Twin VNT 4jj1 - surge

Yo

the setup

i have a stock 2014 4jj1 4x4 - egr removed. Hks fcon ID, plus 30s, front mount , stock manifold with steam pipe Y pipe to a second factory rhf4-VIIO. 15:1 afr at WOT( single or twins)

issue I have is now it’s twinned I have surge at WOT most times , low rpm under 2000rpm I get surge even when it shows no boost and stays there as the revs get higher and higher (dedo deedoodeeeedoooo)Sometimes it pulls with no surge at all . It’s set to 25psi the same as stock and is slower to respond than stock obviously.

I can’t work out how it can surge when it’s not even making boost, both turbo outlet pressures are the same . Then surge on power when it’s the same boost as what it was when single turbo was on it and both turbo outlet pressures are the same.

Tried swapping to twin HPD manual controllers , reset to 25psi, tried 32psi , got worse as boost went up.

I can’t work out why an f55 with 50psi will go yet this with twins won’t take a doughier than stock 25-30psi( I want to run 35-40psi)

I once had this same issue on a twin turbo 2JZ with twin little rhf turbos and they did the same thing and I never got to bottom of it.

I could add anti surge to the comp covers but I just get the gut feeling it won’t fix it fully.

any ideas or theory on the cause ?

yes, I will check drive pressure just haven’t had time but it should be half the drive pressure of the stock single for same boost pressure at engine surely . ( single was 64psi drive pressure)

Jason,

For this explanation I would encourage you to look at this GT12 compressor map which is close enough to the rhf4 for this discussion. Picking a typical airflow of 7 lbs per minute at a pressure ratio of 2 (100kpa/15psig) we can see how we land squarely on a nice efficiency island of 76% right in the meat of the turbos operating range. NOW that you've added a second turbo in parallel we're going to keep boost the same and move our lookup straight to the left until we hit 3.5 lbs/minute to represent the shared load of the turbos. Hopefully you notice how we went past the left-hand vertical boundary of the map, this is the surge line - any time we go left of it we can expect surge. The fundamentals of this discussion are why you rarely see parallel turbos used in diesel applications. My suggestion to you would be to fabricate a compound turbocharging system. This will allow you to retain the factory engine response, eliminate surge, and achieve 40psi with ease.

-Nick