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It is All in the Cylinder Heads

It is All in the Cylinder Heads

Neil Harvey Performance Developments

This paper is not about how to do the work but why you should do it. It will use the Porsche GT3 engine as an example.

I read about all these great engine builds, “I have a 4.0L, I have a 4.2L engine, I have this tuner, I have that Turbo, I have this tune, which one is better?” It goes on and on to the point of boredom. Over the years we have made tuners the rock stars of the engine business. As the “hot rod” world has become more about tuning most end results, regardless of who and what system is used, become about the same.

Often the engine is stressed beyond its internal components ability and parts break. Take the example of the Turbo engine. These were boosted beyond the connecting rod ability without any sort of calculations being done and the result is a slew of bent rods in engines. This is the wrong way to go about “hot rodding” the engine but, unfortunately, quite common. All too often engines are modified by “from the hip” technology and not with any sort of developed calculations. This is all done at the owners expense.

So now the world of tuning is starting to look at the mechanical side of the engine, but a lot of tuners do not understand how to make more power mechanically or where the losses occur and how to take advantage of these losses to gain back power. It should end with tuning not start with it.

To simplify things, an engine is a device that turns chemical energy into mechanical energy with the goal to do it as efficiently as possible. An engine is made up of many parts, crankshaft, rods, pistons, cams but nothing more important than the cylinder head(s). Without a good efficient cylinder head everything else is unimportant. I think you can differentiate certain parts of an engine into parts for reliability and others into performance parts. An oil pump would be more about reliability than a cylinder head is, but both can fail and a good pump can add some extra horsepower too.

If we can turn that chemical energy into mechanical energy more efficiently with a higher percentage of the fuel supplied burned (combustion efficiency) and turned into mechanical energy quicker (burn rate) we will make more power.

Let’s look at this in more detail. You have a 4.0L engine with a longer stroke and a bigger bore. It has the capacity to fill a greater volume of air per its induction stroke than an engine with smaller dimensions. It has to be better, right? Not always. Only if the part that allows air into the cylinder will flow the extra air. This is the efficiency often described as volumetric efficiency. The ability to fill the cylinder to its capacity and more if under pressure. A 3.0L engine may have a higher efficiency if the parts in that engine work better than the engine that has 4.0L displacement. Brake mean effective pressure (BMEP) is often shown as a function of an engines volume (displacement) so that an engines specific output can be compared to others engines. How well it produces power from its displacement. Imagine a 3.6L street GT3 engine and a racing 4.0L RSR engine. The 3.6L street engine may make only 2/3’s of what a race RSR engine makes but it could make its power with the same efficiency. This calculation allows us to look at individual engine components and see where and what we can improve to increase and engines output regardless of the engines size. Simply put, work done as a function of the engines displacement. The biggest gain in these Porsche engines is not the displacement but in the air exchange process.

When you stop and think about it, what is the point of a performance engine if it does not perform? All too often I read and see where the cylinder head is forgotten about and never given the respect it requires in order to make an engine efficient. Why? Because most of the time the person “specing” out the engine does not know or understand what is involved. It is like asking a blind man to feel what an elephant looks like. Which end does he feel and does it matter? Tuners often send out the cylinder head work for reconditioning (performance engine remember) and have them returned all assembled without even understanding what has happened. Even more to the point, does not know how important good work can make the engine perform and how bad work can kill an engines performance. And these are performance engines in sports cars.

My background in Formula One embedded the point that there are only black and white colors here. It is right or it is wrong. Unfortunately the Industry has clouded this business in shades of gray. So many times I read or hear about how there has never been a problem before, never had a problem, cars have run ok, so what is the problem? The problem is you bought a sports car with a performance engine and because the industry does not know or care, they are turning it into a grocery getter.

Typically the cylinder heads are sent out to a production machine shop for reconditioning and the work is based upon time and cost. The shop doing the work has to make a profit and has to be competitive against other production shops to stay in business. So what happens? Corners are cut, good enough is the flavor of the day without any consideration of the final result or customer satisfaction. The “disconnect” is in progress as is the degradation of performance . What matters are the details. The small things really do count. It is all in the details!!

The air flow across the seat and how you manage the air that is getting into the head all makes what a good cylinder head should be. Air management is what it is all about. The angles used, or if radius are better, widths, where the valve contacts the seat, seat heights, valve back cuts, valve back shapes, the seat shape directly under the seat itself and where it blends into the port walls are all extremely critical. Guide clearances for good seat memory, how the air transitions from which port wall to the bottom shapes is critical on the intake and the opposite on the exhaust. These are the details that are very important in an engines performance and so often completely disregarded by production shops and not even considered by the repair shop assembling the engine. These details are common in all heads but each head has its own unique behaviors which can only be found by flow bench testing, dyno testing and in car testing.

It is probably a good time to mention what some of the critical areas are in a cylinder head or maybe more to the point the areas where flow is lost. The biggest loss is across the valve entering the chamber on the intake and across the seat itself. We can help the loss across the valve entering the chamber by the shape of the back of the valve. Across the seat is all about the lower angles on the seat. The area around the guide has some loss but less than the seat. Wall surfaces account for less than 5% of loss, with the short radius having slightly more effect. The GT3 heads are already CNC ported so unless you are prepared to digitize the existing ports and redesign their shapes, reconditioning is typically just about re cutting the seat widths and angles. If done without care you can lose up to 20% of the airflow through the port. Today, unlike yesterday, we look at each individual cylinder and how much torque it produces. We have to look at the small details and areas where an engine loses torque from friction, and other inefficiencies in the hopes of gaining back some of the lost performance. It would be counter productive to lose any performance deliberately by not doing the basics correctly. But first you have to understand what correct is and respect the work required in order to achieve it.

There is an old saying that a 4 cycle IC engines operation is all about “suck, squeeze, bang and blow.” So if we take the parts that are about the cylinder head it would be the suck bit and the blow bit. The gas exchange from outside the engine to inside, then outside again. That really is what the cylinder head does or should do. We should also remember the cylinder head is part of the static compression ratio equation as well. However; more so about the engines dynamic compression, as this is about the actual filling of the cylinders, not the theoretical assumption that the static compression ratio calculation is based upon.

So you may have an engine from the manufacturer that is stated to have a compression ratio of 12:5.0. It may start off with a dynamic compression ratio of XXXX. Why would you not want the same again after spending all that hard earned money having your engine rebuilt or hot rodded? In my opinion way too much importance is placed upon turbo’s, exhaust systems and other parts that are easily sold as upgrades and not enough on the basic parts that make or break engine performance.

All an engines performance is made or not made from the cylinder head. So the most important part of any performance engine is the cylinder head. Sadly, it is the most forgotten, disrespected and overlooked part in the Porsche world.

It takes time and a lot of money to get this all right and most production shops do not have the time to do this. Modern Porsche engines have 4 Valves per cylinder and 6 cylinders. That is 24 valves, 24 guides, 24 valve seats and 24 valve springs. All of which require attention to detail. One must make sure all of the valve seat heights are all the same distance from the chamber roof, all the same widths and angles, the margins on the valves are the same width, valve lengths are the same, spring retainer heights measured and confirmed, valve springs checked for equal rates, seat pressures (installed) are the same, over the nose (open) rates are the same, and coil bind distances measured and adjusted for safety. A quick note to check here, if the shop rebuilding your heads does not ask for the engines camshaft lifts and expected RPM limit, none of these checks are probably being done. If you have cams with higher lift than stock and this is unknown by the machine shop, you could put the valve springs into coil bind.

Camshaft timing or valve timing as it should be known is all about filling the cylinder as efficiently as possible to the maximum as possible. If you rebuild an engine with less air flowing into the cylinders and keep the same cam positions you will have lost some of your volumetric efficiency. When you have all of the parameters correct, airflow, port size, bore and stroke, including all other factors, you will always be able to make the most power. GT3 street engines have variable camshaft control on one or both of the camshafts. Electronically the camshafts can be moved so that they provide the best timing for all RPM’s. Cup engines are fixed typically somewhere around 220 LSA, as their operating RPM range is lot narrower.

Here is a basic model of what happens and why air flow within the cylinder head is so important.

As the piston descends down the cylinder from TDC and the intake valve opens, air (and fuel) are accelerated along the intake runner by the increasing cylinder demand. Typically cylinder airflow demand peaks at 72 to 84 degrees after TDC and then declines to zero by 5 to 10 degrees after BDC. However, air flowing in the intake runner does not decelerate as quickly as cylinder demand decreases. The faster the air flows (without friction or turbulence) in the intake runner and the larger its volume, the more air will fill the cylinder. Miss on the valve seat shapes and some of the other critical areas and you will not have the cylinder filling we chase, to increase the VE over 100%. In the cup world these incremental details do make a difference and are the reason why one car can pull a car length on another down the straight. In the street world it may not be a race track, but the degradation of engine performance is the same.

If you want the best, the quickest car your money can buy, want to win or have the best chance to win, then make sure you have your engine assembler pay attention to what is important. Ask him what he considers important. It is your engine, it is your money.

There are some really good places in the US that do incredible cylinder head work. These places are specialists and have become specialist because they know what is important and how to do it. Compare their costs against your local machine shop. The difference is in the details, the time they spend to give you the very best, not the mediocre or the hidden disasters wrapped up in pre assembled heads given back to the assembler. Insert the elephant riddle here.

I have spent years working towards perfecting the work we do, only to find out we have to keep changing how and what we do to achieve the best performance and results. Engines are changing all the time and so should we in how we approach the “hot rod” business. We have to continually increase our knowledge, upgrade our equipment, open up our minds and accept new ideas. What we used to think was right has been proven since the advent of computers, especially CFD software, to not necessarily be always right. There is no shoe that fits all. An engine is the sum of its parts. Some more important than others when it comes to making power.

If only we could see air. It would be so easy.

Perfection may be unattainable, but trying should not be.