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| Joined: | 05-01-2005 |
| Location: | Minneapolis, Minnesota USA |
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I have noticed that 40s seem a lot cheaper than 45s. On other cars with Webers (similar design) I actually found that it was better to undercarb than overcarb. What are the tuning and performance implications of using 40s?
Andrew,
... and 45's are by far cheaper than 48's. And 52's are pure gold.
For an early, near-stock, street 907 (especially low compression) 40's are adequate. Not optimal, but adequate. Compared to the Strombergs, they're great. The performance improvement offered by 45's (45E's are easier to adjust accurately) is probably not worth the added expense if you intend to keep the rest of the engine stock and don't flog the car.
On the othe hand, Lotus never used 40's on their own versions of the 907. Only 45E's. The best choke compromise for a 907, IMHO, is the 37mm, and that's really just beyond the maximum that should be used in a 40. 37's in 45's work well for a street engine.
I just helped a local owner of a new-old J-H install Lotus Spec-5 DHLA 45's with 36mm chokes on his 907. The only not-spec setting is bumping-up the idle jets from 50 to 55. I think he'll tell you that the car runs great and is very driveable on the street. I have Lotus Spec-9 Dellorto DHLA 45E's on two 2.0 liter 907's (one with 107 cams, one with 104's) and a hot-rod 2.2 conversion. They display no temperment in traffic and pull hard all the way to red line. 40's really tend to drag-out that last bit of the climb to red line.
If you aren't one to use red line often, then maybe that's not an issue. However, if you enjoy a good flogging, then consider going with the 45's.
Greg Fletcher wrote:
If you want to go nuts, a set of 48mm Dellortos might even be desirable. (Snip)...
The Dellortos are an improved version of the Weber (which don't offer the same progression) and I'd avoid Webers for this reason.
48's are track-nutz. Over the top for a street 907/912.
Not all Dellortos have the same progression hole pattern, and most generic Dellortos have 5 progression holes. Lotus worked directly with Dellorto to develop a 6-hole progression pattern for the 907 that really helps off-idle and mid-range performance. If you can find a set of Dellortos from a Lotus engine, take them over a generic pair.
Regards,
Tim Engel