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Twin Pipe 670 -96

806 Views 0 Replies 1 Participant Last post by  Swede670

I've read alot of your posts regarding the 670, there seems to be very experienced people on this board on these engines.

I have a -96 twin pipe 670, it has a full case portjob done, intake matching, piston/cylinder/transfer passage cleanup. It also has Exhaust aux-ports enlarged and cleaned up, not that extreme primarily to avoid short-circuit to transfers via piston wristpin cutout.

Running with 44s and 501 disc, stock pipes, stock silencer.
Gutted airbox at the moment.

Duration is about 203 exh, 135-136 transfer. I've thought quite alot about the transfer duration, it seems a bit too much. This is about what it came stock with duration wise, the cyls are 1,3mm taller than at least the early -93 670 cyls, this is what makes the transfer duration rise.

A few questions arose during the tuning process:

My gut feeleing is that closer to 130 transfer duration is better suited for 8200rpm, just my gut feeleing please comment about this.

I've contemplated using regular cyls with 130 transfer duration and then build from there leaving transfer duration at stock level, what do you guys think about this idea?

The stock pipes, how good are they today, modern pipes are generaly more beefier in mid section.
Are there any proven dyno figures with these pipes?

The stinger dia on the stock pipes seems a bit large, 31mm inside dia.
I'm thinking of going down to 29-29,5mm, or weld in a large nut on the side of the stingers with a bolt that can be screwed in to the exhaust flow creating an adjustable restriction.
Thoughts about this are welcome. Yes i know going too far may cause meltdown disaster...

How is RAVE opening affected by my mods, the softer springs that come stock in the twinpipe 670 implies less exhaust presure with twin pipes than with single.
The opening up of the aux-exhaust port, does it call for even softer RAVE-springs?

The engine hardware setup seems to be quite a bit overkill in many respect, at least when running in stock trim(only 122hp), my feeleing is that it was set up like this in order to have all the right parts stock to create a race engnine, racingrules maybe didn't allow changing carbs, pipes etc.
What kind of races did these sleds compete in and what did the racingrules say about modifications/changing parts?

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