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Ultimate DIY Automotive Modification Tool-Kit, Part 7

Wrapping-up our major series on doing your own car modifications

by Julian Edgar

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At a glance...

  • Selecting the right car to modify
  • Balancing the modification trade-offs
  • Modification workshops
  • Understanding performance
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Over the last 6 weeks we’ve looked in detail at DIY techniques for:

  • Measuring the restriction of intake and exhaust systems
  • Seeing the aerodynamic flow patterns over the surface of the car
  • Measuring aerodynamic pressure variations
  • Testing aerodynamic drag
  • Testing performance
  • Measuring acceleration 
  • Describing and testing ride and handling

Part 1
Part 2
Part 3
Part 4
Part 5
Part 6

All these techniques are able to be individually done for your car, on the real road. Additionally, they all cost little and are easy to do. 

By applying these techniques, you can:

  • Select and upgrade individual parts of the intake system (eg the airbox or intake snorkel) 
  • Select and upgrade individual parts of the exhaust system (eg the cat converter or rear muffler) 
  • Optimise the placement and design of wings, spoilers, undertrays and vortex generators
  • Optimise the location of the engine air intake

  • Improve the airflow through the radiator, air con condenser, oil cooler and intercooler
  • Reduce aerodynamic drag
  • Assess the performance of intakes, exhausts, engine management, boost controls, aerodynamic and weight changes
  • Measure maximum cornering acceleration
  • Indentify ride and handling shortcomings and then assess the affect of modifications

And you can do these on machines as diverse as 6-litre V8 sedans... to two-door hybrids... to pedal-powered alternative vehicles.  It’s easy to underestimate the worth of these simple techniques in making informed, intelligent and very effective automotive modifications, but the results speak for themselves.

But even with these techniques in your armoury, all is not necessarily solved. Let’s take a look at the final ideas that you should keep in your tool-kit. 

The Car

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The cheapest, easiest and most effective outcome is achieved by buying the vehicle that best meets your needs – and then making relatively simple and easy modifications to further improve it to match your tastes and desires. That sentence covers the whole range of cars and incomes!

If you’ve got only a few thousand dollars to spend on a car, fine. If you want to spend $100,000 on a car – again fine. In both cases, before even thinking of modifications, buy what best matches your desires for the budget you have. 

Perhaps that’s self-evident, but lots of people start off by modifying cars that are fundamentally ill-suited to the outcomes they're chasing. If you want a lot of straight-line performance, don’t start with a slow car. If you want a car that handles well, start off with one that’s already good in that aspect. If you want a super fuel-efficient car, buy one – don’t try to turn a gas guzzler into a car that just sips fuel. 

The Process

Modification is about making improvements. The corollary of that is to first identify the deficiencies. Yes, the car that you’ve selected to modify is already well matched to your tastes, budget and desires – but what aspects are not as good as you’d like? They could be as diverse as the sound system, appearance, fuel economy, performance, handling, ride, braking, instrumentation, seat comfort, feel of the gear-change – a myriad of possibilities. 

The next step is to ensure that the modifications do not adversely affect other aspects of the car. But how could modifications make any parts of the car worse?

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Pretty well all modifications have negatives. A cold air intake will usually require that you change the filter more often. A stiffer suspension will usually downgrade ride and increase NVH - and it may also make the car much more skittish in the wet. A new exhaust will result in more cabin noise. Any internal engine mod will usually result in less reliability. Nine times out of ten, aftermarket management won't pass an emissions test. A performance wheel alignment may well result in poorer tyre wear. Higher grip tyres will wear more quickly. Almost any mod will invalidate your insurance unless the company is informed, and any mod is likely to put into jeopardy your new car warranty

So the type of modifications must take into account a balancing act.

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If the car is already uneven in power delivery (eg it’s a turbo engine with a lot at the top-end and not much down the bottom), pick the mods that will improve the bottom end. So in this case, fitting a larger turbo would not be a good approach. Instead, it would be better to make modifications that promote better turbo spool-up – intake, exhaust, boost control, low load ignition timing.

If the ride is already firm, look at upgrading tyres and anti-roll bars, rather than going to stiffer springs and dampers. 

If a better performance / fuel economy compromise is wanted, pick mods that improve engine efficiency rather than simply jamming more air and fuel into the engine. 

It’s easy to modify cars – but it’s much, much harder to modify cars so that compromises in other areas don’t become large. 


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Few car owners have the facilities and skills to do all the work on their cars. That means that modification workshops will become involved. But isn’t that good – getting experts to help you? 

Unfortunately, I can state that the vast majority of workshops that I have been visited in about 20 years of writing about modified cars haven’t been very good. A handful - perhaps at a pinch six or seven – have been excellent, but most have not been the sort that I would trust any of my cars to. Ever. 

Note that I am talking specifically about modification workshops, not normal maintenance and servicing workshops. 

Lots of people will take umbrage at that sort of generalisation, but I can only tell you what I have seen and heard. Workshops that experiment on customer cars – completely unbeknown to the owners of the cars. Workshops that have never read the equipment manuals for the gear they’re using – gear like dynos. Workshops that have such a poor understanding of physics they blandly claim impossible scenarios – like an air/air intercooler that lowers the intake air temp below ambient. And so on....

So while I think it is fine to use workshops for specific modifications, I always pick the workshop very carefully, and agree with them before-hand exactly what the work is to involve. I also ignore it when they tell me that my ideas are stupid, that they won’t work, that they know a better way of doing things, and so on. This may appear the height of arrogance but take a look at the following example. 

Let’s say you’ve done on-road wool-tuft and pressure measurement testing on your car, and have on the basis of this testing decided on the optimal location of bonnet vents. When you take your car to a workshop to have the vents installed, you will know far more than that workshop about the best and worst places to put bonnet vents. You will know far more than they do!

On the other hand, let’s say you’ve identified some specific suspension inadequacies, then taken your car to a suspension workshop where the owner has very carefully listened to your description of the problems. The workshop then makes suggestions for individual changes – eg altering the dampers' rebound stiffness - but nothing else. In this situation, you might be confident in proceeding with that workshop. Or, at least, more confident than if you go in with your list and are told: “Yeah, mate, we just put new shocks and springs on – that’ll fix it.”

I am sure that modification mechanics will be able to tell chapter and verse on botched modifications they’ve seen amateurs make, but in my experience, people who do the sort of careful testing and evaluation that have been covered in this series make far less mistakes than the so-called called professionals. (One reason for this is that the amateurs spend vastly more time on the project.) 

Understanding Performance

Car performance is often expressed in graphical form. In this series we’ve looked at graphs for vehicle acceleration, boost versus engine rpm, and others. The most common graphs you’ll see in the performance car world are dyno graphs – engine power and torque versus engine speed (or sometimes on chassis dynos, versus road speed). 

Dyno graphs are often presented in a deceptive manner.

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Here's the power results of fitting an older model Falcon XR6 with headers and a high performance exhaust. Okay, there has been a power improvement - but it's not huge. Look at the graph and estimate by how much peak power has gone up. If you said about 5 per cent, you're pretty close to the mark - the max gain was actually 4.9 per cent. If this modification was going to cost, say, $1000, you'd be looking at this graph and expressing some doubts about how worthwhile the modification is. 

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But have a look at this graph. This is more like it - look at that towering power improvement!  Except this graph shows exactly the same results as the one above. But it’s easy to look at this graph and guesstimate a power gain, of, say, 10 per cent – isn’t it? Well it wouldn't if you actually read the 'before' and 'after' figures off the graph - but not many people do that at a glance. 

The key difference in the graphs is that the second graph’s vertical axis does not start at zero – it starts at 65. The latter graph is not illegal or even untrue. It’s just that most people would perceive the results differently – much more positively.

In the same way, always evaluate modifications in terms of percentage improvement, not in absolute units like kilowatts or horsepower (or even inches of water pressure drop).  Looking at absolute figures can again be quite deceptive.

I'll give you an example. A fair while ago the lagging of exhausts made a big splash. One of the claims was that the tape insulated the pipes and so caused the gases to remain hotter, apparently increasing their velocity. This resulted in a claimed power gain.

The dyno tests results provided on a brochure said in big letters stuff like "12hp gains!" and "power there for the taking!". Now, twelve horsepower might be a lot... or it might be a very small amount indeed. But twelve horsepower's good though isn't it? It is - but is it much of a change? Close inspection of the pamphlet showed the power gain to have been made on a V8 pumping out about 500hp. So a 12hp power gain represented a 2.4 per cent lift. Sure, that's better than nothing - but 2.4 per cent looks a lot smaller on the brochure than 12 horsepower!

Series Conclusion

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Nearly all the techniques and approaches covered in this series are unorthodox – compared with traditional modification techniques, they’re pursued by very few people. However, they have the huge advantage of being cheap, able to be done on the road under real-world conditions, and applying to pretty well all vehicles. 

They’re also fun.

Finding out how restrictive your car’s intake system is, and then cheaply making improvements that you can actually measure, is exciting. 

Measuring airflow pressures over a car’s body is fascinating, especially when you realise your instruments are sensitive enough to detect when there is another car in front, disturbing the airflow! 

Fitting a new exhaust with cheap, secondhand mufflers taken from a larger car – and then directly measuring the improvement – is rewarding, especially when your financial outlay may be only one-quarter as great as following the herd.  

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Improving the efficiency of the existing intercooler by careful and detailed aerodynamic testing and modification is to take a very smart approach – and not just to throw cubic dollars at a problem. 

Picking the right car and then modifying it is likely to give you the outcome you want without the frustration and pointless expenditure that so often occurs with modified cars. 

In fact, finding the problem, devising the solution and then evaluating the results is to make DIY modification of cars one of the most rewarding pursuits you can follow....

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