Hey all,

I planning on putting and exaust on a 91 civic SI with a jap spec engin. This car is going to eventually be rallyed, what would any of you suggest? I am looking for that rally sound out of it. I cant stand that noise that comes out of those riced up civics.  Thanks in advance

MacArthur

Please, for the love of the sport, keep the stove pipe exhaust at home!  Sorry, I just find them very annoying.  

I'd be careful not to choose too large of a pipe diameter, it will cause you to lose low end torque but you'll gain a little in the high rpms.... for rallying i think you are better off trying to keep some low end torque (not sure if hondas have anything to begin with )so i'd go with something like 2 1/4" piping on that car... you best bet is probably to talk to someone who drives a civic however. Also if you are looking for power equal length headers would help

good luck

MacArthur wrote on 03/24/4 at 04:40:31:


Actually, everything that you've heard about back pressure is a myth.  Here's the facts:
Back pressure is bad <period>.  It robs both horsepower, AND torque from your engine.
So no matter what, you want to have as little back pressure as possible.
However, (you knew this part was coming) there is much truth in the fact that having too large of exhaust pipes can rob some of your low-end torque.  This has nothing to do with back pressure.  The problem with the larger pipes is that they lower the velocity of the gases flowing through them.  It's not that difficult to understand; the actual flowrate is determined by the engine itself and has nothing to do with the exhaust system (relatively).  Therefore the flowrate will remain constant.  So if you increase the volume of the exhaust pipes while the flowrate remains constant, then the air velocity will naturally decrease.  This can easily be seen visually during the cold winter months.  Watch the tailpipe of any car with a stock exhaust system, you'll see the gases being blown out at very high speed even at idle.  Then take a look at a honda (or whatever) with a big phat aftermarket exhaust, and you'll see that it is trickling out at very low speed.
Now, the reason why lower velocity in the exhaust system robs you of power is very complex, but it can be simplied to just this: With a high air velocity in your exhaust system, basically what happens is that when the exhaust valves open, not only does the freshly burnt gases get pushed out by the force of the pistons, but they also get sucked out by the vacuum created by the high velocity of the previously burnt gases.  If you install a large diameter exhaust system, and sacrifice some velocity, you lose this vacuum effect, and it becomes more difficult for the gases to be expelled by the pistons, thus robbing some of the engines torque.
There is a way to calculate the opimum diameter of exhaust piping for your car, but you need to know the exact displacement of the engine, the volumetric efficiency (which you probably don't know), and what RPM you want to be optimised for.  I don't know the formula myself, but I'm sure it can be found on the net somewhere.

EDIT: Wow, what a long post.  Sometimes I just start typing away and don't realise how long I've been going for.  Away, that's about the shortest explianation I can give which still makes sense.

WOW WOW WOW,

THanks alot everyone for there input I learnet alot. I was planning on going with 2 and a 1/4 piping.  And I am happy somone clarified the myth about back pressure.  I live in peterborough so i will have to go to the next PMSC meeting and talk to the guy with the civic. The only bad thing is that i work nights. I will just have to come up with some sort of problem on the day of the next meeting. lol.  

Some one also mentioned suspention (sp) thats also on my list of things to do this year.  That and a skid plate. I went into the ditch hard this winter and every thing was fine, but i went and checked out that spot where i went in yesterday, Had it been in the summer i would have destroyed the underside of my car.  Can anyone suggest what would be a good suspention? If so that would be great.


Thanks again for all the help. I have always found this board very very informative.  Look forward to competing against some of you in the next few years.

MacArthur

Jah, and just to refer to one of the early comments, it is true that an organiser CAN bar you from an event if the car is too loud.  So far I haven't seen it happen.  It's just to protect the sport from the public who don't think loud cars are cool.  Also, sometimes loud is equated with racing, and we aren't racing on public roads.  So, it is something to keep in the back of your mind.  Dats all.  Good luck with it.