Idle speed rpm

[histoy]

We're finishing a '68 Charger 440 engine with automatic transmission and A/C.     It has an an Xtreme Energy cam (Duration 268/280 - Lift .477/.480).   After we broke in the cam, the engine builder said the car idles at 1200 rpm in Park and drops to 650 rpm when put in drive without the A/C turned on.   Does it seem normal for the rpm to be that high in Park in order to get to that idle rpm?    If we were running a stock cam, what would the rpm in Park typically be running?  Thanks for any info you can provide....Brian

[firefighter3931]

What is the stall speed of your torque converter. It sounds like the converter is pulling the engine down. That does seem like a large drop from idle into gear....normally 200-300 rpm is what you should see.

Ron

[histoy]

The engine builder told me that he installed a converter with a 1600 stall.   He said that he thought the Mopar stock unit had a stall around 1200, but I don't really know....Brian

[firefighter3931]

Brian, it sounds like the converter is too tight for the cam profile that's installed in the engine. You can go with a looser converter or swap in a smaller cam. This often happens when parts are mismatched. Personally, i wouldn't idle the engine that low....it should be around 800 rpm in drive. The cam isn't pressure lubed like the bearings...it's splash lubed off the crank and rods. You want to get as much oil on it as possible, especially at idle. If you can live with the big rpm drop from neutral to drive then go ahead and run it. Ideally though, that cam profile needs a 2500 stall for best performance, inmo.

Ron

[myk]

Are we talking 800 rpm just for this application, or everyone in general?  I've always tried to keep the idle speed as low as possible-I hope I haven't been mucking things up any...

[firefighter3931]

Are we talking 800 rpm just for this application, or everyone in general?  I've always tried to keep the idle speed as low as possible-I hope I haven't been mucking things up any...

Myk, i allways idle any flat tappet cam at 800rpm....or more depending on the duration (some won't idle below 1000). The XE grind that History is running in his motor has agressive lobes so the more oil on them  cam, the better it will be protected. Keeping the idle a little higher also increases oil pressure which helps with the main/rod and cam bearing lubrication.

Ron

[John_Kunkel]

Converters often get the blame for poor idle but that cam isn't all that much different than a factory HP cam that will idle in gear all day with the tightest of converters.

[firefighter3931]

Converters often get the blame for poor idle but that cam isn't all that much different than a factory HP cam that will idle in gear all day with the tightest of converters.

That is a good point. The xe grind should behave similar to the factory Hp cam because the seat timing is close....although the XE series grinds have more "high lift" area. Something to look at is the carb and ignition....the engine probably needs some tuning. If the idle speed in park/neutral is 1200 then the engine is idleing on the Carb's main circuit and not the idle circuit like it should be. One quick way to tell if this is the case would be to adjust the idle mixture screws in and out. If there is no change in idle quality the motor is using the carb's main circuit.

What kind of carb is on there now ? How much base timing is there at idle ?


Ron

[Plumcrazy]

The intake manifold type also makes a big difference.  What are we using here?

[histoy]

The manifold is a stock '68 Mopar 440 cast iron.   It currently has a Carter 4401 carb which came from a '68 383 engine with an AT trans.   I don't know how the engine builder has set the initial timing, but I hope to have it in my garage within the next week.  Then I'll start doing some tuning.  I've had to drill holes in the throttle plates on some carbs with higher performance cams in order to get the throttle plates closed enough to get them to idle properly.   I'll keep you posted on my future success...Brian   

[Plumcrazy]

I've done the hole in the throttle blades trick too.    I would try working with the timing as soon as you get the chance.

[Chryco Psycho]

advance the timing at idle & shorten the curve with the avcuum advance & see if it idles better in gear

[firefighter3931]

advance the timing at idle & shorten the curve with the avcuum advance & see if it idles better in gear

Yep, that's where you need to start. You should have a min 16-18* at idle and 36-38* all in by 3000.


Ron

[AmadeusCharger500]

Sorry to jump in here with a question posed to one of the responses, but.
How do you get the car to run on the idle circuit as opposed to the main circuit of the carb?
If indeed it is running on the main circuit at idle.

[Plumcrazy]

Sorry to jump in here with a question posed to one of the responses, but.
How do you get the car to run on the idle circuit as opposed to the main circuit of the carb?
If indeed it is running on the main circuit at idle.

The throttle blades being open too far is what usually causes that.   You'll probably see fuel dripping out of the venturi boosters.
The first thing to do is be sure the problem isn't in the carb, plugged idle passage or bad gasket/o-rings.

Then find out how much initial timing the engine wants by advancing it while watching the RPM's.  You'll get to a point that more advance no longer speeds up the engine.
As a last resort I've drilled small holes in the throttle blades so the engine can get enough air with the throttle blades closed to where they should be.

[histoy]

Some higher performance cams with more valve overlap force you to open the butterfly valves farther in order to get the car to idle.  This takes the carb from the idle circuit to the part throttle position.  In order to get the engine to idle only on the idle circuit, you may have to drill a small hole (0.030 - 0.060") in each one of the primary throttle plates.  This allows more air to enter the carb, and you can adjust the idle speed screw down to idle the engine on just the idle circuit.  I've had to do this with Edelbrock & Carter carbs...Brian

[Chryco Psycho]

Demons have a compensation screw to allow a vacuum leak past the throttle blades so it will stay on the idle circuit , it i sadjusted under the air cleaner stud in the middle of the body , counterclockwise opens the vacuum leak allowing the throttle blades to be closed back onto the idel transfer slots