8 replies to this topic

[Ciccio-kof]

I'm just gonna ask a simple question and see where it takes.

I have a Kent 266 and timed it to 106° inlet full lift as required from manufacturer.

I then checked the inlet valve opening/closing degree with standard rocker and .40mm tolerance and it did match Kent specifics (24/56) when I switched to 1.5 rockers the full inlet lift was still 106° but the timing figures where completely out (around 20/61).

Question is, would you reduce the rocker tolerance with the valve in order to keep it 24/56 or what would you do?

[Midas Mk1]

266 276 286 designed by kent in the 80's for use with 1.5's.

[ACDodd]

Erm, that’s what changing the rocker ration does, it will change the cam timing, because the cam timing is stated as a certain lift. Increasing the ratio means a you get the same lift point earlier.

Ac

[DeadSquare]

Erm, that’s what changing the rocker ration does, it will change the cam timing, because the cam timing is stated as a certain lift. Increasing the ratio means a you get the same lift point earlier.

Ac

I noticed this, when I swapped the Mini Rocker-gear for Triumph, (because it had a higher ratio) on our Downton 850 Mini Se7en

 

Wanting to know if springs would bind and how deep a pocket I would need in the bore for the exhaust valve etc:, I put everything together with an S block and started measuring with a clock gauge and found the anomaly.

 

The engine went better when I had done it, so I didn't worry about the figures, but asking around out of interest, nobody seemed to be aware of the change apart from Jim Whitehouse at Arden.

[Shooter63]

Erm, that’s what changing the rocker ration does, it will change the cam timing, because the cam timing is stated as a certain lift. Increasing the ratio means a you get the same lift point earlier.

Ac

You're supposed to be on holiday, get back to the range and do your best Dirty Harry impersonation.

Shooter

[Ciccio-kof]

Erm, that’s what changing the rocker ration does, it will change the cam timing, because the cam timing is stated as a certain lift. Increasing the ratio means a you get the same lift point earlier.

Ac

And this is where things start to be interesting. The figures from Kent with standard rockers are 24/56 for the inlet. This means that inlet valve should be opening 24 degrees before tdc, right? Fine!! When I swap for 1.5s I would expect this number to increase to for example 26,27 also as per what you said here, but In fact, I measure 20 degrees with same rocker to valve tolerances despite full lift still occurs at 106°

I'm losing it though 😂

[KTS]

not sure what bearing it has on this, but according to Keith Calvers guide the valve clearances should be adjusted for the rocker ratio being used, so possibly could do with opening up a little

 

https://www.calverst...learances-lash/

[ACDodd]

The point is you all think that you install x item and you get y out. That’s just not the case because of tolerances. The art of engine building is to set all 8 valves to give the valve timing and lift figures you want rather that what the as found factory engine will deliver. I can tell you now the factory rocker ratio varies massively due to tolerance and wear. Factory A+ rockers can be 1.17 to 1.22:1 all on the same shaft. That’s the reason I started making ‘0’ tolerance rockers. Checkout the work required to take these factory rockers into high tolerance repeatanility;


https://youtu.be/mir697SuCCU

Ac

Edited by ACDodd, 13 August 2023 - 07:48 PM.

[timmy850]

Erm, that’s what changing the rocker ration does, it will change the cam timing, because the cam timing is stated as a certain lift. Increasing the ratio means a you get the same lift point earlier.

Ac

And this is where things start to be interesting. The figures from Kent with standard rockers are 24/56 for the inlet. This means that inlet valve should be opening 24 degrees before tdc, right? Fine!! When I swap for 1.5s I would expect this number to increase to for example 26,27 also as per what you said here, but In fact, I measure 20 degrees with same rocker to valve tolerances despite full lift still occurs at 106°

I'm losing it though 😂

If you add higher ratio rockers, they will:
Lift more/quicker on the opening side of the cam (so earlier on the cam timing for the same lift)
Give more total lift
Hold it open longer on the closing side of the cam (so later on the cam timing)

Going from 24/56 to 20/61 means you’ve added 4-5 degrees more on each side of the cam profile which makes sense to me?