Back Pressure Tuning in the VW Type 1
Posted: Sun Oct 29, 2017 11:43 pm
Looking down the business end of a stinger on RetroRacing's Blitzwagen
The Perfectionist's Guide To Back Pressure Tuning in the VW Type 1
Exhaust back pressure is widely misunderstood. Is too much or too little a good or bad thing? Is it really that important? Yes. If your back pressure is wrong, there is power to be gained here, so let's get started.
Trick question. Are short straight pipes on a V8 a good example of zero back pressure?
Trick answer. No. The escaping gasses are met with atmospheric pressure. We measure this as 14.7 psi at sea level. This might not sound like much, but firearms designers have long since learned that a bullet fired in a vented barrel has more muzzle velocity than a bullet fired into a rifled barrel filled with sea-level air.
The purpose of an extractor exhaust is to literally extract (pull) exhaust gasses out of the combustion chamber. This works by having all the exhaust pipes merge into a "collector" where the very first pulse of exhaust gasses passing though the collector creates low pressure in the other three exhaust pipes because they are all combined.
The fact a pressure flow across a hole creates a low-pressure suction at that hole is known as Bernoulli's law.
The next pulse drops the pressure even more, and in an instant, as our 4-3-2-1 firing continues, the exiting gasses have greatly reduced atmospheric pressure in the system. Not a true vacuum, but close.
What happens now is when the exhaust valve opens, the exhaust gasses are met with very low pressure in the exhaust pipe and allowed to rush to the collector faster than if there had been more atmospheric back pressure present.
So far so good. Now comes the complications.
When the exhaust valve opens and the exhaust gasses fill the exhaust pipe, the gasses are still moving toward full escape out the tail pipe as the exhaust valve closes. At the instant of closing, the outward bound pressure reverses.
Yes. Reverses. The pressurized exhaust gasses try to run backwards to the sudden super-low pressure area leading to the exhaust valve. This is termed reversion. It happens both in the exhaust and in the intake manifolds. Intake reversion is a similar but different subject.
At this point, the exhaust pipes begin to look like a busy place. Powerful forces, both physical gasses and sonic waves, are beating against each other. An exhaust pipe is not a one-way street.
As the gasses race back towards the closed exhaust valve, the valve opens and a new, more powerful pulse collides with it. The new pulses overpower the old pulses and hammer the exhaust out the tailpipe. Labs study reversion and how to minimize it with exhaust pipe diameter and length.
Racing cams are known for uneven idling, and lack of low RPM torque.
Every cam has a design feature called "overlap" which is a moment when both the intake and exhaust valves are open at the same time. Racing cams have more overlap which is why they idle poorly. But for top end performance, a lot of overlap is necessary. The higher the engine's power band, say power beginning at 3000 rpm and peaking at 7000, the longer the period of overlap.
Overlap is necessary because there is a limited amount of time in a high-revving engine to extract the exhaust, and begin refilling the combustion chamber with a fresh air-fuel charge. Overlap allows both to take place at the same time in the cycle, with the exhaust then closing first, quickly followed by the intake.
Not Enough Back Pressure?
With a good extractor exhaust, the possibility exists of long overlap allowing not just the exhaust gasses to be sucked out of the combustion chamber, but also some fresh (not yet unburned) air-fuel mixture. When this happens, the engine looses power. The problem is not enough back pressure. The most visible symptom is backfires in the exhaust on trailing throttle.
Too Much Back Pressure?
But with too much back pressure, the fresh charge can't easily escape. The issue then is to adjust the amount of back pressure you need.
Optimizing Back Pressure
Muffler-type extensions on merged headers and even "performance" mufflers usually have enough (or too much) back pressure and reducing it means changing to another system altogether.
That leaves the four-into-one stinger as the easiest system to modify.
For testing, tuners typically fabricate some kind of exhaust restriction and see if there is any improvement or loss in performance. A common restriction might be added into the tail pipe itself, such as half a baffle length in a stinger cone, or a metal stinger baffle without Fiberglas packing. You can do this with modified exhaust gaskets.
If you suspect you have power to gain, just experiment.
Recommended Reading: Sir Harry R. Ricardo. He wrote the book on exhaust systems.
(1922). The Internal Combustion Engine. Vol I: Slow-Speed Engines (1st ed.). London: Blackie.
(1923). The Internal Combustion Engine. Vol 2: High-Speed Engines (1st ed.). London: Blackie.
(1931). The High-Speed Internal Combustion Engine (2nd ed.). Glasgow: Blackie. revised by Glyde, H.S.
(1941). The High-Speed Internal Combustion Engine (3rd ed.). Glasgow: Blackie.
(1953). The High-Speed Internal Combustion Engine (4th ed.). Glasgow: Blackie. ...an entirely fresh start.