's maps to start from, i've found they are a good mix of aggressive ign maps without pushing the limit too much.
If you need anymore help, feel free to pm.

 

Thanks Man, I appreciate the help! Especially from a knowledgeable guy like yourself. Once I get to fine tuning this thing I may just drop you a line..

 

All Motor is probably one of this old guys!
Alot of car makers are now using variable exhaust (bmw, ferrari, etc...) for modifying the back pressure in the exhaust in function of rpm. Like i said in another thread, for low gas volume (low rpm) you will want small piping, for high gas volume (high rpm) you will want big piping. The variable exhaust used on most recent car will have a butterfly to create back pressure in order to get the desired volume for the right rpm.

 

honda and every other manufacturer designs an exhaust on many things....ie: decibel level, emissions, performance..etc.

they arent there trying to get the most power out of an exhaust as the know a bigger one will make more power, yet also be louder so they give and take depending on what the engine and car are designed to do.

if you went from a stock exhaust to a 3" without tuning i can almost bet its lean as hell.

hell, the difference between a chambered 2 resonator "performance" exhaust and a straight through 1 resonator exhaust of the same size can make a notable change in air/fuel ratio.

an engine cannot give you performance if there is not enough fuel.

 

So my thinking is also that its lean, the better flowing exhaust is making the car run better i.e more efficiently so now I just need to hook the wideband up and go for a few WOT blasts... I'm a V8 guy and hell we usually only ran 2.5-3" on stuff all the way to 400+CI! I just couldn't fathom putting that big of exhaust on this little 2.0l

 

I laughed when the guy welding my resonator in goes why do you have such a huge exhaust on a little 4 cylinder? I was like uhhh people are running 3" making power.

 

As i said before, smaller pipe diameter will speed of air velocity, but only to a certain point.. after which you want scaveging to come into effect.. which is why we tell alot of people who get headers with 2.5'' collectors to flare out to a 3'' exhaust... there's alot more that goes into exhaust tuning.

 

Believe it or not.. like i've stated quite a few times.. anything pass the merge collector is bad for flow..unless its a str8 through design.
ALOT of honda's show great gains across the board, not only in power but throttle response and efficenty.. truely once you go above 2.5'' loudness gets in the way.. but if you find a good resisnator you should be fine..

 

its not about back pressure...its about exhaust velocity...the bigger piping decreased the exhaust velocity in the low end..its a stock b20 low compression going 3 inch isnt worth it to only rev to 7k....where ur gonna see more low end torque and top end is on a high flowing engine.(b20vtec).high comp, bigger cam primarys better flowing head,intake...

why are u losing torque? cause of lost scavenging....the vacuum created by faster exhaust velocity actualy help pull fresh air into the combustion chamber therefore you make more power all over...but you engine will benefit more from a smaller exhaust...

how tuning will get ut torque back? i have no idea maybe someone else can explain..

 

I have a 2.5" collector on my cheapy 4-2-1 header and then it goes into my OEM cat (which I'm replacing here soon with a good magnaflow 2.5" piece) It then transitions straight into my 3" exhaust which has one straight through resonator about 20" long. Overall sound isn't too bad, really deep and not raspy at all.

 

No, it is about back pressure. It is also about volume and velocity of the exhaust flow. All 3 are used in the physics behind it.

 

Sure its about back pressure. Eliminating it.

 

Again, wrong. When eliminating back pressure at a specific rpm, you won't have the optimized volume and velocity for optimal torque and power.

 

Lets explain what back pressure is. Its the opposing force (restriction) of the exhaust flow. Now what good does holding back exhaust flow do after the last collector. We are talking exhaust design here, not header design. With a header, yes, you have to use proper size and length to optimize flow without hurting velocity given a patricular volume and operating rpm.

Whats the point of an exhaust after the last collector( last chance to achieve a scavenging effect). The point is to reduce noise and to direct exhaust away from the car. If you are trying to get evry bit of power then noise and exhaust smell probably isn't a big concern.

Now scientifically and factually explain to me why you want back pressure in an exhaust sysyem aftee the last collector?

 

The scavenging effect (the vaccum created behind the exhaust wave) will be affected by the speed/volume/density/temperature. Do we agree on that? The scavenging effect, because it is a vaccum effect, will continue in the exhaust. Do we agree on that? I don't know if you did a fluid dynamic class, but if you play with volume, it's going to affect the speed of the flow. If you put a big pipe when the engine is at low rpm, the volume/density of the flow will be able to expand in the pipe and that will lower the speed of the flow. What will happen is that the speed of the flow won't be high enough to create the optimized scavenging effect, ence a low torque/power performance.

 

If you still think that back pressure is bad, i could give you some link of recent motorcycle and car engine that use a variable back pressure exhaust to optimize the speed of the flow (in order to optimize the scavenging effect (in order to optimize the low/high rpm torque and power (in order to make the car go faster)))!

 

I agree to a certain point but only with manifold design and collector design. I do not agree that the area 18"-24 inchs beyond the collector will be effected by scavenging. If scavenging was effected that far beyond the exhaust pipe then why do you see everyone with short race headers making more power than a full exhaust?

Like I said, I agree with you in regards to manifold design. But not after the 18" or so of piping beyond the collector

 

experience vs. reading **** on the internet... which is usually right??

 

don't ever use the term "backpressure" when talking about a performance part unless its going to eliminate it!

a post of mine from 07

"you dont want any at all .. the best would be a vaccuume on the exhaust side ... the problem you are having is due to the basic airflow law of faster moving air has less pressure (how an airplain flys).. if you have a small dia.pipe on your exhaust the air will travel faster out of it and have less pressure than if you had a big pipe with the same amount of air going through it with slower speeds and more pressure... think of it like a paper towel roll (small tube) with marbles in it , you can only add 10 per min. 9 black and 1 red one. you will get to a point where you can watch and find out how long it takes for a red one to come out the top. if you then switch over to a oatmeal box( the round ones with the quaker dude on them) and cut it to the same length as the towel roll.. now do the same 10 per min. with 9 black and 1 red keeping track of how long it takes for the red ones to pop out the other end .. you will notice that the red ones dont come out at the same rate and it is harder to push them through because you are lifting and moving a whole oatmeal box full of marbles( lets just say 500) instead of a tube of them(100?)..

but the exact opposite is true if you push 10 per min. and the tube is like a straw.. you will have a hard time pushing them ..

point is you have to size the pipe to the exhaust output of the motor .."

on a stock motor you need the restrictions to create a "fast/low pressure" zone to suck the exhaust out. on a built motor you need the flow that can be had with a larger tube...

as stated before . forget about adding backpressure and think of it as adding a low pressure zone or fast air...

if you want backpressure weld a washer to your exhaust tip and watch what your performance does

 

People that use short race exhaust will only make power at high rpm. They will kill low rpm torque/power.