Club 80-90 Forums

Cheers Kev - chalk one up for us carb fans! If I'm honest I'm only a carb fan because I know nothing about fuel injection, but it's good to know that we don't need to hang our heads in shame.

I think this 120 jet will fit - it's got the same thread and total length, only the head diameter is 1mm bigger, but I don't think that'll cause a problem. I might buy one to check. Of course, just because the 120 jet is right for my engine doesn't necessarily mean it'll be right if you don't have the same cam and exhaust, but I think it's a fairly safe bet that it'll benefit the bigger capacity engine regardless.

Unleash the full potential of your DJ!

Interesting tidbit I discovered today - the dyno only measures the torque curve, and the power curve is derived solely from that curve. I had always thought the two were somewhat independent, and I wondered how the dyno could measure both simultaneously. It turns out that torque and power are uniquely linked via the rpm.

hp = torque(ft-lbs)*rpm/5252

So the dyno measures the turning force generated by the wheels ('torque') and then converts this to HP via the rpm. I checked it - I can exactly reproduce my measured power curve using just the torque curve.

The facts just keep coming, I hope somebody is going to wiki this

Me too.
I held a spelbinding discussion in the pub.
And gleefully poked an eye.

Printed it and read the very long post.
in summary, with as few words as poss...

DJ on LT carb with DJ dizzy is better than digijet at 4800 rpm
DJ on DG carb is not as good as LT carb but not far off
DG dizzy is not as good as DJ dizzy
Programmable dizzy is better than DJ dizzy but expensive.
You can only measure torque and BHP is derived from this.
On a rolling road you can only measure torque at wheels so Transmission losses are guessed.

A great step forward in knoledge

I can't help thinking this is all at 4800 rpm , full chat, which is not normal driving, can't remember when I last had the DJ at 4800
What is happening at lower revs?
Maybe its directly realated but where is the proof?

That's about it, yes, but you missed one - DJ on re-jetted DG carb is a bit better than on the LT carb. And the word 'guessed' is a bit strong - it was apparently a measurement made by the dyno during the slow down phase, but who knows how accurate it is.

itchyfeet wrote: I can't help thinking this is all at 4800 rpm , full chat, which is not normal driving, can't remember when I last had the DJ at 4800
What is happening at lower revs?
Maybe its directly realated but where is the proof?

Yes, it would be interesting to see the curves for a Digijet DJ - maybe the distribution across the rev range is different. And we shouldn't forget that the accuracy of these results depends on when the dyno was last calibrated, so the absolute numbers need to be taken with a pinch of salt. So the carb and distributor comparisons can be trusted, but the figure of 121 can't. No idea what the error bounds are though, and it's the best measure of my engine that I'm ever likely to get.

Well I can't escape the conclusion any longer - these KS tappets do depressurise very quickly. I hardly notice it from the driver's seat with the windows closed, but whenever I hear the engine outside from a cold start it's quite obvious that at least one of the tappets is rattling away, sometimes quite loudly. This happens almost without fail if the engine is left overnight. I even noticed that the engine was slightly more tappety after the half hour or so that it took me to swap the carb and distributor while I was at the rolling road.

The flipside of them depressurising so quickly is that they also pump back up very quickly. They're always smooth and quiet by the time the choke has come off, whereas if ever the tappets in my old DG had drained down (after several weeks) it used to take a good 15-20 minutes of fairly hard driving to pump them back up.

Like 123-jn suggested, it's probably only the tappet(s) that are under load that are de-pressurising, and to be honest I can't really see how any design could avoid this - under pressure the oil will leak out, since there's an open path through them to allow pressurised oil in and to supply the rockers - but evidently the KS design is more prone to it.

So I'm thinking of swapping them out for a set of the 'other' design. So does anyone know for sure where I can buy a set that are NOT of this design?


I'm looking for set like the ones in this photo from Itchyfeet:


I'd consider a second hand set provided they're all in good condition. I think I could get them refaced if necessary.

This is my understanding, may be wrong, on the latter tappet the cavity in the base has a spring and check ball, thats the cavity that should not leak under pressure because the pressure holds the ball in place, when the pressure is released, then oil can be drawn in , all the holes to outside are in the upper part that can drain with minimal problem. To fit the piston you have to poke a small pin onto the check ball to allow the excess oil through.

Is the other type different in principle?

I wonder if they are leaking down because the ball and seat are badly made or spring not strong enough or tolerances between piston and body not good enough.

Ah yes, you're right, I'm forgetting. The KS ones have the same check ball arrangement. But I remember when assembling them it was quite possible to compress them in the vice (to get the retaining clip in), and that oil leaked out during that compression, and that the plunger did not spring back against the retaining clip. It may indeed be the check ball seating, but perhaps the clearance around the plunger is a little bigger than in the other design, allowing oil to seep past under pressure.

I asked on the FB page, people like Marco mansi builds top quality WBX he will know, I want to change tappets but I have been reluctant to because of your experience.
The only thing I'd add is that perhaps you got unlucky and had a bad batch, no harm asking the supplier to exchange them?

Thank you. A bad batch is possible I suppose. I bought them from Brickwerks, but it was a long time ago (thanks to my slow build). I may try to contact the manufacturer directly to get their opinion.

one more thing occured to me, if VW publish PS and BHP then don't we know transmision losses?

No, that's a misconception about PS I think. Horsepower is measured in different units (imperial HP = 745.7 Watts, PS = 735.5 Watts), but the manufacturers' figures are usually at the engine output. DIN testing specifies this. Apparently there was a change from 'Gross' to 'Net' in the SAE standard, meaning that engine ancillaries (water pump, alternator, exhaust) were taken into account, but the DIN PS is still measured at the output shaft.

CJH wrote:I think this 120 jet will fit - it's got the same thread and total length, only the head diameter is 1mm bigger, but I don't think that'll cause a problem. I might buy one to check.

I bought one - it seems to fit fine, despite the slightly larger head diameter.



The 0.1mm increase in the jet diameter actually makes for a 19% increase in the area - it's just visible in the photo I think, next to a stock 110 jet.

yes I missed the bit about DG carb maybe because it had no graph

Excellent news everybody with a DJ on pierburg is going to wamt one of thoss jets, better get mine now