New Big Block Gran Turismo Kit 200-245cc

[Rich_T]

The GT Big Block kits are now available for sale, Barrel, manifold, read & piston £420 inc VAT or £525 inc VAT with a Porcupine head.

This is, without doubt, the fastest cylinder kit I have ever made (a good 5 bhp on the top of the Small Block for only 14cc more capacity and a very wide power delivery). It has a potential for 20 over-sizes starting at 66mm to 71mm. In the stock 200cc format it is already a pretty astonishing kit. If you thought the GT186 Small Block was good, this kit has much more in every area and every sense of the word.

If you're looking for a powerful engine with wide torque delivery you really need to seek out of of these motors and give it a spin.

[grayspeed]

Are you going to do a 225 off the shelf? And what difference does the porcupine head make in real terms?

[Rich_T]

No, there will be no 225 off the shelf. There are three reasons for this:

1. There is a machined blend radius between the bore and the transfer port wall. This is machined by the CNC as it was not possible to economically put the shape into the casting pattern. If the skirt is too thin then there is a much higher risk of the CNC breaking or stressing the barrel skirt during the machining operation.
2. The kit is already mid 20's bhp at 200cc and still returning excellent mpg...much much better than any TS1. In fact better than any other performance kit I have seen. This is before any tuner has touched it and it's running on stock Indian ignition and Indian 58 stroke crank with Italian rod. If the casings were matched and it ran on a 110 rod with a full circle crank with an RS125 ignition it would do better for sure.
3. The plan is for a long and economic service life, with 20 over-sizes you are very unlikely to wear it out (approx 300k miles service life). A 225 would strip out 80% of the service life for very marginal gain and at a very high risk or damage.

[Rich_T]

what difference does the porcupine head make in real terms?

This is a slight odd one. The head was originally developed because the scrap rate and costs of machining second hand or Indian heads was not economic. To produce your own cylinder head you have to add in additional features (centre plug, CH temp gauge thread insert etc). The only plan was to have a comparable head. In actual fact, the reports go much further than that. I suspect this is down to consistent production dimensions and repeatability but there is strong anecdotal evidence to suggest that the head cools more effectively.

Compared to the OEM design the porcupine head has thicker fins rooted into the combustion area. The theory was that the thicker fin root (and increased number) would provide a faster path to sink heat to the fin extremities. Practically, testing this in a repeatable way in direct comparison with other designs is really quite hard to do. Best I could come up with was a temperature decay test which would require a special jig, heaters, fans and thermocouples and A-D converters. This is all a bit to much when there is a shed load of other stuff to do and develop.

Over a stock OEM 200 head, there is absolutely no contest; the OEM is a compromise and does not perform as well. If you want to take a standard OEM head and machine in a precise semi-toroidal profile and clearances like the Porcupine it would cost the same as the porcupine or as near as. It would also have a very high probability of porosity in the casting, only one plug position nd no provision for a CH temp probe.

[Train Driver]

Will this kit run with longer stroke cranks (64mm)

[mick1]

Looks good. What final gearing is recommended, plus exhaust, carb, timing, etc. Is there a Dyno chart showing what performance is achievable.

[Stampede]

Any pics of the finished kit?

[slangen]

Have only seen these, not much help.

[Adam_Winstone]

Looking great! Not good to have these available now though as it means that the kids may be going hungry this Xmas

Which of the manifolds do you get with the full package (incl. head)?

Adam

[Rich_T]

Looking great! Not good to have these available now though as it means that the kids may be going hungry this Xmas

Which of the manifolds do you get with the full package (incl. head)?

Adam

Who are you kidding Adam, you sold your kids ages ago for a Westlife CD. You chose any manifold you like but I suspect the strongest contender will be the 30mm for L/H side with a PHBH30.

Overall, there are now 4 manifolds. 25mm and 30mm for L/H side use plus 30mm and 34mm for R/H side use (battery tray removed). The 34mm is a genuine 34mm and will not suit a VHSB34 as the diameter is much larger than 34mm. The 34mm would be vest used with a Makuni, I have my mind on converting the jetting from RGV250 VJ22 carbs as they are cheap and the venturi path is angled to the slide and float bowl which results in a tidy install in Lambretta body work (no panel cutting).

These is a further plan for a 39mm manifold which will use the throttle EFi bodies. I'll be working on that some time in the new year.