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Old 05-10-2010, 01:21 AM   #1
907mge
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Default N/A Downpipe Size Question

I have a 1990 N/A with a 3in Hks catback(yes I know it is overkill) with a 3in high flow cat. I now have the pacesetter header that after some minor modifications Iím going to install. My question is if I should make the current 2in downpipe for it larger?
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Old 05-11-2010, 01:55 AM   #2
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I have heard that with the N/A's, you shouldn't go too much bigger with most of the exhaust parts. 2.5 inch cat back is about perfect for N/A's and 3inch for the turbos. Since the downpipe adds some of the most restriction, you may risk losing some power if you go too big, but you should probably get another opinion
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Old 05-11-2010, 03:11 AM   #3
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Quote:
Originally Posted by 907mge View Post
I have a 1990 N/A with a 3in Hks catback(yes I know it is overkill) with a 3in high flow cat. I now have the pacesetter header that after some minor modifications Iím going to install. My question is if I should make the current 2in downpipe for it larger?
it depends on what you are trying to do if you want all out power you should go one full size all the way back actually ending up a tiny bit smaller at exit as the whole point of the exhaust is to keep your air speed up as your exhaust gets further it slows just like an irrigation system so they downsize the hose as you would your exhaust unfortunately this would be extremely hard to do so we just run one full size in our cars

with adding a 3 inch "downpipe" you will get a deeper tone and you will get a steadier fllow than with a 2 inch as with the 3 inch you will only have 1 slow point in your exhaust then the rest of the way it should flow steadily

with keeping the 2inch you will have a slow point at your 2-1 point of your header and then at your 2-3 point of your exhaust you will have plenty of back pressure maybe a bit too much to make full power and you will have a different tone as your exhaust will flow fast at the header and slow in your exhaust

basically the 3 inch dp would be your better way to go depending on the coating on your headers and the way you build it and as for the 3 inch on the n/a its not that overkill its just about perfect
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Old 05-11-2010, 03:20 AM   #4
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3" Is to big for an N/A, Just leave the downpipe the way it is and get a reducer to go from your collector to test pipe. With 3" exhaust you'll have better top end power but your losing lowend power, atleast for an N/A.
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Old 05-11-2010, 03:22 AM   #5
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Making the outlet of the exhaust slightly smaller would be retarted as this would make backpressure you want the exhaust to come out as fast and smooth as possible, if it was a smaller outlet that would hinder the speed of the exhaust.
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Old 05-11-2010, 04:58 AM   #6
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Quote:
Originally Posted by fullthrottlesrqsupra View Post
one full size all the way back actually ending up a tiny bit smaller at exit
Actually, you want to start out smaller and end up larger. After combustion you want a smaller diameter pipe to maintain the velocity of the hot exhaust gasses. The further the gasses travel, the cooler they get. When the gasses cool down, their velocity decreases causing more restriction in the flow. To avoid this, you increase the pipe size, allowing more gasses to occupy that portion of the pipe.

So, start smaller, end bigger.

Let the arguments begin lol
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Old 05-11-2010, 02:53 PM   #7
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^^That was the jist of what I was trying to say.
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Old 05-11-2010, 04:24 PM   #8
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Here is a pic of it now with the exhasut wrap on it.
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Old 05-12-2010, 04:20 AM   #9
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Quote:
Originally Posted by btwilson86 View Post
Actually, you want to start out smaller and end up larger. After combustion you want a smaller diameter pipe to maintain the velocity of the hot exhaust gasses. The further the gasses travel, the cooler they get. When the gasses cool down, their velocity decreases causing more restriction in the flow. To avoid this, you increase the pipe size, allowing more gasses to occupy that portion of the pipe.

So, start smaller, end bigger.

Let the arguments begin lol
yeah true i was thinking backward (pressure rather than flow) my bad but the sudden increase in size gives you alot of backpressure so try and avoid it either way and the header looks like it will be fine the way it is the wrap is gonna void your warranty and make sure you keep an eye on it for corrosion the wrap is known for causing it
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Old 05-13-2010, 01:28 AM   #10
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It is a pacesetter there is no real warranty. I had to modifiy it just to get it to fit. I had to remove the non hightemp paint first then paint it with good high temp paint. After that I took and wraped it and sealed it with the hightemp silicone that came with the kit. I'm hope it still doesn't rust.
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