PUMA RACE ENGINES - PREPARING FOR A ROLLING ROAD TUNING SESSION - PART 1

Most modified cars, and many standard ones, get taken to a rolling road (chassis dyno) for a tuning session at some time in their lives. Many of these visits end up as unsatisfactory experiences for the owner for one reason or another. Often the car comes back still not running properly or the expense exceeds any initial estimates. Most operators are conscientious people but their experience and knowledge varies as does their equipment. Also I frequently hear of cases where the car owner doesn't properly explain what he wants to be done or even understand what should or can be done. To calibrate every single aspect of a fueling or ignition system can be very time consuming. The OE manufacturers spend hundreds of hours dyno testing new engines and making adjustments to optimize power, fuel consumption, emissions and reliability. It is just not reasonable to expect a modified car to behave as well as a standard one in all respects after only a couple of hours tuning work. Weber UK used to reckon on spending a week just to optimize a carburetor for a new application for a major manufacturer. As with all things getting the last 10% right can take as long as the first 90%. A compromise has to be struck in terms of time and money spent to get a setup that the owner is happy with which doesn't break the bank.

The first step is to make sure the engine and certain aspects of the car itself are in good health before any tuning work is done. There is no point trying to calibrate fueling or ignition if a basic fault inside the engine prevents it running properly and the more things the operator has to fix for you the higher your bill will be.

1) Do a compression test to ensure that bores, rings and valve seats are in good order. A compression tester only costs £20 or so and can tell you a lot about the internal health of an engine fairly quickly. It should be a part of every enthusiast's tool kit. There is a more comprehensive article on how to do a test on my website and any gauge will come with instructions which explains most of what you need to know.

2) Check or replace the basic service items. Many rolling road sessions start off with the operator having to find and repair these sorts of faults before he can even get to the calibration issues. The car might run perfectly well in general road use but the stresses of prolonged full power operation on the rollers tend to show up components that are getting to the end of their life. If he is charging you between £30 and £50 an hour you really don't want to be paying for things you could easily have done yourself in advance. He might not even have the right things in stock if yours is not a common vehicle and then even more time and money is wasted trying to obtain them. Spark plugs, HT leads and distributor cap should be renewed if there is any doubt about their condition. If the engine is heavily modified you might want to fit plugs which are a grade colder than standard or at least take a set with you. Check the valve clearances if the engine does not have hydraulic adjustment. Check the condition and tension of cam belts and fan belts. Make sure the cam timing is correct, especially if a non standard one has been fitted. Change the oil and fit a new filter if you are coming up to a service interval or at least check the oil level and ensure there are no leaks. The operator won't thank you for dropping oil all over his rollers and the tyres have a hard enough job finding grip on polished steel anyway without extra lubrication to contend with. If the car has an oil pressure gauge then don't even think about a dyno session if the pressure is much below spec. The crank and bearings might well be on their last legs and in no fit state to take the pounding of full power/high rpm use.

3) The cooling system takes a real hammering on the rollers and engines can get very hot very quickly. With the vehicle stationary you are reliant on the roller's own fans to keep the engine cool. No matter how big these are they can never duplicate the volume or speed of airflow passing through the radiator of a car travelling at high speed. The exhaust system can also get very hot as it is tucked away from the fan's airflow whereas on the road it sees a good flow of cooling air. It isn't unknown for the mounting rubbers to melt right through, even at the very back of the system, leaving you with a silencer hanging on the ground. A spare set of mounts in the glovebox is good insurance. Check the coolant level, make sure there are no leaks and that the radiator is in good condition with its fins reasonably intact. It's common for these to corrode away or drop out on older vehicles leaving just the cross tubes carrying the water. A radiator like this might not cause a problem on the road but it will show up on the rollers in minutes. Replace the thermostat if there is any doubt about it opening properly. It can actually be a good idea to remove the car's own radiator fan. These tend to block a large part of airflow through the radiator and will be far less powerful than the fans in the dyno shop. On some cars it's just a matter of 2 or 3 bolts and an electrical connection to undo and the whole assembly can be lifted out and refitted before the trip home.

4) Ensure the exhaust system is in good condition with no leaks at the joints and no cracks in the manifold. These can play havoc with the calibration of the fueling system and if this is adjusted to compensate for an exhaust leak which hasn't been spotted then it will be wrong again once the exhaust is fixed. An engine which crackles and pops on the overun can be a sign of a cracked or leaking manifold.

5) If the induction system has been modified with twin carbs or throttle bodies etc, then make sure you are getting full throttle with the pedal fully down. This simple fault is one of the most common reasons for modified cars not producing their expected power increase. A good rolling road operator should be so used to coming across this fault on modified vehicles that he should spot it very quickly. That still leaves you to pay for his time though while he tries to get the linkage to work properly.

6) While you are looking at the induction side of things, check the air filter and replace it if it's dirty or clogged. Make sure there are no air leaks and check and adjust the float heights in the carburettor. This is especially important if twin carbs have been fitted in which case the settings need to be the same on both carbs. You might as well check that the jetting is the same in both carbs too and if you know how to balance them by listening through a piece of tube it will save the operator doing this himself with an airflow gauge. He'll probably check the balance anyway as a first step but it saves time if no adjustment is needed.

7) Check the condition of the tyres and standardize the pressures. It can help the tyres to grip on the rollers if the pressure is a little higher than for road use. I tend to use 30 psi for cars that normally run lower pressure than this. If your recommended pressure is higher than 30 psi then round up to the nearest 5 psi. The tyre pressure also makes a big difference to the transmission losses and with low pressures you'll get a lower wheel bhp figure. Using the same pressure every time you go to the rollers makes it easier to compare results from different sessions. Make sure you remember to reset the pressures to standard before driving home or some very strange handling characteristics might ensue. A really good operator should check the tyre pressures before he puts a customer's car on the rollers but I only know a couple who are this thorough. A seriously underinflated tyre, as well as reducing the power figures, can be very dangerous on the rollers just as it can be at high speed on the road. All the extra power that is being absorbed by the underinflated tyres is turning into heat in the tread and in extreme cases they can delaminate or the tyre can blowout.

8) On front wheel drive cars the tracking must be correct. If the tyres are pointing off at right angles to each other they will struggle to grip properly on the rollers and the resulting power figures can be meaningless.

With the above matters attended to the car should in good condition for its tuning session. In future articles we'll look at how to pick a rolling road, what needs to be done during a tuning session and how to understand the printouts or power figures you'll get given.

There's one final matter I'd like to try and set straight. I often hear people say "you can load an engine up much more heavily on the rollers than you can in normal use and that's why they can blow up". In physics terms that statement makes absolutely no sense. I suspect many people think about engines in the same way as they think about themselves - going uphill or carrying a heavy suitcase makes you work harder. Well engines don't operate like people. When you open the throttle fully, an engine is producing as much power, and working as hard, as it possibly can at those rpm. In other words it's as heavily loaded by the simple act of applying full throttle as it ever can be. All the external load does, whether that be vehicle mass, a gradient or a dyno load, is determine whether the engine, and hence the car, will accelerate or not but a bigger applied load can't make the engine work any harder or produce more power, it can only change the rate of acceleration. On a level road at full throttle the car will accelerate; on a steep hill at the same rpm the car might just hold a steady speed or even slow down despite being at full throttle. The engine, because it's at full throttle,  is working just the same under both conditions. The only measure, therefore, of the 'load' an engine is really seeing is the amount of power it is producing and has nothing to do with the external forces being applied to it.

What can happen though, as mentioned above, is that the engine can overheat if it is held at full power for long periods of time with inadequate cooling and on the rollers you can hold the engine flat out for as long as you like. It's hard to keep an engine at high rpm and full throttle for very long on the road because you'll either end up running out of road or having a chat with some gentlemen in blue uniforms about your excess speed. So you can certainly overheat an engine on the rollers but other than this there is no way to make it blow up that wouldn't have happened anyway in similar hard use on the road or race track. An engine that has been built properly will be able to withstand any abuse you can throw at it provided you don't overheat it or exceed its rpm limit. Even the most inexperienced rolling road operator isn't going to do either of those things deliberately. So if your engine has a mechanical breakage during its tuning session then that was probably going to happen anyway sooner or later. It's your own responsibility to tell the operator the maximum rpm you want him to use because he is not going to know how well the engine was built or what special bits are inside it. Provided he sticks to that limit and keeps an eye on the temperature gauge then nothing that subsequently happens should be his fault.

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