Chasing Every Last Horsepower With Porting!

If you read the blog article about my car in October last year, you may recall that I was preparing to install a new cylinder head onto the engine. Well, that work is all complete now. After a day on the dyno, I asked Dan if I could follow up and share the results, as they were pretty surprising, and a strong illustration of the value of increasing flow through head work.

So just a quick recap on the previous specs of the car:

Forged 1.8 bottom end, measured 8.7:1 compression ratio.

Stock Mk2 non vvt head (BP4W)

0.87litre Autorotor twin screw supercharger, 160/64mm pulleys (2.5:1 ratio)

Laminova water to air chargecooler.

E10 petrol (the car does have water injection, but these figures are just on petrol)

This setup made 303bhp / 268whp / 250lbft over at Bailey Performance in December 2021.

What did I do?

On the intake side the work focused around increasing the efficiency of the port – not the size. The key area to work is the ‘bowl’ which is the area just in front of the intake valve. I smoothed the long side turn (top of the port) area out to provide as gentle a turn as possible for the air to turn into the combustion chamber, without any steps or crevices to disturb the flow. On the floor of the port I just took the sharp edge off as the port abruptly turned into valve seat. This area is called the short side radius or ‘SSR’, and particularly on the mk2 heads it is a very sharp turn. Just taking the edge off and grinding a radius will help allow the air to stay attached to this turn for a little bit longer, which increases the flow efficiency of the port, effectively using more of the valve area. 

One area that I did not touch, apart from to clean up any casting flash, was the mid section of the port. This is the area in front of the valve guides. This is the area with the smallest cross-sectional area. This area acts like a venturi to increase the velocity of the air charge. For a given flow rate of a gas if it has to flow through a smaller area, it has to flow faster than through a large area. Velocity is a very important factor for an effective port, as it increases the kinetic energy of the air (KE=1/2mv2) which in turn increases the ability of the air to fill the combustion chamber which is ultimately the goal. The flip side of this is a port that is too small will cause too much of a restriction at high flow rates at high rpm. My engine’s max rpm is only mildly higher than stock, and I determined the port was already large enough for this use.

It’s also worth mentioning that a larger port isn’t necessarily required for a boosted application. Your engine still inhales half its capacity of air per revolution with its normal volumetric efficiency, except the density of this air charge is increased by the supercharger or turbo. It is still important to get this denser air moving quickly and turned gently into the combustion chamber to maximise kinetic energy and chamber fill, just as it was at atmospheric pressure without a power adder.

On the MX-5 exhaust system side, I worked on increasing the flow potential of the exhaust port. Getting the exhaust gases out of the combustion chamber as quickly as possible, with the least pressure against the rising piston on the exhaust stroke. This was done by installing +1mm exhaust valves and increasing the diameter of the port just behind the valve. The area of the valve guide received a significant remodeling to smooth out the port.

The camshafts I fitted were a ‘sports’ cam. 264mm duration and 10mm lift. These aren’t too wild with stock valve timing, but they do offer a fairly significant gain in valve opening area during an engine cycle over the stock BP4W cam.

On to the fun part!

New specs of the engine:

Forged 1.8 bottom end, but with a head skim now a measured 9.5:1 compression ratio.

Upgraded head based on BP4W, ported +1 exhaust valves, 264/264 10mm cams

0.87litre Autorotor twin screw supercharger, 160/64mm pulleys (2.5:1 ratio) – same as before

Laminova water to air chargecooler – same as before.

E10 petrol only – same as before.

Back on the same dyno the car made 350bhp / 310whp / 260lbft. This is an increase of around 50bhp (15%) and 10lbft (4%).

This run made a little more torque and a little less whp, but figures averaged as above over a few runs.

This was achieved at generally a lower boost pressure. Fuelling requirements were up around 10% across the entire full load area of the map, to maintain the same mid 11s air fuel ratio (AFR), see fuel map comparison below.

It seems the increase in compression ratio and exhaust flow also helped. I think this 9.5:1 compression ratio is about the limit on my setup with normal fuel as it was quite sensitive to detonation in the sub 4000rpm area, with a reduction of spark timing required to keep things safe. 

Here is a graph of the difference in boost and fuel requirements between the stock BP4W and the new upgraded head.

Boost is lower apart from one area around 5krpm, but fuelling requirement is significantly increased for the same AFR with the upgraded head.

While I was on the dyno I swapped out the larger intake cam for a stock mk2 intake cam (BP5A).

This cam has less lift and duration, which I expected to make more torque lower in the rpm range, but not flow as much in the upper rpms, but overall produce a quicker car in real life, with more ‘power under the curve’. However installing the smaller intake cam dropped power by around 20bhp and reduced torque to 250lbft, the previous values from Dec ‘21. We did multiple runs at different cam timings but could not get it to make more than 330bhp / 290whp /250lbft. The engine really liked the extra flow area the larger cam offered across the entire rev range. This can be seen in the lower fuelling requirement with the smaller cam, to maintain the same mid 11s AFR.

Faded traces are the stock cam. Boost was higher, duty cycle of injectors lower for the same AFR at each rpm with the smaller cam, indicating more restriction, less air flow and less power produced.

The cam timings we found worked best with the big intake cam actually created quite a lot of overlap, around double that of the stock BP4W cams. Having sufficient overlap, with a low exhaust back pressure allows the combustion chamber to be fully purged of all exhaust gases and totally replaced by fresh air charge. This allows for volumetric efficiencies in excess of 100%.

On my supercharged engine the difference can easily be felt on the road, it feels far more urgent. Datalogs show acceleration is noticeably quicker. I did this with mostly simple, well known port modifications, without the aid of a flow bench. Any further development is going to need a flow bench, but the link between volumetric efficiency and power is very promising, even on my supercharged engine using the same pulleys.

I hope you found this interesting!