Compression Ratio

Hi All

I've had my engine recently rebuilt, it's done under 1000 miles. It has a mild cam with a 276 duration, and is a 1600 twin port, it was originally a

1300 twin port. What has got me more confused than a monkey in a pool of bananas, is that although I've timed at 7.5 btdc, with the light. I'm hearing a slight knock when the engine's under load at low rpms. It's more noticeable when the engine very hot. My only thought is that the compression ratio is higher than the stock because of the smaller heads being used. Does anyone know if this slight increase in compression ratio would cause this knocking? Is it common on new engines?

Many thanks Regards Kevin

Reply to
Kevin
Loading thread data ...

Sureley you would have to change the distributor curve if you change the camshaft duration? - the dizzy and timing are set up for a standard cam. I would say you might hear pinging if the timing is out but not 'banging'. The

1300 heads have to be machined to fit the 1600 barrels. My 1641 has an 8.0:1 comp and doesn't ping when set to 7.5' BTDC, but it has a stock cam and is run on 95RON...

--Steve

Reply to
tunafish

That would be 7.5 at idle. I sure hope that's not your full advance timing which should be closer to 32 degrees. Is your advance working? Knocking is death to the engine. It should never happen. Worse is the fact that it can be knocking at speed and you won't hear it over the din of the rest of the engine.

Better mechanics than I am can address the compression - or rather the deck height, head, piston, lifter and rockerarm issues. It's kind of scarey how badly that can all go when just put together by 'intuition'.

Reply to
jjs

Thanks

It's the 7.5 at idle my full mechanical advance is in the region of 30 degrees. I set the timing without the vacuum cannister attached.

Regards Kevin

Reply to
Kevin

What dizzy are you using? The stock 1300 dizzy for the 30PICT carb won't work for you, if it is the vacuum only actuated unit. You need a vacuum & centrifugal actuated dizzy (single vacuum with centrifugal mechanism) from a later model. Then you time it with the following procedure:

Initially timing: With the vacuum hose disconnected, the advance should be around 30-32 degrees when the engine revs above 3000 rpm. You need a timing light to set it. Then connect the vacuum hose while the engine is running. If the vacuum canister works properlly, the advance should jump about 10 degrees more (around 40). Then note that advance at idle, as a reference for future adjustments.

Now drive the car. If you hear the engine pinging when hot and at heavy loads, you need to retard a bit. Retard by steps of 0.5 degrees, until you can hear no pinging, then 1 degree more for safety margin.

You will loose bottom end performance if you have to retard considerably and you may want to fiddle a bit with the centrifugal mechanism to correct this. What you have to do is to remove the points plate, and twist a bit towards the center the metallic tongue that limits the centrifugal weight movement (determines the max advance limit). If you previously found that you needed to retard 4 degrees for the engine to run properly at speed, then you have to bent this tab so that the max advance is reduced by 4 degrees. You may need several attempts to achieve the correct amount of bending. Bend this tongue a little each time, because too much bending and too many attempts to correct it may break the tongue. Now the distributor will have the desired max advance, and the advance at lower revs will follow again the original curve.

Bill, '67 Bug.

Reply to
Bill Spiliotopoulos

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.