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What is 7th injector?


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#1 ViMaMo

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Posted 03 July 2009 - 04:29

Could anyone explain what a 7th injector is?
Googling didnt help, so looking for help here. :)

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#2 gruntguru

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Posted 03 July 2009 - 05:59

Could anyone explain what a 7th injector is?
Googling didnt help, so looking for help here. :)

What sort of engine? If it is 6 cylinder, the 7th injector would be an additional injector feeding the common air duct prior to the intake plenum. Example would be an injector in a turbo inlet.

#3 ViMaMo

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Posted 03 July 2009 - 06:23

What sort of engine? If it is 6 cylinder, the 7th injector would be an additional injector feeding the common air duct prior to the intake plenum. Example would be an injector in a turbo inlet.


Any sort of engine, what does it do?
So in a 8 cylinder enginer its called 9th injector? Is it applicable only to turbocharges/supercharged?

#4 Bill S

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Posted 03 July 2009 - 06:45

An example might like the one on the Ford Falcon turbo I used to own. They never originally came with a turbo but a company was able to retrofit them from new (before they left the Ford factory in fact) with a turbo kit that made 7psi boost. Made the car pull like a bulldozer and shread tyres at wil, and so was a huge amount of fun to drive.

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Anyway, they did the mods to the EFI system to keep the fuel scheduling about right by adding a small computer that sensed manifold pressure and when it senses positive boost it'd run a 7th injector that sat near the throttle body.

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That version of the Ford six had one injector per cylinder, so the positive boost injector was the 7th injector in this case. Crude, but it seemed to work okay.

#5 gruntguru

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Posted 03 July 2009 - 07:11

An example might like the one on the Ford Falcon turbo I used to own. They never originally came with a turbo but a company was able to retrofit them from new (before they left the Ford factory in fact) with a turbo kit that made 7psi boost. Made the car pull like a bulldozer and shread tyres at wil, and so was a huge amount of fun to drive.

Posted Image

Anyway, they did the mods to the EFI system to keep the fuel scheduling about right by adding a small computer that sensed manifold pressure and when it senses positive boost it'd run a 7th injector that sat near the throttle body.

Posted Image

That version of the Ford six had one injector per cylinder, so the positive boost injector was the 7th injector in this case. Crude, but it seemed to work okay.

Some had an 8th injector too. Driven by a (Callaway?) "Microfueller" from memory. Very crude indeed.

#6 McGuire

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Posted 03 July 2009 - 11:24

Any sort of engine, what does it do?
So in a 8 cylinder enginer its called 9th injector? Is it applicable only to turbocharges/supercharged?


Not necessarily. More commonly, a 9th (or 7th, or 5th) injector is also used as a "CSI" aka Cold Start Injector.... OE on Honda, Toyota, GM, etc. Is mounted somewhere in the plenum where it can feed all cylinders, and is operated when the engine is cold and rich FAR is required. As the engine warms up (or times out) the auxiliary injector is shut off. (May only work when the engine is cranking or for a few seconds upon startup.) Same function as the auxiliary injector on a boosted engine, but is operated when the engine is cold instead of under boost.

Auxiliary injector schemes arose because electronic fuel injectors tend to be more accurate when their fuel delivery is confined to a narrow range. Beyond around 80 percent duty cycle their flow loses linearity and the accuracy goes away. Also, the larger the delta the more expensive an injector tends to be, requiring better parts, more precise assembly, higher rejection rates, etc. So the auxiliary injector became the more precise and cost-effective solution. Especially on aftermarket kit-type stuff as one auxiliary injector is cheaper than an entire set of often-expensive upgraded injectors.

Some years back there were race engines that used two complete sets of injectors because one set that would feed one engine properly was too expensive and troublesome. With a pair of injectors per cylinder they could use production injectors straight off the shelf and still obtain better fuel management. Also employed on nitrous oxide systems where lots of extra fuel is required to feed the additional oxygen.

Today, injector design and manufacture have progressed to the point where auxiliary injector are generally no longer required -- they can get the required accuracy with the primary injector array.


#7 Greg Locock

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Posted 03 July 2009 - 11:24

Early on fuel injectors had a fairly poor dynamic range, that is, if they could flow x g/s at fully open, they weren't so good at small fractions of x, such as what you needed at idle, which is incredibly important for emissions, and idle stability.

So, if you wanted lodesapower, and decent emissions, one quick fix was to run multiple smaller injectors, and switch the extras off at low power.

I can't remember exactly but Lotus ran either one or two extra injectors for the turbo when we went to GMP4.

#8 Dragonfly

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Posted 03 July 2009 - 11:41

IIRC, some old mechanical injector systems had an additional one for cold starting the engine by injecting additional fuel into the air duct and making the mixture rich.

#9 gruntguru

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Posted 04 July 2009 - 06:39

Not necessarily. More commonly, a 9th (or 7th, or 5th) injector is also used as a "CSI" aka Cold Start Injector.... OE on Honda, Toyota, GM, etc. Is mounted somewhere in the plenum where it can feed all cylinders, and is operated when the engine is cold and rich FAR is required. As the engine warms up (or times out) the auxiliary injector is shut off. (May only work when the engine is cranking or for a few seconds upon startup.) Same function as the auxiliary injector on a boosted engine, but is operated when the engine is cold instead of under boost.

The vast majority of these "cold start injector" systems:
- only operated during cranking
- were not ECU controlled (controlled by ignition switch crank position + Thermo/time switch)
- were continuous spray rather than pulsed

I've never heard one of these referred to as a 7th, 5th or 9th injector.

Edited by gruntguru, 04 July 2009 - 06:41.


#10 gruntguru

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Posted 04 July 2009 - 06:46

Auxiliary injector schemes arose because electronic fuel injectors tend to be more accurate when their fuel delivery is confined to a narrow range. Beyond around 80 percent duty cycle their flow loses linearity and the accuracy goes away. Also, the larger the delta the more expensive an injector tends to be, requiring better parts, more precise assembly, higher rejection rates, etc. So the auxiliary injector became the more precise and cost-effective solution. Especially on aftermarket kit-type stuff as one auxiliary injector is cheaper than an entire set of often-expensive upgraded injectors.

Sierra Cosworth with 8 injectors on a turbo 4cyl is a good example. The need for auxiliary injectors is greatest on high-boost turbo engines as they have a much higher dynamic range (small displacement engine needing very little fuel at idle but having very high fuel demand at high output with ultra-rich AFR for detonation suppression and internal cooling)

Edited by gruntguru, 04 July 2009 - 06:47.


#11 Tony Matthews

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Posted 04 July 2009 - 08:45

Sierra Cosworth with 8 injectors on a turbo 4cyl is a good example. The need for auxiliary injectors is greatest on high-boost turbo engines as they have a much higher dynamic range (small displacement engine needing very little fuel at idle but having very high fuel demand at high output with ultra-rich AFR for detonation suppression and internal cooling)

On the RS 500 race engine. The road car was supplied, I think, with the extra four injectors but not activated - the max power output of the standard RS was only 220 hp, 15 up on the base Cosworth, but you certainly needed all 8 to get from 300 - 600+ hp! But - was there a ninth? I can't remember...

#12 phantom II

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Posted 04 July 2009 - 13:32

The extra 8 injectors on the 90-94 4 vale ZR1 Corvettes were only activated at full throttle. It was a Bosche idea first used on Porsche.

#13 McGuire

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Posted 04 July 2009 - 14:46

The need for the 2x injector system was established by Tom Coddington at Specialized Vehicles, Inc. in Troy, Michigan. A 4.0L Lotus V8 was sent to SVI to serve as the mule for the engine management system. There it was converted from carburetors to EFI using an L98 P4 box and they began mapping out the baseline requirements. It soon became apparent that 1x injection had too little effective range to do a good job. Then GM CPC had another added cost (doubled, actually) in the LT5 to consider and no real way out of it. The LT5 program just sort of growed that way.

On the LS9/ZR1 they got around it with two-step fuel pressure. Also, injectors have progressed a good bit since 1985-6.



#14 cheapracer

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Posted 05 July 2009 - 06:00

On the RS 500 race engine. The road car was supplied, I think, with the extra four injectors but not activated - the max power output of the standard RS was only 220 hp, 15 up on the base Cosworth, but you certainly needed all 8 to get from 300 - 600+ hp! But - was there a ninth? I can't remember...


Well about 650hp actually, dont you remember? Oh thats right the English didnt know that was possible, some hick Ozzies had to come to England and show you. :lol:

Dick Johnson/John Bowe bought a Shell Sierra to Silverstone and got pole by 1 second (Dick hadn't even seen the track before) and demolished the lap record in the race. Left the Egganberger and Rouse Teams, as well as the British press stunned how much more power the Oz cars had.

Suprising thing was that in the race the water pump failed (the car was easily leading), never had been trouble before and the car was setup for Australia's heat as well.

#15 Tony Matthews

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Posted 05 July 2009 - 11:14

Well about 650hp actually, dont you remember? Oh thats right the English didnt know that was possible, some hick Ozzies had to come to England and show you. :lol:


Cheeky Cheapy! Must have been a disappointment to see Godzilla running away at Bathurst!


#16 cheapracer

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Posted 05 July 2009 - 13:09

Cheeky Cheapy! Must have been a disappointment to see Godzilla running away at Bathurst!


Oh you mean the fully Oz developed Nissan that also was by far the fastest GTR(s) in the World, in fact the fastest ever Group A car ever built in the World?

By the way, theres a little bit of me in those he, he, he! :kiss:


#17 ViMaMo

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Posted 06 July 2009 - 03:51

Thanks for the replies!

So its mainly used :
1. Aiding cold starts
2. Offset poor dynamic range of older primary injector
3. Extra power delivery

#18 gruntguru

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Posted 06 July 2009 - 03:53

Thanks for the replies!

So its mainly used :
1. Aiding cold starts
2. Offset poor dynamic range of older primary injector
3. Extra power delivery

If it's referred to as a 7th injector, it probably has function 2 and 3 but not 1 - thats a "cold start valve"

#19 slinky

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Posted 07 July 2009 - 11:50

Oh you mean the fully Oz developed Nissan that also was by far the fastest GTR(s) in the World, in fact the fastest ever Group A car ever built in the World?

By the way, theres a little bit of me in those he, he, he! :kiss:


For info, we run 16 injectors on the TT-V8 in RedVictor.. The fuelling requirements mean that we'd rapidly overcome the usable duty cycle on single injectors per cylinder...

It is a bit of a special case though..;)

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#20 McGuire

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Posted 07 July 2009 - 12:43

If it's referred to as a 7th injector, it probably has function 2 and 3 but not 1 - thats a "cold start valve"


Sure, that's the Bosch term for a CSI in the duplex position, for example K-Jetronic. (Which originally had no ECU, thus no ECU control of CSV.) But in other applications the duplex injector(s) will often be put to multiple uses -- as in LT5, mentioned earlier. All that arose in part due to Rudd's thing for air-shrouded injectors, a good idea that never quite worked out. At the same time all that was getting done, The Buick 3.8L intercooled turbo (Grand National) was set up with a single 2x injector for both power enrichment and cold driveability, but as things progressed they were able to get it done 1x. If you look at the intake manifold it's not to hard to see where it went.

I believe the concept of the auxiliary injector was originally Bob Sutton's. He and two other guys at Bendix were essentially the inventors of EFI, circa 1957-8.

#21 OfficeLinebacker

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Posted 07 July 2009 - 14:25

The Buick 3.8L intercooled turbo (Grand National)


Now THAT was a car!