proper rpm for idle and drive

[brentleedragon]

Okay so I have a 79 f150 302 c6 with a 9"rear

I finally installed a tach today, and when in idle (park) I'm about 550/600 rpms and while in gear I'm about 400/500 rpms. My hood says I should be about 600/650 in both...is that right? I have AC...never use (compressor has a leak) I know that makes a difference.


Second as I am driving about 60/65mph I'm running between 2000/2300 rpms. Idk if this is where it should be, being that it is a 3 speed. I am used o my suburban which runs about 1400/1600rpms. Should I be concerned or no?

 

[cipsmithers]

The idle sounds a little lowas for the rpms at speed, it all depends on what gear ratio you got in the rear end

 

[LEB Ben]

I always try to hit somewhere between 8-1000rpm in parked idle and 6-800 in drive idle. As for the rpm, that's completely dependant on gearing and tire size. But I've always read that the rule of thumb is 2600rpm at a cruising speed of 55mph. As a reference, my rigs are all between 1950 and 2400 at 60mph.

 

[Mil1ion]

http://mil1ion.clubfte.com/78CalibrationCoding&Tune-upSpecs.html

 

[brentleedragon]

Sweet thanks Guys

Thanks Dennis for that, so just a little adjustment to get it right.

I have another question, lately I have noticed the truck having some hesitation when going up hills or when accelerating (from like 45 to 60) my foot goes in the pedal and it is basically down and it barely gets up to that speed. I can have my foot in it and it's like it doesnt really respond. Any thoughts?

 

[LEB Ben]

So is it hesitating or being unresponsive. Hesitation could be fuel blockage, accelerator pump issues, carbon build up, blown gasket, etc. Unresponsive, my first guess would be a timing chain issue.

 

[brentleedragon]

Both really. Its hesitating off the line and pretty unrepsonsive when picking up speed. It will get to 60mph but it seems like it is sluggish getting up there. As if there is no power behind it, foot goes almoost all the way down, and no I'm not driving it like a sports car

 

[LEB Ben]

I think I'd start looking for vacuum leaks, check timing, do a compression test and a leak down test. I may be a tad bias though, but that's because those are similar symptoms my 351M had as it was going down the chitter.

 

[Alvin in AZ]

Second as I am driving about 60/65mph I'm running between 2000/2300 rpms.

rpm x tire diameter / 336 x trans gear ratio x axle gear ratio = mph
I multiply the first two and then just keep dividing in the rest of the stuff.

Anyway I don't know anything about tachs. :/
Is it reading right?
If it were mine I'd check it.
I'd first find out how far off the speedometer is when it reads 60mph...
http://www.panix.com/~alvinj/file12/SpeedTable.jpg

Alvin in AZ

 

[brentleedragon]

Well if those speed calculator's are right on the side of the road are right, im about 3mph slower than what my gauge says. so if it says 60, im 57 ish

 

[Alvin in AZ]

Ok, cool.
So what tires you got and what axle ratio?
Since it's a danggedslush box does your particular model lock up?

http://www.panix.com/~alvinj/file12/Ftags.jpg
Top two are off Ford9inch axles both 3.25 ...but one's got Traction Lok.

Alvin in AZ

 

[Mil1ion]

Physically check the distributor vacuum advance. they are prone to seize or tear the diaphram

 

[brentleedragon]

Idk why I didn't check the vacuum advance...it's pretty much shot so I'll replace.