REBUILDING & UPGRADING AN 912 ENGINE

[rgh0]

cant see the images - photo bucket is useless and most historical images in most posts that use it fail

 

cheers

Rohan

[MrDangerUS]

To All,

Would anyone specify for me the differences between  907 and 912 engine heads, valve dia, etc?

Giorgio said (see previous page) that he was able to use S300 valves without changing seats, just machining job.  That's very exciting! It means a significant improvement in engine breathing !

Please educate me.

Thanks.

John

 

[PrecisionMike]

Hi John, my understanding is that broadly speaking the 907 and 912 LC (low compression)are much the same in terms of gas flow but 912 valve stems are longer and the springs I think might be different. The valve spring retaining collets certainly are different but they are interchangeable and can be used to slightly change valve spring closed and open loads. Over time there were I believe slight changes to the head to improve cooling. The 912 has the heater take off at the rear of the head. The 907 took coolant from the inlet manifold. The real differences were in the 912 HC engine which is in the excel SE and Esprit HC which has bigger ports and larger valves. I might be missing something but I never understood the benefit of 1mm larger valves in a standard but re cut valve seat without modifying the seat bore or porting, IF in fact that was what was described. 

The lotus workshop manual does quote valve diameters and the Bell & Colvil web site has full exploded diagrams with all part numbers which I've found useful in finding differences and identifying part numbers.

I'm just completing a full and "special " build myself and one of the many lessons I've learned, is that a with a valve seat cut, valve re face and longer stems you're likely to run out of valve shim range which is solved with new steel tappets of which there are various choices of pad thickness depending on supplier , so do budget for new steel long skirt followers which are a good idea anyway as they are stronger and offer increased stability. If you're interested in that part I do have as work in progress (not yet completed) a spread sheet which takes all relevant parameters and calculates required shim thickness as well as valve spring open and closed loads

I'd recommend a call to either Lotusbits , PNM Engineering or Southwest lotus, all of whom have intimate factual knowledge and real current experience. 

The principle and very major lesson I've also learned is that there are usually knock on effects of any change which require engineering consideration and access to a thick wallet 😁

Good luck

[drdoom]

Just had a quick browse in the final Service Notes which state the 1mm larger inlet valves, fitted to S4S and S300 models, were accompanied by throats also enlarged by 1mm. Makes sense.