How To Determine Possible Cam Angles

[Doof 11]

Ok, so its time to put my engine together which is great news, but im not too sure on how i can determine safe cam angles on a 16v motor.

 

Obviously i can put it all together with some plasticine on the piston and that will tell me the clearance...but do you really have to take it apart, check the clearance, then rebuild with another few degrees on it each way until you run too tight clearance?

 

Any good websites explaining it would be appreciated, i'm going to have a trawl of the net now to see what else i can find.

 

Lewis

[welshpug 1,666]

depends which 16 valve motor, pistons and cam you're using.

 

I'm sure petert or sandy could advise the max lift @ tdc you can get away with and what angle to set peak to.

[Doof 11]

Ok, sounds like a bit more info is needed, although surely i still want to be able to play on the rollers to see how it affects the power curve?

 

Its an S16 with an 88mm crank bringing the deck height to -0.5mm (i.e above the deck). The cams are as follows:

 

INLET

Advertised Duration 272

Duration @ 0.050” 236

Maximum Lift 0.448"

Lift @ TDC

Lobe Centre Angle 106

Part Number 16H419

 

EXHAUST (This is actually an inlet cam so these values might change...)

Advertised Duration 260

Duration @ 0.050” 226

Maximum Lift 0.350"

Lift @ TDC 0.065"

Lobe Centre Angle 109

Part Number 16H426B

 

Might give you a ring this week Sandy for a chat if thats alright?

[Doof 11]

Its also just occurred to me that as im using hydraulic lifters I can't just stick it all together with some clay on the pistons and check the clearance because the lifters will just compress! Have i got to get a solid lifter to test with or something?

 

I'm beginning to wonder if i've underestimated whats involved!

[Sandy 191]

No worries, as long as I'm free!

 

If the head isn't on yet, what you can do is remove one of each of the valve springs on the inlet and exhaust side of one cylinder and leave each valve in there held up by the stem seal. If the stem seal isn't tight enough to hold it, supplement it with an elastic band wrapped around the stem above the seal.

 

With no cams, bolt the head on to about 15lbft with the gasket and bring that cylinder to TDC; make sure the valve is pulled all the way up and use the tail of a caliper gauge to measure the height of the stem above the spring seat (without moving the valve, careful!) Then push the valve down gently until it can be felt to contact the piston and measure the stem height over the spring seat again. The difference between the two measurements will be the total valve clearance at TDC. With standard pistons etc, it's probably about 3.5mm inlet side.

Now repeat on the other valve, so you have figures on both inlet and exhaust.

You will usually need around 1.7mm clearance at TDC, so if you have a total of 3.5mm, that means a maximum safe lift of about (3.5-1.7) 1.8mm.

 

What you have to bear in mind is that the clearance will be tightest around 12 degrees ATDC (after TDC) on the inlet and 12 degrees BTDC (before TDC) on the exhaust. You can measure the clearances at these angles as well, but unless you have measured the cam very accurately, you will have to estimate the lift at that angle from the TDC figures, for most profiles it will be around 1.5mm more than the TDC figure, but it can be much more than that on specialist cams.

At this angle (12 degrees ATDC inlet, BTDC exhaust), you can usually run about 1.0mm clearance, but bear in mind that road/piston expansion and stretch at high rpm will take up almost all of that clearance,leaving no room for valve float!

[Doof 11]

Looks like i have an enormous amount of clearance...i think

 

Clearances:

 

@TDC @12degrees

Inlet 0.275 0.315

exhaust 0.307 0.372

 

Im not convinced i'm doing this right, i mean, i almost have enough clearance on the exhaust to stick it to max lift and turn the engine all the way over!

 

These pistons do have larger valve cut outs so maybe i'll just be ok. Anyone think the clearances sound reasonable?

 

I'm going to double check them before moving on, seems i've got stacks of clearance though!

[Sandy 191]

You're talking inches there presumably? If so,you can time the cams pretty much any way (until the valves clip).

 

I would suggest trying about 3.0mm inlet (0.118") and 2.5mm exhaust (0.099") at TDC as a starting point.

[petert 601]

Looks like i have an enormous amount of clearance...i think

 

Clearances:

 

@TDC @12degrees

Inlet 0.275 0.315

exhaust 0.307 0.372

 

Im not convinced i'm doing this right, i mean, i almost have enough clearance on the exhaust to stick it to max lift and turn the engine all the way over!

 

I'm sure you haven't done it correctly. That seems way too much clearance for an 88mm crank. Also, you should have less clearance at 12 deg., not more.

 

As a guide, the standard 1.9L Mi16 has approx. 0.205" total clearance at TDC, when you drop the inlet valve as Sandy suggested. If you subtract the cam lift at TDC, approx. 0.035", you have a net clearance of approx. 0.170". They're rubbery figures from the back of my brain. So please don't quote me.

 

I would start with approx. 0.095" at TDC on the inlet. That equates to approx. a 108 centreline. Perhaps advancing to approx. 0.105", or 106 deg.

 

You should aim for 0.080" net clearance at 12 deg. ATDC for the inlet, and 0.100" at 12 BTDC for the exhaust. They're well recognised "safe" figures for hydraulic lifters.

 

Definitely get Sandy to help you. The risk of failure is huge.

[Doof 11]

The thing is, i went out and rechecked the figures and i'm certain that they're right. The valve pockets on these pistons are a good few mm deeper than stock...

 

I think actually i didn't explain what i had done correctly as you mentioned you have less clearance at 12 deg. I haven't put the cams in yet (so of course my 12deg quote figure will be larger because the piston is further down the bore). the figures quoted are the maximum valve to piston clearances so cam lift needs subtracting from that to get actual running clearances.

 

In the case of the exhaust: = (0.307-0.065(lift at TDC)) = 0.242 (so a fair bit more than the 0.170" you stated for the std Mi16)

 

If that figure of 0.095" lift at TDC is for the stage 3 cam i have then my clearances at TDC on the inlet are = (0.275-0.095) = 0.180" (still quite a lot!)

 

Thanks again for the info, i really appreciate it.