It's the age old
'airflow vs air velocity' conundrum Rog
You may assume an inlet manifold, inlet ports, exhaust ports, and an exhaust system that flow more air/gas are the way forward, but the truth is bigger is not always better! This is because air/gas velocity is equally important, arguably more important! In reality you need both velocity and flow, but the unavoidable truth is this will always force a set of compromises.
However, velocity is what really allows an engine to achieve high Volumetric Efficiency (VE), largely because even after the piston reaches the bottom of its stroke (BDC) and starts moving back up, the intake valve often stays open. High-velocity air traveling down the inlet runner and past the open inlet valve creates
inertia that continues pushing air into the cylinder even when the piston is on its way back up the bore, while counterintuitive this inertia effect maximises combustion chamber filling with the lovely air/fuel charge and
even when the piston is compressing!.
When tuning internal combustion engines its a classic mistake to focus solely on increasing flow, because any increase in flow will come with a reduction in velocity, this is why when tuning Monty I've always tended to focus on choosing components that promote high gas speeds, flow while also important has always been secondary to me.
What does always need to be carefully considered is, if the air starts moving too slowly, that pressure change reverses the flow pushing air back up the intake hurting power. Longer inlet runners like those found on the Fusion Fabrications inlet manifold generate a higher inertia effect because there's a greater mass of air pushing the column of air harder into the combustion chamber from behind. Making the runners narrower also increases air speed, this high velocity improves Volumetric Efficiency (VE) further while at the same time helping to limit reversion.
The exact same thing happens on the exhaust side, this is why Monty's long primaries work so well. If you're now running retarded exhaust cam timing as with Newman's Phase 3 cams, and you open up the exhaust ports too much, the subsequent reduction in gas velocity can result in reversion, and the fact the exhaust valves are still open just makes this even more likely. This is why I chose to advance Monty's inlet cam by 1 degree and retard his exhaust cam by 1 degree too, if you open the exhaust valve earlier, because the duration is fixed by the grind, it stands to reason it shuts earlier too and hopefully just in time to stop the inert exhaust gasses finding their way back into the combustion chambers. It's also why my decision to open up Monty's exhaust ports from 38mm to 40mm was so controversial.
Shutting Monty's exhaust valves earlier helps limit reversion, this is preferable because when exhaust gasses end up back in the combustion chambers they displace the valuable air/fuel charge, this again kills power and causes poor idle quality and drivability issues. The exhaust ports on a Duratec are 38mm, this is already pretty large for a 2.0 litre four cylinder engine, and for your old 1.8 they are really too large. It's the same on the inlet side, Duratec inlet ports are huge, too big really for good idle quality, low speed drivability and emissions.
This is why the swirl flaps exists, the idea is to create
'dynamic port sizing'. The term 'Swirl' is actually a bit misleading, while I'm sure they do promote some tumble, they should really be called 'Velocity' flaps. At idle the flaps are in their closed position creating substantially smaller exhaust ports, as the air goes through the now flap restricted ports its velocity increases dramatically, this limits reversion and helps to keep the (relatively) heavy fuel in suspension with the air. To some degree its much like a how the venturi works in a carb, although unlike a carb obviously you're not also using the pressure drop (depression) to draw the fuel up as we have pressurised fuel injectors.
What may be a surprise for some of you is the porting work on Monty's head wasn't really about increasing flow, the port re-shaping was actually all about increasing air velocity and so increasing Volumetric Efficiency (VE). To increase flow too, we opted for 1mm larger valves, and controversially we also opened up Monty's exhaust ports from 38mm to 40mm. That's a lot more flow, and with the progressive Phase 3 cams that have lots of overlap the flow mods were certainly risky, the top end performance improvements were more or less a given but the real concern was did we take the elements that increased flow too far, only to discover Monty ended up with a reversion generated splashy idle and poor low speed drivability?
As with all engineering challenges you inevitably end up with a set of compromises, you also never really know how all those compromises will play out until you test them. Given the foundation of my design brief was to retain Monty's driver friendly road manners as best I could, in the end I took an educated gamble and bet heavily on my Fusion Fabrications long runner inlet and long equal length primaries exhaust manifold generating sufficient air/gas velocity to limit the effects of reversion and deliver sufficient Volumetric Efficiency (VE) to produce good low speed drivability too.
I also knew I had one additional ace up my sleeve, my ECU Masters EMU Black ECU, TBH without it I never would have even contemplated putting Monty on Phase 3 cams and a big valve head. Early signs are the increased flow gamble paid off with Monty now idling happily at 1,050rpm, and amazingly retaining his excellent previous slow speed drivabilty too. To my surprise and joy, l can still trickle Monty through town on tiny throttle openings between 1,500rpm - 1,800rpm, I now believe retaining Monty's standard weight flywheel was the right decision, and the fact he only weighs in around 930kg is also a big advantage in achieving the better than expected flexibility.
With the idle and drivability now sorted, moving forward it's going to be all about measuring Monty's peak torque, and more importantly his spread of torque. TBH, if I can match Monty's previous torque results I'll be delighted, only then will I look at the horsepower gains because without a stable idle, good town manners and a broad spread of torque, all the peak horsepower in the world would be for nothing!
