The V8 Owners Forum -

Hi

I have a 1991 3.9 RR which I am going to fit into one my projects, one the eng is under a bonnet so looks are not important and the other is a
T-Bucket where the eng is in full view and I want to retain the looks from how it was in the 70's with twin SU's (twin turbo not an option here)

I have a few choices & questions

For under the bonnet, 2" SU Carbs or Injection?

Can I bolt the Janspeed twin turbos straight on and go & what power gains would I get, I would be fitting twin 2" SU carbs + manifold with this choice.

I also have heads from a 3.5 RR & P6 my question is would any of the heads listed up the compression & performance of the 3.9 and if not how much can I safely skim off the 3.9 heads?


thanks

Brian

Compression ratio is set by the pistons, not the heads. The P6 heads have short reach plugs, so you might as well ditch them.

EFi is far superior to carbs, but of course it's more complicated to get it working properly.

I rather think that with the Janspeed kit, they had different pistons to lower the CR appropriately.

Were there any factory blow-through 2" SU's?, maybe Rolls Royce?

2" SU's foul on the later square rocker covers, I'm not sure if they clear the earlier type.

Chris.

2" SU carbs will fit the later type rocker covers, providing you do not use the later type rubber gasket. I found that there was a couple of extra mm clearance with the earlier rocker covers, watch also the fuel feed pipe does not chaff on the rocker covers.

Toby

Gents the Janspeed kits are suck-thru so no clearance issues with the carbs as they're mounted onto the compressor intake of each turbo. Mine were like that until I went EFi. No special pistons either, as it was/is a low boost kit. I think the original brochure quoted about 250bhp in the SD1s with the kit fitted, but that might be a bit high. My MG was making over 250bhp in a 3.5 with 2" carbs and some mildly ported heads.