GT186 tinkering

[Rich_T]

The recent batch of GT kits has arrived and vanished very quickly. I did manage to make myself a development barrel to test out an modification to include into the big block kit that I've had on the drawing board for a while. Back in the 80's I was told Dave Webster did a bridged exhaust on an iron barrel and CSPE's recent 21bhp GT kitted scooter that used nothing more than a PHBL25 I thought it would be worth developing the exhaust port side of things along the lines of the IAME Leopard engines. This is the result, looks quite fun, retains virtually the same GT186 timings but with a huge exhaust and larger boost ports (ironically, this is how the boosts should have been but the pattern maker screwed up).

It should be running in a test engine in the next 6 weeks or so. This configuration will only be available for a big block GT kit as and when that comes. The big block would be 200cc but have 10 over sizes up to a max capacity of 245cc, all iron cylinder with new 30 mm abd 34mm manifolds.

[cezeta]

so will that be a right hand carb again?

[Rich_T]

I'm going to try and squeeze a 30mm manifold to come out on the L/H side and a 34mm manifold on the R/H side. All manifolds will be reverse compatible. The head for the big block version will be identical to the Porcupine head that is currently being made for the TS1.

[cezeta]

just out of curiosity why is the exhaust port stepped. i thought similar looking at a dr kit for a vespa, the two additional exhast ports are not the full duration. is the a reason?

looks good by the way, the 186 is a good kit and packages unprofessionally so as to not leave the buyer wondering what to do with it so this will be interesting.

[rosscla]

Saw something similar on an old sprinter on fleabay with the bridged exhaust wonder if that might be the Webster version -

http://www.ebay.co.uk/itm/ws/eBayISAPI. ... _923wt_922

[cezeta]

thats the braised in piece if fin method. went for a decent price

[Rich_T]

The large flare in the exhaust profile above the transfer height is to take off as much of the exhaust pressure as possible before the transfers open. The profile drops back to normal width once the transfers are open so that the port area is not too large and taking out the new transfer charge.

I'll run with the standard 350LC piston at first but I'm expecting I might have a problem with the reliefs in the piston above the wrist pin not sealing the closed exhaust port off from the transfer passage. I have a cunning plan to sort this out if it is a problem but we'll see how it goes first before jumping to conclusions.

[Stevo]

21bhp with a 25mm? Impressive to say the least.