Back Pressure Tuning in the VW Type 1

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Back Pressure Tuning in the VW Type 1

Post by FJCamper » Sun Oct 29, 2017 11:43 pm

Image
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.

Cam Overlap

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.

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Re: Back Pressure Tuning in the VW Type 1

Post by tencentlife » Sun Nov 05, 2017 10:16 am

Nice to see that popular articles on exhaust theory still amount to a pantheon of confusion.

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Re: Back Pressure Tuning in the VW Type 1

Post by FJCamper » Sun Nov 05, 2017 11:04 am

Hi Tencent-

Yes, back pressure is as vague a subject as handling.

I'm sort of a tech myself, with many patents on scientific processes and devices and extensive pro and club racing experience on VW's and Porsches since the early 1970's.

If we had an instrument that gave us a number, we'd be able to be be more straight forward.

There is optimization only within an individual configuration on an individual engine. For the individual tuner, the bottom line is better or worse performance, and aside from combustion chamber design, ignition, cam design, ad naseum, the only thing to do is increase or decrease back pressure.

I'm sure you are well informed enough to know what a scientific endeavor it is for a manufacturer to build a tuned intake and exhaust system. We VW guys are limited to what we can get from vendors.

FJC

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Re: Back Pressure Tuning in the VW Type 1

Post by ps2375 » Tue Nov 07, 2017 12:22 am

FJCamper wrote:
Sun Nov 05, 2017 11:04 am
f we had an instrument that gave us a number, we'd be able to be be more straight forward.
If only I had the time and money to visit/own a dyno (chassis or motor) anytime I need/want to. Life would be a lot more fun and definitely more interesting.

Or is that more interesting and definitely more fun? Either way, more of both.

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Re: Back Pressure Tuning in the VW Type 1

Post by alsoran » Fri Nov 24, 2017 6:00 pm

FJCamper, have you experimented with super traps? They are tune-able.

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Re: Back Pressure Tuning in the VW Type 1

Post by FJCamper » Sat Nov 25, 2017 5:48 pm

Hi--

No experience with them since they were more for the off-road crowd, but I see your point and I'm going to get one!

FJC

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Re: Back Pressure Tuning in the VW Type 1

Post by Ol'fogasaurus » Sat Nov 25, 2017 7:09 pm

https://www.bing.com/images/search?q=su ... &FORM=IGRE

I just gave away my supertrapp and yes, you can do a certain amount of noise suppression is done by removing discs; as I remember stock the dB range was about 108 (as tested) when used on an off-road header. Mine was one older short units (6" or 8" body) from the '90s that was fiberglass packed and had, I think, 8 discs. To quiet them down (we have a 93 dB limit) you removed discs. The more discs you remove the more restrictive it is: after meeting the 93 dB noise limit (the guy doing the checking was kind enough to wait while I made changes... several times; when all was said and done I could barely climb some, but not all, of the dunes so I went to a turbo exhaust system and had no problems with noise or loss of power. The more discs you add (up to a point and I don't know what that is) the louder it gets. They now have longer body 'Trapps that are much more quiet than what I have and are also less restrictive.

If you are being monitored (like we are) they measure the noise limit 90° from the opening. The sound sensor has a predetermined length of rope/string attached to it that they go to the end of the exhaust (90°s) with to get a measurement. The trouble I had with that is on the 'Trapp, the exhaust comes out the sides not straight out the end so the measurement taken from the side I don't think are accurate.

I hope this helps some.

Lee

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Re: Back Pressure Tuning in the VW Type 1

Post by FJCamper » Sun Nov 26, 2017 12:31 am

Hi Fog,

Sound reduction and backpressure are not academic to us at RetroRacing.

We need the power because we really run a 85.5 x 74mm engine against high-dollar full race prepared MG's, Triumphs, Spitfires, etc. in Historic Sportscar Racing, and at some tracks, sound is an issue. This even applies to our LeMons racing with a hefty 2.2 litre.

We have four separate exhaust systems, open stinger, baffled stinger, hollowed-out cherry bomb, and a straight pipe curved 90 degrees off to the right off the collector.

We're in both series representing Vee Dub and do well enough to aggravate the front runners and keep them looking over their shoulders.

We run Road Atlanta (LeMons) on 8-9 December and we're going all out to win our class.

FJC

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Re: Back Pressure Tuning in the VW Type 1

Post by Ol'fogasaurus » Sun Nov 26, 2017 12:41 am

"FJCamper, have you experimented with super traps? They are tune-able."

"No experience with them since they were more for the off-road crowd, but I see your point and I'm going to get one!"

Sorry, I thought the question had to do with Suprtrapp mufflers and back pressure; that's what thought I was giving :shock: :oops: . As I remember, up here, since the tracks have now become surrounded by houses, sound is off and on in the courts just like off-roading or sand riding is.

Lee

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Re: Back Pressure Tuning in the VW Type 1

Post by FJCamper » Sun Nov 26, 2017 1:17 pm

Hi Fog,

We're on the same page. Noise abatement is strict at some of our tracks. I wrote my back pressure post from a power improvement slant rather than a complaint about trying to muffle a race.

The SuperTrap suggestion interested me because it's tunable.

The LeMons races run on Saturday and Sunday, and on Sundays the races start before local church hours, and we actually have to have a one hour "quiet time" in which the race is paused. All the cars return to the paddock. No engines can be run at all.

This led me to write this stanza in my racer's epic The Black Flag:

Have you ever passed a Gullwing with your tires a wearing thin?
Have you held your throttle open, just to hear the screaming din?
Have you tried to do it lately, better not, noise is a sin!
And then you’ll sure catch hell.

FJC

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Re: Back Pressure Tuning in the VW Type 1

Post by Ol'fogasaurus » Sun Nov 26, 2017 1:51 pm

Good, I wasn't sure if I misunderstood or what.

"The SuperTrap (sic) suggestion interested me because it's tunable."

If you are talking about the removing of plates at the end of the Supertrapp, then yes you can but, as I said, only to a point before you start chocking things down. You can easily go too far removing plates as I said; by the time I met the sound limit the power was down to where I could barely make it up the hill from the parking lot/camping area too the entrance of the dunes... it was that restrictive. This was back in the mid-'90s so I don't remember just how many plates of the 8 in the stack at the end of the muffler I removed to hit the dB limit but it was either 4 or 5. I haven't played with the longer ones that they now have but I am pretty sure that the longer ones muffle better w/o having to remove so many plates.

Maybe this will help some with your decision: http://www.supertrapp.com/resource/faqs.asp

Lee

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