Thread: Anyone done a Flapper 3.9 ?

yes and yes, 3.9 auto was reluctant starter on cold Canberra mornings, replaced the thermotime switch with one with a longer time / temp value. still takes a couple of seconds cranking but much better and runs well once started.

Thanks Dave,
that sounds just the ticket. Do you recall where you sourced the longer duration thermotime from ?

What possible good reason is there for going backwards????

The flapper system is neanderthal in comparison to 14CUX and will not do a thing to improve on it.

Sorry if my statment is a bit silly, I am a novice when it comes to EFI systems. If the 'flapper' system was designed for the 3.5L and has now been installed on a 3.9L won't it be delivering the same amount of fuel and therefor be running lean? Running lean is bad and causes overheating does it not?

It's like putting a 350holley with jets for a smaller 6cyl engine onto a much larger V8 which requires larger jet to deliver more fuel otherwise it will run lean and overheat. I learn't thin the hard way on older commodores

Originally Posted by Newbs-IIA

Sorry if my statment is a bit silly, I am a novice when it comes to EFI systems. If the 'flapper' system was designed for the 3.5L and has now been installed on a 3.9L won't it be delivering the same amount of fuel and therefor be running lean? Running lean is bad and causes overheating does it not?

It's like putting a 350holley with jets for a smaller 6cyl engine onto a much larger V8 which requires larger jet to deliver more fuel otherwise it will run lean and overheat. I learn't thin the hard way on older commodores

Not really. Its just the process it runs to get the fuel and air there.

A flapper uses a separate cold start injector and thermo coupler to operate the choke system. The cold start injector is a fogging injector that injects directly into the plenum when cold and shuts off via the coupler once a certain temp is reached. These also run a much lower fuel pressure than the 14CUX giving not quite as good fuel atomisation.
The flappers ability to measure the airflow is based on the amount of effort required to lift the flap in the airflow meter which corresponds to an angle sensor supplying a resistance to the ECU, indicating air flow rate. Not very accurate not to mention the flow resistance caused by the flap directly in the airflow. Great in its day, but very low on the technical scales.

The 14CUX in comparison, although now old but still very flexible, uses the hotwire principle MAF which is still the basis for EFI systems today.
The only thing that limits it is the chip that is installed.

The choke system on a 14CUX is nothing more than a fuel map that increases the injector pulse time slightly to increase the fuel delivery at each injector based on engine coolant temp. As the temp increases, the ECU reduces the injector pulse width leaning out the fuel. The higher fuel pressure in a 14CUX negates the need for a cold start injector as its atomisation is far superior.

There are a lot of improvements and tweaks that can be done to a 14CUX system with nothing more than a multimeter and a screwdriver plus there are numerous chips available these days too. Then you can add oxygen sensors also.

The Lucas injectors are another area that can have significant improvements by simply removing them and replacing with something a little more updated as I have just done.
Biggest problem with these is the spray pattern they produce. Look inside any 3.9 head intake ports with these and you will see perfectly clean aluminium where the fuel gets sprayed onto the intake wall and then varnish deposits from the PCV vapours.

With the flapper, your stuck with what you got.

I could go on and on but......

Cheers

Andrew

Thanks Andrew, very infomative for a cave man like me

Originally Posted by LOVEMYRANGIE

What possible good reason is there for going backwards????

The flapper system is neanderthal in comparison to 14CUX and will not do a thing to improve on it.

My reason for not using the hotwire system is that the system I have is an unknown quantity. I purchased it second hand and whilst the previous owner said it was all good, I've since conducted some simple tests as suggested by Des Hammill on the AFM to confirm it's condition, and the results were not good. Incidently, Des in his book "how to power tune rover v8 engines", says that you will be "hard pressed to tell the difference" between a well tuned 4CU system (flapper) and a well tuned 14CUX system (hotwire) on the same engine. He doesnt say what engine. Noting the title of the book, I'd suggest this comment is from a purely performance point of view.

That said, I'm not entirely happy with the cold start mixture the flapper system is providing the 3.9. I think it's too rich. It runs brilliantly otherwise. But I cant tune it for good all round performance and good cold start idle. I've tried enlarging the air bypass valve however the fast idle does not seem to kick in until after about a minute of rich running. ie it idles at around 500rpm for a minute before increasing to around 900 after 3-5mins, then droping back down to around 600 once warm.

So, I'd be very gratefull if anyone in perth with a 3.9 would be kind enough to try conducting a quick test using my ECU and AFM to ensure they are OK ? If so I would replace the flapper system with it.

Originally Posted by Taz

My reason for not using the hotwire system is that the system I have is an unknown quantity. I purchased it second hand and whilst the previous owner said it was all good, I've since conducted some simple tests as suggested by Des Hammill on the AFM to confirm it's condition, and the results were not good. Incidently, Des in his book "how to power tune rover v8 engines", says that you will be "hard pressed to tell the difference" between a well tuned 4CU system (flapper) and a well tuned 14CUX system (hotwire) on the same engine. He doesnt say what engine. Noting the title of the book, I'd suggest this comment is from a purely performance point of view.

That said, I'm not entirely happy with the cold start mixture the flapper system is providing the 3.9. I think it's too rich. It runs brilliantly otherwise. But I cant tune it for good all round performance and good cold start idle. I've tried enlarging the air bypass valve however the fast idle does not seem to kick in until after about a minute of rich running. ie it idles at around 500rpm for a minute before increasing to around 900 after 3-5mins, then droping back down to around 600 once warm.

So, I've be very gratefull if anyone in perth with a 3.9 would be kind enough to try conducting a quick test using my ECU and AFM to ensure they are OK ? If so I would replace the flapper system with it.

Not a problem on the test for you. I'm in Alexander Heights. I cant this weekend as I have stocktake at work, but next weekend should be fine.
I can fill you in on the finer details of tuning it when its done.

I'm sure Mike90RR will be more than happy too also, right Mikey??!!

PM me when you want to.

Cheers Andrew