Exhaust restriction equals more torque?

[140Air]

Yes, another exhaust back pressure question On another site a guy is testing Harley Davidson exhausts on a dyno. The engine is moderately tuned, not a race engine. Open pipes or minimal baffling produces a sag in the torque curve at lower RPM's. The guy doing the testing says there's not enough exhaust restriction....He adds more restrictive baffles and the torque dip disappears with a slight drop on top end..... I call it exhaust reversion, something my highly tuned 650 race Triumph has.......Perhaps Harleys have unique tuning problem...but... there are better ways to eliminate power curve dips than exhaust restriction?

Restricting the flow de-tunes the exhaust. Detuning does indeed fill in any dips and flattens peaks, so it can increase flow in some ranges, but ultimately it should diminish flow overall throughout the whole range. It sounds like the engine's top end is not at a high enough rpm to see a loss of flow.

[hoffman900]

Yes, another exhaust back pressure question On another site a guy is testing Harley Davidson exhausts on a dyno. The engine is moderately tuned, not a race engine. Open pipes or minimal baffling produces a sag in the torque curve at lower RPM's. The guy doing the testing says there's not enough exhaust restriction....He adds more restrictive baffles and the torque dip disappears with a slight drop on top end..... I call it exhaust reversion, something my highly tuned 650 race Triumph has.......Perhaps Harleys have unique tuning problem...but... there are better ways to eliminate power curve dips than exhaust restriction?

Restricting the flow de-tunes the exhaust. Detuning does indeed fill in any dips and flattens peaks, so it can increase flow in some ranges, but ultimately it should diminish flow overall throughout the whole range. It sounds like the engine's top end is not at a high enough rpm to see a loss of flow.

At that point, wouldn't it be worthwhile to try to go down a size in exhaust diameter?

[Cogburn]

I have a 86" shovelhead. I made a stepped open pipe baffle type thing and picked up a BUNCH of torque right at cruising speed. It now has 85 lb/ft at 3500 with no dip. It sounds like it has plain open drag pipes.

Before with the open drags it would not even accelerate through 60mph in high gear. It would blow gas out the carb onto my leg from the severe reversion.

The common fix is a 1/4" eye bolt in the last few inches of the pipe. The eye bolt can be turned to tune out the reversion. At certain speeds you can hear a whistle from the eye bolt.

[HDBD]

Cycle Shack whipped this many years ago (and supplied their pipes to HD at the time). By using a deflector in their baffle and a very open design exhaust pulses could reflect back and aid in scavenging yet still remain a very low restriction setup. In my opinion you need an adequate volume exhaust system to handle the displacement. Restriction is not needed or wanted.

[twl]

IMO, for a twin, the exhaust should be a 2-into-one system. Take advantage of that.
If you must use individual pipes, a simple formula is to put a substantial step larger(1/8"bigger or more) at the appropriate length for reinforcing peak hp rpm, and make the total length of the pipe the appropriate length to reinforce torque peak rpm.
This solves the typical wave problems encountered in single-pipe single-diameter fixed-length systems. Make the I.D. of the headpipe enough to properly handle the exhaust flow for the displacement and rpms involved.

[HDBD]

Agreed
Packaging and sound control become factors on street riden motorcycles

[CamKing]

Something to think about.
An exhaust restriction is only a restriction above a given RPM. Below that RPM, it's not a restriction, and could be more efficient.

[lorax]

Something to think about.
An exhaust restriction is only a restriction above a given RPM. Below that RPM, it's not a restriction, and could be more efficient.

I've seen the word restriction used when the engine pickuped power when the so called 'restriction' really reduced or halted over scavenging in a given range.

[kirkwoodken]

[quote="Cogburn"]I have a 86" shovelhead. I made a stepped open pipe baffle type thing and picked up a BUNCH of torque right at cruising speed. It now has 85 lb/ft at 3500 with no dip. It sounds like it has plain open drag pipes.[/quote]

ANY gas engine should make one foot/pound of torque per cubic inch. For a given engine size and rpm range: A pipe that is too small in diameter will tune at some speed. A pipe that is too large will not tune at any speed. One of the greatest exhaust myths is: "Its gotta be big to breathe." Specifically, it needs to be the RIGHT size to breathe most efficiently in a given rpm range.

[Belgian1979]

I have a 86" shovelhead. I made a stepped open pipe baffle type thing and picked up a BUNCH of torque right at cruising speed. It now has 85 lb/ft at 3500 with no dip. It sounds like it has plain open drag pipes.

ANY gas engine should make one foot/pound of torque per cubic inch. For a given engine size and rpm range: A pipe that is too small in diameter will tune at some speed. A pipe that is too large will not tune at any speed. One of the greatest exhaust myths is: "Its gotta be big to breathe." Specifically, it needs to be the RIGHT size to breathe most efficiently in a given rpm range.

Then the question remains : what is the right size ?

[HDBD]

Until we know the specifics on the motor there is no way to tell what is ideal
Rpm max =?
Displacement =?
Camshaft duration =?
Compression =?
Head information
Most harleys are done at 6200 rpm unless it is a sportster

[Krooser]

Years ago I mandated mufflers on all the cars running at my dirt track. The high dollar (for the time) teams bought 'race mufflers' and bitched that they all lost HP and torque. The low buck guys bought $12.00 spiral tractor mufflers from the farm store and they all reported more torque off the corners… take this for what it's worth.

[DCal]

I think some "restricted" motors were by design. Think about the nail head Buick, small ports,1.500 exhaust valve, gobs of torque down low, all in by 2500rpm and just enough to pull your 5K lb Electra convertible across the intersection with the rear tire smoking.

[140Air]

I think some "restricted" motors were by design. Think about the nail head Buick, small ports,1.500 exhaust valve, gobs of torque down low, all in by 2500rpm and just enough to pull your 5K lb Electra convertible across the intersection with the rear tire smoking.

I think the gobs of torque was a result of being forced to tune the engine for torque since high end breathing suffered so much from the misguided port design. The Nail Head chamber was a pent roof with only two valves on one side where one was an intake and one an exhaust. The pent roof had a centralized plug and good combustion. The valves, being pent roof valves, were unavoidably small and the exhaust port, being leaned 45 degrees over the wrong way, was torturous with about 135 degrees turn angle. Look up the Buick LeSabre Dream Car (or the Buick XP-300) of the very early 1950s with the supercharged pushrod 4-valve head with 90 degrees included angle between the intake and exhaust valves. Then you'll know the why-in-the-world of the Nail Head.

[Barry Burch]

If I live to be 300 I guess I'll never understand how so called over scavenging
can hurt power at any rpm. Even with a restricted 2 barrel engine. Lets park the
piston at tdc overlap for a couple days. Pull a vacuum on the exhaust and flow a few barrels
of fuel/air mix out the exhaust. When the ex valve is finally closed and the piston starts down the bore
what happened before is irrelevant. It's true to much ex duration can hurt power both on opening
and closing but that has nothing to do with scavenging. I'm not alone. Jere Stall of stall headers had
his doubts if over scavenging even happened.