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 Posted: 10-04-2014 09:13 pm
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Esprit2

 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 289
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Shorter air horns to optimize higher rpm performance,
Longer air horns to tune for low rpm performance.

Given the range of available air horn lengths, and the space available within the airbox (or even in a K&N air filter), the degree of impact that longer air horns will have on low end performance won't be significant. If you're not pleased with the current low end torque with stock air horns, then you won't be pleased with the result with longer air horns. The change isn't that dramatic, so apply your budget where it will have more impact.

Air horns, with their rolled and tapered inlet shape are important (!) to smooth airflow into the carbs, so don't delete them. Or if they're currently missing, install some. Even if they're the wrong length, the smooth entry is worth having even if the length isn't to spec. Something is better than nothing.

On the other hand, the top end performance when you're bouncing off the rev limiter is noticeably better if the correct, tuned length air horns are installed. With a near-stock engine, that would be the stock length air horns.

The "Torqueless Wonder" tag really applied to the Federal emissions spec 907... it really was weak. The compression was low, the cams were timed for emissions (can easily be set back to the correct 110 MOP for both intake and exhaust), the ignition timing was really lame (dial in more static advance, and/or re-curve the centrifugal advance), and the Stromberg carbs were tuned "emissions-lean" (strangled).

The Strombergs could be re-tuned for performance, but most owners just converted to Dellortos. J-H used DHLA 40 carbs on all their engines, while Lotus used DHLA 45E carbs on all of theirs. Different goals.

The J-H Dellorto spec 907 used the same 8.4:1 compression ratio in all markets (emissions & non-emissions); however, Lotus' own Euro/ Dellorto spec engines used 9.5:1 compression. That made a very worthwhile difference. If you're going to invest in new pistons, then I'd recommend 11:1 on pump premium fuel.

With the exception of compression ratio, all those changes can be made to a J-H 907 for a noticeable improvement in performance.

The single largest improvement you can make to any 907 is to convert it to 2.2 liters. It's a small change in stroke, but it makes a big difference in low end torque.

Last edited on 10-04-2014 09:19 pm by Esprit2