Can turbo engines even run at 60 deg BTDC?

steptoe · Post by **steptoe** » Wed Jul 18, 2007 11:20 am

Got Cheap Grief going this time with a battery booster hooked up and dizzy installed according to ambiguous instructions from the manuals. After loosening acc cable off idle came down to 1200 rpm and through matching up a spare drive plate and my marks on fitted drive plate established the EA82T is running at 60 degrees before TDC at 1200rpm

Figuring things are out by a tooth (lift dizzy out, wind back - clockwise- one tooth) so and will need to rectify to drive run right

BUT can a low compression motor such as these EA82T even run at 60 degrees Before Top Dead Centre ? - at idle.It would seem they can idle and rev up no load. Just never come across this before, always owned High Comp motors, so brain has troubling believing what it is seeing. Also allowing for the fact 1200rpm is faster than the 750 rpm idle it should have and that 1200 would have SOME advance on the 20 for idle .

with acc cable too tight it was idling at 1800 then what sounds like pinging activating knock sensor to kill engine till it touched 1200rpm then back up to 1800rpm and back, just cycling. next start when warm...needs more fiddling

BUT ITS GOING

130psi in all pots too and the lifters quietened up pretty quickly too !!!

Yee haa....

discopotato03 · Post by **discopotato03** » Wed Jul 18, 2007 1:19 pm

Its very hard to believe any piston engine could run with the spark plugs arcing at 60 deg BTDC at such low revs . When you think centre stroke is 90 deg BTDC creating combustion very early makes for cylinder pressure in a shrinking volume (piston rising up the cylinder) so I tend to think the pressure created would want to fight the thing revolving at all .

If all else fails I'd be going back to basics . By this I mean finding TDC on no 1 cylinder by placing something soft like a pencil through the spark plug hole and getting close to the pistons highest or TDC . To establish if its on the power stroke the no 1 cylinders valves will both be closed and the rockers against the back of the lobes or the base circles if you like . If the engines on the wrong stroke (one complete revolution out) both valves will be open because they're trying to scavange the cylinder at the end of the exhaust stroke and the beginning of the induction stroke .

If its right . Then you wind the engine by hand around almost two complete revolutions to get back to just before this point say a rough gestimate of 10-15 deg before TDC . Now you need to look at the distributors rotor button and where the contact arm points to in relation to the No 1 spark plug leads contact inside the dizzy cap . Basically if you mark with a permanent texta where the no 1 pole is on the outside of the dizzy body you have a reference as to where the rotor button should point to which you can see with the cap removed . I would turn the body one way or the other so that the approach side of the rotor buttons contact arm just lines up with the texta mark .
Now provided the electronic part of the dizzy is ok and the mechanical/vac advance is working properly the engine should be set to roughly 10-20 deg BTDC and should run ok - provided everything else is within spec .

I had a muck around with my 3 plug RX dizzy to make sure that the rotor would turn and spring back ok . These also have an unusual serated cap on the back of the vac advance diaphram can and this has a seal behind it and a spring . It may pay you to pull its vac line off the engine a suck on it (clean it first !) to see if 1) it holds vacuum and 2) the dizzys breaker plate actually moves .

This is old school stuff but then so is the basic 4 stroke piston engine .

Have fun cheers A .

WARNING: Be very careful putting anything in the cylinder via the spark plug hole because if it falls in there's often only one way to get it out . Make sure its not hard or sharp enough to damage things like cylinder walls or valves , and don't get it caught between the valves and their seats .

Gannon · Post by **Gannon** » Wed Jul 18, 2007 5:10 pm

Im curious how you worked out it was at 60deg, i thought the timing marks only go up to 20deg.
DP is right, im surprised it even runs, a spark at 60deg before the piston reaches the top of the cylinder is gonna make the piston go back down almost.

steptoe · Post by **steptoe** » Fri Jul 20, 2007 1:33 am

Glad I am not the only one with raised eyebrows. I used a spare drive plate, made my own numbered marks on fitted drive plate and used timing light to establish just where ignition was, marked it again comparing to spare drive plate. Three times the 20 deg distance from TDC.

Now i fixed it. 360 deg divided by 13 teeth on dizzy cog equals about 27 deg per tooth. I lifted dizzy enough to feel one tooth, two tooth retard as i turned rotor anti clockwise. Stuck her back down, started and showed up as just after TDC. So reduce 60 deg by two teeth (54 deg) got it close to TDC ! Advanced her up to 20 and sounds fine, still 1200 idle though. who knows what has been touched on this in twenty years !!