Re: modified merlin heads
Hi guys, first post for me on here, I'm the guy doing the modifications to these heads, thought I should get on here & give some insight into the work & the gains expected.
I'm going to keep this in laymans terms as much as possible - I don't know how much you guys in general know about engine design & modification, so I'll keep it in simple terms, if anyone want more in depth info just ask.
First up, the stock port design is very poor, not really suprising considering they were designed in the 20's before anyone really knew a lot about airflow through cylinder heads.
There are a few important things to consider when designing a port - flow is only one of them, also of critical importance is port size (or cross sectional area), the size of the port determines air speed through the port. Too large a port has very low air speed, low air speed does not produce any significant inertia ram effect - air has weight, like anything else, inertia is a function of weight & speed, increase the speed & you increase the inertia. Get the speed 'right' & the effect is that once on the induction stroke the inertia of the air built during the downstroke of the piston allows the air to keep flowing into the cylinder even after the piston has reached bottom dead centre & pressure in the cylinder is actually higher than that in the intake runner - the inertia of the air keeps it moving into the cylinder. This allows for greater than 100% fill of the cylinder.
Another critical dimension is intake runner length, the intake valve as it shuts creates a high pressure pulse that runs up the intake runner & is reflected off the runner entry (a pressure pulse will reflect from a significant change in cross section - like a runner entry), if the intake runner is 'tuned' to the right length you can set it up so that that reflectedhigh pressure pulse arrives back at the intake valve just as it is opening, giving the intake charge a head start on cylinder fill.
Both the runner cross section & runner length (& taper) can be calculated for any engine, this is common practice in modern competition engines, merlins are no different, they will respond in the same way as any other engine.
Now I'm gonna talk about flow, flow is measured in cubic feet per minute on a flow bench - a depression is pulled across the port & the valve/s opened at set intervals (usually .100" steps), the amount of air moving through the port is measured by the flow bench.
As well as measuring raw flow on a flow bench you can also look at the quality of the air flow - is it turbulent, does it separate from the port short turn at higher lift, how much does it swirl into the chamber etc etc.
The stock merling ports are very turbulent, no need to even use a velocity probe to determine the pulsing, you can hear it - very loudly! They have an ear piercing screech, the worst sound you can hear when flowing a head, that noise says the port is fighting itself & air is bouncing around where it shouldn't, the manometer on the flow bench was bouncing up & down to the point where actually taking a measurement was difficult & ended up having to be averaged from several tests.
In contrast the modified ports on these head have a nice stable sound & the manometer remained steady.
The average intake velocity on the stock heads was down under 200ft/s - hard to measure with the port behaving so badly, but somewhere around there.
The modified heads were much higher, around 280ft/s, very close to ideal for inertia ram.
So basically the new ports are much smaller, faster & a massive amount more stable than the stock ports, they flow a massive 23% better than the stock ports despite being around 1/2 the cross section.
In simple terms more intake flow means less boost is required to move the same amount of air, or more air can be moved at the same boost.
This has further knock on effects, the lower boost requirement means the source of the boost (be it a supercharger or tubocharger) is not working as hard to move the air, which for a supercharger means it does not require as much engine power to drive & for a turbo means less backpressure (which means less pumping losses & residual ex gas left in the cylnder), both of which will see significant net gains in engine power above the gains see by improved air flow & quality of air flow, so you basically double dip - you get more power from better flowing heads & reduce parasitic losses into the bargain.
All this means that these heads along with new intake & ehxaust manifolds *should* produce power gains in the region of 35-40%, provided the turbo's do not run out of puff - which they may well do.
Hi guys, first post for me on here, I'm the guy doing the modifications to these heads, thought I should get on here & give some insight into the work & the gains expected.
I'm going to keep this in laymans terms as much as possible - I don't know how much you guys in general know about engine design & modification, so I'll keep it in simple terms, if anyone want more in depth info just ask.
First up, the stock port design is very poor, not really suprising considering they were designed in the 20's before anyone really knew a lot about airflow through cylinder heads.
There are a few important things to consider when designing a port - flow is only one of them, also of critical importance is port size (or cross sectional area), the size of the port determines air speed through the port. Too large a port has very low air speed, low air speed does not produce any significant inertia ram effect - air has weight, like anything else, inertia is a function of weight & speed, increase the speed & you increase the inertia. Get the speed 'right' & the effect is that once on the induction stroke the inertia of the air built during the downstroke of the piston allows the air to keep flowing into the cylinder even after the piston has reached bottom dead centre & pressure in the cylinder is actually higher than that in the intake runner - the inertia of the air keeps it moving into the cylinder. This allows for greater than 100% fill of the cylinder.
Another critical dimension is intake runner length, the intake valve as it shuts creates a high pressure pulse that runs up the intake runner & is reflected off the runner entry (a pressure pulse will reflect from a significant change in cross section - like a runner entry), if the intake runner is 'tuned' to the right length you can set it up so that that reflectedhigh pressure pulse arrives back at the intake valve just as it is opening, giving the intake charge a head start on cylinder fill.
Both the runner cross section & runner length (& taper) can be calculated for any engine, this is common practice in modern competition engines, merlins are no different, they will respond in the same way as any other engine.
Now I'm gonna talk about flow, flow is measured in cubic feet per minute on a flow bench - a depression is pulled across the port & the valve/s opened at set intervals (usually .100" steps), the amount of air moving through the port is measured by the flow bench.
As well as measuring raw flow on a flow bench you can also look at the quality of the air flow - is it turbulent, does it separate from the port short turn at higher lift, how much does it swirl into the chamber etc etc.
The stock merling ports are very turbulent, no need to even use a velocity probe to determine the pulsing, you can hear it - very loudly! They have an ear piercing screech, the worst sound you can hear when flowing a head, that noise says the port is fighting itself & air is bouncing around where it shouldn't, the manometer on the flow bench was bouncing up & down to the point where actually taking a measurement was difficult & ended up having to be averaged from several tests.
In contrast the modified ports on these head have a nice stable sound & the manometer remained steady.
The average intake velocity on the stock heads was down under 200ft/s - hard to measure with the port behaving so badly, but somewhere around there.
The modified heads were much higher, around 280ft/s, very close to ideal for inertia ram.
So basically the new ports are much smaller, faster & a massive amount more stable than the stock ports, they flow a massive 23% better than the stock ports despite being around 1/2 the cross section.
In simple terms more intake flow means less boost is required to move the same amount of air, or more air can be moved at the same boost.
This has further knock on effects, the lower boost requirement means the source of the boost (be it a supercharger or tubocharger) is not working as hard to move the air, which for a supercharger means it does not require as much engine power to drive & for a turbo means less backpressure (which means less pumping losses & residual ex gas left in the cylnder), both of which will see significant net gains in engine power above the gains see by improved air flow & quality of air flow, so you basically double dip - you get more power from better flowing heads & reduce parasitic losses into the bargain.
All this means that these heads along with new intake & ehxaust manifolds *should* produce power gains in the region of 35-40%, provided the turbo's do not run out of puff - which they may well do.
Comment