Actually, that is a common misconception. Backpressure is not needed for a car. What you need is exhaust velocity, if you have a good velocity in the exhaust, it will draw a vaccumme in the heads and instead of just the intake pushing new air in and the old air out, it will pull the old air and help pull the new air in.

With out getting to complicated, where people lose power is they put a huge diameter exhaust on their car with some big headers, all this makes the air move slower and thus yo don't get the "pull" effect from the exhaust. On the other hand, if you make the exhaust to small you will create to much backpressure because the resistance to push the air out goes up with velocity, total flow rate and diamter of the pipe.

So if you put the bigger pipe, say a 3 inch on our car, you shift the power band (from the exhaust portion of it anyway) up, because it takes more velocity to get to that optimal flow rate now. So now you have a power band that is probably good for about 7-10K RPM rang (keep in mind these number are only for a small 4 cylinder). If you put on a 1.75 inch exhuast, you shift the power band down and will get more torque, but you kill your topend.

Anyway, a 2.25-2.5 inch exhuast is optimal for the EX motor, and a high flow cat is always a good thing, because it drops the overall resistance of the exhaust flow with out changing the velocity, same idea for the muffler. Now a cost and emissions versus power gains comparison is a little different, and up to each person and what they want.

An old trick I used on my old camaro, put some break fluid into a spray bottel. Since the line usually draw a vac from the engine's momentum in the intake, the air being draw in usually ends up in the piston. So if you spray the break fluid around a leaky vac seal, when the break fluid burns, it will make a really high pitched sound, won't hurt anything though. I don't know if our cars work the same as an '71 camaro for vac lines, but you won't hurt anything.

Go back to 87, save some $$$$$$$$$$$$$$$$$$$$$$$$$$, no difference anyway, this has been proven
And, I agree with this post - mods = waste of $$$, I am stock and quick with manual trans and lighter wheels, good tires, k&n drop in (don't know if the drop in does anything)

Spraying brake fluid around your engine compartment is a great way to destroy painted surfaces. Use propane and listen for a change in idle speed. It's a lot cleaner. Just don't blow yourself up...

1. I would say that, having zero back pressure from exhaust will send your engine to graveyard. I was told by my ME professor about this.

2. Changing the muffler does not PRODUCE more power, but just helps to restore some power loss due to restriction(3-passes vs. 1-pass)

3. Having large diameter header and large diameter pipe down, then switch to narrow pipe all the way to the tip of the exhaust will give the most horsepower and torque. (It is how the stock EX setup)

Conclusion: The stock EX has already been tuned to its best possibility from Honda...Any mods will just create problem, not horsepower.

I was thinking in the same way as gearbox was thinking....He had his head milled, and I have exhuast gas mixing with fresh air(valve advanced)...Should I go with 93? I guess not, because the engine wants to have a 30 deg igintion advance when I use 93....That's a killer of power...

Do you guys know the newer nissans with twin pipe exhaust single muffler has a valve in it that allows only one pipe to flow at low revs and at high revs the valve opens to allow both pipe to flow. So that gives you good power both low and high end. I think the current sentra, altima 2.5, and the old maxima has it.

Swap

Ask your ME professor if he knows what a Renyolds number is... if he does I and he realizes that you have to achive a certain range of renyolds numbers to have the best possible flow, I'll be impressed. Why would I be impressed? Because fluid flow (and yes air is considered a fluid, just with low denisty) is more so a Chemical Engineering thing than a Mechanical Engineering thing. A mechanical engineer will be more worried about viscous disapation and all that stuff where as a ChemE is better equiped to handle fluid flow and temperature change. How do I know all this, I'm teaching that class right now

When I say make more power, my reference as at the tires, since that is what really matters, so with my basis being that, any improvement on the exhaust will increase the power, irregardless of if the power was already being made by the engine or not (it's the black box theory and you and I simply have different boxes)

But I do agree that our civic already has the best setup, maybe a little better cat and muffler, but don't change the heads or piping.

^are you a professor?

[QUOTE=82801BA]1. I would say that, having zero back pressure from exhaust will send your engine to graveyard. I was told by my ME professor about this.
[QUOTE]


As stated previously, it's not backpressure, but exhaust velocity that matters. Reducing backpressure in an exhaust without making any other changes can lean out a cylinder, but it's the lean condition that hurts the motor, not the lack of backpressure. The old adage that you need backpressure to make torque is .

I'm a TA that is teaching a class because my advisor went on savitacal (sp?), he took a semester off. I'll have my PhD in about 3 semester, then I'll be a proft, but I'm making the test and grading and all that.

That's cool... wish I had that

As for the gas thing, I use high octane because it burns hotter which keeps the engine cleaner, it won't damage anything by running hot (unless you are constantly in stop and go traffic). But by running hot it will clean the carbon deposts off your valves and what not. As for a difference, I normally run 93, and decided to run 87 the other day. For that whole tank my motor was loud (not knocking) but just seemed more noisy when I asked it for more power, went back to 93 and it went away. It also seemed to have a smoother power curve. But that's just my .02, notice I didn't say it made more noticable power, just it seem to run better.

A TA? Cool, but I am in aerospace engineering, which always have to deal with Reynold's number(fluid dynamics), sucks.....

As far as Reynold's number's concern...Low velocity usually have lower Re, and high velocity have higher Re...If Re is too high, the flow will become turbulent, and it will flow everywhere under boundary layer...For that matter, if the pipe is straight and narrow, it is the same effect as having enough back pressure(if I still remember fluid mechanics)...Anyway, changing for a big pipe and having straight thru muffler will just make things worse...

By the way, Jrfish007, my ME professor's research is in internal combustion engine and low emission vehicle, and he is selected to be in the EPA research for fuel-cell vehicle...

[QUOTE=SMX][QUOTE=82801BA]1. I would say that, having zero back pressure from exhaust will send your engine to graveyard. I was told by my ME professor about this.
I am not saying that lack of back pressure will lower torque, but the car is not jet engine, still.

You know, a pipe is still a pipe, and that everything in between is OUTSIDE of combustion chamber, means that it's after the end of work-producing process. Whatever mass get in the header is whatever mass gets out of the tail pipe(Control volume). Does velocity at the tail pipe really matters? I would say no, but it should be designed so that the mass flow rate(dm/dt, for a derivation, dv/dt---- specific volume flow rate) is sufficient for the exhaust gas to flow through. If the allowable dv/dt is higher than what is needed, then the velocity will slow down due to volume increase.

That's why in ASE, I need to design the exit pipe to have everything consistence...

So will 93 octane reduce power? I dont think I noticed much difference, but maybe 89 was better. Stupid new computers control everything.

Your right about the Re stuff and the pipe, a bigger pipe is generally worse for our cars, you want the max speed Re you can get with out hitting the " exesive" torbulent regime.

My primary research is in catalsys, we just got done desing a new high flow and ULE emmisions cat for Toyota and are currently working on a simliar project regarding diesel motors that is sponsored by the DOE and DOD. I may not know as much about motors as your professor, but I probably have the edge when it comes to exhaust.

Actually, higher octanes better not reduce your power. According to the ECE depart (electrical engineers in my college) most OBDII computers have a progressive timing mechanism. If you continually put good gas in, they can actually advance the timing a couple of degrees, which of course give a couple of extra ponies, but not that much (note the new independent coil ignition system in our cars versus older civics). Over all, the octane is not going to make you car into a rocket, but it will help you burn cleaner and will not harm your car. Old car's a little different story and that's where most myths come from. Look at the new WRX, it cna change it's timeing by upto 15 degrees of timing depending on the air temperature and fuel, our cars is more like 2, and 5 if denotation occurs.

Gearbox, I would recommend you to stay with a higher octane, by milling your head down, you have raised your compression (that's the idea behind milling). When raising the compression, you raise the chance of denotation, this can be corrected with a fuel that is more resistant to denotation (aka higher octanes). Now if you need 93, or if 89 is good enough, that's up to you. But 93 will not hurt anyting.

dv/dt=a, so "the allowable acceleration of the exhuast is higher than what is needed"?

what is the allowable acceleration of your exhuast?

and how does that relate to a volume increase, where is the volume increasing?

Of course the velocity will slow for a bigger volume (I think you really mean cross sectional area, but whatever).

Seems like you are thinking in terms of a PF, or plug flow, but really you should try to model the exhuast as continous system with flux (flux for a change in throttle).

My dv/dt comes from v=V/m, which is specific volume...
Sorry that I don't have the symbol for specific volume...

So I should model it with (phi) instead of using specific volume?

Jrfish007:

So you said that when engine detects knocking, it will advance timing....Then why my car advance timing by 28 deg?

I'm confused as well. I know this sounds stupid, but the car felt faster with 89 octane instead of 93. So I'm just gonna switch back and see.

When you say it advances timing, I'm assuming you're talking about ignition timing right?

ok i think i ll read those fluid **** cuz i am starting a class in october called fluid mechanics.

Cool. The thing with different octanes still gets me. Usually with higher octane, the ignition timing can be raised. Whether this helps power I don't know. Also if you're running lean, you need higher octane. I guess the only way to find out would be to dyno different gas in your car.

Don't forget we have long stroke....you probably will have the spark force pushing against the piston force(produced by another piston that is going down)...also advacing too much will lower the compression ratio dramatically...

Try to imagine a piston in linear constant velocity(although it's not that linear and constant) in a cylinder(also, not really a flat-top cylinder)...

A compression of 9.9:1 means that, for the 95mm that goes up, the fully compressed height will be 9.6mm, yielding the cylinder height to be 104.6mm....The cylinder is traveling at 0.528mm/deg, which makes up 180 deg(again, assume linear, although it's not)....In a 10 deg timing advance, the spark occurs when the piston is at 89.72mm from BTC, at an compression of 6.02:1...20 deg, @84.44mm and 4.19:1...For mine, 28 deg, @80mm, 3.25:1....

As I said, it is not really a linear motion(the numbers are given in linear approximation and is NOT an accurate result), but it gives you better idea for how the timing advance will do....

omfg i read everything time to post....

damn it i forgot what i was gonna say like 3 pages ago.

oh

i got a 2.5 inch cat back left the cat stock for now. just echaust till i get my turbo. k laters

No, sorry if it was confusing, but I was right fast before I left. The engine can advance the timing, like I said, around 2 degrees, which usually can yeild 2-4 horse at the flywheel. If the engines detecs a knocking sound, it will retard the timing by 5 degrees to prevent denotation and blowing your headgasket.

Have you done actual reading by ODB-II?
Try and see if you can handle the result