View single post by Esprit2
 Posted: 11-18-2015 05:59 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 341
stevegarnjobst wrote:
Not sure that conversion is allowable under the SCCA rules. If it were, is the update worthwhile?Yes. The round-tooth HTD timing belt has much deeper tooth engagement, and is far less likely to jump timing. On an interference engine, that's great mechanical insurance to have.

Also, all trapezoidal timing belts are made with 1960's technology. No new engines have been manufactured using that belt since the introduction of the HTD belts circa 1985-86, so the rubber industry has not invested in upgrading the old trapezoidal belts. They still use old HCR rubber and construction.

The only exception is JAE's Lotus belt. They work with Gates Racing to make trapezoidal 9XX belts in HSN rubber (~1995 technology). It's better than the standard HCR belts, but still not modern day HNBR rubber.

Cheap HTD belts in the Lotus size are still available in old-school HCR; but I strongly recommend that if you're going to make the upgrade to HTD, that you should spend just a few bucks more and use the later 1995-spec HSN HTD belt.

No major rubber company markets a HNBR belt that will fit the Lotus 9XX. However, once again, JAE works with Gates Racing to produce a special run of HNBR HTD belts. They have Gates Racing's signature blue outer layer, so they're easy to spot. It's the best belt you can put on a 9XX. The downside... $165 each. If you're going to use the engine in competitive autocross, consider it mechanical insurance. If you go there, talk with me about how to tension it. The book spec doesn't work for the blue belt.

Having said all that, I autocrossed my Eclat and Esprit S2 with the stock trapezoidal timing belt and never suffered a failure. However, I checked/ re-set the tension religiously, and replaced the belt often. I was always doing 'something' on the engines, so every time the belt had to be slipped off the pulleys for something, I replaced it. My belts didn't get very old.

For trapezoidal belts, the standard service interval is 24,000 miles or 24 months, whichever comes first. Time is just as important as miles. Don't cheat on that.

Tim Engel

Last edited on 11-18-2015 06:38 pm by Esprit2