16 replies to this topic

[maccers]

As above, I am about to install a trigger wheel for a mega jolt kit.

Just want to make sure 100%.

TDC is when the piston it right at the top of the engine block before it starts to fall back down into the bore?

TDC is measured on piston number 1, the water pump end of my A+ engine?

TDC is as above but when the two rockers arms above piston number 1 are both rocking?

I think the trigger wheel I have ordered has 36 teeth and i need to remove one so want to get this correct, as there are 4 pre drilled holes for it to seat into the pulley. I need to install it so the missing tooth is 90 degrees before TDC, assume thats like qtr to the hour and not qtr past the hour looking at the pulley/wheel when fitted.

Cheers!

[Artful Dodger]

Everything you said is correct apart from the rockers bit.

Tdc on number one cylinder is when the rockers move on no1 cylinder, but this is the exhaust / inlet stroke. You want it on compression stroke so it is when no4 rockers are rocking.

You can find accurate TDC by putting a DTI gauge in the spark plug hole

And 90 deg before tdc would be quater to the hour, as you put it

Edited by Artful Dodger, 23 October 2012 - 01:21 PM.

[maccers]

thanks for that

[dklawson]

When discussing TDC I find the term "rocking" confusing so I don't use it. I use more mundane terms.

As stated above you want TDC at the end of the compression stroke (beginning of firing stroke). That will be when piston #1 (water pump end of block) is at the top of its travel AND you can grab its 2 rocker arms and wiggle them. If the rocker arms are tight, you need to turn the engine through one more complete revolution to reach the correct TDC.

I don't know MegaJolt so forgive this question/comment if it seems silly. How accurate do you need to mount the trigger wheel? I assume the proximity sensor that counts the trigger wheel teeth is on an adjustable bracket. You should be able to move the sensor to compensate for any minor errors you make mounting the trigger wheel.

[R1minimagic]

Setting TDC from the piston crown is not very accurate, it can be quite a number of degrees out doing it that way as the crankshaft moves quite a lot but the piston doesn't (as it is reversing direction). Best way to do it is as above, then put a dial guage on one of the cylinder 1 pushrods just as you would do timing a cam in. A few degrees difference of ignition can make a big difference to the running of an engine...

Edited by R1minimagic, 23 October 2012 - 05:53 PM.

[maccers]

Hello Doug - the trigger wheel is essentially the bottom pulley all in one, and bolted to this is the super charger pulley. Links:

Pulley - http://www.med-engin...lley-management

Pickup - http://www.med-engin...and-bracket-kit

It looks quite rigid to me, ie no compensation. Perhaps the compensation is done on the ECU end of things, I dont know, I will receive all the bits very soon.

I assume the pulley fits one way as its keyed. I dont know if I will need to remove one of the teeth as the locator, this is the all crucial locator according to the people that sell the ECU. I suppose looking at the above, if the pulley already has the teeth integrated, and the pulley is keyed and can only fit one way, it will be quite easy to get the right tooth to remove, if needed, maybe MED do this already I dont know.

It should arrive tomrorow so I'll find out.

[dklawson]

Well... since this pulley is made specifically for this task and already has the teeth machined in it... did MED not machine away the reference tooth for you already? It seems odd that with the pulley being keyed that you have any latitude in what you do and/or how this pulley mounts. It would have been easy for MED to remove your reference tooth while the pulley was being roughed out.

The bracket for the sensor does have mounting slots but I assume those are to move the sensor in and out to get the right gap between the sensor and the gear teeth. It does not look like you have much adjustment of the sensor "left and right" which would be beneficial to get the timing set right.

Again, I don't know MegaJolt. Perhaps they do have a software method of compensating for the sensor position by entering an offset variable. If that is the way it is handled you are probably going to have to carry out the equivalent of degreeing a cam (like R1 said above) to determine at exactly what crank angle the sensor detects TDC.

[Ethel]

KAD's piccies do suggest you have to remove a tooth to suit, guess it allows for other configurations. What Dilligaf says is right, but you could also compensate for the sensor location by manually adding/subtracting the discrepancy for all the advance "cells".

Since it's a wasted spark system it doesn't matter whether 1 or 4 is starting its power stroke, both plugs will fire together. TDC is only an artibary datum (reference point) anyway, set it by your timing marks (ideally with a strobe, as Dilligaf explained) so you have a reliable way of keeping a tab on the advance, even with the engine running. Since you'll be losing your timing marks with the old crank pulley, set them at 10 degrees and make a mark that won't be lost with the pulley. On the flywheel, like it was on earlier engines would do - or mark the new pulley without disturbing the crank rotation.

[maccers]

Thanks all for your support here, I did expect there to be a degree of fitting accuracy and not just bolt it on and let the ECU work it all out. Agree on the TDC position too, when I had a go at setting the rocker gaps on my old setup, I was quite suprised at how much "slack" there is between the piston reaching the top of the block, crank still turning but piston not moving.

I'll find out today on the missing tooth too when it all arrives, agree it would be a bit odd to develop it for the A series, 4 cylinder engine, key the pulley and leave it up to the end user where to remove the tooth. I think 6 and 8 cylinder applications the tooth is removed at 60 or 70 deg BTDC. Some trigger wheels are tottally bespoke and need welding on and positioned correctly. Not having that skill or confidence, I thought buying a matched setup from a good workshop for the mechanical side would be better..!

Cheers again all.

[samsfern]

Youll need to find true tdc, not just when the pistons at its highest point. It stays at its highest point for a period of about 8degrees, true tdc is in the middle of that period.

[maccers]

Yep, agreed. Although, if the pulley is keyed and will only fit one way, and either the tooth has been removed already, or as has been suggested, I will need to remove a tooth 90 degrees BTDC, the teeth are quite coarse and there is a big gap between them too, maybe too coarse that the 8 degrees as above will sit in only one tooth gap if that makes sense?

I'll check with MED but I am assuming that each tooth length is 5 degrees and its gap is another 5 degrees, seeing as the wheel is a 36 tooth wheel, so (36 * 5) * 2 = 360 degrees

Interesting about the EDIS module, I wander how well it would run on that alone?

[samsfern]

Still worth finding true tdc and marking it on the crank pulley and timing cover, so you know exactly where it is.

[SolarB]

To find TDC I fit a protractor, measure piston height a set amount each side of TDC and then divide the figure by two.

[R1minimagic]

It will use a leading edge on the teeth so doesn't matter what the width and gaps are, still needs timing in properly

[maccers]

The bits from MED have literally just arrived. I will have to remove the tooth. The pulley has a key on it so it will only intsall one way onto the crank. I cant change this. I am assuming the pulley on there now is fitted correct so all I do is swap them over.

The pickup from MED also bolts into a rigid bracket so I cant change this either. The brakcet fixes to the timing chain bolts, which I cant change the location of.

I guess the point I am making is that like other wheels where you manually fix it to the pulley you need to be bang on. The one I have is a pulley with teeth cut into it and keyed so it will only fit one way.