Basic V8 tuning....The inlet system
Rover's Vikingship V8 Tuning.....The Inlet Rover's Vikingship

The inlet system...For tuning this is one of the most important details of an engine for more power. The inlet is even more important than the outlet. The amount of air and fuel drawn in at the inlet stroke determines the amount of power that can be produced at the power stroke.

So what determines how much air and fuel can be drawn in? The engine can only get in an amount of air which equals its capacity..right?? Well not entirely so. It is usually a bit less. There is always some resistance in the inlet piping which increases with engine speed. A good engine usually gets about 70% to 90% of its capacity. The ratio between the theoretical amount of air/fuel and the amount in reality is called volumetric efficiency. So if you add a turbo or supercharger you can put in more air/fuel than the engine's capacity and thus get a volumetric efficiency which is higher than 100%.

Inlet restrictions
  • Inlet opening (grille, scoop on the bonnet)
  • Inlet piping to the air filter
  • Air filter housing
  • Air filter
  • Inlet piping to carburettor, or Flow meter
  • Carburettor, air Flow meter, throttle body
  • Inlet manifold
  • Inlet piping in the cylinder head
  • Valve stem and valve
  • Combustion chamber

As you can see this is quite a list. And a lot to go wrong and thus to be improved upon. Let's see how it works with our SD1.

The inlet opening
The air needed for the combustion is drawn in from the engine bay. But this air has to come from somewhere. The main area where the SD1 gets it's air is from the area beneath the bumper. Because it is a wide area it is not very restrictive.In fact as speed increases the air pressure in the engine bay increases, not a bad thing. However there is one drawback. The air which enters the engine bay has already been partly heated up by the radiator. For power the air must be as cool as possible. This is why you often see intake systems which protrude above the bonnet. (Ram air intake). But this is only for serious power. Normally taking in air from under the bonnet will do nicely any alterations won't get you a lot of horses.

The inlet piping to the air filter
Ever looked at the air inlet piping of a carburetted SD1? be it a 2300/2600 or a 3500. Boy, is this bad! it is long....very long, and it even takes the air in from near the exhaust where the air is the hottest....Emissions, I know, but it is far from ideal from a performance point of view. Getting rid of all this piping will already help a bit. For a Vitesse things are a lot better in this respect as the air filter housing takes in the air in front of the car.

Another important point is the way the inlet is formed. When using a straightforward inlet the air will contract while entering the inlet system. If a concave inlet shape is used the contraction will be less or reduced all a lot less resistance

Supercharged Rover V8...Raising effective pressure means power!!  18,5 kB

The airfilter housing
A small filter housing can create a lot of resistance. Fortunately the Vitesse and carburetted SD1's aren't too bad in this area. The filter housings are of a reasonably good design.

The airfilter
For everyday use the air filter on the carburetted and EFI V8 engines has quite a large filter area. As a result the air resistance isn't exceptionally high. However the air filter is one of the main restrictions in the inlet system so any improvements here are worthwile. K&N has a nice line up of performance filters for the carburetted and injected Rover engines. Even the 6 cyl. can be fitted with one. The K&N filter not only has considerable less resistance but it also lets you get rid of all the inlet ducting and air filter housings. It makes the engine bay looking a lot better too.

How much can be gained with a K&N filter? Well the diagram shows that the K&N filters flow significantly better than standard items. Be careful when replacing your standard filter for a K&N one. Because the resistance is less the vacuum in the carburettor will be less at the same air speed then before. As a result your engine will run leaner. So you have to adjust your mixture. Check the plugs! (see maintenance page).
This is not critical with EFI engines because the air flow meter measures the amount of air entering the engine and doesn't work on inlet manifold pressure like the carburettor does.

One carb feeds one cylinder 5,9 kB

But this is not all. Because the inlet resistance is less the volumetric efficiency of your engine rises. This means a higher effective compression ratio than before. In the page about basic tuning we already mentioned that flame speed increases with the compression ratio. So decreasing inlet resistance also means increasing flame speed. As a result your ignition can do with less advance. See the ignition page for more details (not up yet).

So you can see just bolting on a K&N isn't all to it. The rest of the engines systems must be altered too to get the most performance. Only with these adjustments can K&N filters give the advertised 5 to 10 hp increases.

The carburettor/injection
With all its jets, butterfly valves, etc. the carburettor is one of the largers restrictions in the inlet system. There is a lot involved in tuning a carburettor or injection system so we have another page on it soon.

Any comments, or do you want to ask or give additional info or do you have questions, just let us know!!

The influence of the inlet shape  5,5 kB
Inlet shape
Using a pipe with sharp edges for the inlet isn't the best solution as the picture above shows. Because of air turbulency contraction occurs and the effective inlet diameter is greatly reduced. Making the inlet wider with a tapered design makes matters even worse!. Flow is reduced no less than 5,5%. If the inlet has a 270 degrees roll back with a 1/4" radius flow improves with 5,2%. Vitesse owners can see this in the shape of their inlet pipes under the inlet plenum box.
Long, long standard inlet design 21,6 kB
Inlet length
The picture above shows clearly the very long route the air has to follow before it enters the engine. Here is a standard 3500 V8 set up. Look at the following points which are certainly not good from a performance point of view
  • The inlet air is preheated by the exhaust manifold, hotter air means less air mass to enter the engine.
  • A long ducting to the top of the engine.
  • A 90 degree turn towards the air filter, lots of turbulence at the T-section because it isn't styled to give low resistance flow
  • Passing through the air filter, which isn't that badly shaped
  • A 180 degree turn to the carburettor
Now with a K&N instead of all the inlet plumbing above it will be no surprise things can only improve. However it isn't original.....But it's your car.

Why More is not always better!

If two carburettors are not enough then let's take four....Four SU's or Webers give a very impressive appearance but do they help? Let's assume you buy a special manifold to bolt on four standard SD1 HIF44 SU's. This gives quite an improvement in breathing doesn't it?

Well, it does improve only a small bit on a reasonably tuned engine but can give some nice gains on all out racing engines. How does this happen, Well it gets a bit technical but the story behind it is as follows. The firing order of the V8 is:


Normally there are two carburettors one for cylinders 1-4-6-7 let's call it carburettor A

and the other one for cylinders 8-3-5-2 let's call it carburettor B

If we look at the interaction between the carbs and the firing order we see this:


In the diagram we see that there is a 50% overlap between carb. A and B but no overlap at all for one carb. This means that effectively carburettor A. and B. are delivering air/fuel to only one cylinder at a time, not more !

With four carbs there is still one carburettor delivering the air/fuel to one cylinder only. Only the time between the carburettor delivering it's mixture increases. Thus no improvement! Then why do you see them?. The answer lies not in the carburettor but in the manifold. When using four carbs the inlet manifolding can be made very straight with little resistance, but that's not the main thing. The big gains can be had because the length of the inlet manifold can be excactly matched to the engine characteristic. This gives the tuner the ability to extract the maximum power in a certain rpm band. However this only has real effect on serious high revving engines and thus only of use in serious racing applications.

mainpage © A3aan Dec 2000