Better MPG thru restriction.

[Anonymous]

You can improve your MPG if you're not a hot-rodder by increasing (slightly) the exhaust backpressure. This is especially true if you've already installed a hi-perf cam with lots of overlap.

Here's why, based on Ford's FE engines (big blocks):
The exhaust manifold can be tuned to cause some backpressure during certain RPM ranges. This backpressure reduces the "feed-thru" of unburned gases that happen during overlap. If a very low-restriction exhaust is used, the lack of backpressure improves the breathing at higher RPM, like over 3000 RPM, but some of the gases slip thru easier at low RPM. (Witness the newest hi-perf computer-controlled engines with "X" exhausts: they use gates to block a whole exhaust pipe at low speeds just for this reason, forcing both sides of the V-type engine to use one pipe below 3000 RPM. It opens above that for better HP.)

Ford used the "log" manifold on these I-6 (and FE engines) to improve backpressure and lower production costs. On the FE, the pulse timing of the exhaust is such that the exhaust manifold is "restricted" at low RPM by high pressures from the last 2 cylinders on each bank. At higher RPM, the overlap causes inertia in the gases and the pulses smooth out, becoming a hot flow. This is one reason why the old 390 delivered surprisingly good MPG at around-town speeds, while the later 400/429/460 did not: there is always a slight "flat spot" in the log-manifolded FE engines, right where the transition is happening from pulsed flow to hot flow. On mine, it's at about 1500 RPM. On my 3.3L I-6, it's at about 2200 RPM.

On the I-6, low backpressure for HP is helpful above 3300 RPM, for sure. But, low backpressure at 2000 RPM allows about 3 degrees of overlap to go to waste. Adding a slight restriction (like a standard muffler instead of a straight-thru type) will improve both the torque and MPG available in the range of 1500-2000 RPM, assuming a standard cam. This is because the 3 degrees of overlap will have to push against the higher backpressure, and it won't flow so well.

It's likely also true with long-overlap cams, but I don't have one in my cars for testing. As an example, though: in my motorhome (a log-manifolded Dodge 360) I changed the straight-thru 3" muffler to a conventional 2-chamber side-by-side oval muffler. My steep-mountain climbing speed improved from 38 MPH to 44 MPH in high gear and the MPG on the highway went from 9.4 to 10.1, steady. My hill-climb MPG went from 7.8 to 8.8 MPG, consistently.

So, if you're in the market for a muffler and better MPG, look into a nice oval-shaped Walker or similar type.

 

[LaGrasta]

oval-shaped Walker

That's what I'm running. I haven't monitored my gas for awhile though. I'll have to check it out.

 

[Anonymous]

Im not doupting what you say but when I put on a straight through muffler in place of the stock standard one on my 3.5 litre Mits Magna my economy went up from a consistent 470 km a tank to 550 km a tank. The exhaust pipe and the stock manifold on these flow fairly well standard so replacing the old muffler was like pulling out a cork. The stock mufflers are apparently very restrictive on these.

 

[Anonymous]

Very interesting information...
I kinda agree with it, but backpressure is always a waste of energy. The fact that such loss can be recovered by reducing the waste of unburned gases at low rpms, can be better corrected in my opinion by recurving the distributor (basically advancing the timing) and getting the appropiate cam. In my personal experience, I was able to almost double the output of a pushrod inline 4, while mantaining gas mileage, basically by removing restrictions and backpressure, compensated with the right ignition timing and increase in compression, while using a cam with substantially more lift than stock, and a very conservative overlap.
Backpressure might help you with the MPG at low rpm ranges, but it will kill it once you step on the throttle, even mildy. The 221 SP got about the same MPG rating on the highway than the stock 221, with a substantially higher power output, thanks, in my opinion, to this concept.

 

[mustang6]

I'm with Southern Cross Racer on this one. I've heard of situations where too little backpressure caused over-scavanging problems when the cam and timing were optimized for an exhaust with higher backpressure. That said, my experience on the FE engines has always been that they respond to tri-Y headers and dual exhaust with increases in HP, torque, and mileage. When you get all 3, I think you are improving efficiency.

 

[Anonymous]

Certainly on the 4-bbl FE engines, the improved exhaust also improved the MPG, but in a subtle way: at normal driving conditions, the secondaries stayed closed a little longer because the engine was giving about 200 RPM more at that same throttle setting before the secondaries opened. But, under heavier loads, like pulling mountain grades, the MPG went down with the lowered backpressure. The only way to improve MPG while opening the exhaust under these "steep" conditions is to install a 2-bbl carb. That's happened a lot around here, before the days of smog police.

I'm not referring to WOT driving, just around-town, soccer-mom and go-to-work stuff, 45MPH and under. Increased backpressure WILL reduce top end, but if you never use it anyway, like my wagon, it's a good tradeoff.

I also agree with Southern Cross about the changes he's discussing, and they would certainly be more desireable: problem is, here we are not allowed to do that kind of changing. A single modification, be it the exhaust, the cam, the carb or a recurved distributor, will cause failure at the local smog police shops as a "modified engine" - heck, even removing the front driveshaft from a 4-wheel drive Jeep causes failure as a "modified vehicle" here. It's very strict. My 360 Dodge is just now beyond the time limit and weight size limit, so I can mod it a little and get away with it. But, the Fairmont is still under "their" thumb, and even exhaust changes must be done by "an approved shop", whatever that is... When I go in for the test, I have to either reinstall the old distributor or retard the timing some 5 degrees to pass. Strangely, they will let you retard timing and pollute more air by volume or lean out the engine until the valves cook, yet arriving with a K&N monster filter or the like will trigger an instant FAIL when they open the hood.....morons. The modified distributor, with less total advance and more initial, yields LOWER HC PPM numbers than the original one, with better MPG, but it's not allowed.

 

[mustang6]

You reminded me of a funny experience I had years ago getting a 1977 F150 with an FE to pass emissions (they aren't so picky about visual stuff in my state, but you still have to pass the sniffer). I got this truck without an engine, and the reason I bought it was because I had a friend who had a newly rebuilt 1962 352 that was bored .080 over and ran too hot for the radiator in his car to cool. I got the engine for $200, the truck for $500, put them together and had a sweet runner, that big old truck radiator kept the engine cool enough.

Since the truck was a 77 it was subject to emissions. The engine had a new Holley 500 CFM, stock exhaust manifolds, and new dual exhaust. It failed miserably on HC the first try, running way too rich and ignition too far advanced. So I picked up a stock 77 2-bbl Ford carb and put that on and backed off the advance for a 2nd try- this time it ran lean enough to pass on the HC but the CO was now about double the acceptable levels. I talked to the emissions guys about what could be done but they were convinced there was no way a 1962 engine could pass 1977 emissions no matter how fresh it was.

Then I got an idea. The way they make sure the test is valid is by measuring CO + CO2 as a % of exhaust. It has to be at least 6%, and my reading was 20%. Since I needed the CO to go down without raising the HC, I asked the guys what would cause CO + CO2 to be too low to qualify the test. They said a leaky exhaust sytem sometimes disqualifies people because not enough exhaust gets to the end of the pipe to be sniffed. I asked them if there were any rules about leaky exhaust, they said not really, as long as the CO + CO2 was above 6%.

So I pulled out into their parking lot and loosened my exhaust pipes at the exhaust manifold flanges- made about a 1/4 inch gap on both sides- exhaust still came out the pipes but with not nearly as much pressure. Then I went back around and got back into line for my 3rd try. Everyone else in the other lines were looking at me- my engine was running crappy from being so lean and retarded, and my exhaust sounded almost like it was open, it was so loud.

The guys in the emissions station were shaking their heads as I pulled in and they hooked everything up. But when the test was run, my CO was now under the acceptable limit and my CO + CO2 had only dropped to 9%- still high enough to be valid! I passed the test!

After I received the passing paperwork I pulled back out in their parking lot and tightened my exhaust back up, advanced the timing, and drove away.

 

[xctasy]

Awesome :rolflmao:

I'll give you a Spot prize for working within the letter of the law!

Sniff Dawg Eat Dawg Award Goes to you, Mustang 6!

 

[Anonymous]

Now, THAT's thinkin' with your wrench!
Bravo!

 

[American Thunder]

If increasing backpressure helps efficiency in any way, it's a bandaid, not a cure.

Edit: Oops, I just realized this is an ancient thread. I guess there's no delete post feature here?

 

[james singleton]

I do not know about the MPG angle, but this morning I was reading this article in the Tech section of a recent magenzine from Sanderson Headers, that I thought might be of interest to some. Granted the article seems to be talking about V-8's, but I believe the concept would be the same for the inline sixes; here is part of the aricle:

"LET'S LOOK AT HOW HEADERS WORK, AND CLEAR UP A COUPLE OF COMMON MISCONCEPTIONS. FIRST IS THE SIZE OF THE PRIMARY TUBES. IT'S EASY TO ASSUME THE BIGGER THE TUBE THE BETTER, BUT THAT'S NOT THE CASE.
THE FACT IS, PRIMARIES THAT ARE TOO LARGE ACTUALLY COST YOU TORQUE AND HORSEPOWER BY SLOWING DOWN THE RATE AT WHICH THE EXHAUST TRAVELS THROUGH THE SYSTEM. THINK OF YOUR ENGINE AS AN AIR PUMP. EVERYTIME THE EXHAUST LEAVES THE COMBUSTION CHAMBER, IT'S BEING FORCED INTO THE PRIMARY TUBE FOR THAT CYLINDER.
SMALLER DIAMETER PIPES FLOW LESS VOLUME THAN LARGE ONES, BUT THE EXHAUST IN THE SMALLER PIPE FLOWS FASTER. UNTIL YOU REACH THE RPM WHERE THE SHEER VOLUME OF EXHAUST GASES REQUIRE BIGGER PRIMARIES, SMALLER TUBES SCAVENGE FAR MORE EFFIECIENTLY. IF YOU'RE USING THE ENGINE IN THE 1,500 TO 3,500 RPM RANGE, WHICH IS TYPICAL FOR A STREET DRIVEN CAR, YOU DEFINITELY WANT 1 1/2" TO 1 5/8" PRIMARY TUBES FOR A SMALL BLOCK AND 1 3/4" TO 1 7/8" FOR A BIG BLOCK. ANY BIGER AND YOU'LL LOSE A LOT OF LOW END TORQUE. BEYOND 3,500 RPM IT'S A QUESTION OF WHERE YOU WANT THE POWER PEAKS. AS YOU CAN SEE FROM THE CHARTS BELOW, SMALL TUBES DON'T LOSE THEIR EDGE IN HORSEPOWER OR TORQUE UNTIL YOU GET ABOVE 5,500 RPM.

So it sounds like to me like bigger is not always better, and that it depends on what you are after! Jim

 

[fordconvert]

Here ya go just what you need for 100% back pressure, 0 emissions, could be yours for only $99.95.

You've heard of the Y pipe.. the X pipe.. the H pipe.. nothing can touch the O pipe!!! It provides 100% back pressure feedback on ALL engines!!...

http://kalecoauto.com/index.php?main_pa ... a4cfcbed47



On the serious side, back pressure is part of the 'system'. Changing only the back pressure is going to change the way the whole system/ engine operates. Weather or not its a good or bad change depends on many factors and your goals. Can you make other changes to benefit from lower back pressure? Yes. Are they easy changes? Sometimes. Practical? Sometimes.

 

[American Thunder]

haha! You might be able to market that thing to the ricer crowd. (As long as it comes with a big fancy sticker.)

 

[American Thunder]

I do not know about the MPG angle, but this morning I was reading this article in the Tech section of a recent magenzine from Sanderson Headers, that I thought might be of interest to some. Granted the article seems to be talking about V-8's, but I believe the concept would be the same for the inline sixes; here is part of the aricle:

Only the primary tubes of the headers matter to proper scavenging. It's critical to match the size and length of the primaries to the engine's needs.
Once beyond the collector, however, the less backpressure present, the better. Always.
If performance drops as a result of the reduction of backpressure after the collectors, then you need to retune the fuel/ignition curves to better take advantage of the reduction in exhaust pressure. (The engine will tend to run leaner with better exhaust flow.) In certain dyno tests I've seen, flowmaster mufflers actually used sound wave technology to increase the exhaust flow to the point where they created a negative backpressure in the pipes! In those cases, the flowmasters were making more hp and torque than open headers. Other than for those cases, open headers always make more power than closed exhaust, assuming the engine is tuned properly for the exhaust.

 

[wallaka]

In those cases, the flowmasters were making more hp and torque than open headers. Other than for those cases, open headers always make more power than closed exhaust, assuming the engine is tuned properly for the exhaust.

That just meant that the collector was too short, and the mufflers were acting like a collector extension.

 

[Bort62]

In those cases, the flowmasters were making more hp and torque than open headers. Other than for those cases, open headers always make more power than closed exhaust, assuming the engine is tuned properly for the exhaust.

That just meant that the collector was too short, and the mufflers were acting like a collector extension.

Not necessarily. A properly tuned muffler could create a standing pressure wave which could increase scavenging.

 

[wallaka]

True enough.

 

[grocery getter]

Here ya go just what you need for 100% back pressure, 0 emissions, could be yours for only $99.95.

You've heard of the Y pipe.. the X pipe.. the H pipe.. nothing can touch the O pipe!!! It provides 100% back pressure feedback on ALL engines!!...

http://kalecoauto.com/index.php?main_pa ... a4cfcbed47



On the serious side, back pressure is part of the 'system'. Changing only the back pressure is going to change the way the whole system/ engine operates. Weather or not its a good or bad change depends on many factors and your goals. Can you make other changes to benefit from lower back pressure? Yes. Are they easy changes? Sometimes. Practical? Sometimes.

that is very true. put a header on a motorcycle, if you don't re-jet the carb, performance will be lower than with the stock pipes.