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The Parts Guy
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Discussion Starter #1
I'm just about ready to have the exhaust done on my 4.6L DOHC powered 90 XR7. The car has the 92-97 style fuel tank and Kooks headers (1 3/4" primaries and 3" collectors). For now the engine will remain a pretty stock 4.6L DOHC, but once all of the bugs are worked out with the car I will be installing a very strong SC'd 4.6L DOHC (if everything goes right I hope to make around 500 rwhp). I'm planning on running duals with high flow cats, into an X-pipe, to dual mufflers in the rear.

So here's the question:

What size pipe should I use? I'm leaning toward compression bent 3". Also, will dual 3" pipe fit past the fuel tank? I was thinking that it would if the pipes were stacked vertically as they pass under the fuel tank, as opposed to side by side. I'd like to do the exhaust once, so it needs to be able to handle the amount of flow that the second engine will need.

That's it, just looking for some confirmation that dual 3" will be the way to go.

Thanks,
Rod
 

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3" will be a bit too big for a stock motor.

Because your system will likely be bent using a pipe bender (not mandrel), the diameter in the bends will be narrowed a bit.

Factory is 1 7/8ths I think with a Y pipe? I just had a 2.5" dual exhaust system built last summer, and I think the car has actually lost a bit of torque because the system is a bit too big for my current needs.

2.25" should be plenty for stock or 2.5" max if you are planning on heads or something.
 

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Jedi Master
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a 2.5 set up is my recommendation if your going with true duals, with a x-pipe with the current set up

that 2.5 set up should hold up for a great while

true 3.0 dual will probably be tight fit so maybe when you SC it a 2.5 to 3.5 then a 2.5 should be cool

how far down the road do plan to sc it??
 

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The Parts Guy
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Discussion Starter #4
Kevin said:
3" will be a bit too big for a stock motor......2.25" should be plenty for stock or 2.5" max if you are planning on heads or something.
mn12xr7 said:
how far down the road do plan to sc it??
It won't be stock for too long, maybe a year. It isn't a daily driver either, I built this one as a toy that I can have fun with at the track and do well in car shows with. The reason why I'm leaning toward dual 3" is so that I won't have to replace the exhaust when I swap in the second engine. I still have a few weeks to think about what I will do.

Here are a couple pics of the car's current state. I should have it running within the next two weeks, but there is still a lot of work to be done after that.









I don't have a good picture of the underside at the moment, but it is painted flat black.

Thanks for the replies.

-Rod
 

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There is this race team that I read an article from. They say the max to put on for a dual exhaust should be 2.25" They have done testing on different sizes and determined this for both the DOHC and SOHC. I'll see if I can find the article so you can see what kind of power they are working with.
 

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If he is going with a compression bend exhaust he could use the 3 in. duals. We had 3 in. duals on a '95 camaro w/383 and only lost like 5 ft/lbs down low compared to the stock ls1 exhaust that went on it later (which is 3in. pipes(hooker y-pipe) that Y into a 2 3/4 and into dual 3 in. tips). He had about 375 ft/lbs of torque, so I think you will be okay. It will NOT be excessive for your setup. Depending on what gear you go with right now...it could cause a bit of low torque loss. I say this but I just plan on having the 2.5" duals for about the same power.

Russell
 

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What gear do you plan on running?
 

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The Parts Guy
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Discussion Starter #8
Wow, this is an old thread.

To tell the truth, I've been sidetracked with other cars lately, and haven't gotten around to putting exhaust on this car. I'll probably stick with something around dual 2.75" pipe. The car has the FRPP 3.73 setup in it at the moment, but I'll probably step it up to 4.10's once I start driving it.

As far as torque losses go, I'm running the following setup on my 3.8L splitport 94LX:

Ported manifolds
dual 2.25" Catco high flow cats
18" long Magnaflow mid mount muffler
single 3" to dual 2.75" turn downs

Between installing that exhaust (over the stock single exhaust), and an ASP crank UD pulley, and stepping up the timing with 93 octane (over the 87 octane setting) the car gained 20 rwhp and 10 rwtq all the way across the rpm range (2500 - 5200 rpm).

-Rod

EDIT: Here's the dyno sheet. Remember, this is from my 94 LX with the splitport 3.8L N/A, not the car pictured above.


 

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When you say timing do you mean spark timing or cam timing?
 

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Ha..I just noticed how old this thread is....I just posted on it because that other guy posted the biggest to go with should be 2.25 duals. I was about to post that I would stick with no bigger than 2.75" duals, but then I decided to see what gear you were going with. You'll be fine with 4.10s and the big exhaust. That's crazy that the 3.8 gained that much.lol

Russell
 

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The Parts Guy
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Discussion Starter #11
funnyfingers said:
When you say timing do you mean spark timing or cam timing?
Spark timing.

-Rod
 

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The Parts Guy
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Discussion Starter #12
96bird said:
That's crazy that the 3.8 gained that much.lol

Russell
What can I say, she really likes to breathe. :D

-Rod
 

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All that tells you is to go with properly sized pipe for your horsepower level. :D
 

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So here's the question:

What size pipe should I use? I'm leaning toward compression bent........
Correct me if Im wrong but wasn't that the original question of this thread?



http://www.sandersonheaders.com/completeperftech2.htm

One of the most critical decisions to make when you're building a performance exhaust system is pipe size. Bigger is not always better (see tube size information on the "Lets Get Technical" page). It's very important that the pipe diameter be sized to the application and that it remains the same from the collector to the end of the tailpipe. Changing the diameter up or down within the system will "confuse" the exhaust by creating turbulence and excessive backpressure. These sizing guidelines are reprinted by courtesy of Flowmaster, one of the most respected exhaust technology companies in the industry.
 

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You didn't answer the question though...you just posted a link that tells that you need to size your exhaust right for the volume. He should be fine with 2 3/4 duals. There are horsepower charts on the internet.
 

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Fusion0507 said:
It's very important that the pipe diameter be sized to the application and that it remains the same from the collector to the end of the tailpipe. Changing the diameter up or down within the system will "confuse" the exhaust by creating turbulence and excessive backpressure. These sizing guidelines are reprinted by courtesy of Flowmaster, one of the most respected exhaust technology companies in the industry.
So I guess that means anyone with 2-1-2 exhaust are doing it wrong then.....:rolleyes:
 

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Ok, I don't see what the confusion is here. Your collector size is dependant on the size of the primaries of your headers. Pick your headers for your applicatation, and fit the rest of the exhaust the same diameter pipe of your collectors.
It read pretty straight forward to me.

You didn't answer the question though...you just posted a link
Exactly. From the links I'm sure he can make a decision regarding whats best for his application/rpm range.


Also, I saw nothing that mentioned crossover pipe size on the site I linked to.
 

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I think that chart is easily misinterpretted. 2.5 may be best for peak horsepower over 2.25 which would be great in a circle track but not in drag racing. I guess you would also have to consider that also.
If you have 300 ft lbs torque all the way accross your rpm range the horsepower at 3200 rpms will only be 180 horsepower.
Honda has a special exhaust on a motorcycle that takes advantage of this and switches exhaust paths for best horsepower at a given rpm.
That is why when I read the article from that racing team I can easily believe that 2.25 inch exhaust will perform the best on a drag strip and at red lights. However, yes at 6,000 rpms 2.5" will give better results.
 

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The Parts Guy
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Discussion Starter #20
Easy guys. I asked the question months ago, and I've already made my decision.

Just let this thread die out, it was on it's way there until tonight.

-Rod
 
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