36 replies to this topic

[87hfrex]

hey guys.. im new here to the forum.. well ive been reading for a long time but finally joined.. i have a question i haved looked around for but cannot find.... is it alright on my car (87 crx hf) to straight pipe it?... the number one thing iam worried about is with it being carbuerated is it going to cause problems with the restriction and backpressure it needs?..

any comments will help
-thanks guys

[nivek2002]

The car need backpressure or you will loose horsepower, especially at low end. Just stick a glass pack on it and it will be good to go.

http://www.redpepper...allery/album543

Kevin

[1stgencrxsi]

the more back pressure u have on a 4 banger the more power u have i have a highflow cat and a muffler on mine

[bigperm]

the more back pressure u have on a 4 banger the more power u have i have a highflow cat and a muffler on mine

<{POST_SNAPBACK}>

what?!?!?! too much back pressure will prevent your car from running at all. back pressure does not = more power. i agree that you don't need a monster 3" exhaust for that engine but a 2" crush bent cat back exhaust with a $36 dynomax super turbo muffler will definitely open things up a little without hurting your low end. It'll work just as well as a straight pipe for that car and be much easier on your ears.

[1stgencrxsi]

well yeah but im saying that back pressure doest help 4 bangers

[rickshank]

well yeah but im saying that back pressure doest help 4 bangers

<{POST_SNAPBACK}>

There's nothing magic about 4 cylinders. Exhaust size (within reason) can move the powerband around a bit.

[87hfrex]

just make a custom cat? or are there any companys that make a cat for anything besides the si's?

[twilightprotege]

i have to comment on this.

NO CAR WANTS BACKPRESSURE!!!!!! backpressure = loss of power. there are no two ways about it. backpressure is an evil word that was invented by HOS exhaust shops to sell dodgy systems.

what NA cars want is exhaust gas velocity. the faster you can make the gas move the better. this is why an exhaust too large for the car is bad. the gases move too slowly. this is also why stock exhaust systems make good power down low - the pipework is small and the gases are moving very quickly already - it's the same reason the car dies above peak power. you want to have an exhaust large enough to have a peak flow rate just above your peak power point. in a perfect world, the exhaust should be large enough to be at 100% flow capacity at redline

a well designed muffler will not slow down the exhaust gases

[RFL85CRXSI]

BUT you do have to stop and think about the back pressure needed to cool your exhaust valves, with a 3" free-flowing exhaust, you will end up burning up your valves. The best thing to do is just test things out, I am running a Chikara 4-2-1 hedder with the stock diamiter pipe (which is more than enough that my 1.5 needs) with no cat (as of now). I have a muffler located directly under the passenger area, and a glass pack- type tuner muffler.

[CRXdan]

ive never heard of anyone burnign valves from too large of an exhaust./

[twilightprotege]

you do not want backpressure!!!!! i cannot say that more than enough.

what keeps the exhaust valves cool is gas velocity. the quicker you can get those gases out the better. backpressure = pipe too small for the amount of gas coming out, ie the gas backs up. nasty nasty nasty. kills power big time.

[Surestick]

Exhaust valves will have to deal w/more heat if you add an exhaust that has low restriction & causes you to lean out because you made no adjustments to your carb. If you are making extreme modifications to your exhaust you will probably want to get a wideband to make sure you aren't too lean.

Backpressure isn't all that bad as long as it doesn't occur during the wrong part of the exhaust stroke (overlap). This is the purpose of the exhaust valves you see on motorbikes - they may produce back pressure but more importantly they change the length of the system & therefore the time when backpressure reaches the exhaust valve.
Generally you can tune an exhaust to behave well at part of the rev range & not so well at others. What you pay for with a high quality exhaust system is the knowledge & experience it takes to get the tradeoffs to be less than they would be with an inferior system.

Edited by Surestick, 11 October 2005 - 09:18 AM.

[MadMax]

ive never heard of anyone burnign valves from too large of an exhaust./

<{POST_SNAPBACK}>

My 86 DX was driven for a year without a muffler, #3 cylinder has a burnt exhaust valve. My exhaust currently is stock with a coffee can muffler (I got for free) mounted slanted near the rear axel so you can't see it

Oh yeah nivek2002 your sedan looks freakin beautiful with those phone dials on it. Sorry I had to point that out

My .02 = exhaust gas velocity = what you want to worrie about
Straight threw you only see on cars at the race track that rarly get under 4,000 rpm. I highly doubt anyone on here drives at 4k normally. So like others have said stick to a smaller size, close to oem but have it as straight as possible. nivek2002 has a simular plan that I have for an exhaust, and a few others on RPR have the same setup. It has one major bend from the collector to the tip of the exhaust, other than that its straight. Not to mention you have less weight from extra piping to the rear.

[mesean13]

you do not want backpressure!!!!! i cannot say that more than enough.

what keeps the exhaust valves cool is gas velocity. the quicker you can get those gases out the better. backpressure = pipe too small for the amount of gas coming out, ie the gas backs up. nasty nasty nasty. kills power big time.

<{POST_SNAPBACK}>

That's right, you don't want any exhaust gasses going back into the combustion chamber. It's called reversion and it is an evil power robber. If you do an internet search on anti-reversion chambers you might learn a little more about it. This is also why it is good to bench flow cylinder heads and match exhausts accordingly to adjust torque not with backpressure but good flow on our smaller 4 cylinder engines and the reason why top fuel dragsters with insane torque run flame throwers that can allow the exhaust to exit equally.

[twilightprotege]

Exhaust valves will have to deal w/more heat if you add an exhaust that has low restriction & causes you to lean out because you made no adjustments to your carb. If you are making extreme modifications to your exhaust you will probably want to get a wideband to make sure you aren't too lean.

Backpressure isn't all that bad as long as it doesn't occur during the wrong part of the exhaust stroke (overlap). This is the purpose of the exhaust valves you see on motorbikes - they may produce back pressure but more importantly they change the length of the system & therefore the time when backpressure reaches the exhaust valve.
Generally you can tune an exhaust to behave well at part of the rev range & not so well at others.  What you pay for with a high quality exhaust system is the knowledge & experience it takes to get the tradeoffs to be less than they would be with an inferior system.

<{POST_SNAPBACK}>

motorbikes get their "backpressure" from an exhaust expansion chamber which throw back a sound pulse

ive never heard of anyone burnign valves from too large of an exhaust./

<{POST_SNAPBACK}>

My 86 DX was driven for a year without a muffler, #3 cylinder has a burnt exhaust valve. My exhaust currently is stock with a coffee can muffler (I got for free) mounted slanted near the rear axel so you can't see it

Oh yeah nivek2002 your sedan looks freakin beautiful with those phone dials on it. Sorry I had to point that out

My .02 = exhaust gas velocity = what you want to worrie about
Straight threw you only see on cars at the race track that rarly get under 4,000 rpm. I highly doubt anyone on here drives at 4k normally. So like others have said stick to a smaller size, close to oem but have it as straight as possible. nivek2002 has a simular plan that I have for an exhaust, and a few others on RPR have the same setup. It has one major bend from the collector to the tip of the exhaust, other than that its straight. Not to mention you have less weight from extra piping to the rear.

<{POST_SNAPBACK}>

if you are going to put an exhaust system on a car why would you want max power under 4k? anyway, exhaust systems dont rob THAT much power down low. i have a 2.5" mandrel exhaust on my mazda protege5 (2L I4). dyno results show a gain in power over the stock 2" crush bent system from 3k upwards. i'm only loosing a maximum of 2-3whp below 3k. would i ever notice that? nope. is the loss of 2-3whp below 3k worth the gain of 20whp at 7000rpm? yes.