Hi Guys
The engine original spec was as follows:
2x 1.5" SU constant depression carbs.
Balance pipe fitted to heated manifold
2 litre straight six engine
CR 9.5/1
Cam 25/65 65/25
As now:
3x 1.5" SU constant depression HIF38 carbs.
Was an in-appropriate balance pipe arrangement.
Large bore cold air fed to air plenum box housing bell mouths.
Was an unheated manifold arrangement.
2 litre straight six.
CR 9.5/1
Cam 37/63 74/28. Correctly intalled now!
Has shallow depth MGB style bell mouth 'stub stacks.
6-3-1 exhaust manifold with un-evenly matched primary pipe lengths.
Mapable Megajolt ignition.
Gas-flowed cylinder head.
Unless I go for Alfa Romeo Spider style bent stub stacks, there is no space available to fit longer stub stacks due to the wheel arch being in the way.
I have been very careful to ensure the carbs are balanced at and off idle properly (i.e. gauges prove they simultaneously draw the same volume of air) and the float heights are correct to book. The carb pistons are all weighed and evenly matched. This includes the piston in dashpot drop time test and they all rise/fall at the same rate.
Where-as twin carbs draw evenly through each alternate carb, triple carbs exhibit a longer drawn out cycle where-by the outer two carbs draw a different sexuence to the middle carb. My outer two carbs(1+3) exhibit pronouned fuel vapour stand-off throughout the rev range, to the point that RR sessions are a definite health hazard!
I can't help but think that as the twin carb set-ups don't do this to anywhere near this extent(if at all), it frustrates me that the current set-up is busy pushing fuel vapour out of the carb throat when it should be drawing it in.
These cars put out an alledged 105bhp in the 60's. The car was putting out 120bhp at the rear wheels on the last session. It did drive much better than had been the case post build a few years ago.
Initially the camshaft wasn't timed in correctly.
The mapable ignition also greatly improved the torque and drive-ability of the car- far more torque.
The gas-flowed cylinder head was inadvertantly left with a CR of only 8.3/1! This was corrected with skimming it to 9.1/1, but that was ll it would do due to an inability to skim the head any further. The improvement to the CR also improved the performance incrumentally.
Getting better needles and lighter carb piston springs improved the situation too, to such an extent that after the last rolling road session, it was a dramatically improved car to drive.
A recent Trackday session at Cadwell Park showed the car to be very nice to drive- a good spread of torque, though even though the cam is only mild, it does have a marked powerband after the initial low down torque.
I've ditched he exhaust wrap. Unless there is an issue with under bonnet temperatures(there isn't at present), i'm not convinced of its merit for my installation.
I can see the relative merits for outright performance of side draught carbies, but the installs on GT6' are fraught with dreadful after market linkages and then there's the question and cost of finding the correct fuelling and getting a knowledgeable RR operator to achieve that throughout the rev range and not leave me with a gas guzzling monster that's got a miniscule power band at 4000+rpm!
I am happy to fit twin 1.75" carbs and a gas flowed inlet manifold in due course, but I want to just try to sort these out to a point i'm happy with before then.
My initial plans are a new cylinder head with a CR of 10.5/1.
Trial the much enlarged (and improved) balance ppe arrangement, with the newly installed heating arrangements for the inlet manifolds.
Try longer ram pipes on the RR- irrespective of them not being able to be fitted with the bonnet down!
The, have a power mapping run to maximise the power throughout the rev range.
So, any thoughts on my plans?
Regards
Bruce.
