I have a theory that EVO's develop funny vacuum issues when horsepower exceeds 450awhp. Specifically, maybe there is some technical issues with the check valve or booster. Nothing obvious, but minor changes only felt at the limit.

Anyone have the expertise to explain to me how vacuum works on our EVO's? I want to track down this process. Also, is there ever a need to upgrade our check valves, brake booster, or master cylinder?

 

It's a known issue, a couple of seconds are required for the boost to be evacuated and negative pressure resumed. I would start by putting a boost sensor behind the check valve that can be logged along side your normal sensor to prove the behavior and determine the cause.

 

Turbo brings the motor to positive pressure rather than vacuum. Large turbo under full throttle more so. You can see that positive/negative pressure on your boost/vacuum gauge. Hot cams will also decrease vacuum on an engine.

If the motor does not produce sufficient vacuum, some accessories that rely on vacuum like brake booster may be affected. Diesel trucks often run vacuum reservoirs or even vacuum pumps as they do not produce sufficient vacuum - no throttle butterfly.

http://en.wikipedia.org/wiki/Manifold_vacuum
You get the same effect by running a gasoline engine at full throttle, more so with a large unrestricted throttle body, a large unrestrictive turbo and cams with lots of overlap.

There is some suggestion that lack of sufficient vacuum may be involved in erratic ABS operation when coming off full throttle under certain conditions.

 

Thanks guys. Very helpful. The symptoms are exactly what I experience when I run my high boost tune. Combined with all the internal upgrades etc, the effects are exaggerated.

Solutions?

Anyone added vacuum reservoir or electric pump to an Evo?
How hard would this be of diesels are already running similar set up?

 

This may not be the total solution even if vacuum is involved in the ice mode phenomenon. In the 370Z community results seem equivocal with some reporting success with a vacuum reservoir others not. In the Porsche community the focus for a solution seems to be on the Teves motorsport ABS for $8500 plus .... some others suggest less harsh brake application and threshold braking, less aggressive pads etc.

 

I have put bigger GT3 boosters in a few Caymans with good results.