PUMA RACE ENGINES - PREPARING FOR A ROLLING ROAD TUNING SESSION - PART 2

Choosing a Rolling Road to Use

Choosing a rolling road to go to means making sure that the operator is familiar with the requirements of your car, has the right parts and equipment in stock and understands what you want to achieve. Some places specialise in carbs and others in fuel injected cars and you need to find someone who is expert in your own particular application. The first step is to discuss full details of your engine type, fuel system and any tuning modifications you have made. If your car has carbs then ask if he has a full range of jets in stock including idle jets, main jets, air corrector jets and emulsion tubes for that model of carb. Most places will be well stocked for the common carbs like Weber DCOEs and twin choke downdrafts. That might not be the case for less common carbs like Dellortos, Solex etc.

If your car has fuel injection then ensure he is familiar with your particular system and is confident it can be adjusted to get the fueling correct, bearing in mind how heavily modified the engine is. The fueling needs to be correct both under cruise conditions, and at full throttle. This can be hard to achieve without a fully mappable interface such as the Dastek Unichip or K Star type systems. Any off the shelf "performance" chip that has been designed for a standard engine is unlikely to be of any use if your engine is heavily modified, so beware of attempts to sell you one as a cure all. It may be necessary to have a custom chip burnt to suit your engine and not many places have the equipment to do this. If your car has an aftermarket mappable fuel and ignition system then he needs to be familiar with its software system or he'll be learning it as he goes along at your expense.

Agreeing The Scope Of Work To Be Done

Fueling

This needs to be correct at all rpms and throttle positions. The first step is to get the idle and cruise mixture correct. On a carb the single most important jet is the idle jet as this not only controls fueling at idle, but at low rpm and part throttle as well. Once this is correct the main jets, air corrector jets and emulsion tubes can be changed as required to optimize high rpm and full throttle operation. For a race car the CO reading should be about 5% at full throttle. This corresponds to an air/fuel ratio by mass of about 12.6:1. For a road car a slightly weaker setting of 4% (13:1 A/F) will give better economy at the cost of only a tiny amount of power. Most standard production cars are set up to deliver 3% (13.4:1 A/F). Many rolling road operators think that rich mixtures are better (or safer) and aim for 6% or 7% CO. This is not only terrible for economy, but also hurts power and can be damaging to the engine. Ask in advance what CO reading the operator thinks is best at full throttle and be wary if he thinks rich is good.

At cruise the CO reading should be about zero or even slightly weaker still (15:1 A/F).

Ignition

Getting the ignition timing right at all rpms and throttle positions can be very time consuming and most places don't bother unless you specify it in advance. On cars with distributors the only simple adjustment is to advance or retard the distributor which affects the timing at all rpms by the same amount. To change the timing at one particular rpm means rebuilding the distributor with different advance weights and springs and this is not usually possible or cost effective. As a minimum you should expect the operator to try advancing and retarding the distributor by a couple of degrees at a time at peak power rpm to find the best setting and you'll then have to live with what this does to the timing at other rpms. Make sure he checks there is no detonation in the mid rpm range though - the timing will have to be retarded again if this is the case.

On cars where the timing is controlled by the ECU it will have be adjusted by burning new chip settings. Unless you are prepared to pay for many hours of his time you can't expect this to be done perfectly at every part of the rpm range. It is up to you to agree in advance how much detail you want him to go into.

In all cases you want the minimum amount of ignition advance that gives best power. More advance for its own sake is never a good thing. For safety it can be best to find the optimum setting for power and then back off a degree or two.

Cam Timing

If you have a non standard cam and an adjustable pulley the cam timing can be adjusted to find the best setting. Be clear as to whether you want this done and make sure the operator knows how to adjust it. Advancing and retarding the setting in steps of between 2 and 4 degrees and rechecking the power output is the usual way. Hopefully you will have already set the timing to the cam manufacturers recommended figure as a starting point before you go the rollers though.

I remember my own first trip to the rollers many years ago and I certainly didn't know enough at that time to query, or even understand, what the operator was doing. I imagine most people are in the same boat. Just watching and hoping he knows what he's doing isn't really the ideal way to go about things though. It pays to learn enough to let you keep an eye on things yourself and like any other contract it's essential to have everything clearly agreed in advance. Driving away and saying to yourself "well I thought he might at least have done X and Y" when he didn't bother is leaving things a bit late.

Power Figures

The end result of any successful rolling road session is hopefully a gain in bhp. The ONLY meaningful figure from any rolling road is the bhp at the wheels and this is the only thing the rollers actually measure. No matter what type of rolling road is used and regardless of what the operator says about his computer generated figures or suchlike, the flywheel bhp figure is ALWAYS just an estimate and often far too high. Make absolutely sure in advance that the operator will use the wheel bhp to guide him as he performs the calibrations and that any printouts you get show the wheel bhp figures as well as, or instead of, the flywheel bhp estimates. Also ask in advance what bhp at the wheels he would expect to see from a standard car. Any improvement your tuning mods have made must be judged against this figure. As a guide, the wheel bhp will be about 15% lower than the true flywheel bhp. So divide the wheel bhp by 0.85 to get a reasonable estimate of your true flywheel bhp. If the estimated flywheel bhp printouts you get given are significantly higher than this then ignore them because they won't be right.

If you want to get a bit more complex with your power estimates then use the following equations:

FWD cars - estimated flywheel bhp = (measured wheel bhp + 10) / 0.9

RWD cars - estimated flywheel bhp = (measured wheel bhp + 10) / 0.88

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