Question:
I have just bought a standard XE engine and I want to tune the engine further as time & funds allow. Would you recommend for my initial spec carbs or injection?
Answer:
I would always recommend fuel injection over carbs. For example you are looking for between 180 – 200bhp, whereas a standard XE engine on carbs would produce in the region of 180bhp. A standard engine on tapered throttle bodies would produce 200 – 210bhp. With the advantage of improved response and driveability, not to mention very good fuel economy.
In theory, you should be able to get the same amount of air through a carburettor and you could do through a throttle body of a similar size. There are many reasons why in reality it does not work quite as you would expect. One of the simplest being, that if you fitted a choke size to your carburettor the same as a throttle body you may well be able to get fairly close, but when you are not trying to achieve peak bhp the gas speed is slower. A fuel injection system is programmed to supply the fuel as the engine requires, whereas a carburettor can only supply the fuel that the vacuum of the engine can draw through it. This is why carburettors have different choke sizes fitted. You have to compromise on peak power over power through the entire rev range. There are many other reasons that come into play, I could literally spend days describing some of the tests we have carried out over many years.
One of the tests we carried out was to take a standard 2.0L XE engine, and place it on the dyno, we initially ran the engine on carburettors and optimised the settings, we gained to sets of results. An engine that was drivable throughout the entire rev range obtained approx. 180bhp. We experimented with much larger choke sizes to see what could be achieved, we managed to push the horsepower up to 190bhp but the engine was completely undriveable on anything other than peak rpm. We then took off the carburettors and fitted multi-throttles, the engine instantly produced 195bhp and was even more drivable than the carburettor engine using the smaller chokes. After carrying out all the tests on multi-throttles, we then changed to taper throttles, we managed to produce 208bhp, the engine had slightly more bottom end torque than the multi-throttles, a small increase in the mid-range torque and the torque hung on much longer up the rev range, which was why it produced 208bhp.
On each of the systems we tested, we found it necessary to optimize the exhaust manifolds as well, as each system had a slight variation in its requirements to produce it’s best.
Over the years we have experimented with various engine specifications and have never found that even if an engine does produce, let’s say 20 horsepower more than another but over a very small power band, that the engine with the wide power band makes the car quicker round the track.
If you wish to progress slowly, which is what many of our customers do, we use the same throttle bodies on most of our fuel injection kits and as I am sure you can appreciate, as you try to obtain more bhp, you get smaller returns for ever increasing cost. As a rule of thumb, if you convert from carbs to fuel injection, we would normally expect to see between 10 & 20 bhp gains (this depends on carburettor and choke sizes) with a huge improvement in driveability. You could then improve your engine in stages as your budget allows. The system could then be programmed on the road, rolling road or dyno to achieve the optimum from your current engine.
2021 Update
This FAQ was quite written quite a few years ago and effectively nothing has changed, the only thing that has made it more likely that you would go for throttle bodies is that carburettors are now considerably more expensive than throttle bodies because they are manufactured in small quantities since they are not used as often. Also when you get them to a rolling road, they take a lot longer to set up than fuel injection and it is extremely likely that you will need to change jets, chokes and possibly other parts within the carburettor to optimise your settings, these are not cheap and since most rolling roads do not deal with carburettors any longer, the parts may not be available.
Category:
Fuel Injection