Hi everyone. I wanted to start this thread to see if anyone has any info regarding the throttle response of the MCS. As we all know, it's pretty bad. If you give it gas, there is a noticeable delay between your foot going down and acceleration beginning. I just installed a boost gauge and have noticed some interesting things. The gauge moves perfectly with the pedal. There is no delay whatsoever. I had always thought that the problem was in the programing and the time it took for pedal movement to correspond with the throttlebody opening. This must not be the case. The throttlebody must be opening with the pedal in order for the gauge to follow it. There's no other way to reduce vacuum than opening the throttle...is there? If this is the case, the lag must be found elsewhere. I've heard that a free flowing intake helps with throttle response. I never really bought into this but now, I'm having second thoughts. I plan on testing it out myself when my Sportbox gets here. There could also be lag within the intake tract, intercooler, or maybe the designers just weren't able to tune the intake manifold to the right length because of space restraints. I don't know.

One other thing I've heard (and am able to confirm) is the reason for loss of power around 3k rpm at half throttle. If you go to half throttle at low rpm, boost shoots up to 4-5 lbs but as you hit 3k rpm, it tapers off and within a few hundred rpm, it's actually back in a slight vacuum. I don't have a way of measuring throttle position so I have no way of knowing if the ECU is actually reducing throttle or if something else is happening. One possibility is that the quck throttle action snaps the bypass valve closed as if you're floored (5psi as about all you can make at 2k rpm). Then at 3k rpm, the spring force pulls it back open.

Any comments would be appreciated.

 

Very interesting thread.
Just to clarify, the BPV spring is for closing the valve, not opening it.
If you have a older valve with a worn spring, you may see the fluctuation in boost you are describing. Another possibility is a small leak in the diaphragm.

 

I'm also interested in seeing what info the more technical-minded might have to offer. I've noticed exactly the same things you've mentioned. In normal driving, it seems you almost never see boost - maybe a few lbs. Unless I really floor it, I rarely go over about 8 lbs, and I have to be in it to get that. The good news, I guess, is that the car is scooting pretty well with just that 8 lbs, but of course I want more. For a car with a supercharger, it doesn't seem to be boosted very often, and I don't necessarily want to drive around with the pedal mashed (throttle wide open) all the time.

I've got a DT bpv, which should help the situation, but I was having drivability issues with it in, so it got taken out (Chad knows all about it, and he's as confused as I am about why my car freaked out - not trying to bash his part).

 

Here is a thought....air, abeit the greater of the two halfs, does not fire the engine alone....what else must happen other than the throttle body opening?

 

This is true of any supercharged street car - The vast majority of the time, you'll be producing little or no boost.

 

Yeah, and it's probably for the best, BUT one of the great things about a sc over a turbo (traditionally) is the instant boost - no spool-up time. Sticking a bypass valve into the mix kind of takes that away. These cars still jump to boost really quickly when you stab the pedal, but it would be nice if they were already at the edge of boost when you're cruising (instead of being at 15lbs or so vacuum) and then jump instantly if you press down a bit. Yes, gas mileage would suffer, but.....

 

Thanks for the replies guys. Trickle X, you have a good point

 

Also keep in mind, if you have a stock flywheel, that sucker is HEAVY! The inertia from that thing alone will significantly hinder the engines ability to rev quickly under no-load. Couple that with a 7 pound crank pulley, and overenthusiastic emissions-based drive-by-wire mapping, and you have the makings of a small engine with a proverbial elephant on its back.

 

ok one thing i know is that throttle body is before the supercharger. not next to intake manifold. now think this.... if you have throttle open at half and rpm at keep raising, then you will have vacumne rather then boost, because air did not have enough space to go in and sc is still sucking what ever is inside the intake to creat vacume.

 

i thought the pistons created vacuum but whatever . i can say for a fact that when i opened up a hole (4" ) in the back of my intake to the cowl , my throttle responce off idle was much worse. however at higher speeds it was seemingly better . seems that small hole from the factory is there for a reason .

 

I'm not sure I quite follow you. There will be vacuum between the throttlebody and SC but there should be boost after SC in the manifold (as long as the bypass valve is closed).

The way I see it (which may be the same thing you're saying), the lower the rpm, the less throttle angle is needed to reduce vacuum. This is because the engine is sucking less air and a partially opened throttle doesn't really impede anything. As rpm's increase, that same throttle angle will result in higher vacuum because the throttle plate is starting to restrict flow. I'm not sure how this goes with the supercharged engine in the MINI but it probably has something to do with it as the bypass valve is all about manifold vacuum.