How to test on your own using a 'highway dyno'

http://www.bgsoflex.com/dyno.html

make runs both with and against the wind

 

CAI - What I was asking is if there was a decent heat barrier sealing off the filter from the rest of the engine compartment. Too many short ram intakes are sold as "CAIs" when really they're just grabbing whatever air happens to flow the easiest.

Dual exhaust - I'm not sure I completely agree with Flowmaster's statement based on the fact that they're trying to sell stuff, and they want to sell what the public wants, and the public wants shiny chrome tips. In theory anyway the whole exhaust system should be tuned for the best compromise between the low-end velocity and the high-rpm flow capacity. In practice though, they're probably right - there are so many other factors limiting power that the difference probably isn't significant.

Pipe diameter - We're talking dual exhaust here, your truck came with a single pipe. A single 2" pipe wouldn't be nearly big enough - in fact I'd recommend a single 3" for medium sized v8's with moderate power levels. Yeah, I think the OEM diameter is too small. 2.5" pipe has a cross section of what, 4.9 square inches. 3" pipe is about 7 square inches, dual 2" is almost 6.3 square inches, and dual 2.25" is almost 8 square inches, and of course dual 2.5" would be just shy of 10 square inches. The name of the game here is compromise - we're not driving top fuel dragsters, we actually want low-rpm driveability, which is why we put longer pipes on. The pipe diameter plays a role in how well the exhaust gasses are scavenged from the cylinders - too big on the pipe and the gasses slow down and cool too fast, causing poor scavenging which means less fresh air/fuel is able to enter the cylinder, and without the fresh stuff you're going to make less power. On the other hand, if the pipes are too small then they won't allow enough gasses to escape (backpressure) and the engine will have to "push" the fumes out the pipe causing valuable horsepower to be used in pushing exhaust instead of turning the tires. If you want to get technical here we could start discussing the extra surface area that dual exhaust has over single exhaust with similar flow capacity - more surface area means more drag.

 

i hear ya Chris. Exhaust diameter is part of it. For the most part stock exhaust has alot of bends, mostly crush-bent. Any decent exhaust shop will use a tube bender making flow that much better. given that, and a larger tube and more free flowing exhaust, sound like you are flowing better on the exhaust side. Remember that the exhaust system has a scavenging effect in order to draw exhaust out of the engine. Headers will only increase flow on the exhaust side further having a negative effect. I would open up the intake side and increase head flow. also you will want to upgrade spark as well. then go headers.

 

So horatio, do you think they went too big on the exhaust??? They took the stock 2.5", sized it up to 3" to get the muffler on, and then 2/75" on the way out. Dual of course...

Ever since they did the exhaust work on it, just runs kinda funky on the low end. Of course the CAI just made it worse. Yeah, it has a heat shield, and like i said i moved it up to the front of the truck where i could get some ambient temps.

 

Could death flash be the problem. that will limit the potential of upgrades, eh??

 

Cool on the CAI. [8D]

Dual 2 3/4" is pretty darned big. I'm a huge Subaru nut too, and with a N/A 2.5 liter the biggest you want to go is 2.25", stepping up to 2.5" starts to hurt low-rpm power. With a 5 liter v-8, the same can be said for each bank. A single 2.25" for each bank is about as much as you'd want to go unless you're extremely high compression or forced induction/nitrous - and in that case you need the added size to move enough air.

I don't know what the exhaust manifolds and Y pipe look like on a 4.7 with 2wd, but the Y pipe on my 5.9 4x4 is WAY too small and crimped going under the front shaft. Headers won't hurt performance unless you're running inappropriately sized pipes - 1 5/8" primaries and a 2.5" collector is about right for a N/A medium sized v8.

 

i think im just going to have the exhaust redone. I think i agree with you. Its just too big

 

Even the hemi isnt largely restricted by its stock air box. Im sure i will take alot of flack for this statement but an intake is sort of a waste of money. I wasted my money on one, and I wont ever do it again. Websites like BITOG love to prove just how these things are a waste of money unless you got a turbo or supercharger that actually truely leads to the airbox making a restriction.

Alot of people always say start with an intake and an exhaust system. I'd say unless your just looking for the sound, on one of our v6/v8 NAs, dont bother.

 

As regards the myth about exhaust pipes being too big:
----
as info to your question about backpressure always being a horsepower robber -
here is a quote from page 139 of the Austrailan book "21st Century
Performance"

begin quote

"Few tests have been done that clearly show the effect of changing back
pressure. Most muffler and exhaust comparison tests change more than one
parameter simultaneously, making the identification of exhaust back pressure
as a culprit difficult. However, Wollongong (Austraila) mechanic Kevin Davis
has done extensive testing of varying back-pressure on a number of performance
engines. These range from turbocharged Subaru Liberty (Legacy) RS flat fours
to full-house traditional pushrod V8s. In not one case has he found any
improvement in any engine performance parameter with increased exhaust back
pressure.

The tests came about because Kevin has developed a patented variable-flow
exhaust that uses a butterfly within the exhaust pipe. He initially expected
to use the system to cause some back pressure at low loads 'to help torque.'
However, he soon changed his mind when any increase in back pressure proved to
decrease torque on a properly tuned engine. What increasing the back pressure
does do is dramatically quieten the exhaust.

One of the engine dyno tests carried out by Kevin was on a modified 351 4V
Cleveland V8. Following the extractors he fitted a huge exhaust that gave a
measured zero back pressure. Torque peaked at 573Nm (423 ft-lbs) at 4700 rpm,
with power a rousing 329 kW (441 hp) at 6300 rpm. He then dialed-in 1.5 psi
(10.4 kpa) back pressure. As you'll see later, very few exhausts are capable
of delivering such a low back pressure on a road car. Even with this small
amount of back pressure, peak torque dropped by 4 per cent and peak power by 5
per cent. He then cahnged the exhaust to give 2.5 psi back pressure. Torque
and power decreased again, both dropping by 7 per cent over having zero back
pressure.

Figure 6.1 {which is a graph with three lines showing HP against rpm} shows
the power curves gained in the tests. These results were achieved on a large
engine with a large overlap cam - one of the type some people suggest is
'supposed' to like back pressure.

If, in fact, power does increase with increased exhaust back pressure, it is
most likely the air/fuel ratio and/or ignition timing that are no longer
optimal for the altered state of engine tune."

end quote
-----------
Want to prove this to yourself?
It is easy to increase the backpressure of an exhaust,
simply by using a vice grip to clamp sockets inside the end of the tailpipe.

Get yourself a stopwatch, a vice grip, and a set of half inch or three quarter drive sockets and convince yourself. This will also work at chassis dynos.

 

Thanks everone for your input. I really appreciate it. But to be honest, now i am more confused about what could be robbing me of my low-end torque. If it is not exhaust, or intake, then it's back to square one for me.... [:@]