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Doing the Impossible

Mods so cheap you won't believe them.

by Julian Edgar

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The small bald man grinned. The master of his exhaust shop, surrounded by piles of old mufflers, new ones hanging on the walls, hydraulic bender at the ready and friction saws lined up along the bench, the grin was really more of a confident sneer.

"These kids, they come in here," he said. "They get all their information from the Internet, an' they want me to put a three inch exhaust on their Gemini."

He shook his head.

"But I know what's right for each car - V8, six cylinder or four. Bloody Internet!"

There was a pause. I looked out the open door of the workshop, staring at the blue sky. A Monaro idled by and I admired the stance of the car, so curvaceous yet so visually at one with the road. There was the sound of traffic passing. The radio played classic gold.

"So waddya do?" he finally asked me.

"I write for an Internet car magazine," I said...

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What you can read below is the sort of Internet information that the muffler man was referring to. It goes completely against the prevailing wisdom and covers modifications that many people would consider to be lunacy. Specifically, this story is about how I modified the intake, exhaust, ignition timing, boost control and auto trans of my V6 Turbo Nissan Maxima for an all-up cost of under AUD$100. Yep, boost, intake, exhaust, timing and trans.

Does the car now go harder? Yes, much harder.

Does the car now have better driveability? Yes, much better.

Is it on the ragged edge, about to blow up? No, why should it be?

The modifications that are covered here are obviously specific to the car on which they were done. However, the general lessons in the approach are common to many cars - in fact nearly all Japanese turbo cars from the mid-Eighties to the mid-Nineties. (It's important to note that as covered in "DIY Budget Intercooler Fitment" the Maxima has been fitted with an intercooler - another very cheap job.)

The Intake

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The Maxima uses a pressed metal airbox fed by a duct that disappears into the inner guard and picks up air nearer the front of the car. This duct feeds the top half of the airbox, which uses a filter panel common to nearly all Nissans of that era. The lower half of the airbox feeds a hotwire airflow meter that in turn connects to a resonant chamber. From the chamber a long duct connects to the turbo intake.

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Inside the cabin measuring the restriction of the intake system is a negative pressure gauge. This particular gauge was picked up secondhand in poor condition and rejuvenated with some polish. (See "Pressure Gauges and Making Power" for more on this approach to acquiring gauges.) The gauge cost AUD$10. To measure intake restriction all that needs to be done is to plumb the gauge to the intake as close as possible to the mouth of the turbo.

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The peak intake restriction that ever showed on the gauge was 30 inches of water, quite a lot for an intake system - although not a disaster. The first step was to remove the upper half of the airbox lid, exposing the full face of the filter. In this form the maximum pressure drop decreased to 20 inches of water - the intake flowed much better with the lid off. I then removed the airfilter completely and did another full throttle run, but the pressure drop remained the same, indicating that (as is often the case) the filter element itself had virtually zero restriction.

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With the filter out, it could be clearly seen that the design of the box blocked quite a lot of the filter area from direct access to the exit. In fact it appeared that at least a third of the filter area was being wasted.

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To see if an improvement could be easily made in this area, I made a cardboard spacer that lifted the filter above the box exit, allowing airflow across the whole area of the filter. However another full-throttle run showed that the restriction remained the same, at 20 inches of water.

Obviously, to reduce the restriction still further I'd need to make changes downstream - to the airflow meter, resonant duct, etc. However, I decided that cutting intake restriction by one-third was a sufficient gain at this stage.

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The easiest way to leave the full face of the filter exposed (while at the same time keeping it securely held in place) was to cut out the centre section of the metal lid, leaving only the rim that clamps the filter down. The cutting was carried out with an angle grinder and a hacksaw, making for a very ugly job when viewed half way through the process!

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However some further work with a file and a quick spray with some black paint gave a very neat final result.

But what about sucking hot air from within the engine bay? (This should always be avoided as power loss will result and detonation will more easily occur.) In the case of the Maxima, the bonnet scoop already installed for the intercooler was able to be used to supply cold air to the filter. In fact the intake air temp (being measured with an LCD probe - see "LCD Temp Display!") seemed to be lower in temperature than before, perhaps partly because the big black metal upper lid was no longer under the bonnet being heated.

Total cost to me: $0 (I already had a can of black spray paint, with just a tiny bit left in it)

Quantified Improvement: Intake restriction dropped by 30 per cent

Driving Improvement: A little quicker to come onto boost

The Exhaust

The standard exhaust system on the Maxima uses a pipe that varies in diameter - being about 2¼ inch near the front of the car, 2½ inch in the middle and 2-inch at the back. A single cat and a large rear muffler are used. The muffler looks remarkably like the one fitted to local 2.4-litre U12 model Pintaras; not surprising since the two cars are relatives despite their different appearance and engines. (See "Lung Transplant" for more on exhaust work on that car.)

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No back-pressure measurements were taken, but previous flow testing has indicated that the muffler on these cars is quite restrictive, something confirmed when we later cut it open. It was therefore decided to replace only the cat-back section of pipe and the muffler. This pipework has two 90-degree bends in it - it is very tight for a modern car.

Going up a pipe size but then using press-bends is a cheap way of avoiding the use of mandrel bends. In other words, using 2½ inch pipe with press bends costs vastly less than using 2-inch pipe with mandrel bends, yet the flow results are much the same (depending a little on how good or bad the press bends are). So I wanted 2½ inch press-bent pipe and a 2½ inch muffler to suit.

But I didn't want to pay anything for the muffler....

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Well, people throw away heaps of things that are still useful to others, and I figured it wouldn't be too hard to find a good, straight-through 2½ inch muffler. (I wanted straight-through because all other designs are more restrictive - you gotta be able to look in one end and see out the other.) In fact the first muffler shop bin I accessed (ask permission first) had the perfect muffler - a 2½ inch (or near metric equivalent) straight-through design off the back of a diesel Landcruiser. (I think, anyway. It had 'Toyota' written on it and the body and attached tailpipe looks just the same as on a Landcruiser...)

Cost? Nah, there wasn't any....

The muffler was in good nick (to find out, tap it on the ground and see if anything falls out, as well as giving it a very close visual inspection) and it was sized to easily fit in the requisite space. The next step was to get it fitted, along with that new cat-back section of press-bent 2½-inch pipe. That whole job cost AUD$80 at a (different) muffler shop.

With the muffler fitted, it's obvious from the sound that the Maxima has a modified exhaust, but there are no drones or booms - in fact the note is pretty good. And the car goes noticeably harder...

Total cost to me: $80

Quantified Improvement: Not done

Driving Improvement: Much quicker to come onto boost, especially at part throttle. Stronger full-throttle performance.

Boost Control

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The boost control is simplicity itself. It comprises a T-piece inserted in the wastegate hose with the unused port of the T venting to atmosphere. With the T-piece orientated one way, boost rose to 1 Bar but also varied a fair bit on the gearchanges. But with it orientated like this, boost rises to 0.6 Bar, which is what was wanted. It also varies less than 0.05 Bar (under 1 psi) through the rev range.

And that's all there is to it - no adjustments, no restrictors, nothing.

Total cost to me: $0 (I already had the T-piece and clamps and hose)

Quantified Improvement: Peak boost increase from 0.45 Bar to 0.6 Bar

Driving Improvement: Quicker to come onto boost, especially at part throttle. Stronger full-throttle performance.

Ignition Timing

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The Maxima uses a distributor that can be rotated, conventional style, to globally advance the timing. After I bought the car I had the cam belt replaced and the ignition timing was then adjusted - it had apparently been well advanced, even though I never heard the engine detonate. After the timing had been returned to factory specs, the car was slow as a dog off the line, so I soon undid the bolt and advanced it again. How much? I don't know; I haven't a timing light as none of my cars for a decade have had adjustable timing. However, what I do have is an excellent ear for detonation and I am quite sure it has never detonated with the premium fuel, added intercooler, and a close eye being kept on real-time intake air temp.

Total cost to me: $0

Quantified Improvement: Not done

Driving Improvement: Much sharper throttle response, especially off the line.

Auto Trans

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With the very major improvement in mid-range torque I found that the auto trans was a little too eager to kick down - for example, it would go for second gear which immediately took the revs to 5000, needing to then change only 1000 rpm later at the redline. Better to have the trans stay in a taller gear and then use that new-found torque to get the acceleration.

The Maxima uses a throttle cable to tell the trans how much throttle is being used (as opposed to a trans that uses a vacuum modulator, or a fully electronic trans which takes this information direct from the engine management ECU). So it was dead easy to loosen the adjustment nut and move the cable so that the trans sees less throttle in any circumstance. The result? Full-throttle kickdown is still fine but the car holds onto lower gears and 'torques its way' in mid-throttle situations much more readily.

Total cost to me: $0

Quantified Improvement: Trans doesn't kickdown when it will result in only 1000 rpm left before the redline

Driving Improvement: More refined, punchier performance, especially in urban driving


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If I told you that I'd done the boost, exhaust, timing, intake and trans on my turbo car you'd be asking: "Fantastic? Go a lot harder now?"

And I'd be saying: "Sure does..."

Which, if you follow me, is exactly what I am saying!

So how much faster is the car? My timing indicates that 0-100 km/h now comes up in about 8.1 seconds, but it hard to say with certainty because my new alloy wheels came with slightly different profile tyres and so the speedo accuracy's changed. (New alloys? Ah, didn't I mention my four Silvia alloys that cost me $150, complete with two good tyres?) The car is now seriously faster than it ever was before, evidenced in part by the torque steer that I can now feel as well as the much more urgent push in the back.

The results are similar to many people's turbo car modifications, but the difference is that most people's modifications cost way more money....


Magazine stories are always full of absences - like where did that bit come from at that price, or, who did that work for nothing. This story hasn't got any of 'em - except for one. During the modification process covered in this story I was able to (for other reasons) borrow an accurate air/fuel ratio meter and confirm that on the road, the Maxima wasn't running lean at any point. If I'd had to go for a dyno to find this same information, the overall cost would obviously have gone up. You shouldn't try to extract more power out of any car without knowing what the mixtures are.

Death and Glory?

One reason that I am quite confident that the Maxima will not blow up (although I am sure that plenty of people reading the story will think otherwise) can be summarised in two words: instrumentation and experience. As evidenced in this article, boost pressure and intake air temp are constantly monitored. Air/fuel ratio has been measured under load. Also, and as I have said, I have a very good ear for detonation, and the experience to know when it is most likely to occur.

None of this is a guarantee against destruction, but it's pretty good insurance.

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