Club 80-90 Forums

Well that was an expensive afternoon
Carparts for less also have 20% off some very good prices already

CJH wrote:When I started the engine this morning all the tappets were nice and quiet. There may be something in this extra pre-load after all. Made me doubt whether I should touch them again, but nevertheless I warmed up the oil and then backed off all the adjusters to let the tappet plungers spring back to their natural position. The trouble is, whatever position the crank is in, two valves will always be under load, and even with the adjusters wound right out the rocker arm still rests on top of the valve and therefore puts some load on the corresponding tappets. I may have to do this in two stages - once to do the six tappets that are fully unloaded, and once more to do the last two. I guess I could have removed the rocker assembly.

I have always followed the Richard Atwell guide when adjusting lash

kevtherev wrote: I have always followed the Richard Atwell guide when adjusting lash

Good advice to go back to the guide. There's a tip for dealing with my compressed plunger:

Richard Atwell wrote:Remember that when turning the adjusting screw on the engine you are working against the valve spring if the lifter is full of oil. You can test this by pressing on the knurled base of the rocker arm with a screwdriver. If there is any movement, then the lifter is soft and your adjustment will be compressing the spring inside the lifter instead of moving the valve spring (normal). If this happens, you should run the engine with a 0.006" (0.15mm) clearance on that lifter for 10-15 minutes then re-attempt a hydraulic adjustment when the hot valve cover cools enough for you to touch it again.

CJH wrote:
Good advice to go back to the guide. There's a tip for dealing with my compressed plunger:

Richard Atwell wrote:Remember that when turning the adjusting screw on the engine you are working against the valve spring if the lifter is full of oil. You can test this by pressing on the knurled base of the rocker arm with a screwdriver. If there is any movement, then the lifter is soft and your adjustment will be compressing the spring inside the lifter instead of moving the valve spring (normal). If this happens, you should run the engine with a 0.006" (0.15mm) clearance on that lifter for 10-15 minutes then re-attempt a hydraulic adjustment when the hot valve cover cools enough for you to touch it again.

you can set preload on a tappet with air inside but you need to be gentle and dexterous, you can feel the point it is just touching before compressing the internal spring but its subtle, you do need to make sure pushrod is seated correctly which is always the tricky bit.

itchyfeet wrote: you can set preload on a tappet with air inside but you need to be gentle and dexterous, you can feel the point it is just touching before compressing the internal spring but its subtle, you do need to make sure pushrod is seated correctly which is always the tricky bit.

I don't think I've necessarily got air inside any of the tappets. I think the problem is that when one of the tappets is sitting on the top of a cam lobe the plunger inside is under pressure, and becomes depressed - i.e. the oil in the cavity leaks past the plunger or the check valve. When the cam is rotated to release that pressure (e.g. at TDC, while adjusting the pre-load), it's only the small internal spring that can then push the plunger out again, against some resistance from cold oil, so it doesn't return quickly.

I had a go on Friday evening, after letting 6 of the 8 tappets relax all day. On those 6 the touch point was easy to spot, and I set 2 turns of pre-load. On the other two, I *thought* I'd got it right, but when I tried to start the engine I had no compression. I didn't get to the bottom of that because it was cold and dark and I needed the van over the weekend, so I simply backed them all off 1 turn. I suspect the compression problem was because the 6 needed time to settle (letting them relax all day was like fitting them fresh), and the 2 had too much pre-load. But I've got two approaches now - either repeat what I did, but unload all 8 by removing the rocker shafts, or pre-load 6, gap the other 2, run the engine and pre-load the last two. The latter seems simpler.

If the tappets work well you won't have compression for hours after setting preload, if they are leaky tey will return to equilibrium much quicker

I'm hoping that was the reason. And since these tappets tend to de-pressurise quite quickly I don't think it'll take very long to get compression back.

Hi CJH.
when you were doing the rolling road tests you mention drive train losses of 18.3hp which is well worth knowing.
Did you get any idea of air resistance/drag losses for a T25?
Ie: hp required to maintain a steady speed, say 60mph, on a flat road.
I know this is not something that's measurable on a rolling road but would be interesting to know.

I didn't, but it would be interesting to know. I've thought about this before, and I think it must be a huge number. Top speed is reached when air resistance (+ rolling resistance) equals the force being applied by the wheels. If you think about the force that the engine produces at the kind of engine speed associated with top (vehicle) speed, air resistance must be huge. This is why the tyres on the driven wheels wear out - at speed they are clawing away at the road to overcome air resistance. I'm sure it would be possible to do some basic sums, based on the cross-sectional area.

Good to know it's not just me that thinks about stuff like this while at work,
I'm guessing about 45BHP, based on a 1.6 petrol doing a top speed of what? 65mph? from it's 50BHP engine.
Pure guess work but I'll have a think about the maths.

https://www.thesamba.com/vw/forum/viewt ... ght=visual" onclick="window.open(this.href);return false;

Reproducing the image from that Samba link:



And of course there's an online calculator

Using a panel van (unladen) for the figures (weight = 1395kg, frontal area = 3.11sqm, coefficient of drag = 0.44, and a guess for the Coefficient of rolling resistance of 0.01), the online calculator gives, at 60mph:
21.69hp aero resistance,
4.92hp rolling resistance,
so a total power requirement of 26.61hp.
On top of that there's the 18.3hp drivetrain losses, giving a total of ~45hp just to maintain 60mph.

Not a bad guess Bob!

The calculator also gives some estimates of fuel consumption. They depend on some more guesses, such as fuel energy density, drive train and engine efficiency. Fun to play with nonetheless.

Vielen dank Marlin und CJH.
Meine Deutschkenntnisse sind schon etwas eingerostet, aber Ich glaube zu verstehen!
Interesting stuff, I wasn't sure if the 18bhp transmission loss included rolling resistance or not.

bigbadbob76 wrote:I wasn't sure if the 18bhp transmission loss included rolling resistance or not.

No, nor me, but I think rolling resistance is a function of the weight of the vehicle, which transmission loss would not be.

So factoring in headwinds and hills..... it's amazing a 1.6 petrol goes anywhere.
And an electric conversion would never get further than the bottom of my street.