When I went for T5.5 I had type 7's which were nice for the LUCAS system and APC control. But felt a bit short when going to T5. Then I went to type 8's which made it feel like the 7's without T5. But on longer cruises I still found the rpm a bit high (we have highways which are rated at 130kph, so cruising is usually a little higher than that.
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Then the type 9's came out which are around a 450rpm drop in 5th (with later style diff) compared to the 8's.
Here's a vid I did with them.
https://youtu.be/qncfUXRiKHE I had a lot of mixed feelings when I first drove with the 9's. The pro's were the rpm drop while cruising in 5th and 1st and 2nd gear that were actually worth something.
On the other hand they were very noisy, the triplex chains and my solid front mount made it sound like a supercharger almost the whole time. Especially on lower speeds when there is less wind and tire noise
The lack of downlow torque made quick take overs to need downshifs. Cruising at 100kph in 5th and "flooring" it would result in protest from the engine. Which wasn't a problem with the type 8's.
The big problem was that after 1200kms or so the chains started to slip.. (turned out the chains are too long for the tensioner to work with) I'm still waiting on the verdict of RBM, but I doubt they will admit that that is the problem and it was to blame on bad installation.
I then had contact with Jörgen Eriksson for a 8.5kit which was easier to source than a type 8 and basically cost the same, besides they were new instead of 30 years old and with miles on them. Also Jörgen was very helpful and kind in trouble shooting the problems I had with the type 9's
Long story short, I should've gone for the 8.5's from the get go.
They are just a little longer in ratio than the type 8's but the chains are a lot tighter so the engine and gearbox feel like they are directly connected instead of that jerkiness you usually have. Also they are a lot quieter due to the original offset 3 chain design.
On the 2.1 head matter. I just picked it up from the rebuilder, it will be installed next week, right in time for Intsaab which is here in the Netherlands this year.
The offboost drop in power should be minimal since it has been shaved to almost max tolerance so the compression drop should be minimal. If it's noticeable I might be able to play with the ignition timings to regain some power back. :)