Bucking when engine braking

[Husky65]

Hey fellas,

Quick one. I have noticed that if I'm cruising around the 1800-2000(or lower)rpm mark and let of the gas, instead of a nice engine brake I get a bit of bucking. I just down change a gear which helps by raising the rpms over the 'bucking zone', however, I was wondering if this may have anything to do with the cam and tuning. It's is a bit annoying when getting of the gas at 80kph in 5th and it bucks a bit.

I have quickly read that different more aggressive cams may cause this, and it can be tuned out by taking out some timing on light load driving up to 2000rpm.
The cam is a comp cams xe264hr 264/270 .544/.544 with around 9.5 comp/.

Anyone have any info/experience. I am getting the car tuned tomorrow so might bring it up with him.

[Shaunp]

Maybe you're getting out of the rpm band, of the cam it maybe wanting a little air, is there way you can keep the IAC open a bit so it will free run /float a bit perhaps,

[Husky65]

Yeah not sure about the IAC, I'll ask the tuner. Need to be careful with the ford IAC's as they are really touchy feely.

[lukep6470]

Hi,

I'm getting a similar thing now with my 230/240 cam at around 1100RPM which is a real PITA with an auto (AOD).

I never had it with the same setup on my mild 289. I have been improving it a bit with tuning so that might help for you.

[lukep6470]

Whoops I just looked up your cam and my 230/240 is at .050. Yours is only 212/218 at .050 so is a lot milder than mine.

Maybe your motor is running way rich at the revs you are getting bucking?

[Husky65]

Yeah mate it could be. Couldn't get it tuned today so it will have to wait.

[lukep6470]

Interesting thread here:

http://eectuning.org/forums/viewtopic.php?t=5434

Basically retard ignition advance at low load to decrease bucking.

[hybrid]

Do you guys know what advance you're currently running there?

[Shaunp]

Im not sure about that, if you get off the throttle you should have max vac,I would have thought. It could be a reversion thing , but retarding it will lower the vac

[hybrid]

Yeah I think it sounds a bit strange as well. Sounds more like the idle valve is open more than it should be.

[lukep6470]

The theory is that with higher vac you are getting exhaust into the intake due to overlap. Retarding the timing at the points it is bucking decreases vac and helps with misfiring.

I will play with this on the weekend. I will datalog where it is bucking and pull some advance out in those areas. It seems to make sense for me as the bucking goes away immediately I give it a little throttle. With ITB's it doesn't take much throttle to radically decrease vacuum.

[hybrid]

For comparison, I've got 23-24 degrees at light load at 1500rpm, and 17.5 at 1200.

[Husky65]

At idle I'm at 16dg, and and 32 at full advance. Interesting you mention the IAC, I thought that. However with it unplugged it still does it. But as mentioned above, if I lightly press the throttle no bucking,

[Shaunp]

Unplugging it wont prove anything as it will just be where it was prior to unplugging it. Most modern cars allow the idle to float a bit for smoother gear changes and less emissions

[hybrid]

It should work on the 2-wire ford valves Shaun... they aren't a stepper motor.
They work against a spring. The higher the pulsewidth, the more they open.