Engine Knock

~~~

Several people have described a detonation problem ("pinging") -- when going up a hill and giving it too much gas, a loud pinging is heard in the engine.

In a VW engine, pinging/detonation is a mainly a function of three things -- compression ratio, spark timing, and running lean.

  • Compression ratio -
  • If your engine has been rebuilt correctly, it should have a compression ratio of about 7.5:1 and this should run OK on 91 RON octane -- normal unleaded in Australia (87AKI is the equivalent in the USA). But if the 1600cc heads were used without modification and without putting spacers under the cylinders (to increase the head volume), then your compression ratio could be as high as about 8.4:1, and this would need at LEAST 95 octane (LRP or premium unleaded in Australia) - 91AKI is the US equivalent. The VW engine can run very happily on unleaded, in case you were wondering -- it doesn't NEED lead in the fuel. There's a lengthy article on this web site about fuels and VW engines if you are interested.

  • Timing -
  • Too much advance can cause detonation when you floor the throttle.

    The popular 009 (centrifugal-advance) distributors vary a lot in how much maximum advance they make (cheap build), so they should be set at 28-32 degrees BTDC at 3000+ rpm. THEN you can check where the idle advance lies -- it could be anywhere from about 5 to 10 degrees BTDC, so your 7.5 degrees BTDC could be over-advancing it slightly, but you wouldn't know until you've first set it at 3000 rpm using a timing light. As a test, try reducing it to about 5 degrees BTDC at idle and see if this makes a difference. 5 degrees BTDC is about 3 mm to the left of the 7.5 BTDC notch. If you do decide to get a timing light, 30 degrees is 46.5mm to the right of TDC on the standard VW pulley.

  • Running too lean -
  • The other thing is that when using a 009 distributor you need to run the carburetor a little richer than normal, to make up for the missing vacuum advance, otherwise you may be getting flat spots on acceleration as well as the detonation. The 30PICT/1 carburetor usually copes better than most with the 009 distributor (for flat spots), but still needs to be run rich. The other problem is the larger capacity - you also need to be set richer for the extra air it's drawing for the larger capacity engine. So you COULD be running the engine very lean, and this will cause it to run hotter than normal, which makes detonation worse than otherwise.

The single vacuum distributor will usually provide better overall performance than the 009 distributor. The 009 works very well where the engine is operating at high power and high rpm most of the time (great for VW racers for example) but most cars operate over a wide range of throttle setting and speeds (city and highway, heavy acceleration and cruising) so the vacuum distributors do better.

For a 30 sized carburetor, 1776 and vacuum distributor, you need a 55 idle, X127.5 or maybe X130 main, and an air correction jet of about 110 or maybe 100. So try the X127.5 main and 110 air correction jets first, then X127.5 and 100, then try X130 main and 110 air correction, then X130 and 100 air correction. Each of these steps will richen the mixture at medium and higher airflows.

For a 30 sized carburetor, 1776 and 009 distributor, you need a 55 idle, X130 or maybe X132.5 main, and a 100 or maybe 80 air correction jet. Same deal with the steps - X130 main and 100 air correction first, then try X130 and 80, then X132.5 and 100, then X132.5 and 80. When using the 009 distributor, you should also make sure the accelerator pump is set to full squirt - there are tiny holes in the operating arm under the coil spring (right side of carburetor) to adjust the stroke on the accelerator pump - more spring compression means a bigger squirt.

Each larger size in the main jet (they come in 2.5 steps) adds about 2% fuel flow. Each reduction in 10 number difference in the air correction makes and additional 2-3% increase in fuel flow (at high airflows/throttle only).

The air correction jet supplies AIR, so smaller means richer. The air correction jet works mostly at higher rpm (about 4000rpm upwards) to prevent the carburetor going over-rich at high airflows.

The older 30 series carburetors usually cope better with the 009 distributor than the larger 34PICT/3 carburetor does because they are less sensitive.

Still, I'd be checking the main jet and air correction jet sizes, because the 30 series carburetors were originally jetted for 1300/1500cc engines, and the 1776 makes it pull through a lot more air, which can result in running a little lean. Lean means hot, so if that's happening it would exacerbate any tendency to detonate.

 

* * * * *

 

 

 

 
 
Design by Erin