 |
PM |  | |
Quote | | |
Reply | | |
Full Topic | |
| Joined: | 05-01-2005 |
| Location: | Minneapolis, Minnesota USA |
| Posts: | 572 |
| Status: |
Offline
|
So it sounds like shimming the liners up is the way people usually go, but is there any reason why milling the block down should be an issue?Anything that reduces the distance between the crank and the cams (milling the head, decking the block, etc) introduces that same amount of extra slack into the timing belt. By definition the cams are timed with the crank at TDC, and the crank cannot be allowed to drift off TDC. So as the tensioner pulls the new slack out of the run of belt between the crank and the exhaust cam pulley, it must draw it up and over the cam pulleys. That turns both pulleys counter clockwise, retarding the timing. How much? It depends upon how much the parts were machined; but something less that a full tooth pitch, so you can't do anything about it with stock parts. Aftermarket adjustable pulleys would be required in order to zero in on perfect timing. IMHO, that's worth it for a racing engine, but not for the street.
On the other hand, Lotus introduced the current composite head gasket in Jan 1993. It's compressed thickness is about 0.5mm / 0.020" thicker than the original steel-asbestos-steel gasket, so it lifts the head that much higher off the block deck. Besides 'advancing' the cams a bit, raising the head also increases the combustion chamber volume, and reduces the compression ratio by about half a point... 8.4:1 becomes 7.9:1.
When the new gasket was introduced, Lotus revised the machining on all new 910 & 912 engines (the 907 was long out of production), cutting the block deck that much lower, and seating the liners a like amount deeper to preserve the Nip. So engines built since Jan '93 have the correct compression with the new gasket, and all older engines have 0.5:1 less compression that they started with.
If you're ever faced with decking the block a few thousandths for whatever reason, consider taking the full 0.5mm / 0.020", just to get your compression back.
Alternatively, if you ever buy new pistons, order half a point more compression that you really want.
DonBurns wrote:
I have a 2.2 liter, 104 / 107 cam combination and 9.6 compression. Are the valve clearances the same for any cam? So .006 In / .011 Ex regardless of cam?The 104 and 107 cams both use the same valve clearances:
0.005 - 0.007 Intake
0.010 - 0.012 Exhaust
If you converted a 907 to 2.2 and are using the stock 907 head with the 104 & 107 cams, the stock valve springs will go coil bound due to the extra lift. The 910 & 912 heads are built from the start to work with the high lift cams.
For a 907 head, there are special thin-wire springs available from Dave Bean Engineering, and probably some of the other USA Lotus specialists, like JAE. I don't know if Delta carries them; but they sell the cams, so they might have the springs to go with them.
In the UK, it seems more common to machine the valve pockets in the head a little deeper to allow for the extra lift. Either method will work... just be sure that you do 'something' to allow for the lift. If you don't, something is going to break the instant you try to start the engine.
Regards,
Tim Engel