"Tuning out" cam surge?
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Joined: May 2000
Posts: 799
From: Redmond, WA
"Tuning out" cam surge?
I saw the following post on the Impala SS forum. Is it really possible to (somewhat) tame a big cam by cutting timing at low rpm or throttle percentages, as explained below?
In general cam surge, aka cam chug, aka "fish-biting", is caused by too much timing at too low of an rpm. Technically ANY cam will do that if the timing is too high, but it's common with "big" cams. My 396 with the Comp Cam 306 (230/244 duration) did it when under 2500 rpm. Every time the torque convertor locked I went chug chug chug down the road...
To fix it I went into the timing tables and reduced the timing under 2500 rpm and at low (<25%) throttle angles. It really wasn't that hard to tune out -- I got it on the first try for the most part. Leave the timing alone at high throttle percentages as you'll want that power when you ask for it.
Keeping the torque convertor unlocked also helps mask it as the link between the rear wheels and engine is damped by the fluid in the TC. It's also mitigated by a higher gear rear-end as that gears down the motor.
Now that I've got the six speed I'm really glad I tuned it out. A manual trans is like a locked TC -- the engine is always directly linked to the tires. I can, and do, use 6th gear as low as 1200-1400 rpm with no "fish-biting".
To fix it I went into the timing tables and reduced the timing under 2500 rpm and at low (<25%) throttle angles. It really wasn't that hard to tune out -- I got it on the first try for the most part. Leave the timing alone at high throttle percentages as you'll want that power when you ask for it.
Keeping the torque convertor unlocked also helps mask it as the link between the rear wheels and engine is damped by the fluid in the TC. It's also mitigated by a higher gear rear-end as that gears down the motor.
Now that I've got the six speed I'm really glad I tuned it out. A manual trans is like a locked TC -- the engine is always directly linked to the tires. I can, and do, use 6th gear as low as 1200-1400 rpm with no "fish-biting".
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Joined: Nov 2001
Posts: 3,515
From: Kansas
i only have the tiny xe224/230 110 lsa but that is how i did it. people told me to increase timing down low which i did and every time i increased it a degree or 2 in the low rpm/low map ranges i noticed no change in cam surge just idling along in 1st gear. so i then went back to the original file and took out 2 degrees for the hell of it... worked like a charm! though i still cannot run 6th gear at 1400 rpm but i dont think id lug my motor that bad even if i could.
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Joined: May 2000
Posts: 799
From: Redmond, WA
I have a 260/267 on 112 lsa. The fact that it's a solid roller should help somewhat, and the high compression should help also, but I'm told that the lightweight rotating assembly could make it worse. I'm just wondering how livable I can make it. If I can make it behave like a CC306 (which Mindgame described his solid roller as behaving like) that'd be fine. I have a 6-speed, so it's not going to help soak up any of the surge.
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