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 Posted: 11-27-2015 08:25 pm
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Esprit2

 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 289
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Steve,
The number cast into a Lotus part is the raw casting number, not the finished part as used on the engine, or that you could buy. Besides the casting number, there was a machined bare head part number, and a fully built-up, ready to install part number.

Lotus didn't fully machine the ports from small-as-cast to various finished sizes. They were used pretty much as cast.

The original AP head's intake ports (AP was the first castings vendor) did go through a series of evolutionary changes (at least three that I know of) before the vendor change to the Zeus head with it's final port config (Zeus is the later castings vendor, circa 1993). The Zeus head is superior, but didn't come along until well after JH was out of production. It would require some fast talking to justify using one on a JH autocrosser.

The early JH Mk I 907 did have the small ports, however, the later Mk II did get larger ports... the ones I believe are 1mm taller. Lotus produced the 907 through 1980, and late 907/ early 912 heads had ports that were larger yet. That may (??) be what Mike Taylor showed to you. The big port head will bolt right on.

I don't know enough about JH to know how far their 907's ports went down that evolutionary path. But the last of the JH engines were pretty much the same spec as the Lotus 907s of the day, with the exception of carbs and distributor advance curves. JH & Lotus Strombergs used different needles and tuning. JH used DHLA 40 Dellortos, while Lotus used the larger 45s.

Do you have pictures you can post of the two heads with small and large ports?

Regards,
Tim Engel

Last edited on 11-27-2015 08:26 pm by Esprit2