My race car uses a BMW S52 (3.2L out of an M3) that I just had to replace a head gasket on. Whilst it is partially disassembled - I decided to reduce some clutter. I got all the heater hoses out, re-routed the wires and trimmed the connectors for things I don't need... to that end - I'd like to ditch the big Idle Air Motor that sits under the intake along with it's irritating hosery.
Since this is a race car - I have no issue holding it just off idle to warm it up or using one of the small tits on the manifold to route a little air around the butterfly at idle. I have done it on older E30 M20 motors with no problems that I was aware of. My only real concern is that a little bit of extra air leans me out under high load conditions (that is how the HG went pop in the 1st place... fuel pump failed mid-session at WOT).
The way I figure it though - the only time a consequential amount of air will use the bypass is when the butterfly is closed or very close to closed. At anything past a quarter throttle there won't be any vacuum to draw it. So much in proportion goes thru the TB that it will make no difference to the ability of the O2 to compensate with fuel.
Anyone done this and had issues?
On Dr.Linda's megasquirted 883, I used the intake manifold and throttle body off a Buell. It has no IAC, just a cable attached to the idle screw on the throttle butterfly. Want a faster idle while warming up? Twist the cable. When warm, twist it back.
I did this on my Jetta. I replaced the IAC that was NFG with a piece of hose and a mini ball valve. I previously replaced the dash in my car with an un-cracked one from a diesel car, and it has the pull-out cold start thingy on the dash. I ran a cable for a manual choke from the ball valve to the pull out handle. Pull out the handle in the morning for fast idle, then push in when warm. My car doesn't have A/C, and the IAC wasn't really needed. Works awesome.
Whatever you do, make sure you have decent idle control under all conditions. Its actually pretty important on a race car - when you have the inevitable spin & push both feet in you'll want to have the engine running when you get the point to go back on track.
Kendall
Kendall_Jones wrote:
Whatever you do, make sure you have decent idle control under all conditions. Its actually pretty important on a race car - when you have the inevitable spin & push both feet in you'll want to have the engine running when you get the point to go back on track.
Kendall
Oooooh. Good thinking... I did not really consider the "both feet in" situation. I was picturing warming up, strapping in, etc... but things do occasionally pop up... and I have already found out that the best way to get stuck in the gravel trap is to come to a complete stop so you can re-fire the engine.
doc_speeder wrote:
I did this on my Jetta. I replaced the IAC that was NFG with a piece of hose and a mini ball valve. I previously replaced the dash in my car with an un-cracked one from a diesel car, and it has the pull-out cold start thingy on the dash. I ran a cable for a manual choke from the ball valve to the pull out handle. Pull out the handle in the morning for fast idle, then push in when warm. My car doesn't have A/C, and the IAC wasn't really needed. Works awesome.
A spring-loaded ball valve so it needs a little vacuum to draw air? Any idea what sort of spring that takes? Maybe use a PCV valve backwards?
Is it using a MAF to meter air, or some type of speed density engine management? With a MAF, the lean-out condition can be handled by plumbing your controlled vacuum leak to a point between the MAF and throttle body. (A speed density system wouldn't care.)
In reply to Giant Purple Snorklewacker:
I just used a plain old 90* sweep plumbing ball valve. On the VW's they use a flapper type air meter (on my Digifant car anyway) and the IAC is just a throttle bypass. The air that goes through it is already metered. There is a separate idle control screw on the throttle body (not the throttle stop, an actual adjustment screw) that I set so that when the car is warm, it idles at ~900 rpm with the ball valve completely closed. When it's cold, I open the ball valve a bit so it will idle by itself.
MadScientistMatt wrote:
Is it using a MAF to meter air, or some type of speed density engine management? With a MAF, the lean-out condition can be handled by plumbing your controlled vacuum leak to a point between the MAF and throttle body. (A speed density system wouldn't care.)
Yes, MAF. That makes sense - a smaller hose from the original location to the old fuel canister vac line opening ought to do the trick. I'll give it a try tonight - I just got the intake back on and the harness connected. All I need is a radiator and a fuel pump installed to try it out.
I was going to suggest drilling a very precisely-sized hole in the throttle plate, but all these other suggestions are much more scientific, and less redneck. And adjustable.
For posterity:
I took the car to NJMP for some track time after removing the IAC motor and putting a controlled leak between the MAF and the intake. I tested this at low RPMs around my neighborhood and it was swell... cant really rev an open exhaust around here for any real testing...
Guess what? If the leak isn't big enough... the throttle body will slam open when you get past half-way and it will stick there until you hit the kill switch. If the leak is big enough... it runs like a bag of prune E36 M3 until the revs are way up. So... I effected operation "Idle Air Motor Undelete" and everything was spiffy.
I am not finished - oh no... now its personal. But I'll test my theories before ending up parked in turn 3 trying to put the kill switch key back in and refire.
I'm baffled...
First, by the concept that an undersized bypass from just after the MAF to the intake manifold could suck the throttle open (am I totally misunderstanding what's going on here?)
Second, by the notion that it would run crappy with a larger bypass...
Am I understanding correctly that the original system was
MAF -> Throttle body -> manifold
and that your bypass went from between MAF & Throttle body to the manifold?
Where is the bypass entering the manifold? Is it right at the throttle plate?
I figure I must be missing something in a big way; if so, I apologize.
