Correct Vaccum Gauge reading on a four pot

[Ian Vincent]

Many years ago I acquired a vacuum gauge as a tuning aid and I recently came across it whilst looking for something else entirely.

I have since fitted a vacuum gauge take off to the inlet manifold and run the engine with the gauge attached.  It is showing about 15 ins Mercury which from my recollection is a tad low.

The needle has a very slight vibration (about + or - 1" Hg).

The engine is idliing pretty smoothly at around 800 rpm once warm.

I have checked both carbs and the pistons are moving freely and they are balanced.  Timing is 12º BTDC at idle.

Does anyone out there have any suggestions as to what the issue may be or is 15" about right for a four pot TR engine?

I haven't done a compression test yet but the last time I did one they were all pretty consistent at about 165 psi.

Rgds Ian

[harlequin]

Hi Ian

Your question got me wondering what sort of readings my TR3A would give and as I have just had to pop down to Tescos and the car was fairly warm I connected up my cheapo Gunsen vacuum gauge to a nozzle that is on the centre of the manifold balance pipe.

At 800rpm I also have 15Hg with the needle flickering +-1Hg the highest steady reading was18Hg at a bit over 2000rpm

I understand that a flickering needle is a sign of valve guide wear but I think in my case its probably a lack of damping in my cheap guage. I have no idea of the history of my engine but it pulls strongly, uses next to no Oil, and fires up immediately. 

From what I have read 21Hg is about max from an engine in a low state of tune like a Ford 100e or an A35 this will drop if the cam is designed to give more performance so 15 is probably not so bad,. I should add that a TR6 with pi has a reading of 12Hg  but that may be influenced by something else

Hope this helps 

George 

Edited September 29, 2019 by harlequin

[stuart]

Depends on what cam you have, hotter cams often have less vacuum.

 

Stuart.

[harlequin]

Stuart

You beat me to it while I was typing my edit and fighting the dog off of my bacon sandwich 

George 

[stuart]

1 hour ago, harlequin said:

Stuart

You beat me to it while I was typing my edit and fighting the dog off of my bacon sandwich 

George 

Who won?

Stuart.

[Ian Vincent]

Hi George & Stuart,

I am much reassured by your responses.  I suspect my flickering needle is down to the gauge being an old and cheap one.  I had the misfortune to replace my head gasket recently and whilst I had the head off, I took the opportunity to get a local engineering shop to check it for flatness so I had to dismantle it.  The valves & guides were all spot on so I am not worried about that as an issue.

The cam is a Newman fast road one so that may account for the reading being on the low side.

My vacuum nozzle is also on the balance pipe on the inlet manifold (a TR4a one).

Rgds Ian

[harlequin]

Stuart 

He's a Labrador so he's rubbish at typing but always wants to help, however when it comes to hunting down a bacon sarnie he is world class! We can probably put today's encounter down to a draw,  Fido got the slippery bits.

George 

[Lebro]

I had a Redex vacuum gauge in my TR back in the 70's As I recall you could vary the damping of the needle by adjusting a clamp on the flexible pipe.

Bob.

[harlequin]

I have it on good authority that a vacuum gauge can be very useful when tuning an engine, but I think like most of these tools one has to spend serious money and buy quality otherwise you are wasting your time.

George