Twin throttle plenum
Discussion
I was working out area and a twin 50mm diameter gives a slightly bigger opening than a single 70mm butterfly.
So to create a Venturi into the inlet manifold I assume we want a bigger throttle opening allowing a larger volume of air into the plenum down the flared trumpets into the narrower inlet manifold and into the cylinders. In effect each part that narrows helps to speed up the air into the engine. That was my thinking of the reason for bigger area throttle not reserved just for big bhp engines?
So with a bigger dia afm I should get a reasonable bhp increase without going on a bonkers expensive build. So a big valve head matched ports to the inlet manifold to keep the air moving with a slightly higher lift cam keeping hydraulic lifters and standard valve springs.
Looking at the original sd1 twin plenum it used 60mm diameter butterflies on a 3.5 v8. Would twin 50mm be too small for a 4.2?
So to create a Venturi into the inlet manifold I assume we want a bigger throttle opening allowing a larger volume of air into the plenum down the flared trumpets into the narrower inlet manifold and into the cylinders. In effect each part that narrows helps to speed up the air into the engine. That was my thinking of the reason for bigger area throttle not reserved just for big bhp engines?
So with a bigger dia afm I should get a reasonable bhp increase without going on a bonkers expensive build. So a big valve head matched ports to the inlet manifold to keep the air moving with a slightly higher lift cam keeping hydraulic lifters and standard valve springs.
Looking at the original sd1 twin plenum it used 60mm diameter butterflies on a 3.5 v8. Would twin 50mm be too small for a 4.2?
Isn’t the valve size really the restriction?
No amount of throttle butterflies will compensate for how much air can actually be sucked in via the heads.
Even on Phazed 5.5 with massive everything I think on the rolling road max power was achieved with only 3/4 throttle as the engine is held back because of the heads even with stage 4 porting and biggest valves possible. His engine produced 400 hp I seem the remember so that’s proof his ACT plenum was providing all the engine could suck in at 3/4 throttle!
I’m sure that’s why the “wild cat” heads were conceived and why turbo charging became popular as that’s a far cheaper way to force air in.
Smaller butterflies do offer a more responsive engine due to the faster air speed created at lower revs as you mention but get towards 4500 revs and that becomes a minimal gain due to the valve size restriction.
It’s also why most good gains on acceleration on an already good engine are usually achieved with a modern Ecu system.
Being able to program the Ecu to give more advanced ignition and bespoke fuelling to your engine at lower revs gives the engine a noticeable difference in instant acceleration and add loads of torque.
You also remove the restriction of the AFM which helps get more air in. . Mine was much louder on induction roar from the front of the car after having a new Ecu. I think that can only be created with a wide bore pipe right to the butterfly and maximising valve and ignition timing via the mapping.
That’s my understanding of the science.. whether that’s correct is another question.
Overall I think twin smaller butterflies can only be a good thing.
Interesting project for sure.
No amount of throttle butterflies will compensate for how much air can actually be sucked in via the heads.
Even on Phazed 5.5 with massive everything I think on the rolling road max power was achieved with only 3/4 throttle as the engine is held back because of the heads even with stage 4 porting and biggest valves possible. His engine produced 400 hp I seem the remember so that’s proof his ACT plenum was providing all the engine could suck in at 3/4 throttle!
I’m sure that’s why the “wild cat” heads were conceived and why turbo charging became popular as that’s a far cheaper way to force air in.
Smaller butterflies do offer a more responsive engine due to the faster air speed created at lower revs as you mention but get towards 4500 revs and that becomes a minimal gain due to the valve size restriction.
It’s also why most good gains on acceleration on an already good engine are usually achieved with a modern Ecu system.
Being able to program the Ecu to give more advanced ignition and bespoke fuelling to your engine at lower revs gives the engine a noticeable difference in instant acceleration and add loads of torque.
You also remove the restriction of the AFM which helps get more air in. . Mine was much louder on induction roar from the front of the car after having a new Ecu. I think that can only be created with a wide bore pipe right to the butterfly and maximising valve and ignition timing via the mapping.
That’s my understanding of the science.. whether that’s correct is another question.
Overall I think twin smaller butterflies can only be a good thing.
Interesting project for sure.
RobXjcoupe said:
I was working out area and a twin 50mm diameter gives a slightly bigger opening than a single 70mm butterfly.
So to create a Venturi into the inlet manifold I assume we want a bigger throttle opening allowing a larger volume of air into the plenum down the flared trumpets into the narrower inlet manifold and into the cylinders. In effect each part that narrows helps to speed up the air into the engine. That was my thinking of the reason for bigger area throttle not reserved just for big bhp engines?
So with a bigger dia afm I should get a reasonable bhp increase without going on a bonkers expensive build. So a big valve head matched ports to the inlet manifold to keep the air moving with a slightly higher lift cam keeping hydraulic lifters and standard valve springs.
Looking at the original sd1 twin plenum it used 60mm diameter butterflies on a 3.5 v8. Would twin 50mm be too small for a 4.2?
I wrote all that waffle before reading this. So to create a Venturi into the inlet manifold I assume we want a bigger throttle opening allowing a larger volume of air into the plenum down the flared trumpets into the narrower inlet manifold and into the cylinders. In effect each part that narrows helps to speed up the air into the engine. That was my thinking of the reason for bigger area throttle not reserved just for big bhp engines?
So with a bigger dia afm I should get a reasonable bhp increase without going on a bonkers expensive build. So a big valve head matched ports to the inlet manifold to keep the air moving with a slightly higher lift cam keeping hydraulic lifters and standard valve springs.
Looking at the original sd1 twin plenum it used 60mm diameter butterflies on a 3.5 v8. Would twin 50mm be too small for a 4.2?
What size are the butterflies on the ACT twin plenum, as that might be a good guide.
No idea but it doesn’t sound far off.
Valves size on the heads I have are slightly bigger with 41.4mm inlet and 35.5mm exhaust, bullet valve guides fitted also. Heads have had additional work with machined valve throats and then blended by hand. And a 0.5mm skim to accommodate the later composite head gasket.
Interesting you say max power at 4000rpm as my engine is going into an auto so probably spot on for that.
I have a opportunity to produce a twin butterfly plenum so wondering if it’s worth doing really
Interesting you say max power at 4000rpm as my engine is going into an auto so probably spot on for that.
I have a opportunity to produce a twin butterfly plenum so wondering if it’s worth doing really
Valves size on the heads I have are slightly bigger with 41.4mm inlet and 35.5mm exhaust, bullet valve guides fitted also. Heads have had additional work with machined valve throats and then blended by hand. And a 0.5mm skim to accommodate the later composite head gasket.
Interesting you say max power at 4000rpm as my engine is going into an auto so probably spot on for that.
I have a opportunity to produce a twin butterfly plenum so wondering if it’s worth doing really
Interesting you say max power at 4000rpm as my engine is going into an auto so probably spot on for that.
I have a opportunity to produce a twin butterfly plenum so wondering if it’s worth doing really
Valves size on the heads I have are slightly bigger with 41.4mm inlet and 35.5mm exhaust, bullet valve guides fitted also. Heads have had additional work with machined valve throats and then blended by hand. And a 0.5mm skim to accommodate the later composite head gasket.
Interesting you say max power at 4000rpm as my engine is going into an auto so probably spot on for that.
I have a opportunity to produce a twin butterfly plenum so wondering if it’s worth doing really
Interesting you say max power at 4000rpm as my engine is going into an auto so probably spot on for that.
I have a opportunity to produce a twin butterfly plenum so wondering if it’s worth doing really
std 65mm plenum will take you up around 280 before it starts to become a restriction - and that assumes you are not running an air flow meter which is only 60mm diameter and the rest of the inlet and engine for that matter is 500 spec. (this was based on comparing atmospheric pressure and plenum pressure at wot/max rpm on a 500)
I believe (but dont know for a fact) that the twin plenum is 2x65mm
a bored out 72mm will take you beyond 280 bhp, but only in conjunction with griff 500 trumpet base (44mm id ) and associated ported manifold. (i have a 72mm plenum and 500 trumpet base available) - but as above, only suits bigger power cars running aftermarket ecu ditching the afm, mapped ignition and better injectors.
I believe (but dont know for a fact) that the twin plenum is 2x65mm
a bored out 72mm will take you beyond 280 bhp, but only in conjunction with griff 500 trumpet base (44mm id ) and associated ported manifold. (i have a 72mm plenum and 500 trumpet base available) - but as above, only suits bigger power cars running aftermarket ecu ditching the afm, mapped ignition and better injectors.
So got this afm. Its internal dia is 76mm so larger than the standard plenum throttle. So if I used twin 50mm diameter throttle butterflies that’s a tad smaller in area of the afm so it’s all getting a little narrower as the air is sucked into the cylinders and hence speeding up as it goes through the cylinder head.
Looking at the act site they offer the same size afm with an appropriate chip for the original management system.
Cylinder head has clear had work. So using the original ecu set up I think this should pep up the standard power output.
My griff 500 has had engine work (previous owner) but on its standard chipped ecu makes 300bhp at the flywheel which is considerably down as I’m reading from others using a modern management hitting that figure at the wheels!
Seems incredible getting that much extra by changing the management system.
Valves size on the heads I have are slightly bigger with 41.4mm inlet and 35.5mm exhaust, bullet valve guides fitted also. Heads have had additional work with machined valve throats and then blended by hand. And a 0.5mm skim to accommodate the later composite head gasket.
Interesting you say max power at 4000rpm as my engine is going into an auto so probably spot on for that.
I have a opportunity to produce a twin butterfly plenum so wondering if it’s worth doing really
Interesting you say max power at 4000rpm as my engine is going into an auto so probably spot on for that.
I have a opportunity to produce a twin butterfly plenum so wondering if it’s worth doing really
RobXjcoupe said:
92 Range Rover auto
Waste of time IMO - all that extra breathing is only required at high rpm in a lightweight car, RR auto is the complete opposite of that and will change up around 5k anyway.Some ACT trumpets in the standard base is probably about as far as you need to go with that combo and engine size.
For a rangie, i would consider the THOR manifold as fitted to the later P38's - as that has long narrow runners which give good low down torque over the predessor GEMS manifold (which is almost identical to the 14CUX/Hotwire manifold). This would require an aftermarket ECU and the fuel rail modifying to returned (from returnless) and still probably not worth the effort unless you want a project.
eliot said:
RobXjcoupe said:
92 Range Rover auto
Waste of time IMO - all that extra breathing is only required at high rpm in a lightweight car, RR auto is the complete opposite of that and will change up around 5k anyway.Some ACT trumpets in the standard base is probably about as far as you need to go with that combo and engine size.
For a rangie, i would consider the THOR manifold as fitted to the later P38's - as that has long narrow runners which give good low down torque over the predessor GEMS manifold (which is almost identical to the 14CUX/Hotwire manifold). This would require an aftermarket ECU and the fuel rail modifying to returned (from returnless) and still probably not worth the effort unless you want a project.
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