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The Audi's DIY Boost Control - Part 2

Adding some further refinements to our new DIY boost control.

by Julian Edgar

Click on pics to view larger images


This article was first published in 2000.

Using cheap off-the-shelf pneumatic engineering products, this new DIY boost control system allows you to set how quickly boost comes on as well as how high it goes. If you want, you can reduce wastegate creep to zero, giving extraordinarily quick boosting.

This week we change the pressure reg and add a new valve to give even better results.

The complete boost control system was covered last issue, so it's important that you read that article before you look through this one - especially so that you understand the details of the tuning process. You'll find Part 1 at "The Audi's DIY Boost Control - Part 1". While that system worked very well in giving an adjustable wastegate anti-creep function and then holding boost fairly constant, I thought that the boost regulation could be improved a little - and that boosting after each gear change could also be made to occur faster.

Refinements

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To improve full-load boost regulation, I sourced a slightly more expensive - and sophisticated - pressure regulator. Made by Italian company Metal Work Pneumatics, the MR BIT SR ¼ Micro Regulator (with mounting bracket and gauge port plug) cost A$77 (including GST) from Metal Work Australasia Pty Ltd (Metal Work has outlets worldwide). While not a very expensive instrument level precision regulator, the BIT SR regulator has better regulation and faster blow-down than normal miniature regulators. When you order the reg don't forget to let the supplier know that you want to regulate low pressures - up to a couple of Bar max.

With the BIT regulator in place, the system held boost more accurately. A side benefit of the more expensive reg is that it is also nicer to use, with a higher quality feel.

After driving around for a few days with the new boost system, I decided that my personal preference was to have boost come on a little more gently. Certainly in a high power two-wheel drive car, if you set the system to give no wastegate creep at all, it would be easy to break into wheelspin when exiting corners - with a well-matched turbo, the boost comes on just so hard! To reduce the surge of power that even little throttle twitches gave, I opened (ie turned anticlockwise) the pressure relief valve a little, until the boost response was as I prefer. This setting still gives faster boosting than the factory system, but also retains good throttle control, with the engine providing a fairly linear torque delivery.

The beauty of this boost control system is that the speed with which boost builds up and the height to which boost rises are both independently adjustable - within the physical constraints of the turbo, anyway.

After Gear Change Boosting

With the IMI Norgren pressure relief valve (see Part 1) and the Metal Works pressure regulator in place, the boost control worked very well. However, after a few more days of testing, I noticed that an odd characteristic occurred in the Audi. In first and second gears, boost quickly built to the set 1 Bar of pressure, but after the full-load change to third gear, boost took much longer to rise to the max level. After the third/fourth, fourth/fifth and fifth/sixth changes, boost rocketed straight back up to its peak. So on just the third/fourth change it was almost as if some wastegate creep was still occurring.

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I considered what might be happening to hold some pressure on the wastegate during gear changes. What if the boost was being trapped between the pressure relief valve and the wastegate, holding it open by a small amount? This could occur if the pressure after the relief valve fell below its unseating pressure, and was also below the pressure at which the regulator starts to 'relieve' boost (ie vent it to the atmosphere). After all, if on a quick gear change any pressure were trapped in the system for even half of a second, it would negatively affect the boost build-up following that gear change.

Adding another valve to the system removed this problem. The new one-way valve is plumbed into place so it connects the outlet of the pressure regulator to the inlet of the pressure relief valve, as is shown below. The valve is orientated so that flow is permitted from the wastegate line to the input of the pressure relief valve.

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While initially it looks as though it will act as a bleed (ie lowering the pressure being seen by the wastegate under boost conditions), this isn't so. Why? Because whenever boost is occurring, the pressure pushing against the normal flow direction of the one-way valve is greater than the pressure being regulated by the pressure regulator, so the valve stays shut. But when boost falls (ie you've taken your foot off the throttle to change gears), any residual pressure that might be trapped against the wastegate is immediately exhausted through the new valve. This closes the wastegate and so makes the next boost event occur more quickly. Note that this system assumes that you have a throttle-closed blow-off valve working on your engine. If you haven't, there will still be boost present in the feed hose and so the one-way valve may not open.

The one-way valve that was used is an Italian-made brass York 23151-06 3/8-inch check valve, fitted with 5/16th inch barbed hose fittings. While an in-line brake booster valve would probably be adequate, the York valve has an extremely low opening ('cracking') pressure - just 7 kPa or 1 psi. It is also suitable for temps up to 80 degrees C and uses a 304 stainless steel internal spring. Complete with the barbed hose fittings, it was just A$14 (including GST) and was purchased from my local pneumatics supplier - BL Shipway & Co Pty Ltd.

With the one-way valve installed, the wastegate anti-creep behaviour was now consistent from gear to gear, allowing very good throttle control after every gear change.

Installation

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With the system working as I wished, I mounted the two adjustable valves and the one-way valve on an aluminium bracket under the bonnet. (You may be able to mount the regulator in the cabin, but expect the system to behave slightly differently with the extra air volume contained in the longer hoses.) Because in the Audi the only free location was near to the turbo, I added to the mounting bracket a polished stainless steel heat shield and a layer of high temperature ceramic fibre insulation to protect the components. Note that the Norgren valves are rated to 65 degrees C, while the Metal Work reg is rated at only 50 degrees C (but that's when running max pressure of 145 psi!). Either way, you should take care to heat shield the valves or mount them on the cool side of the engine bay.

I also insulated the boost control hoses, using Pyrotek Pyrojacket sleeving, which comprises a layer of high-grade silicon rubber over knitted glass sleeving. This stuff is so good that you can place some of the sleeving over a finger - and then for a minute or so, aim a heat gun set on 'high' straight at the insulation. Oh yeah, and not burn your finger.... The sleeving is available in a number of internal diameters, with PJ8 (12mm ID) being a tight fit on the 5/16 rubber fuel hose used in this application. However, at A$28 a metre, the insulating material is quite expensive. Besides hose durability, another reason that I fitted the sleeving is because I suspected that the heating of the air in the hoses was giving variable boost behaviour in some situations.

On-Road Performance

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As can be seen by this second gear acceleration graph, the wastegate anti-creep setting that I finally chose provides more of a boost rush than the standard electronic system, but not as much as it's actually possible to get. (Note that this full throttle graph is a little deceptive, in that it is the part-throttle boost behaviour that is changed by the greatest amount when adjusting the anti-wastegate creep pressure relief valve.)

The chosen setting allows good drivability, without boost suddenly booming in at half throttle. In fact, to get positive boost in normal driving, it still takes a deliberate attempt to put your foot down a bit further than normal. However, the way in which the torque then increases is very satisfying indeed - at part throttle the car feels so much stronger and more driveable than in standard form. An example - when cornered very hard, the Audi can be made to four-wheel drift a little on the corner exit. With the boost control set to give this very throttle-sensitive extra mid-range torque, the car can be cornered in second gear with both the front and back progressively sliding - without having to nail the pedal to the floor to get full boost and while still retaining good throttle control.

Conclusion

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The final system gives excellent control over the rate of boost increase as well as over maximum boost. Boost increases after gear changes are always quick, and the rate of boost increase in each gear is consistent. Finally, all the parts are readily available (and are cheap) and installation is straightforward.







Contacts:

IMI Norgren Pty Ltd

(branches in Melbourne, Sydney, Adelaide, Brisbane, Perth and worldwide)

Australian site: http://www.au.norgren.com/products/products.asp

Metal Work Australasia Pty Ltd

02 9725 3599

Manufacturer's Italian site: http://www.metalwork.it/eng/bit_gruppi.html

Pyrotek

02 9631 1333

Branches Australia-wide

BL Shipway & Co Pty Ltd

08 8443 6327

www.blshipway.com.au

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