boost spike

[Gannon]

Boost Spike vs. Boost Creep

Boost Spike:
Boost spike is when the boost level initially "spikes" up to higher than the preset boost setting, and then quickly settles back down to where it should be. As most people with turbos know, once the boost pressure in the intake starts to rise, the rate at which it rises quickly increases until the pressure is increasing at a phenomenal rate. This means that, if your boost is set at 12 psi, when it reaches that point it will be increasing so quickly that it will go higher than 12 psi and then drop back down once the boost control system can correct it, which is within a half second or so.

Boost Creep:
Boost pressure is controlled by the wastegate, which allows exhaust gasses to bypass the turbine wheel. In effect, it creates an alternate route for the hot exhaust coming out of the motor to take, which means that any gas passing through it will not spin the turbine wheel.

Now, if this wastegate cannot flow enough to bypass the required amount of exhaust, then that means that too much of the gasses are going to go through the turbine wheel, meaning that it will have too much energy imparted on it (it will be spinning too fast). As the excess exhaust gas amount gets greater and greater, the turbine wheel spins faster and faster, and the boost level rises.

Brumbi,... what you have is boost creep.


In the image below, the round thing on a hinge in the right hand side of the turbine outlet is the wastegate. Notice how much bigger the TF035 wastegate (left turbo) compared to the little wastegate of the VF4 (turbo on right)

[BRUMBI]

thanks yeah thats what i mean but because i have a manual boost controller ive herd they spike a fair bit, so minimal spike would be good

also dose 10psi sound like a good pressure to run stock with free flowing exhaust and waic, engine has done approx 50 000kms

[discopotato03]

Long post alert , don't say you weren't warned ....

I'll get to the TF035 in a sec because its not really a good example in this instance .

There is a bit of history with automotive turbocharging systems and it was all gaining momentum from the early 80's on . It was a fairly steep learning curve for the manufacturers who's initial involvement was motor-sport , the pinnacle was the close of the Formula One turbo era in 1987 . The Rally world was following close behind and AWD and turbocharging was going gangbusters right through the middle 80's and is still up there today .

Subaru's toe in the water was the L Series RX Turbo but don't forget they are pretty crude by today's standards so don't expect anything too enthralling from a factory std example .

The EA82T . As we know the headers on these cars are absolutely RS - rat do do . The turbocharger itself , typical of the era , uses a turbine larger than they would have in this day and age for the power output and get it all happening with a tiny turbine housing . The compressor is smallish and being 5/10 bladed has low power requirements which helps give a VF2/4a lowish boost threshold .
The "T" at the foot of the up pipe is woefully "designed" from a performance perspective and small internally . Just about every bend in the header and down or cat pipe has a smaller diameter tube inside it so don't be fooled by what these horrors look like on the outside .
The first "joint" at the south end of the cat pipe necks down to form a slight choke point and the bit behind it isn't much better .
The last muffler in the system also isn't anything to write home about .

So rev head racing rips everything off behind the turbo's outlet and fits an exhaust with 1.5 to 2 times the flow capacity of the std straw (this is just an example , no slur aimed at anyone here) .
So we now have a massive reduction in pressure behind the turbine meaning a greater pressure drop is now possible across the turbine itself .
Mr Subaru's straw exhaust is no longer there to artificially cap the exhausts mass flow so the turbine can spin faster and make usable airflow further up the engines rev range . The trouble is that to do so means the turbo's rotating group (compressor and turbine) increase in speed beyond that which the engine and turbos designers recommended .
The engine management computer wouldn't know this (on a 3 plug , I believe 4 plugs fuel cut beyond a certain AFM Voltage level) so it can't be expected to regulate AFR's properly .
Most flap AFM systems of that era went to a flat line 10 to 1 AFR with the vane opened fully so all the computer could see to run the show was the rpm signal so 1 dimensional and very crude . If the turbo could force enough air down the engines throat the 10 to 1 climbs to the danger point because even at 100% duty cycle they can't supply enough fuel for suitable AFR's . Not having any form of intercooler means inlet charge temps are getting toasty and usually the pistons (not oil cooled in EA82T) overheat and collapse their ring lands - that's if they haven't detonated to death .

Boost Spike . This happens because the waste gates swing valve is operated by a spring loaded diaphragm actuator which has atmospheric pressure and the spring load on one side and manifold pressure on the other . When the turbocharger positively charges the inlet manifold pressure rises on one side of the diaphragm and starts to overcome it at about 3/4 of the springs loading . As soon as it does the rate of boost rise slows as the swing valve opens and bypasses an increasing portion of the engines exhaust gas flow around the turbines blades .
Turbocharger boost control is literally turbine speed control which works because the compressor is at the other end of the turbine shaft and revolves at the same speed .

Boost creep . This occurs often because once the waste gates swing valve is fully open (this on std turbo's not backyard hybrids) no more exhaust gas can bypass the turbine so the system runs out of regulation . This usually won't happen if the whole system is std because manufacturers can't afford the engine to go bang and have to cough up for expensive warranty repairs - and dents to their reputations . Publicised failures cost sales .

Out of time and fingers for today .

More tomorrow , cheers A .