sm botes

New to this forum , hoping to get a explanation on turbo surge

built a blow through turbo system using twin dellorto sidedraughts , utilising a garret t34 hybrid t3 .48 hot . t4 0.60 cold dash 23 wheel (had a straight t3 .48 hot and t3 .42 cold , but was unusable in the first three gears)
flow and ported head , cvh22 cam , 6 to 1 cr(Mitsubishi 4G62B 1800cc pistons) , log style cast manifold , 63mm xaust , single backbox , turbonetics evolution wastegate and screamer pipe ,large FMIC , - 2 bar boost no problem . Boost pulled clean in all gears

broke this motor running with the fuel pump off at the local dragstrip - time for build #2

rebuilt it with an EFI head from and xr3i (efi system from the newer CVH as fitted to the Fieta RS Turbos , not the mech inj system of the Escort RS T's) , 500cc injectors , Gotech MFI ECU , cr 7 to 1 , ARP rod bolts.

picked up surge with this motor - under load in 4th gear low RPM , boost fluctuates , makes a noise like the turbo cutting back on itself when you close the throttle without a DV , not pleasant

somebody explain - i have all the thoery in the world including both turbo bibles by Bell and Bell , this confuses the crap out of me

busa4

surge is caused by the backward motion of the air in the charge piping when the throttle is closed. the compressor is trying to push air forward at the same time air is being forced backward. this is what causes that distinct noise we all know.

the higher the gear, the more engine load. is your bov opening? is your wastegate working correctly?

sm botes

nothing to do with the BOV , wg is a new turbonetics evolution

efi motor built with an RS turbo cam , thought it would give me better power delivery - not so sure
measured the upper half of the inlet plenum , around 1 liter on the original Fiesta RS Turbo
made up a new plenum in ms , about 3.5 liters now , still with rs turbo cam , bottom plate of airbox radiused and match ported into lower half of the alum runners - getting better
better in fourth gear now , still have bad surge in fifth

Were my carbs able to flow more air than my efi setup?

Carbs felt stronger than my managed system

as i understand it , the motor cannot cope with the high pressure ratio and the low flow rates of WOT in high gear or load.

so ive improved the engines breathing ability with the larger plenum , would putting the cvh 22 cam (11mm lift with 276' duration) further improve the engines ability to breathe and move the air flow away from the surge line on the left ?

serious techies only , the above rubbish answer is the same as ive gotten on other forums

just to mention , the motor is a 1650cc 8 valve , 158kw ATW 282NM

time seems to be a factor in surge?

Turbo has enough time under load in WOT to build boost past the motors ability to take it in and goes to the left on the curve into surge.

air cant get into the motor quick enough (high load/gear WOT) , and since the turbo has done work , its expecting hot exhaust gas to keep it going.
The produced air is not used , expected exhaust gas does not materialise , assembly stalls - chatter
turbo slows down , pr ratio and air flow drops within the compressors ability , boost stabilises.
boost builds up quickly again , high pr and low air straight to the left and into surge - chatter
this cycle continues until the motors rpm rises to the point where it can handle more air , away from the surge line.

With careful throttle control and avoiding high load WOT , this can be avoided

in the lower gears the rpm rises quickly in relation to the boost and you avoid the surge line

So is surge then the oppsoite of turbo lag?

Lag = turbo catching up with motors rpm
therfore surge = turbo working to fast for the motor

And remember , lag just reminds us what happens on a NA motor between 0 and full rpm......

busa4

[QUOTE=sm botes;1302691]serious techies only , the above rubbish answer is the same as ive gotten on other forumsQUOTE]



great! another know it all.

you have two possibilities to solve your surge problem. since your boost curve is rising too quickly which would indicate surge you will need to prevent the quick rise of boost. the only options you have to do this is to get a larger exhaust a/r housing or lower your boost.

busa4

surge is the effect of aerodynamics of the compressor. its also called stall. if the compressor is trying to move air beyond its aerodynamic limit then surge will happen. the noise heard is the compressor wheel going in and out of stall rapidly. surge almost always happens at low rpm and goes away with wot.