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Sabre Heads
Sabre Heads

To many of us it seems a dark art and to many who have some knowledge- wether in or outside the motoring industry- it's all about opening up the ports and polishing. All very wrong, what creates flow in a head is it's valves and the seats!! all the component parts around these items effect this flow but do not create it. This is the point at which it gets complex, or maybe not!!?? For many years the knowledge has been about to design superb heads and engines, yet until very recently many have been dire. The single cammed pushrod engine had many limiting factors, a touch better was the single overhead cam design, going twin cam helped further but the advent of 16 valve, twin cam heads going into production cars, has made life significantly better. Not many of the ideas here and now are that new, 3 valve heads were around in Rudge motorcycles in 1904? as were compound angle valve heads, even rotary valve designs tried- what has changed is the materials/parts that make them reliable, as well as a far greater understanding of whats going on, this aided by ever more sophisticated machines to gauge and even emmulate/visualise these processes. But no matter what, flow comes down to those poppet valves and their seats!!

The valve seat has three purposes- 1) seal the port/combustion chamber, 2)cool the valve, and 3)guide the air through the valve. Multiple angle to fully radiused seats are essential for good air flow, plus using the flowbench often shows a particular valve/seat shape to suit a particular head. A three angle seat will outflow a simple 45 degree seat by up to 25% at lower valve lifts. The 45 seat top edge should be set to the valve O/D for maximum flow, the width of the seat has quite an effect on flow, thinner seats offer more flow but one has to be sure to not effect longevity, small gains from ultra thin seats are not worth reliability losses. 1mm INLET seats work on all heads well, one can drop to .8 of a mm and remain safe. Exhausts are a complicated issue, 1.4mm works well on modern 16V engines but many manufacturers use far less with no issues, commonly 1.2mm on small capacity engines but surprisingly to me, many Mitsubishi's as in the V6 GTO/ Evo have 1mm exh seats on a turbo engine! I've seen similar work on Mercedes. Rather unusual when turbo's tend to 'Trap' heat in the engine, admittedly I haven't spent millions on development and as these engines suffer no problems it leads one to rethink commonly held knowledge!!

The flow through an engine is ultimately determined by the valve diameters. While well designed smaller valves will outperform larger valves on occasion, a good, big valve will always outflow a good smaller valve. Common sense!

Valve size is limited by a No. of factors, first the engine bore diameter- the bore also effects the combustion chamber size as the combustion chamber is usually 1mm smaller in dia. than the bore, this allows for casting/alignment irregularities. Then your valve is affected by it's proximity to another, and in the case of unleaded heads- restrictions induced by the need for a seat insert for the exhaust valve. In the case of alloy heads , an insert is needed for each valve- these generally 1mm larger than the valve dia and often far greater. Also the angle of the valves from the vertical- which will also effect lifts at overlap/tdc. Thus the three/four and even 5 valve head can utilise this space more effectively than any two valve, with the added advantage of greater flow.

A particular point most people do not know is that the Exh. valve is more efficient than an Inlet- this is why the exh. valve is smaller than the Inlet!! As soon as you start adding components- such as inlet manifolds/plenums, you begin losing all that lovely/often impressive bare head flow- this is why a single barrel/throttlebody per intake port will release alot of horsepower. While the opposite often occurs with the exhaust, put on a manifold and the flow will often remain the same or go up!! This is why a well designed exh. manifold will make good gains- particularly torq. Add to this the fact that Intake filling relies on atmospheric pressure to fill the void being created by the dropping piston on it's downward stroke- whereas the piston forces the exhaust gases out and as these lose heat/area, they literally create a vacuum behind them drawing more gas from the cylinder- one can see it gets complex tuning an engine- particularly for more.

Back to flow and how in this instance it can be done wrong, highlighting the value of the flowbench in development, as well as getting it right!! In this instance, an Xe 16v 2ltre, brought to me by a new customer who felt he had been fiddled on a 'Professionally produced Race engine'. After the cust. having found fault with the bottom end, I was asked to give/provide a rundown of faults (if any)etc on the head. Without going into all the faults found, the following is the result of the flow tests done, which show how it can be done wrongly and how it can be reworked to do as it should have from beginning!! In this instance the inlet was ok but the exh. a disaster but we will go over both.

Standard Xe 16v headflow, 33mm valves, all std parts- 86mm bore, 50 thou/1.25mm increments of lift

23.4cfm  47.7   72.3  94.8  104.9  111.97  114.77  116.87  118 97

The 'Professionally' race modded Inlet, std 33mm valves- 86mm bore. Perfectly reasonable.

22.45    46.9   70.7  93.3   111.3  125.3   131.6    134.4    138.6

Reworked and fitted with Race quality 34mm R.e.c stainless valves- 86mm bore.

23.9      49.6    75.2   99.5  118.5    130.7   139.8    143.9   147.6

Now the exhaust. As with the inlet, the MODIFIED HEAD ENTRY/EXIT SIZES DID NOT CHANGE from how the head came in, the rework/flow changes were within the ports/seats/valves. First figs are a std head, 29mm valves.

20.4cfm  45.9   73.4   89.4   91.9   97.3    98.4    99.3   99.8

This is the supposedly 'professionaly' race modded exh. flow, std 29mm valves. See how all the valuable low/mid flow has been destroyed, then achieving a puny improvement at the point where the lame lift cams fitted- stopped!!

  18.9     39.2    59.3   77.2   89.6   97.3   102.2  105.4  108.2

All is not lost, using 2mm larger valves- the only way to reintroduce certain seat/port features removed by the previous work- the head was saved. 31mm R.e.c stainless valves-86mm bore.

  22.9    47.9   75.95   95.3  109.7  114   117.3   118.1   118.1

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