Confusion over EGBV function

EGBV

This allows some exhaust gas to bypass the EGCV and join the output of the first turbo. It serves two purposes - one, it prespools the second turbo by allowing some exhaust gas flow through it. Two, it controls the amount of exhaust gas going through the first turbo in much the same way as a wastegate - therefore controlling the maximum boost pressure generated by #1 turbo.

The above is a description of the EGBV in the FAQ. bit of the Technical Section. Now I don't have a problem with the bit about providing pre-spooling of the no.2 turbo but what I just can't see is how it controls the amount of gas going through no.1 turbo. Looking at the line drawing provided with the description I just don't see how it can effect no.1 turbo at all.

Can anyone enlighten me ?

My interpretation is that when the EGBV is open it is robbing the 1st tubby of some exhaust gases from the manifold and exiting them downstream of tubby 1. With the EGBV closed then all the manifold gases are forced through tubby 1. (just remembered that this is wrong since the wastegate will be open anyway by this point for tubby 1)

Another thought too is that it acts as a wastegate to tubby 2 since it will be open when 2 is online.

Merry Christmas:)

Mike

Hmmm, from the line drawing ,as far as I can see the only exhaust gases that will ever go through the EGBV will be those from the no.2 turbo.

And as for the EGBV acting as a sort of waste gate for no.2 turbo well I don't see how it could as the valve is situated on the exhaust side of the turbine rather than engine side.

The reason I would like to sort this out is because I have just fitted a full decat pipe and would now like to increase the boost a little. I bought myself a couple of Manual Boost Control valves, one for the Waste Gate and one for the EGBV.

I have fitted one to the waste gate (see pic's in my garage) and set it to 1 bar and am very happy with the results. It's like driving a completely different car now at full throttle.

I have also plumbed in the other MBC into the pressure side of the EGBV but not adjusted it to any set pressure yet.

Before fiddling with the MBC I have over the past couple of days tried to read as many article's as I could about mod's to the EGBV, of which there are many, so as to do things correctly and not destroy anything in the process.

However, the more I read and the more I look at pictures of the sequential set up the more I think intercepting the pressure signal to the EGBV actuator is a waste of time. As far as I can see the function of the EGBV is to pre-spool the no.2 turbo and that is it's only function and as such should be left alone to do it's job.

Am I missing something ?

Right, I think I have seen the light regarding the fitting of an adjustable mbc to the EGBV.

I think that what we are trying to achieve is a "delay" to the pre-spooling of no.2 turbo.

The longer you can delay the EGBV opening the more exhaust gases will have to flow through no.1 turbo as engine rpm increases, spinning the turbine faster thus producing more power before no.2 turbo starts to join in.

I guess that little drop in boost pressure you see at about 3800 rpm is when the EGBV opens and starts to pre-spool no.2 turbo.

By fitting the mbc you will in fact raise the point of this slight boost drop further up the rev range.

I guess the danger here is if the EGCV opens and you have had no pre-spooling you risk damage to the turbo as it will have to spin up to very high revs from a virtual stand still.

Thats it, isn't it ?

Hi Mike M , having re read your reply I am now in agreement with what you say here.

My interpretation is that when the EGBV is open it is robbing the 1st tubby of some exhaust gases from the manifold and exiting them downstream of tubby 1.

However this bit I don't agree with.

"Another thought too is that it acts as a wastegate to tubby 2 since it will be open when 2 is online"

I'm pretty sure that once the EGBV has opened to pre-spool the turbo it's all over for the EGBV and it serves no other purpose due to it's position in the system.

Hi Mike M , having re read your reply I am now in agreement with what you say here.

 

 

 

 

However this bit I don't agree with.

 

"Another thought too is that it acts as a wastegate to tubby 2 since it will be open when 2 is online"

 

I'm pretty sure that once the EGBV has opened to pre-spool the turbo it's all over for the EGBV and it serves no other purpose due to it's position in the system.

Yeah your right mate just checked the drawing and the EGBV is on the turbo outlet making no difference whatsoever.

Cheers

Mike

Have a look at my website, I've got a few words on the subject

Hi JohnA, over the past couple of days I have read your description of the sequential system about 10 times, and each time I have not been able to picture just what we are trying to achieve with the EGBV mod.

I now know why, for some reason every time I read your description I missed one very important sentence........

"2. The fact that the EBV also controls the prespooling of turbo #2.

This is more serious, because in order to get more boost out of the whole system at low revs, you need this valve to open later."

God knows why but that sentence just never seemed to register in my brain. If it had I would not be asking these stupid questions.

Right, at last I can go and twiddle with things safe in the knowledge I won't destroy anything.

Someone has been having a few wine glasses too many eh?