14 replies to this topic

[hughbertmungo]

Gidday, I was having a play at timing my 1275 today as per you tube instruction with dial gauge and crank protractor. The cams a reground one to kent 276 profile. I found proper tdc in middle of dwell gap.

I got a timing card with it that states 36 deg before tdc at .010 inlet lift and 9 deg at .050, I've got 26 crank deg before at .010 and 0.5 deg at .050, I think this means I'm 10 deg of crank rotation advanced so retarded on cam if I'm thinking it through correctly. Does this mean I need a 5 deg offset cam key to get 10 deg of crank rotation? Also should I allow for some initial chain stretch with my duplex chain so maybe a 6 deg key?

Simple geometry has got me confused.

Cheers.

[GraemeC]

10 degrees seems a lot - are you sure you're not one tooth out on the chain?

[hughbertmungo]

Na, set it up dot to dot, I tried advancing cam 1 tooth and came out around 8 deg advanced. I'll advance it a tooth and again and remeasure it a couple more times to be sure. Do the degrees of offset on those cam keys refer to cam or crank rotation? I'd presume 10 deg crank would be 5 deg cam?

Cheers.

[GraemeC]

The keys will be crank degrees as that is where you are doing the measurement.

[jaydee]

Double check your measurement first, checking max lift this time (nominal timing for a 276 you have inlet full lift at 106 degs after TDC).

Since those woodruff keys obviousely come at max 9 degs increase/decrease, set it so you read 8 degs advanced then adjust using a 8 degs woodruff key.

I'd reccomend advancing a couple degrees more for a 276 cam than the nominal 106°.

[hughbertmungo]

Beaut, cheers. I'll have a check at middle of max lift and see where I'm at. The deg at .010 and .050 that I've been working off on the timing chart from the cam grinder seem to suggest a 10 deg offset, which seems like alot to me.

[Guess-Works.com]

There are 40 teeth on the larger of the two timing wheels, which means each tooth is 9 ( 18 degrees at the crank ) degrees. If you're more than 9 degrees out the move it around one tooth, offset woodruff keys only come in 0 to 9 degrees.

The dot to dot is only a setup guide, and will do for most engines, but when running a performance cam, by all means start off with dot to dot as a reference but don't assume they will be anywhere near correct, as you've found out, 10 degrees out..

When you do measure lift on a DTi, one ever measure in engine direction, so turn the engine clockwise to a point before max lift and then continue to the same point after max lift, then add the two degrees and divide by two, if you want to check again, turn the engine anti clock wise well past the first point and then clockwise back to that point.

Clockwise is as looking at the front of the engine which is actually the side which points to the radiator.

Edited by Guess-Works.com, 09 October 2015 - 04:29 PM.

[hughbertmungo]

shudent I go back and fowarth  a bit to find my proper max lift centre? So if I advance a tooth then retard with a offset key shall I allow for a couple of crank degrees of chain stretch?

[hhhh]

No, as Guess-Works said: "When you do measure lift on a DTi, only ever measure in engine direction, so turn the engine clockwise to a point before max lift and then continue to the same point after max lift, then add the two degrees and divide by two, if you want to check again, turn the engine anti clock wise well past the first point and then clockwise back to that point."

 

Going back and forth will do nothing constructive. A couple of degrees extra advance to allow for chain stretch is a good idea.

[gazza82]

Or fit a new chain?

[hhhh]

Fitting a new chain is never a bad idea, but in this context, "allowing for chain stretch" is for when your new chain is no longer new yet still installed and slightly longer than it used to be due to wear which will retard your timing at least a degree and likely two.

[hughbertmungo]

Brand new duplex chain and sprockets, It was fairly cheap so I imagine it's not of the best quality. If I advance a tooth gives me 8 deg + - advanced as guess work said. So if I whack a 6 deg key in at a tooth advanced to retard cam back to 2 deg ish advanced from spec will that be about right?

Cheers.

[hhhh]

One tooth is 18 crank degrees (which is the only thing we should ever refer to to avoid confusion.) If you advance a tooth and put in a 6 degree key in the retard direction, you'll be advanced 12 degrees from where you started, which I don't think is what you want.

 

What you want to do is find where full lift is with the timing dots aligned. As an example, say it's 115 degrees ATDC and you want 106 net after assumed stretch of 2. So you need to advance it (115 -106) + 2 = 11 degrees. In this case, you'll want to punch a new dot for alignment after advancing the crank one tooth which will bring you to 97. Install a 7 degree key (or 8 as a second choice) in the retarded direction. That means when viewing the end of the crank the part of the key in the crank sprocket is offset in the counter clockwise direction. Your cam will now be dialed in at 104 degrees ATDC, eventually to be 106 as planned once the chain wears a bit.

 

Another example, say it's 90 degrees (extreme advanced example) with the dots aligned. In this case, you'll want to punch a new dot for alignment after retarding the crank one tooth which will bring you to 108. Now if you install a 4 degree key offset the opposite to the above, you'll be at your installation target of 104.

 

I hope this makes sense. Good luck, but double or triple check your TDC first with the same technique as finding full lift, a few degrees before and aft and split the difference.

Edited by hhhh, 10 October 2015 - 03:34 AM.

[hughbertmungo]

So, I advanced the cam I tooth and got 97.5 atdc degrees several times in a row turning engine clockwise. Had a check of no 4 as well and got same result. I'll order a 6 deg woodruff key which will retard me back to 103.5 which sounds about bang on. 

Cheers for your advice.

[hughbertmungo]

Advanced 1 tooth and retarded back 6 deg worked out perfect, 103.5 deg atdc. Cheers for your help.