EFI Intake ideas - any input?
Posted: Wed Jun 25, 2025 10:08 am
Hello,
Since i have a working throttle body (ITB) build on my type1 mild engine (using motorbike ITBs, injectors, fuel rails), which is working okay, I’ve been thinking about possible future designs.
I will be making an intake for my type4 project but may consider another type1 prototype for fun.
My preference is for a single throttle body and 4 port injectors but i’m not convinced the CB end casting style is the best approach (& no good for T4)
Design1
would have a head mounted section with injector bosses, that would rise up each side & for a type1 would split so there are two runners. There would be a joint in those runners so the upper section could be quickly removed for easy spark plug access etc. the upper section would go into a good sized plenum (one per head). Both plenums would be joined by a large bore cross pipe above the alternator (type1 & type4 cali), with a central mounted throttle body. With a decent length of primary runners, it should provide good port inertia. Overall, it’s similar to the intake on a modern 911 but obviously with only 4 runners & some packaging problems to fit around the upright doghouse shroud.
Pros: primary ID can be sized for purpose & not excessively so (killing airspeed & inertia). Large bore cross pipe avoids nasty harmonics, hopefully . Single throttle body means no linkage balance issues, good throttle progression and easy Map based tuning.
Cons: lot of elements to design. If the lower intake is 3D printed, it needs isolation from head. The throttle will be lazier than a set of ITBs. Lower section could be made in metal but at higher cost, and rest could be printed.
Design 2:
This could use a set of generic IDF manifolds as the base (per head). On top a printed section in place of the IDF carbs, which has injector ports but no throttle butterflies. That section would have some length of primary which then combines into a plenum (per head), similar to design 1. Again, a large bore cross pipe with central throttle body would link the intake sections on each side.
Pros: less to design & print. Lower section are easy/ cheap to source for T1 or T4. No modifications needed to the metal for injector bosses. As above, simple tuning & linkage.
Cons: injectors higher in the port. Carb manifolds are typically larger bore than needed for most engines, reducing the efficiency & airspeed in the primary runners. More likely to have fuel fog in the plenum section at certain rpms, perhaps causing imbalance.
So, any input on the ideas above?
Since i have a working throttle body (ITB) build on my type1 mild engine (using motorbike ITBs, injectors, fuel rails), which is working okay, I’ve been thinking about possible future designs.
I will be making an intake for my type4 project but may consider another type1 prototype for fun.
My preference is for a single throttle body and 4 port injectors but i’m not convinced the CB end casting style is the best approach (& no good for T4)
Design1
would have a head mounted section with injector bosses, that would rise up each side & for a type1 would split so there are two runners. There would be a joint in those runners so the upper section could be quickly removed for easy spark plug access etc. the upper section would go into a good sized plenum (one per head). Both plenums would be joined by a large bore cross pipe above the alternator (type1 & type4 cali), with a central mounted throttle body. With a decent length of primary runners, it should provide good port inertia. Overall, it’s similar to the intake on a modern 911 but obviously with only 4 runners & some packaging problems to fit around the upright doghouse shroud.
Pros: primary ID can be sized for purpose & not excessively so (killing airspeed & inertia). Large bore cross pipe avoids nasty harmonics, hopefully . Single throttle body means no linkage balance issues, good throttle progression and easy Map based tuning.
Cons: lot of elements to design. If the lower intake is 3D printed, it needs isolation from head. The throttle will be lazier than a set of ITBs. Lower section could be made in metal but at higher cost, and rest could be printed.
Design 2:
This could use a set of generic IDF manifolds as the base (per head). On top a printed section in place of the IDF carbs, which has injector ports but no throttle butterflies. That section would have some length of primary which then combines into a plenum (per head), similar to design 1. Again, a large bore cross pipe with central throttle body would link the intake sections on each side.
Pros: less to design & print. Lower section are easy/ cheap to source for T1 or T4. No modifications needed to the metal for injector bosses. As above, simple tuning & linkage.
Cons: injectors higher in the port. Carb manifolds are typically larger bore than needed for most engines, reducing the efficiency & airspeed in the primary runners. More likely to have fuel fog in the plenum section at certain rpms, perhaps causing imbalance.
So, any input on the ideas above?
