Lifting the boost on a turbocharged car is always gonna give you a real kick in the pants. But unfortunately lifting the boost in a non-intercooled turbocar can cause the engine to instead do you-know-what in its pants...
So increasing boost can be a process fraught with danger: fuel lean-outs, increased intake temps... and detonation! But what about simply bringing the boost up more quickly to the standard level? That way you're not placing the engine under any more stress, it won't need more injector duty cycle to supply greater amounts of top-end fuel, and the increase in intake temp will be unmeasurable.
Sound too good to be true? It isn't.
Sound expensive and difficult? Again, it isn't.
After completing the intake and exhaust mods (Modifying the VL Turbo Intake) and (Pure Pipe Perfection - Part 1) on my Commodore VL Turbo, I next wanted to lift boost pressure. At first, I thought I'd simply squeeze in maybe another 20 kPa (around 3 psi) throughout the rev range and try to avoid detonation by always running the car on 98-octane fuel. I guessed that - maybe - the ignition would also need a very slight retard as well.
However, I soon became concerned that this boost mod would be treading very close to the line of detonation - especially given the large amount of heat soak that affects the VL's alloy intake manifold. (The highest intake air temps in the VL occur when idling on a hot day in city traffic. I can easily see this by watching my dash-mounted LCD temp display - we covered the instrument here LCD Temp Display!.) I could just imagine nailing it from a set of lights after the car had been moping around in its own heat - and then hearing that dreaded tink, tink, tink...
But, this one particular weekend, I wanted to go faster straight away. There was no time for stuffing around installing intercoolers, brackets, pipes and hoses: an instant speed fix was required.
So my aim was to lift the boost - but not the maximum boost level, just the rate of boost increase. This means the turbo would spool up to its factory boost limit quicker; giving improved "squirt" and on-and-off the throttle performance.
Stock as a Rock
My first step was to see how quickly the standard turbo arrangement spooled up. With an assistant reading off a boost gauge (measuring from the inlet manifold) we soon painted a picture of how quickly - or slowly, rather - boost rose to its peak value. Note that my VL Turbo boosted to a limit of 50 kPa - other standard VLs may vary up to 5 kPa from this figure.
This graph shows the boost curve - which is probably a little better than achieved in a dead standard car, my intake and exhaust changes having already improved this aspect. From the graph, you can see that it takes a fair while for boost to climb to its maximum 50 kPa value. And when max boost does eventually arrive at around 3000 rpm, that leaves only about another 3000 rpm of full boost performance before you hit the rev limiter.
And I wanted a meatier boost spread than that!
Off with the Hose!
With our baseline established, my next test was to see how fast boost could be generated without the wastegate connected at all. That is, I wanted to see the absolute fastest that boost could come up. (Note that other than a full anti-lag system, no boost control can bring up boost any faster than will be seen in this test - not even a $2000 electronic system!) So to find the very fastest rate of boost increase possible, I pulled off one end of the factory hose that connects the compressor outlet to the wastegate actuator. The open end was then temporarily plugged with a bolt so that boost wouldn't escape.
The reason that I expected this to alter the rate of boost increase is because all wastegates start to creep open, even before full boost level is reached. In other words, at 25 kPa (just half of the eventual full boost), the wastegate will have already started to move, bypassing exhaust gases around the turbo and so slowing the rate of boost increase. Manufacturers deliberately take this approach so that the boost comes on more gently.
(At this stage I also decided to replace the factory intake manifold over-pressure relief valve with a screw-in blanking plug. This ensured that the maximum available boost pressure reached the engine, and - as a bonus - we could see at what stage the turbo is capable of producing more than the standard 50 kPa boost.)
So - running dangerously unwastegate'd - we took to the road to find out the absolute fastest that the standard T03 turbo could spool up.
If you try this experiment, be very careful not to accidentally over-boost. Keep a careful eye on the pressure gauge and also make sure your mixtures are okay.
As you can see here, boost came up much faster in unwastegated form. The factory boost limit of 50 kPa arrived at 2250 rpm - considerably earlier than the standard 3000 rpm. So the standard Commodore boost control system lets boost develop a lot slower than the turbo is actually capable of. Or, to put it another way, bottom-end torque is much reduced of what it has the potential to be.
Note that I backed off at 2500 rpm - no intercooler is fitted, remember! - with boost still rocketing.
So how did the car drive without the wastegate attached? Let's just say its pick-up was improved monumentally and wheelspin was much easier to induce...
Changing the Wastegate Volume
So, guided by our slowest and fastest boost rise figures, we set about changing the airflow dynamics of the wastegate line. Note that the following two mods only affect the dynamic response of the turbo system. That means they'll work only when you're accelerating - they won't show up at a constantly held load.
Both of the approaches change the volume of air in the hose leading to the wastegate. This means that there is always a slight delay in the reaction of the wastegate to an increase in manifold boost, giving less wastegate creep and so faster boosting.
The first quick-and-easy mod was to replace that short factory hose between the compressor outlet and the wastegate actuator with a very long length of new hose - around 2½ metres in fact.
From this graph, you can see that the extra time that it took for pressure to reach the wastegate actuator did have a positive effect. Boost increased up to twice as fast, and - looking at all of the data - 50 kPa maximum boost was attained slightly earlier than in standard form. On the road, the improved spool-up time was noticeable - especially after quick gear changes.
Increase the Volume a Bit More
Continuing on the "slow what the wastegate sees" theme, I then grabbed a ¼-inch T-piece and plumbed an overboost reservoir into the new 2½ metre hose. What's an overboost reservoir, you ask? Put simply, it's a sealed empty volume that can withstand internal pressure. I made mine from a 10cm length of 2¼ -inch exhaust tube with two ends welded on. These are actually fence post caps. One of the end caps had a small hole drilled into its centre and a small ¼ inch hose fitting welded on. The whole thing cost less than $10 in parts and labour.
Curiously, adding the overboost reservoir to the long hose gave a mixed bag of results. Improvements were made over just the 2½ metre hose at both 1500 and 2500 rpm sites. Furthermore, boost response was as strong as it was unwastegate'd until the engine hit 1500 rpm. That bit can't be bettered.
Unfortunately, though, overall the reservoir didn't prove as successful as those that I'd fitted to previous cars. On those other vehicles, boost would actually over-shoot the maximum wastegate setting for a moment before returning back to normal. I'm guessing that the approx 250cc volume of this particular reservoir wasn't enough to suit the VL. Perhaps a variable restriction (eg a ball valve) could be inserted on the turbo side prior to the T-piece? (This would slow the rate at which the air could fill the wastegate line and canister, so increasing the wastegate lag and letting boost shoot higher until the pressurised air filled the control system.)
Nevertheless, there was now an overall improvement that satisfied my "needs" for one Saturday. The combination of the 2½ metre hose and the reservoir gives noticeably better boost build-up, and the car is more eager to chirp its tyres after a brisk gear change...
It's an approach that with a little experimentation and care, can yield performance results quite out of proportion with the money spent....