Drawbacks to rough idle?

streetstang67 · Feb 5, 2004

The tech guy at comp has suggested a specialty cam the 260AH. He said its very similar to the XE256H, just has tighter lope separation which makes a rough idle, while the XE256H has a smooth idle. I'm trying to decide cams. Is there anything else to the rough idle besides the sound?

SuperDave · Feb 5, 2004

Less vacuum is available at idle.

6Stang7 · Feb 5, 2004

That rough idle or lumpy idle is due to valve overlap. Valve overlap is when both then intake and exhaust valves are open, which allows your engine to breath at the higher RPM range. When the exhaust is forced out by the piston it creates a vacuum, which will help drawn in more air/fuel. The downsides is that not only do you have less vacuum available at idle like SD said, but you also will have horriable gas mileage(at idle the gas is pretty much getting blown right out the exhaust port, starving the engine, hence the rough idle) plus a slower throttle responce off idle.

-Shaun

gp001 · Feb 5, 2004

and your coffee will spill

2nd Mustang · Feb 5, 2004

That lumpy idle is oh so sweet! Throttle response is pretty good too. I vote for lumpy.

6Stang7 · Feb 5, 2004

2nd Mustang said:
That lumpy idle is oh so sweet! Throttle response is pretty good too. I vote for lumpy.

I have yet to hear one with good throttle responce off idle...unless it had some type of forced induction of course. Instaed you get a big hesitation, to the point were the engine sounds like it has died. But you're right, that lumpy idle sounds sooooooooooooooo good.

-Shaun

2nd Mustang · Feb 5, 2004

6Stang7 said:
I have yet to hear one with good throttle responce off idle...unless it had some type of forced induction of course. Instaed you get a big hesitation, to the point were the engine sounds like it has died. But you're right, that lumpy idle sounds sooooooooooooooo good.

-Shaun

I blip the gas once before letting out the clutch from a dead stop. Holding the gas down at 1500 to 2000 rpms just spins the tires for me.

brianj5600 · Feb 5, 2004

I find it hard to believe that 260 deg advertised duration will have much roughness at idle. I don't believe that any of the above things will even be an issue.

67GTA-FB429 · Feb 5, 2004

Big Lumpy cams ROCK!! There are so cool sometimes I feel like me fillings are coming out. I have had no problems with vaccum or throtle response with my cam (check sig).

calpolymustang · Feb 5, 2004

i wouldnt be concerned about the idle on the XE256H, its basically an RV cam. its also about the biggest i would run with stock springs. if this is a daily driver, then you should be ok. if you want a stong cam, get smaller durationw hile maintaining high lift. this way you get good torque and high top end. a 289 doesnt make much torque, so dont plan on wheelstands. and remember, a cam is only one part of the machine. it takes more than just a cam to make horsepower

i have a single pattern crane cam with .488" lift, 110 lobe and .218 duration at .050 and mine has a wide range of power. however, ive done more than just a cam swap. ive ported the heads, engineered an exhaust system, as well as modify the intake and ignition system. my biggest bottle neck right now is the small valve sizes and small runners. i imagine that 330 hp is the most ill get out of my engine until new heads are put on

SuperDave · Feb 6, 2004

If you have a weak blader, a "lumpy" cam would cause DISTRESS when stuck in traffic! Ohhhhhhh!

6Stang7 · Feb 6, 2004

brianj5600 said:
I find it hard to believe that 260 deg advertised duration will have much roughness at idle. I don't believe that any of the above things will even be an issue.

Why do you think that?

-Shaun

TireSmoknWindsr · Feb 6, 2004

SuperDave said:
If you have a weak blader, a "lumpy" cam would cause DISTRESS when stuck in traffic! Ohhhhhhh!

If this is the case though at least vinyl is easy to clean

SuperDave · Feb 6, 2004

TireSmoknWindsr said:
If this is the case though at least vinyl is easy to clean

I just couln't resist interjecting a little "bathroom humor"! This forum often becomes so serious!

GaPonyFarm · Feb 6, 2004

I'm not sure that cam will cause a super rough idle, but it may well reduce your vacuum. If you have power brakes that could be a major issue.

D.Hearne · Feb 6, 2004

6Stang7 said:
That rough idle or lumpy idle is due to valve overlap. Valve overlap is when both then intake and exhaust valves are open, which allows your engine to breath at the higher RPM range. When the exhaust is forced out by the piston it creates a vacuum, which will help drawn in more air/fuel. The downsides is that not only do you have less vacuum available at idle like SD said, but you also will have horriable gas mileage(at idle the gas is pretty much getting blown right out the exhaust port, starving the engine, hence the rough idle) plus a slower throttle responce off idle.

-Shaun

Speak for yourself, my lumpy idle motors all have excellent off idle response. It's all in the carb tuning.

68 daily driver · Feb 7, 2004

slapstick humor

SuperDave said:
I just couln't resist interjecting a little "bathroom humor"! This forum often becomes so serious!

SD;
Thanks for the good old slapstick humor, you have earned the right to do that in here. I just mention in another post about missing hungrymonkey and his slapstick humor, and now you take up the slack. Keep it up, a good laught does good for all of us. you have helped alot of us on our serious questions.

68 dailydriver

gp001 · Feb 7, 2004

68 daily driver said:
SD;
Thanks for the good old slapstick humor, you have earned the right to do that in here. I just mention in another post about missing hungrymonkey and his slapstick humor, and now you take up the slack. Keep it up, a good laught does good for all of us. you have helped alot of us on our serious questions.

68 dailydriver

What no one thought my spilled coffee crack was funny

D.Hearne · Feb 7, 2004

If it'll make you feel better, I did.

zookeeper · Feb 7, 2004

There's not much chance of any cam with that little advertised duration causing you any greif at all. I'd be shocked if you could even tell it had much lope to it at idle. You'd need to get around 230 degrees duration @ .050 lift before you'd need to be even a little concerned about vacuum accesories, etc. My '65 had a hydraulic flat-tappet cam with 236 degrees@.050 lift and still did just fine with power brakes. My opinion about Comp is that they make THE best street cams there are, and you'll be just fine with that one. OK, now back to the potty humor...

Drawbacks to rough idle?

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