Can one have too big of an exhaust on a turbo vehicle? Specifically the Ecoboost F-150. I'm planning on installing a 4" system, (modifying the y pipe), but thought I'd ask first. The headpipes from the turbos/cats are 2 1/2", so the rest of the system needs to be at least 3 1/2" single pipe to equal that. Seems to me one would want the least restriction possible downstream of the turbos/cats. Please correct me if I'm mistaken, I have no previous experience with turbos.<!-- google_ad_section_end -->

 

I wouldn't go too big too fast. Larger pipe means lower velocity. For a NA engine scavenging is important and if you go too big you don't scavenge well.

There's a practical limit to larger exhaust and I think for a 3.5 engine at normal (not 8000 rpm race) operating speed 4" might be beyond it.

 

It's not n/a, it's twin turbo, that's why I asked. Exhaust already taking pressure drops through the turbo & cats, seems low restriction would be better, but what do I know?

 

On "Gearz" TV show on Speed channel they put the Heartthrob 4" exhaust on. It increased 38hp and said it would increase gas mileage as well. I am more interested in the mileage increase myself as the truck has plenty of power. They also remapped the computer and added the cold air intake saying it really improved the truck.

 

What about switching the truck to a real dual exhaust? Too expensive or not practical? Impossible?

 

That's a good idea, I looked at that but the fuel tank blocks the left side, not enough room between it & the driveshaft. I don't really want to run two down the right & then switch it over. Right now I've replaced the 2 1/2" section with 3" tubing, have a 3" Magnaflow coming to replace the stock muffler. Then it will be 2 1/2" from each bank to the 3" y connector, then 3" the rest of the way. Btw, a 3" system has more area than a dual 2" system.

 

I replaced the 2 1/2" resonator section with 3" pipe, then replaced the stock muffler with a Magnaflow 12641, 30" body length, 6" body diameter, 3" pipe size. Kept the 3" tailpipe. Here's a short video, it's pretty quiet.
http://www.youtube.com/watch?v=UvHoxc0Ow

 

Link no worky

 

I just clicked on it in your reply & it worked.

 

I heard above someone stated that when the diameter is too big velocity slows. That is so true. People always think that you want big pipe after a Turbo to reduce backpressure, the exhaust already spiked in pressure going through the turbo. Well that whole theory makes sense when you just dump the exhaust to the atmosphere. But when you run long tubing after the turbo, you have to consider all the same things that you consider on a non turbo exhaust, to a certain extent.

If you size the exhaust too large you create a huge expansion chamber that fills with slow moving stagnant exhaust gases. Now your engine has to pump out this exhaust gas. If sized properly the flow will literally 'siphon' itself out at your engine's peak volumetric efficiency. Ford engineers chose 3" because they knew that this 'siphon' effect would happen right at peak volumetric efficiency, even with the restrictive muffler. Knowing this, we just need to help Ford where their hands were tied. They had to use that awful muffler to make the truck quiet enough to sell to the public and pass federal drive by standards.

Here's a thread that I wrote which goes into a little more detail:

How a properly built exhaust system works

As you can see, the right kind of muffler, properly sized will make the best sound, without losing power and maybe even gain some.

Keith

 

A friend had a eco boost truck and he got an MBRP system put on, he said it feels more powerful now and he gets 2 litres per 100 better than he did before!
I did the quick math and would save the money for the new system (I will guess cat back) in about 1 year approximately with the gas savings.
Of course I just read that Ford doesn't suggest changing to a free flow to avoid spooling??
Comments?

 

To give you an idea I have a modded 7.3L Power Stroke. I added a 4" exhaust and lost a bunch of low end power. Better on top end where I seldom go. I plan on downsizing the pipe as the 4" is too large.

4" on a EcoBoost should not even be considered. On my Eco I removed the silencer because of the shorter wheelbase. I plan on keeping it stock other than that.