How to Modify
AussieSpeed performance inlet manifolds - Tech Tip
Isn’t it tempting when you get your new parts home to think how can I squeeze every last bit of horsepower out this new engine I am building and get the die grinder out and go nuts opening up the manifold ports ……………. Well don’t even think about it ….. just read on here and find out why.
I am guilty of it, thinking I know what I am doing and have completely cocked it all up, from engines to body work you name it I have had to rework it so listen to this old fool  tell an old tale that I am sure will be repeated again and again by others…………………
When a performance manifold is designed properly like the inlet manifolds from AussieSpeed the plenum & manifold runners are sized and shaped for high velocity yet big enough to breath at the upper rpm. One thing that most racers don’t pick up on, is evenness of flow from port to port so that your cylinders are filled with the same amount of charge, of the air fuel mixture. Too little in one and too much in the other will have one cylinder running lean and another running rich. The lean one will be burning head gaskets regularly, misfiring and detonating while the rich one will be fouling plugs, coking up and be contributing stuff all power to the engine by the middle of a 40 to 50 lap race.
Straight six engines which have all 6 cylinders in a line are probably the hardest inlet manifolds to balance flow. The short runners to cylinders 3 & 4 usually have them running rich while the end cylinders 1 & 6 lean out. Dyno tuners sometimes run different jets in the inner and outer barrels of the carbie in an attempt to make it better. Some engine builders have gone to the trouble of having a camshaft custom ground so that each lobe profile is different enough to get the evenness of charge into each cylinder.
The ideal inline six manifold would be a meter long [3 ½ ft] or more for equal length runners, however this would place the carburettor in a position beyond the width of most cars and in a perfect posi’ to wipe out dudes standing at the bus stop on the side of the road as you blast past ……………….. not good.
The 6 cylinder manifolds that come closest to this are the older slant 6 manifold and the later Hemi Hurricane manifolds for the Chrysler engines shown above and below.
In most cases we are pushed for space so the manifold places the carburettor close to the head thereby making the manifold design itself extremely important. As an example the following tale is of an experience I had with a customer who went and port matched their manifold to the head port opening, which is quite a common thing to do and regarded as usually what has to be done for performance applications however this was far from the case and it really opened my eyes to how much work has gone into the AussieSpeed performance manifolds on the flow bench. My customer may recognise their handy work here, believe me I am not picking on you, I am just trying to help others and I admit fault too because I thought it would do no harm so don’t get offended mate  ………….. it is as much my fault for not pulling you up.
The picture above shows pre production testing of an AussieSpeed manifold being flow bench tested. On a new design manifold AussieSpeed tests a prototype and alter the casting cores as needed and test again and again for evenness of flow until satisfied before a product is released. So you can be assured they are ready to work right out of the box. The feedback I have received from dyno tuners is that it takes very little time to get the fuel mixtures right on engines running AussieSpeed manifolds. However in the picture below shows the one we had problems with after the customer modified the ports.
The manifold on top is the out of the box casting and the manifold on the bottom has had its ports modified at the head end. Click on any of the photos to see a larger version.
If you look closely the porting is quite rough and not blended smoothly back into the runners at all, so one cannot assume all ports jobs are executed well for a start, however if you have a keen eye you will notice all ports are slightly different in size too on the modified one.
Look closer again at the ports to 1 & 6 and there is a very sharp inside edge that has taken all of the nice radius away and left a turn that is more than 90 degrees, so this is a good example of how not to port in the first place however the point I want to make is this;
Notice in the standard AussieSpeed casting the ports have gradual radius and a nice gradual tapper into the head. These ports are shaped and sized in a particular way that works well for both air speed and volume for the plenum shape of this particular manifold.
Different plenum shapes and sizes for the various different brands of engines behave differently so what may work for the Ford EA-AU manifold shown here does not work for the Holden blue motor and definitely not for the Chrysler sixes. Only flowing the manifold on a flow bench will tell you if you have made it or modified it right.
The bigger modified port has that wow, bugger me, that’s big look, grouse more power? But in actual fact it made insignificant difference in power on the dyno, however the tuner could not get the air fuel ratio right with the modified manifold.
What was happening was there was an imbalance in the charge filling each cylinder, when we flowed the ported manifold, cylinders 1 & 6 were not even and running less cfm than the out of the box manifold and cylinders 3 & 4 were not even and running slightly more cfm, it was now way off the mark.
Once we bolted on an out of the box AussieSpeed manifold the fuel mix came back and was quickly set after a run or two on the dyno. In the end it could be leaned out quite a bit and we actually learnt a bit about the behaviour of the OHC Ford straight six engine during these tests which we will elaborate on in another tech tip article, certainly not what we expected to see stay tuned.
So to recap, the best way to modify an AussieSpeed manifold is DON’T …….. Ron.
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