Slopers on the 3ltr

John Murch · Posted: Mon May 08, 2023 7:20 pm Post subject: Slopers on the 3ltr

On the twin SU sloper set up on the 3ltr, the front carb is very close to the bonnet side, I have only just noticed that now that my car has a 4.5 block the carb is actually in contact with the bonnet hinge. So the space between the air intake of the carb and the bonnet is perhaps 1/2 inch maximum. One would have thought that this would affect the air flow, but I don't seem to have a problem.
On a 4.5 the radiator is wider so there is more space for the carb.
Any views on this?

JohnWilliams6437 · Posted: Wed May 10, 2023 5:46 pm Post subject: Slopers on 3 litre

John
Interesting -I hadn't realised the carbs were that close-The Alloy piston housing on the front carb just touches the bonnet hinge.
I suppose in time both would show more than a mark but mileage dependent!
As to the engine getting enough air there have been so many conversions to 4.5 that I think we would have heard about it by now,
Particularly as I believe the bonnet was supplied by Bentley themseves so there must be similarity.
But something else to look for !
Thanks for the tip. JW

JohnWilliams6437 · Posted: Wed May 10, 2023 5:47 pm Post subject: Slopers on 3 litre

John
Interesting -I hadn't realised the carbs were that close-The Alloy piston housing on the front carb just touches the bonnet hinge.
I suppose in time both would show more than a mark but mileage dependent!
As to the engine getting enough air there have been so many conversions to 4.5 that I think we would have heard about it by now,
Particularly as I believe the bonnet was supplied by Bentley themseves so there must be similarity.
But something else to look for !
Thanks for the tip. JW

Stephen Blakey · Posted: Thu May 11, 2023 11:57 am Post subject:

John,

This is a bit like the valve lift conundrum. The text books will tell you that (ignoring the valve stem) there is no (flowbench) benefit opening a valve by more than a quarter of the port diameter, as at that lift the area of the curtain around the valve circumference becomes equal to the port area. This is quite an easy thing to prove using the quations for area and circumference of circles which we learnt in junior school. Maybe you would like to give that a go?

Hence we can reckon that if the distance between the carb and the bonnet averages more than a quarter of the choke diameter, you should be alright.

I agree with John that if there were a problem, somebody would have been on to it by now.

Stephen

John Murch · Posted: Fri May 12, 2023 5:46 am Post subject:

Thanks Stephen,

That makes perfect sense, I will not now worry about air flow!

John