PDA

View Full Version : Does anyone truly know?



Planetwarmer
01-14-2009, 11:51 AM
What causes a high-horsepower blown motor to sure the way they do at idle? I hear a lot of people give different reasons, so I want to hear what it causes it from someone who really knows what causes it. I figured this would be a good place to ask.

BlownCrewCab
01-14-2009, 01:44 PM
Since big displacement blowers allways make more air than your motor needs at any givin moment. even at idle it's making positive pressure before the carb (or what ever your using) and the motor uses all this air until it runs out, then it slows down and the cycle repeats it's self again. It's this "make a bunch of air,use it all up" that make it seem like it's surging.

Boatless
01-14-2009, 02:07 PM
I believe, it's due to a lean to rich condition at idle.

The blower as BCC noted produces more air at idle than is required making a lean condition and the motor accelerating up in rpm.

Once the vacuum pulls fuel through the carburetors the mixture goes back to rich and the rpm drops to the point where the air mixture goes back to lean and thus the cycle repeats itself.

On injected motors this is more controllable as the computer can compensate for the lean and the fat spots. But they can still surge also.

Now the real question. Why do they whine when they surge?

Kevlar belts have produce a whiney sound that is far more pronounced than a standard fiberglass reinforced belt.

BigGrizzly
01-14-2009, 05:05 PM
The term is blower surge, it is caused by a rich condition. This is caused by improper setup and can usually be cured. Several down sides to blower surge. !1) if gone unchecked it causes oil dilution-not good. Next there is usually a sooty transom. We do a lot of blower motors and Garry and Johnny won't let a motor leave if it is that way. My little procharged engine doesn't do it. Some unknowing people think the engine is cammy.

Mr X
01-14-2009, 07:15 PM
Most common on "roots" type blowers with carburators only. Not EFI.
I have had them do it tuned right and wrong. It has to do with the larger
amount of vacuum created by the blower at idle.....stock carbs get confused and think the engine is reving and dump more fuel in, then it gets too rich and tries to stall, then the cycle starts all over again.
An easy fix is to modify the carb with a small tube, drilled right into the powervalve section of the carb, and run a small 3/8 hose from that tube to a 3/8 fitting installed in the intake manifold UNDER the blower. Then it will not surge any more.

BigGrizzly
01-15-2009, 10:15 AM
Ted, good solution, and works well. There are other ways too. I have seen the fixes. I still call this tuning. I know you know what I mean because you too have been there or you would not have known of the balance tube fix.