Calculate the Power Potential of an Engine


The first two articles have covered the main items that need to be considered when trying to evaluate the power potential of an engine. All that needs to be done now is to look at the equations that turn valve area into potential bhp. We will assume that all ancillary parts of the engine design such as induction system, exhaust system, compression ratio etc can be modified such as to impose no further restriction on the power potential. What we are going to calculate is the potential peak power of a fully modified engine in race tune with excellent port work and “perfect” induction and exhaust system design. This analysis applies mainly to engines designed originally for normal road use but we will also consider briefly how a custom designed race engine like an F1 engine might fit into this scenario.

Step 1 – calculate the valve area

The area we need here is the total area of all of the inlet valves in square millimetres. Hopefully everyone reading these technical articles will know how to do this but I suppose for the sake of completeness…

Valve Area of each valve = diameter squared x pi ÷ 4 – then multiply by the number of inlet valves in the engine.

For a fully modified engine we must consider not the standard valve sizes but the maximum valve size that might be fitted. As a rule of thumb, most road engines have enough space to enable valves 7% bigger in diameter to be fitted into the combustion chamber but of course this varies from engine to engine.

Step 2 – Adjustment for the type of engine design

We have seen that different engine designs have different power potentials for a given valve area. A basic adjustment to “weight” the valve area for these considerations needs to be done.

2 valve per cylinder, parallel valve – reduce area by 10%

2 valve per cylinder, inclined valve – leave area as is

4 valve per cylinder – increase area by 10%

Custom designed, money no object 4 valve per cylinder race engine – increase area by 25%

This rather rough and ready “weighting” gives us a broad brush approach to refining the power prediction based on just engine type and valve area. We make no consideration as yet of engine size or the effect of the camshaft design and valve train type. Some common sense is going to have to be applied if it can readily be seen that a particular engine has a severe design limitation in some particular area such as valve lifter diameter. The venerable MGB engine for example is so limited by its Siamese port design and pushrod valve train that it doesn’t get anywhere near the power potential predicted here. Some of the parallel valve engines with overhead cams can rival the inclined valve engines in power output though.

With enough development work on port design and cam profile the bhp targets can eventually often be beaten. In the USA the Chevrolet V8 engine has been refined over the years by dozens of engine tuners spending thousand of man hours on research. Its power output, given the age of its initial pushrod design, now beggars belief and rivals that of some OHC 4 valve engines.

Step 3 – Predict the flywheel bhp

Take the adjusted valve area from step 2 and divide by 30

This now gives us our predicted flywheel bhp in full race tune. In other words, high compression ratio, top notch carburation or throttle bodies, good exhaust system, race camshafts and fully ported, flow bench developed cylinder head.

For fast road tune a good target would be 75% of the above figure and for rally tune about 85% to 90% of it.

Note that engine size has never entered this analysis at all. In fact engine size does play a role in potential power output but nothing like as much as is commonly believed. That’s a story for another article though.


1) – The venerable 2 litre Ford Pinto engine has been tuned by so many people that its power potential is pretty well known. A really good one can just beat the 200 bhp mark and David Vizard in his excellent book on the engine achieved 212 bhp after years of development work. Lets see how the numbers stack up.

Inlet valve size is 42mm but the normal big valve used in race engines is 44.5mm. Engine type is SOHC with inclined valves so no adjustment to the base valve area is required.

Valve area is 44.5 x 44.5 x 3.1416 ÷ 4 = 1,555.3 sq mm per valve x 4 valves per engine = 6,221 sq mm

Power potential = 6,221 ÷ 30 = 207 bhp. So not a million miles out then. Copyright David Baker and Puma Race Engines

2) – Let’s try a little comparison between two different engine types – the 3.5/3.9 litre Rover V8 and the 1905 cc Peugeot 405 M16. At first glance you might think there would be no contest. The Rover has twice the number of cylinders and twice the engine capacity but does it work out that way?

The Rover V8 has 8 cylinders and 40mm inlet valves – total valve area 10,053 sq mm. Being a 2 valve parallel valve pushrod engine we need to reduce this by 10% though and end up with 9,048 sq mm for a power target of 302 bhp.

The Peugeot is a 4 valve per cylinder engine with 34.6mm inlet valves. Total valve area 7,522 sq mm. We need to add 10% though for the 4 valve engine type to arrive at 8,274 sq mm and a power target of 276 bhp. Rather less in it than might otherwise have been thought. Of course the 1.9 litre 4 cylinder engine is going to need to turn some pretty serious rpm to develop the power its cylinder head is capable of supplying though.

3) – and finally for a bit of fun. See if you can guess whose engine this is. 3 litre, all aluminium V10, 4 valve per cylinder and 35mm inlet valves – I think we can also safely say that money was no object here.

Total valve area is 19,242 sq mm and we need to bump this up by 25% for the engine type to arrive at 24,053 sq mm

Power target is therefore 24,053 ÷ 30 = 802 bhp. Hmm, not too shabby for a 3 litre engine. Vorsprung durch technik as they say in Germany just before sliding into the tyre wall and breaking both legs.


Please don’t get the idea that from one simple measure like inlet valve area we can arrive at a definitive power target for an engine. What this analysis should have done is give you the basic tools for understanding how to evaluate an engine and arrive at sensible power targets which will be at least “in the ball park”. It hopefully shows though just how important valve area is compared to other factors like engine size or number of cylinders. Actually achieving the power targets depends primarily on the skill of the cylinder head modifier though and that’s why most engines never get anywhere near their full potential. The difference between a well modified head and a poor one can be 20% of the engine’s power potential.

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