21 replies to this topic

[Mini and Puck]

Hi there, I'm building an engine for a fast 998, and I'm fitting a mg metro cam, and a 12g295 head. My question is, what should I set the cam timing to? What is a standard 998 when lined up dot to dot? I'm assuming o will need an offset woodruff key, maybe five or six degrees? Assuming a suitable carb and maniflow lcb exhaust, electronic ignition, what bhp could I hope for after rolling road? Would 60 to 65bhp seem achievable?

[Cooperman]

The nominal timing for an MG cam is 110 degrees ATDC. setting dot-to-dot is intended to achieve this, but in practice the cam timing using this method can end up several degrees out either way due to manufacturing tolerances.

So an offset woodruff key can be used to correct any error and it is bet to measure the actual figure before buying any keys as it may be accurate enough. When a new timing chain is also fitted it is best to set 2 degrees advanced from nominal to allow for initial chain stretch, so set to 108 degs.

[Spider]

When the Dots are lined up, Cam timing could be anything from 105 to 115 degrees. There are normal manufacturing tolerances that makes just about every engine different. They get you in the ball park, but no cigar (well, not too often!), other than using them as a starting point, don't rely on them as a true reference. If the CR of the engine is anything respectable, then 2 degrees will make a noticeable difference, so I would suggest not just brushing it off.

 

Get yourself a Dial Gauge on a Magnet Base and Degree Wheel, that's about the only way you'll be able to measure how the Cam is currently timed in and what adjustments maybe needed. There are zillions of guides on the internet these days as to how that is done, here's one I found;-

 

 

Although it's a V8 engine, the procedure is much the same for our engines.

 

Coming to the MG Metro Cam, No. 1 Inlet Full Lift - as per published factory specs - is at 110⁰ ATDC and I'd suggest you start with that, or perhaps (me being me) I might advance the Timing by 2 to 4 degrees. It's usually not a bad idea to advance any chain drive Cam by around 2 degrees to allow for stretch and wear, in any case.

[Mini and Puck]

Many thanks! So if the timing gear is not new but has covered just a couple of thousand miles, I think I will just set it to 110 degrees. I have the dial gauge and the pointer and cam timing disc. I have done it a couple of times now, so hopefully I can get it right this time!

[KernowCooper]

The chain will stretch, I'd set it advanced as mentioned by Cooperman and Moke Spider above, you dont want to be in a boat in 5000miles time its lost some of the bhp you set out to achieve

[ACDodd]

104 is my advice.

Ac

[Cooperman]

I always set BMC/BLMC/Rover cams to 108 degs ATDC with a new chain.

[Mini and Puck]

So it's not a new chain. And dot to dot turns out to be 109.5. I've checked it twice. Let's face it, these cam timing discs are not exactly vernier accurate are they? Anyway, I think it sounds from the above advice that I should go with what it is.

[Cooperman]

That should be fine.

[jaydee]

No matter its a new or used chain, it will stretch few degrees simply due to not being under load when setting timing with the pushrod method (almost the same if not more as taking the timing at the valves)

The chain will have some 'slack' that wont have when under load, it wont run back and forth while running.

[Mini and Puck]

Okay, so a two degree woodruff key then. I want this to be right.

[Mini and Puck]

Okay, I have my woodruff key, and I will be doing this tomorrow. Assuming this will then work out to 107.5, I now need to turn to the interesting issue of CR. I am running with a MG Metro cam, standard Pistons, and a 12G295 head. I have calculated the CR to be 9.74 as follows|
Swept volume = 1014 divided by 4 = 253.5

Compressed volume =

Cylinder head gasket 2.7cc

Plus

Block volume at tdc 3cc

Plus

Cylinder head volume 23.3cc

Equals 29cc

Am I working this out correctly?

I am thinking that 9.74 is too low compression for a MG metro cam in this engine with this head. I will be adding a MG Metro electronic distributor and electronic coil. With vacuum advance.

Any opinions on this?

Thanks!

Edited by Mini and Puck, 26 July 2015 - 10:06 AM.

[Cooperman]

When you say you have standard pistons, are they dished or flat top.

Also, how far down the bores are the piston tops from deck level?

Need that info to check the figures.

[Spider]

I'd just like to clarify;-

 

Head volume 3cc

Plus

Cylinder head volume 23.3cc

 

 

Head Volume?   Is that the volume in the Block, with the piston at TDC?

 

 

As for a final Static CR, I'd be looking to at least 10.5:1.

[Cooperman]

Standard pistons in a 998 are normally dished, which means a volume in the bore at TDC of more than 3 cc. If they are flat-top pistons are these standard?