[FAU Engineer]

I just went to the track last weekend and I saw a Chevelle that sparked a question that I have had for a while. It had a roots blown BBC, and when the car idled it would sound like it would in about 1 second intervals idle, rev, idle, rev, idle, rev, and each time it would catch a little rpm the car would lurch, and I have seen other roots blown cars do this also. It's weird though that the real high end roots blown cars like pro modifieds and the street driven kind of "tame", respectively, blown cars don't do this, but a lot of the racecars in between do. I am dying to know why they idle like that

 

[sandmann120]

It's because the idle needed to be turned up. The blower was trying to suck air past the butterflies, and couldn't get enough, so the butterflies will 'give' a little, and the engine will surge. A roots blower can be made to idle smoothly, it's all about the tune. Hell, our AA/FD idles fairly smooth , at least without any surging, but it also 'idles' at about 2700 RPM.

 

[66 283]

A big part of it is idle mixture. A friend's 600 inch BBF with 14-71 and twin dominators did the surge but once he put a wideband and fine tuned the idle mixture (each of the eight screws 1/16th of a turn would make a big difference!) the surge is gone. It idles relatively low despite a big roller cam.

 

[supersport396_2000]

I like the surge myself..kinda like it wants to move.

 

[Hi-po SS 454]

Can't miss that Bitchin Idle being a huffer, even better when you hear the wine on each surge......

 

[blown70]

What actually causes the idle surge is the blower has so much draw that it will actually pull the idle circuts dry so what you are getting is normal/lean/normal/lean or idle/rev/idle/rev.This can be easily cured with a little patience and some very small drill bits to enlarge the idle orfices in the bowl.The orfices are so small that with a blower pulling on them they just can't keep up.When the engine slows down the circuts fill back up then it speeds up and pulls them lean.
As far as race cars?anything with a mechanical fuel injection will surge at idle.ie blown alcohol,street car with enderle injection.

 

[jocww]

he just wanted to sound badass. i want my car to sound like that on purpose, who cares if it doesnt get that off idle trq bwapp papapa bwapppa papap bwaaaaaaaaaaaaaaaaaap bwaaaaaaaaaaaaaaaaaaap bwaaaaaaaaaaaaaaaaap bwaaaaaaaaaaaap thats all your gonna hear from me

 

[FAU Engineer]

That definitely makes sense...
It is wierd to hear it running like that but it does give the car some attitude, like someone has to hold it back or something, everytime it surges it jumps, its pretty cool anyways. Thanks guys

 

[1bad67]

I thought the surge was improper bypass tune on injected cam driven pumps and carbed guys tune em to surge LOL!

 

[lemonice]

speaking of blowers, does anyone know a place in louisiana that can tune a bbc with a 671, 2 holley 750s.?????

 

[68ssvelle]

speaking of blowers, does anyone know a place in louisiana that can tune a bbc with a 671, 2 holley 750s.?????

i thought it was the cam beening to big causen the surge...i may know someone that can help you tune ..pm me where you at in la ..im in baton rouge..theres a place here..o i see less ville i now someone you can call