K-tec`s "group A" carb

[Junglist]

Why dont we like them ??

This is what K-tec say about them;

This is a fully modified carburettor body which benefits from a larger venturi and is fully re-jetted to suit the greater airflow through it. The machined venturi bore is 30% larger than the standard item which results in more boost pressure making it into the engine. This means power is increased without increasing the boost pressure. This is achieved because the standard carb can cause a pressure loss of up 5psi through it due to the bore size of the standard venturi (i.e. 15psi boost pressure entering the carb can result in 10psi boost pressure exiting the carb and entering the engine = 5psi pressure loss) whereas the free flowing Group `A` carb will only 1-2psi across it resulting in more boost entering the engine. MORE BOOST = MORE POWER. Obviously the carb is fully re-jetted to cope with the extra airflow and has been used successfully on cars running over 35psi with no change to the supplied jetting. It can be fitted to standard cars but is a must for cars running over 20psi as the standard carb becomes far too restrictive.

Any truth in that or a load of bull ?.

Whats the pros(if any) and cons ?

[Scoff]

There are more cons than pro's unfortunately

1) Those pressure differentials are extreem cases. Often the 25mm choke is not that bad.
2) Regardless of the change in pressure differentials there is nobody that I know of that has seen a performance gain from the bigger choke
3) The bigger choke means less of a venturi effect. That in turn means that the main jet has to be a lot larger because the system has become less sensitive and can't meter fuel as well as before. That's fine to a point except that fueling gets sloppy, so in order to maintain acceptable AFR's in places it inevitably gets too rich in others.
4) Fully re-jetted suggests that all jets have been optimized. Not usually so. Typically just the main jet and air corrector are changed, sometimes the 1st stage enrichment is enlarged too. (Which is a bad thing, they really ought to have enlarged the 2nd stage bush. I don't know what exactly K-tech do, I can only talk about the old BB and Europarts carbs I used to correct a lot)
5) "35psi with no change to the supplied jetting" is a crazy thing to claim. Just because the engine did not explode on the dyno does not mean that the carburettor was fit for purpose ! I can almost guarantee that the fueling will go horribly wrong flat out in 5th gear at probably much more than 20psi on a good engine, let alone 35psi! They often leave the standard 1.7mm needle valve in. We all know that the 1.7mm need valve maxes out at about 180hp. After that you are running on the contents of the float bowl and it's a gamble how long that will stay full before your AFR's go dangerously lean and you melt an engine.

Anyone who's experimented enough with these things will tell you that a well jetted 25mm will usually perform as well as a Group A and you'll have better fueling as a bonus.

Quote from "four stroke performance tuning" by A. G. Bell:

"If you take a carefull look at the venturi you will find that it has a radiused inlet and diverging tail section. Designers worked the venturi shape to create the maximum pressure drop (at the aux. venturi) with minimal flow losses. Frequently, when the venturi is bored the basic internal shape is not changed to suit the larger internal diamater. Boring really requires that both the entry radius and the divergent angle of the tail be changed, otherwise the increase in bore size will not increase airflow, but that will result in a less effective metering signal" - (Hence the requirement for over sized jets in the main circuit)

[Junglist]

Thanks Scoff .

[Wallace]

Interesting

[Thundercat]

I fitted a k-tec group a carb to a friend car and found it trouble some to say the least at idle. Only know after carrying out the cup mod breather system does it idle solid. Does seem to handle boost well but not running an afr gauge would worry me!

[G-man]

New poster! (less than 10 posts)

Nice one scoff was looking into one of these but don't think i'll bother now! I'm looking at making it more mechanically solid so waht would u suggest if not a "group A" carb?

[Scoff]

The standard carb, rejetted to match your engine

[G-man]

New poster! (less than 10 posts)

The standard carb, rejetted to match your engine

nice one cheers for the advice bud

[6FOOT6]

is it 2003 again? Im must have boosted so much that i hit 88 MPH and went back in time.



Hope your Well Scoff!