Hi Mag10, got your PM. First, is your Speedie one of the last 790's or an 865?
The rev limit on the early 790's was 7,500rpm and on the later 790's 7,400rpm. The 865's have always been 8,000rpm.
At what revs do you hear knocking?
An 865/904 with 790/813/Thunder cams doesn't run great with 865 maps. I have dyno developed a mapping on a 360° 865 with Wiseco HC pistons and 813 cams. Started out with 865 maps at 70.19bhp @ 8,500rpm / 49.07ftlb @ 6,200rpm and ended up with the custom mapping at 74.60bhp @ 8,420rpm / 52.28ftlb @ 6,200rpm with gains all the way up from 2,500rpm. (the dyno's and a writeup will be published on my site shortly)
Although the above development is on a 360° engine, it works the same on a 270° in the same tune.
The low rev igniton timing is very different on the 270° 790's and 865's. If I load standard 790 maps to an 865 with 790 cams, the normal overrun snap crackle and pop turns in to loud bangs which sets off car alarms and makes small children cry. This has been set right on the 270° custom mapping.
So as to your knocking. I haven't experienced or heard of a modded twin knocking, except on occasion in high temp weather, but there again I haven't worked with the JE pistons and your petrol, which I believe is slightly different to ours. TPUSA have much experience with BB HC engines and haven't suffered knocking with a modded igniter. It may be worth speaking to Carlos and asking his opinion.
On a side note while I'm here, sorry to go off subject, but with regard to the early 270° 790 igniter's that cut-out when you hit the rev limiter. With the one's I have modded the cut-out goes with the rev limit. So if the rev limit is raised to 8,500rpm for example, the cut-out doesn't happen until 8,500rpm and as useful power drops off before this level, it should virtually eliminate the problem. (unless you like the sound of a screaming 790 of course).
Sorry for the long post, but I don't get over here as often as I might like.
