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Old 04-26-2011, 03:15 PM   #11 (permalink)
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Getting a higher flowing exhaust should not hurt MPGs but it is not likely to help. If I were to do anything I would replace the CAT with a quality highflow unit (some flow well but after a year degrade and flow worse than stock) and replace the muffler only if it is rusting out and needs replacement.

Just for the record you can NEVER have to little back pressure you can only have a lack of exhaust scavenging.

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Old 04-26-2011, 03:23 PM   #12 (permalink)
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Quote:
Originally Posted by comptiger5000 View Post
I'd be surprised if the silencer makes any noticeable difference on vs off. Backpressure isn't the key (unless it's excessive), fast, linear flow is what matters.

There are calculations that can be done to find the optimal header size and length, optimal pipe size, etc. Autolounge.net | Tech | Exhaust
Cool. So my takeway from that was that adding backpressure (the silencer) won't help, I need the correct sized tubing. Probably better to take the silencer off.

Sound right?
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Old 04-26-2011, 03:24 PM   #13 (permalink)
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Any mod that allows more air and fuel to pass through an engine means you have to increase the restriction of same air flow to produce the lower levels of power that are required in most normal cruising operation.

Which engine would get better mileage, all other things being the same?

1. A 20 cubic engine running at 80% of max load without fuel enrichment.
2. A 1000 cubic engine running at 5% load.

Alfa Romeo grand prix engines circa 1950 produced 390 HP at 10k RPM, out of 90 cubic inches, but got 2 MPG when racing. Compare that to a restrictor plate NASCAR racer getting close to 6 MPG at Daytona at 180 MPH, out of a basically 60+year old engine design.

Low RPM, low manifold vacuum, no fuel enrichment is where you get the most power PER UNIT OF FUEL CONSUMED.

The key component is the per unit of fuel consumed, which will seldom be decreased by allowing more air in and more exhaust out in the same size engine.

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Old 04-26-2011, 04:32 PM   #14 (permalink)
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I would say a higher flowing exhaust will help unconditionally if you keep your foot out of it.

It has already been mentioned that reduced back pressure means a higher manifold vacuum to maintain the same power output. A higher manifold vacuum means that you will have a greater crank case vacuum via the PCV valve. This will reduce the amount of air being pushed around in the crank case and reduce drag on the engine. A higher manifold vacuum also means better fuel vaporization (but this results in negligible improvements to the percentage of it burned).

As to whether it will give a 3 to 5% mileage gain, I doubt it. When cruising at speed flow rates through the engine are low, and thus the back pressure is low. Improving the flow of the exhaust will lower back pressure a tad, but squat minus squat is still squat.

It should help but I doubt you will be able to measure it with any certainty.
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Old 04-26-2011, 06:49 PM   #15 (permalink)
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An engine is a complicated lever. Higher manifold vacuum means a shorter lever. When the volume of air inducted is maximum the "lever" is the longest it can be.

That is why best BSFC is achieved at lower RPM and low vacuum which equates to highest load without enrichment, or about 80% of max load.

It's really simple. if you compress a gas to 10 times its atmospheric pressure and ignite fuel in that gas, its pressure increases by the temp difference between induction and combustion.

The ratio is about 7 times the compression pressure, so if you have 200 pounds of compression you have 7 times that or 1400 pounds of combustion pressure.

If you reduce the compression pressure by restricting the intake air to 50% of atmospheric then you cut the 200 PSI to 100 and the combustion pressure to 700 PSI.

An engine "idles" because you have starved it for air until it can't run any faster than idle speed, at which point your efficiency is 0 since you are using fuel just to spin the engine over, which is the most extreme example of manifold vacuum. You would not have enough power to move your car unless you reduce the manifold vacuum by increasing the throttle position.

Most power per unit of fuel consumed 1300-2000 RPM with no enrichment and no manifold vacuum. You can achieve 0 manifold vacuum with very little throttle applied at low RPM as long as the load is high enough.

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Old 04-26-2011, 07:12 PM   #16 (permalink)
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This has been studied and formulated and there are calculators out there like this Autolounge.net | Calculators | Exhaust that tell you exactly what size exhaust pipe you need!
Back pressure is caused by a blockage and not by a properly sized exhaust pipe.
To take it to the extreme if you take your exhaust pipe off your car all together your engines power output will drop, an exhaust pipe is not a duct to the back of the vehicle that is made small to reduce cost, it is a part of the tuning of the engine, that is why the exhaust pipe calculators ask for the speed that you are running your engine at and the displacement, because if you have a huge V8 running at 8,000RPM it is fallowing the same rules as a 1.5 liter engine running at 2,000 rpm.
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Old 04-26-2011, 07:51 PM   #17 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
An engine "idles" because you have starved it for air until it can't run any faster than idle speed, at which point your efficiency is 0 since you are using fuel just to spin the engine over, which is the most extreme example of manifold vacuum. You would not have enough power to move your car unless you reduce the manifold vacuum by increasing the throttle position.

Most power per unit of fuel consumed 1300-2000 RPM with no enrichment and no manifold vacuum. You can achieve 0 manifold vacuum with very little throttle applied at low RPM as long as the load is high enough.

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In actuality your manifold vacuum is reduce quite a bit due to EGR.

Also any time you increase the air induced to the engine you increase fuel since mixture is maintained by the carb or injectors via the computer (assuming a gas engine of course). If you didn't your engine would run lean.

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Old 04-26-2011, 09:39 PM   #18 (permalink)
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Backpressure is not what boosts low end performance, rather its the effect of having higher exhaust velocity at lower voulme. Certainly introducing deliberate restriction into exhaust by introducing some kind of contstriction further down the exhaust will cause loss of efficiency at all speeds.

I think some exhaust mods do gain MPG, most exhausts are set up from the factory to resonate and muffle effectively at cruising speed, this assuredly does sacrifice some efficiency. This is partly why swapping mufflers around and removing the resonator can lead to a droning 'fart can' noise - even if you haven't picked noisy performance mufflers, the factory exhaust had that sound tuned out.

My car (same as a US domestic A32 body Nissan Maxima) has a restrictive U-bend in the y-pipe between both banks of cylinders and the inlet to the cat, this is designed to keep the car quiet and muffle the V6 booming sound accross wide range of speed. It keeps the exhaust cabin noise quiet even with a rather feeble rear silencer. Replacing this alone with a better collector on this car is known to be single-mod gain of 10-15hp and sometimes more (not necessarily mandrel bent nor equal length even!). I'm planning to do this mod, as it is so close to the engine and is bound to be reducing performance at all speeds. Again it's a resonance thing, not a exhaust flow thing. If it's silencing the car at a given speed then I suspect there may be gains to be had.

Not expecting much though, maybe 1-2% in typical driving? I'd see better if towing or climbing hills though - and indeed my daily 20km round trip commute includes 500ft of evelvation change mostly being in one steep fast run.

(Do they make multi-lane highways climb such steep hills at open road speed limit in other countries or are our highway engineers just a special kind of mad?)

Last edited by womprat; 04-26-2011 at 09:40 PM.. Reason: like mistake in grammar
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Old 04-26-2011, 09:52 PM   #19 (permalink)
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Quote:
Originally Posted by ConnClark View Post
In actuality your manifold vacuum is reduce quite a bit due to EGR.

Also any time you increase the air induced to the engine you increase fuel since mixture is maintained by the carb or injectors via the computer (assuming a gas engine of course). If you didn't your engine would run lean.

Regards
Conn
No egr at idle.

regards
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Old 04-26-2011, 10:10 PM   #20 (permalink)
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I am eating this stuff up. Great info guys. (Only problem, now I want to build a custom exhaust optimized for low RPM. :P )

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