I've asked this question before, but can't find the thread to bump it back to the top: After installing the Flowmaster dual 2 1/2" in 3" out "Y" pipe, I experienced a loss of power going up hill, (hills are hard to find here). I want to reduce the pipe size after the muffler, (currently 2 1/2"). My question is, to lower the torque curve, do I reduce the pipes to 2 1/4", or 2"? Hopefully, I can get a pro exhaust person to answer, or someone who has experienced the same situation.

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Not an answer to your problem, but you have helped confirm something I've asked before and had someone else say the same thing too........

I still have my FM "Y" in the box. I've heard that you can lose some low end. My muffler shop guy said the same thing too. He actually refused to install it because he said he didn't want to be responsible for making my truck drive like crap.

I have a Gibson SS with a 3" tail pipe. While I feel I actaully GAINED some low end from that mod alone, if you lost low end with the FM Y and 2.5" pipe, then I'd probably lose a ton!!

It still sounds so "backwards", that you would have to restrict the exhaust so much to maintain low end torque. That must be why Ford made their Y pipe with the jagged 1.5" hole from the drivers side?? Really kinda defeats the whole purpose of adding a nice flowing Y pipe (or exhaust system) then have to just restrict it farther down the system.......

I'm guessing that since all my restriction is at the stock Y, that's helping with the scavanging effect that is supposed to be occuring to help with the low end. Then my Gibson is just letting what does get past the Y get out easier since it's already done it's job of scavanging?

I don't know if you've seen the post about the guy that did an X pipe mod and had really lost power. He had it dynoed and everything. He had to add an exhaust trap setup to his tips. Basically, he had to add a plate inside the tip to make the hole like 1.75" or something like that. He got the power back.

 

Geez, thanks for not answering my question.

 

Oh, you had a question?? That's right..... Well, it seems you have to "plug up" the system at some point. Smaller dia. pipe would probably accomplish that. The trucks that go with the SI/DO mufflers seem to go for 2.25" tail pipe.

 

What I'm looking for here, is a definitive answer, not idle speculation by someone who hasn't done the "Y" pipe mod.

 

okay, stupid point time. and i think i learned this from another post on this site, but due to my reckless youth, and current beer comsumption rate, i can be way out of line here.

say you have two 2 1/2 pipes flowing into the one 3 inch out.
at that point your combined diamater to vent the fumes is limited down to 3 inches. two combined pipe totals after that would be

5 inches for the dual 2 1/2 pipes, or 4 1/2 inches for the dual 2 1/4 dual pipes.

at this point those last diameters are both bigger than the 3 inch tube prior so you would get no additional restriction.

make sense?

(i think my head hurts)

i am curious about this, would love to know the answer.

 

billycouldride

It's about cross sectional area

area = Pi*([diameter/2]^2)

Simple example:

You have two 2.5" pipes going in and a 3" coming out.
Total area in is 2*Pi*([2.5/2]^2)= 3.125*Pi
Total area out is Pi*([3/2]^2) = 2.25*Pi

Ipso Facto, the afore mentioned Y-pipe is acting as a restriction.

 

edit

 

And the question remains without a definitive answer.

 

Ooops, thanks Max, i clicked onto that link and now see what ya mean.

 

hey max, what link?

corpsie,

thanks for the science

 

SOTP results and talkin on this site don't mean a thing. Only a dyno will tell you for sure. Changing out 1 Y for another of the same in's and out's should be no big deal. I didn't loose any low end with my true dual conversion with X pipe. That's the way to go, not fake duals. You want your question answered, do a dyno before, change it out, do one after. They you will know for sure instead of taking someone elses word for it.

 

bump!

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BE THERE!!!!!

 

Lookeehere

 

the flow of the gases at high velocity through a small pipe create a suction that pulls the burned gases out of the cylinder

before a cylinder intakes air, the cylinder that fired before it just cleaned out the its old gases with the suction and sent them down the pipe and the cycle continues for the next cylinder.

the more resistance, the more pressure is built up in the system, and more pressure in the cylinders and the less oxygen that can go into the cylinder.

resistance is not needed for torque
suction by the velocity of gases is needed to clean the cylinders
bigger pipe can flow more and create less resistance, but it can not create the velocity to pull out gases

what is perfect is no restrictions and high suction to clean the cylinder.