controlling VNT boost spikes

[ewenmaclean]

Hello,

a couple of people asked me how you could use a boost device in a VNT turbo system - usually this is for electronic tdis. The VNTs suffer a bit from boost spike - especially with shorter and possibly straight through exhausts with no cat like on the T3s. If you get boost spike you risk the turbo itself and can easily go into limp mode which can be really annoying.

I use a slightly modified boost valve designed for a wastegate systems where the wastegate gets actuated at the desired boost. An example of one of these is this:

http://www.ebay.co.uk/itm/UNIVERSAL-MAN ... 336abf5216" onclick="window.open(this.href);return false;

nothing to do with me - just an example.

When designated boost pressure is reached (adjusted using the screw) it passes boost pressure to the output. These have vent holes drilled in to release any pressure in the boost pipe to the wastegate actuator. This hole needs to be blocked up to make it work with a vnt otherwise the system will not hold vacuum. It is plumbed in as in this amateurish drawing I knocked up:



once preset boost is reached (23 psi in my case), boost pressure is released into the vacuum system momentarily and the vnt mechanism reduces boost - this is immediately resumed once the boost falls below the preset.

The duty cycle of the N75 valve which controls the acts at least 10 times a second so this valve will only be depressed momentarily when activated.

I put this up as I think it's worth considering with the AFN engine for example - especially if people are experiencing limp mode.

Ewen

[syncroandy]

Top tip ! I have a ball-valve left over from my 1.6, may well try this out on the TDI.

[..lee..]

well my erritic boost issue has gone now that i`ve bled a bit of the map sensor. its now far more than i ever expected. temp fix i know untill propper remap can be sorted. but for now i`m a very happy bunny.

[ELVIS]

Big part of an article in this months PPC was about these/this.

Chap was still having issues so went for an EBC in the end.Cant remember the exact issues but was still having problems with manual controller.

[lloydy]

How many people actually get this problem? And what is actually causing it? Is it happening to people who are using jx manifolds more than people using the ones that come with the engine?

[ewenmaclean]

Hello,

thanks for the replies - I may not have explained properly - this is meant to be used with electronic boost control. It is designed for wastegate turbo system to keep boost on longer, but this is modified (by welding up the vent hole) to control overboost in vnt vacuum driven systems.

Even with a really good map the vnt will still spike a bit at wot - for example this is an image of my first attempt at remapping for a bigger turbo and bigger injectors on a 3rd gear wot run - you can see it has a bit of lag and some overboost - not too much but a bit:



it's less laggy now and overboosts a bit less, but the spike is still there and almost impossible to remove - this partly due I think to low back-pressure on the the T3 short exhaust with no cat, but all VNT turbos exhibit this behaviour a bit - especially modded engine setups with bigger injectors and turbos. The more stock the less it happens it seems. I'll try and put up a plot soon of a plot with the modified boost device put in.

The reason for posting this was to be used in conjunction with an electronic map, so that boost spikes can be eliminated. They can be a real pain if the difference between actual and requested is too much, or is different for too long - this will kick the computer into limp mode.

Ewen