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Old 21-05-2014, 10:40 PM   #1
Cornally
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Cat cam 743 timing

As above lads, I looked on the cat cam website, and this is what it says,
Inlet 5/45'
Exhaust 41/9'

But it says in the description it says about using standard timing marks? I've never timed an engine on verniers before so I'm bringing it to a guy to do for me, so I'm just wondering what I should tell him to set it to?

Any help appreciated,
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Old 21-05-2014, 11:12 PM   #2
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Find the lift at tdc figures, much easier and accurate way to do it.

1.55mm on both btw.

Last edited by welshpug; 21-05-2014 at 11:12 PM. Reason: Automerged Doublepost
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Old 22-05-2014, 01:14 AM   #3
Cornally
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Quote:
Originally Posted by welshpug View Post
Find the lift at tdc figures, much easier and accurate way to do it.

1.55mm on both btw.
So do I tell him to set it up for 1.55mm lift at tdc? I don't really a have a clue, hence why I'm gettin someone to do it!
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Old 22-05-2014, 08:25 AM   #4
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Yep, that's pretty much it. I'd never done it before, but the process is intuitive once you have done it a few times and got your head round things. I know the below link is for a different engine, but the principles are the same for timing cams using the lift @ TDC method.

http://s262612653.websitehome.co.uk/verniers/

I've found that figures that people give for lift @ TDC are a good starting point, but a session on the rollers swinging them around and adjusting the mapping can give some gains over the book timings.
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Old 30-09-2017, 12:53 PM   #5
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I'm in the process of fitting these cams but my head has been skimmed so the piston to valve clearance is very small. I was going to dial them in using the lift at TDC method but the company who supplied them said it's safer to run them on the standard timing marks.

What is safest? Standard timing or as per the CatCam data sheet?
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Old 30-09-2017, 02:16 PM   #6
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depends how accurate the lobe to keyway angle is, doing it to pins is total guesswork tbh

personally I'd machine the pistons so you can time them properly, its rather pointless changing cams if you cant fit them properly, youre more likely to have a better spread of power with a slightly advanced standard cam than poorly timed aftermarket items
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Old 30-09-2017, 06:24 PM   #7
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By pins are you talking about the locking points on the standard cam pulles into the block?

Do you suggest machining cut outs in the pistons?

Last edited by Temple; 02-02-2018 at 06:57 PM.
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Old 01-10-2017, 06:52 AM   #8
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The total valve to piston clearance in the standard J4 engine is only about 3mm inlet and 3.7mm exhaust, JP4 slightly less inlet, more exhaust. A safe minimum for J4 clearance over lift at TDC with hydraulics is about 1.5mm inlet and 2mm exhaust (the exhaust runs closer in action because it's closing over TDC and any stretch in the belt etc builds the risk on that side); that only leaves about 1.5mm inlet and 1.7mm exhaust for lift at tdc. 708s and 743s push this to the edge to either make the most of it or to get the best headline improvements to help sell more cams, whichever way you want to see it. It's not unusual to see the valves marking the pistons with these fitted and in extreme cases valve heads will drop from fatigue, if the head has been skimmed, then obviously you have to back the cams off to get the safe clearance over LTDC.
The only way to know is to check the inlet and exhaust valve drop at TDC with the head dummy fitted and no valve springs, then you can qualify what safe working range you have in your particular engine, don't let anyone tell you that you can turn the cam until it stops (valve touches piston) to see, 6mm stems bend easily and turning the cam you have a lot of purchase on it, plus the give in the hydraulic lifter when static is unknown and gives a false reading.
Add to all of this that people generally time the cams retarded without realising (due to belt take up) and you have a minefield.

The moral of the story is, make measurements, not assumptions and don't try to max everything, unless you really know what you're doing!
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Old 01-10-2017, 09:55 AM   #9
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Thanks for the detailed information guys, much appreciated!

Would it be prudent to increase the cut outs in the pistons on the inlet side slightly to allow for more clearance?

Am I correct in thinking that if I dial the cams in accordance with the CatCam specs then advance them slightly then I will increase the valve to piston gap?


Cheers,

Chris
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Old 01-10-2017, 05:44 PM   #10
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more clearance cannot hurt, im sure sandy could say how much could be taken safely out of the standard piston, advancing makes the valve open earlier and closer to the piston at tdc.

measure many times beforehand!
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