I have been doing some experiments recently and am willing + ready to share some of what I have learned if anyone is interested
This is if you want to gain an advantage over the competition, forget about off the shelf parts everything is custom when you want the best
The main area that creates power in any engine is the head, if you can flow X CFM @28" at 90% of your maximum valve lift you have the potential to make 1.03 x the CFM @28" if the inlet valves can reach and go beyond this lift flow point to create adequate "inlet flow area"
So if your max cam lift is .500" multiply this by 0.9 = 0.450", let us say the flow is 190CFM @28" at this lift point that equals a max power of 195.7bhp if everything else is maxed out including the full inlet system, exhaust, chamber pistons, ring seal, AFR etc which is a realistic power figure when using 48's or 45mm TB's completely unrestricted apart from the head flow itself
The injection head has a far superior inlet port shape having a better higher short turn and also much more room for improvement when squeezing all of the power you can get from a pinto as you will see later in this thread, carb heads can also be made work well but more work is involved and there is less potential working within the casting limits
First of all let us look at the limitations of the inlet ports and where the biggest improvements can be made
Although the injection ports have a higher short turn the port angle is still very low and is close to 83* measured from the valve guide centreline to the port centreline, however this isn't as big of a limitation to flow as one might expect, the real limitation is the distance/vertical height between the inlet valve seat and the port roof + port floor especially
An ideal port would be at least 2" taller creating much needed space between the inlet valve seat and the floor + roof of the port, this in turn creates a straight lead up to the valve throat and seat area so that the airflow can use much more of the valve area instead of the air skipping the short turn and flowing right across the back of the inlet valve but also restricting some of the flow around the long side of the port's turn into the throat area
To sum up the better we can make the short turn flow the more power we can make
What about optimal valve sizes?
Converse to what has been written in may books the exhaust port does not need to flow 80% of the inlet flow to make best power, for a NA Race/Rally engine with over 11 to 1 CR 65% exhaust flow vs inlet is about ideal when using the same cam lobes for inlet & exhaust, with even higher CR over 12.5 to 1 we can successful use as low as 60% exhaust flow vs inlet simply by using longer exhaust duration, opening the exhaust valve a little earlier as with high CR most of the cylinder pressure (what moves the piston and translates into torque & HP) is made in the early stages as the piston moves away from TDC, when the exhaust valve opens with high CR most of the power has already been made so opening the valve a little earlier isn't going to hurt power But what it does do is to change the "optimal inlet to exhaust valve diameter"
A good rule to follow for a 2V per cylinder engine such as the Pitno for optimal valve diameters is to make the inlet valve 50% of the bore diameter and to make the exhaust 40% of the bore diameter, with a 93mm bore that equals a 46.5mm inlet valve and 37.2mm exhaust valve as a good working estimate to follow without running into bore shrouding issues, and this will give a little over 65% exhaust flow vs inlet flow which can be used with an available cam
This is very different than the usual 44.45mm and 38.1mm valve combinations, with NA there is Never any need to use over a 37mm exhaust valve, even a 36.5mm exhaust valve is more than adequate for a high rpm heavy breathing roller cam monster
Is there really any advantage in using a 46.5mm inlet valve over a 44.45mm or 45.7mm inlet valve? - YES
Up to 46.5mm every bit of extra valve area counts as it gives increased inlet flow all the way from 0.050" / 1.27mm valve lift up to maximum lift and up to this valve diameter bore shrouding is not an issue, fitting a 48mm inlet valve might sound like a good option at this point but with a 93mm bore size it will not give any more flow than a 46.5mm valve unless you could move the valve away from the bore
Depending on the port shape used as a good estimate a 46.5mm inlet valve is worth 10 CFM @28" vs the same port with a 44.45mm inlet valve due to the larger throat area, the throat area is the area just under the valve seat and ideally it needs to be make 90% of the valve diameter, so with a 44.45mm inlet the the throat diameter will be 40mm ID and blended into the rest of the port, and with a 46.5mm inlet the throat would be 41.85mm or a 4.63% increase in throat diameter not to mention the increase in Flow Area at All valve lift points which shows up with flowbench testing
Remember for every 1CFM the head flows @28" at 90% of the max valve lift we are using the engine can make an extra 1.3bhp
However we should not get too greedy searching for extra air flow by making the ports excessively large especially for a low cc (not stroked & bored) engine
Port Velocity Is Important, the std inlet ports are already 38mm in diameter which is much too large for a 42mm inlet valve and ideal 37.8mm throat diameter, even with a 46.5mm inlet valve a 38mm diameter port CSA is about right for best torque and HP, the reason for this is that at mid to high rpm's on the inlet stroke before the valve closes the inlet system if working well can ram in more fresh inlet charge before the valve closes, remember the inlet valve of a race engine does not close until the piston is AFTER BDC, the piston actually travels up the bore 80* on average before valve closure Almost Half Way up the Bore!
If the inlet system has a minimum CSA that is too large the air velocity will be too low and some of the air will be able to escape out of the cylinders, but if the minimum port CSA is optimally sized the intake system will keep ramming in air up until valve closure giving a small supercharged effect from peak torque rpm upwards
On the other hand if the minimum port CSA is too small the peak air flow will be reduced, it will create great low end torque as the inlet system will have more ramming energy at low rpm than a larger port but the high rpm airflow will be restricted if the port is not made large enough it will flow "super sonic" and choke the top end especially at and after peak hp
So as with everything there is a balance between one extreme and another, a good inlet port air speed to aim for is 123CFM @28" per square inch of minimum port CSA = approximately 295 FPS Average port velocity at the minimum CSA
If we had a well developed inlet port flowing 217CFM @28" at maximum valve lift for a race/rally engine we can say that best power will be made if the port is:
( 217 / 123 ) / 3.14159 = 0.56157
Square root of 0.56157 = 0.749
Multiplied by 2 = 1.498" or 38mm
So a port flowing 217CFM @28" at maximum lift should have a minimum port CSA the same as a 38mm round port for an unrestricted top end and good peak torque
Which is more important mid or high lift flow?
Again it is not all about high lift flow, if the port flows big amounts of air at 16mm lift but the cam only lifts 12.5mm and at this lift point the flow is only average then the shape is not going to work well with the cam
Ideally we want to "tune the short turn shape" to match the camshaft, focusing on improving the mid lift flow as much as possible and having good flow at 90% of max lift for decent peak power, but the Average flow numbers added together are much more important than just the peak lift flow
By adjusting the shape of the short turn, lowering it down to get "a better look at the back of the valve" the high lift flow may increase by 3CFM but if the mid lift flow decreases by 7CFM we would then have made made a step in the wrong direction!
Remember the valve reaches peak lift for only an instant, but is reaches mid lift twice in the lift curve
In the next post:
What port shapes work best for the pinto engine and why?
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