What is the 'correct' boost?

[Harrie Harrison]

First post!

I recently bought a 5GT and would like to ask what the normal position of the turbo boost needle should be when straight and level on the motorway? The reason for asking is I cannot remember what my previous car showed and I think the current car is too high. At 70 mph at a steady speed the needle on the boost is vertical - is this normal? The car has been previously fitted with a non-standard exhaust which is slightly louder than the original version so would this result in the turbo spinning-up more? On hard acceleration the needle very rapidly moves round to the 2 o'clock position and while I am very pleased with the resultant acceleration, I am concerned I might damage the turbo or engine. Many thanks, Harrie.

[francob80]

That sounds like standard boost, if the dial isn't going all the way off the gauge I am sure it will be fine. I think from memory standard boost is 7/8 psi and about the 2 o'clock position.

Your exhaust won't effect the amount of boost you are running.

[Trevhib]

Welcome along Harrie.

Take a read of this thread, which I found using the search function:
http://www.rtoc.org/boards/showthread.php?t=27443&highlight=standard+boost

Taken from that thread:

http://www.renault5gtturbo.com/pics/reced/BOOSTGAUGE.jpg

A=8.25psi
B=9.75psi
C=10.5psi
D=12psi
E=13.5psi

A to B is roughly std.

On a standard turbo, assuming the car has no problems, you're going to be ok running to E.

[Trevhib]

As for where the needle should be when travelling at 70mph on the flat: assuming the car is standard in every way, if I remember correctly, you can expect to have to push some boost through the turbo to retain that speed. I can't remember exactly where the needle on mine used to go to. Vertical (A), seems a little high for 70mph. I would have guessed a couple of marks before that.

You could check for boost leaks.

[Ian S]

It was long time ago but, IIRC, my E reg was about 10 to 12psi as standard.

On a standard car, with all the standard boost compensation pipes, if they are all functioning, an exhaust that flows more freely (which may be what you now have) than the standard exhaust will cause about 1psi less boost because is defeats the boost compensation.

Those pipes are there to create more boost to compensate for the increased back pressure of the standard restrictive exhaust. As back pressure rises, the pipes shut the wastegate a bit, increasing the boost.

[Trevhib]

It's true to say that over the years, different people running the standard set-up have reported their gauges going to anywhere between A and about halfway between C & D, with folk arguing over what pressure the cars came out of the factory with, between about 8psi and 11psi (which tallies). It wouldn't surprise me to discover that there was indeed a range of 2-3psi between different cars.

IIRC my three standard cars were approaching C, about 10psi, so I'm in the middle.

[Matt Cole]

The question is, how were they set up from the factory? I imagine at the actuator. Does the technical manual not detail the setting up methods?

[Trevhib]

Yes it's done at the actuator rod. Was that by the turbo manufacturers or by Renault though?

[Harrie Harrison]

New poster! (less than 10 posts)

Many thanks for the very helpful comments. HH