Update: I went to GSF today and looked at a few relays - thanks to Anil at GSF for indulging me while there was a bit of a queue building up! The 1.6 GTI relay is 93590 - 19.80 + VAT, less member's discount. The early 1.8 relay is 93592 - 23.20, etc... I compared them both and the pins are different, so went for the 93592, as the pins match the one in my car. On the drive back from GSF, I saw 7000 rpm on the 'sensible scirocco' (stock 1.8 8v on Kjet). I'd tested it on the way there, and it was cutting in at about 6700rpm. I'll test it on the track car later. Important lessons: 1. Rubjonny is always right!! 2. The rev limiter on later cars (without the fancy rotor arm) is in the fuel pump relay. Swapping the relay will save me 4 gearchanges in a lap of Curby! 3. Rubjonny is ALWAYS right
I'd be happy to have a look at one of these 'rev-limiting' fuel pump relays, with a view to working out whether there's a simple component-value change which would change the rev limit, if anyone has one they don't need?
Soooo... Following my earlier post, Mike H was kind enough to send me his 'low rev limit' fuel pump relay for investigation. It's taken a bit of getting around to It looks like this from the outside: And its innards look like this: After a little work I decoded the circuit which is like this: There's a dozen discrete components, a chunky open relay, and an 8-pin DIL chip which is marked up "VW739" and BPR/1. It wasn't readily identifiable from these numbers; I suspect that VAG had the chip manufacturer put their own ID markings on it, BPR possibly being "Benzin Pumpe Relais" which is German for petrol pump relay I think. If 739 means anything to anyone in this context, please let me know. One thing stood out on examination of the little PCB; R7 (right in front of the relay itself in the above pic) was mounted higher than all the others, and had been soldered in after everything else (only solder joints with flux still present). This made me think that this was the only component that would need changing to change the rev limit, and that this PCB could therefore be used across a range of applications with different values placed here to suit. The not-everyday colour code on this resistor is brown/black/green/red (105 x 10), and it measured 10.5K with one end lifted from the board. Yours may be different, and measuring it with both ends connected may not give a true reading. The driven side of the ignition coil connects to pin 31b of the relay module and goes through a 301K resistor onto pin 5 of the 8-pin. With oscilloscope and plumbing the necessary connections into my car, I was able to see that the signal at this pin 5 is a series of sharp spikes of 5V pk-pk size. The frequency of these was related to rpm as rpm/30, as you might expect for a 4-cylinder 4-stroke. I initially confirmed Mike's observation that the relay switched at around 6400, using an LED (cheers for the idea Trev) to show me when the fuel pump would be on and when off ('cos my car has a mechanical fuel pump, I couldn't wire that side of things in). Wasn't keen to rev my engine much higher than that so I substituted in a different value for R7, guessing that higher might give a lower rev limit. It didn't. So I put in a lower value, bingo. This time the relay switched the 'fuel pump' off at around 5500rpm. I can now confirm that this limiting is definitely only done by cutting power to the fuel pump, having no effect on the ignition, as I could freely rev my engine past this point. If you find this hard to believe, imagine how fast the fuel pressure will drop with no pumping, and a fast flow out of the injectors at the time. Fuel isn't really compressible, so there'd be no gradual 'unsqueezing'; just a plummet in pressure. Anyway, I thought you guys might like some info on what resistor value gives what rev limit, so I set the thing up on a PSU and signal generator to simulate the coil signal after the 301K resistor, and plotted the following graph. At three points where I tried fixed resistors in the circuit, on the car, the graph agrees quite nicely with reality. Obviously, if you decide to use this info to alter the rev limit of your engine, you do so at your own risk! A bit of 'selecting on test' may be required to get exactly the limit you want, due to component tolerances from one PCB to the next. You could even use a double-pole, double-throw switch and two different resistors to have two settings, e.g. track and road, if you wanted... Graph Mike: When I return the relay to you, what rev limit would you like on it? If different from as supplied, you could test it and check what happens on your car?
they got these on ebay, only problem is its the american ebay http://cgi.ebay.com/ebaymotors/Vw-M...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
Top Man!! As I've already said, I thought it was cutting the ignition somehow, but given all the effort you've put in, I'm more than happy to believe your analysis. The RPM limit... I'm currently using a relay without a rev limit (Mk1 1800 GTI type) but how about 7000 rpm - it would give me a bit of safety in case of over-excitement! You should start selling these!
x 2 .... thanks Mike - do away with all the OEM stuff and use an OMEX adjustable unit, soft and hard settings - connects to coil / earth. I`ve a spare one but it`s rather isolated right now
I have a soft limit on my roc,not 100% sure when prob at about 6500 then a hard limit at about 6700,it cuts off the fuel pump. I know this 1000% as the pump sits behind me in the back of my car,where the fuel tank is. The pump makes a racket until you hit the limiter and it stops dead,i use the soft limiter as a shift light . Would deffo be interested at shifting the limit to say 7000rpm,think thats when my cam peaks out ? gs2h sports r
1800 8v - 285` - solid lifter Pack D head Ran soft limit of 7200 / hard limit of 7400 Safer to cut ignition than fuel ?
Standard PB block apart from balancing, lightened flywheel. Block was selected from a well used delivery van. TSR built unit, superb - wish I`d kept it - development advice was to fit ARP fasteners and twin carbs for + BHP END OF HIJACK
I'll pop in the closest resistor(s) I can to give you 7k limit, and stick it back in the post to you. Looks like 18k or 20k will be the nearest easily available value, but one can always cobble a couple of resistors in series to get a nearer fit. Easily DIY-able for anyone who owns or can borrow a soldering iron. Standard W or W resistors will do fine.
There's a lot of useful information there, will save me having to search around for old mk1 relays. Thanks Ez Pete.
