FIM 853 Chip

[Shazaam!]

50mm throttle bodies are used on both 748's and 916's. They're different in that in the bore below the throttle plate the 748 tapers to 44mm and the 916 TB tapers to 47mm (I think.) This changes the air flow velocity and fuel mixing characteristics to better suit the performance differences for the two engines.

[CrimsonCloak]

Mine appeared identical to the 916 pair I bought, so perhaps the previous owner had already been in there.

[Ducshop]

Yes, the 748's are 2mm smaller and easy to bore as it's only the straight section below the butterfly.

[Old Baldy]

Originally Posted by CrimsonCloak

Mine appeared identical to the 916 pair I bought, so perhaps the previous owner had already been in there.

They look similar, until you measure them. The 748 TBs have a 33MM long x 44MM ID straight (non-tapered) restrictor built into them (some where machined and some where cast, according to Sigma).

For an 853, it would probably benefit you swapping out the TBs. (Unless you have better luck than me with getting someone to bore them out - because I contacted all the major tuners here and elsewhere, and no-one was interested in boring my old 748 TBs for me.)

[huit]

Would you need more fuel pressure if you go to a larger throttle body?
Paul

[jhord]

I have bored the restrictors out in the past, you don't even need to disassemble the TB's to do it. (just clean the h*ll out of everything when you're done) Problem is it doesn't make all that much power diff. from what I have seen, on stock-cammed, stock-valved 853's anyway. Maybe a couple hp up top. Those stock tiny little valves and low-lift cams are the roadblock.

[huit]

I am running 2mm over intake and 1mm ex, any justification to go bigger?
thanks paul

[CrimsonCloak]

I'm pretty sure the two sets I had were identical in all respects, but I'll check again when I put the pro-thunder heads with the oversize valves in since that's probably when it will start to matter. Now to find some affordable SPS cams...