Cold idle speed

[Dave Kitch (33108)]

The Operations and Performance Verification Manual lists cold idle for a 327/300 hp engine as 1600-1800 rpm. Is this a hard and fast number for the operations part of Flight Judging? I noticed that my choke closes when the room temperature is less than 50F but when temp is 60 or higher will the choke thermostatic coil completely close the choke? If not, than I would expect the idle speed may not be as high as 1600 rpm?

My interpretation of the 1600-1800 rpm range is it is just a guide and during operations the judge is more interested in the idle being higher than normal and when the engine warms up the choke opens. The judge is not to look for an absolute rpm at cold conditions but that the engine rpm is higher than normal (depending on room temperature)and as the engine warms the choke opens. Also, how hard and fast is the 400-600 rpm range at hot conditions for judging?

Regards,
Dave K.

[Duke Williams (22045)]

Re: Cold idle speed

Year??? AFB? Holley?

What does the applicable service manual say about setting the cold fast idle and hot idle?

Duke

[Terry Robertson (43499)]

Re: Cold idle speed

Hi Dave I just P.V.'d my 300 HP '62 at Las Vegas. The judges checked my idle speed pretty closely for the 1600-1800 and 400-600 hot idle. I think it has to be there. When my engine is cold (no matter the ambient temp) the choke closes completely. Yes the choke is a pain to get everything set but when it's right it works. Have fun

[Dave Kitch (33108)]

Re: Cold idle speed

1966 327/300 hp, Holley list 3367.

Dave K.

[Dave Kitch (33108)]

Re: Cold idle speed

Terry,

The PV and Ops Manual states that the cold idle speed could be less than the 1600-1800 rpm depending on the temp and altitude.

Dave K.

[Clem Zahrobsky (45134)]

the specs in the GM manuals for fast idle

is on a completely warmed up engine. you warm up the engine,set the fast idle cam in the specified step and then adjust the fast idle speed. you never do it on a cold engine.

[Duke Williams (22045)]

Re: the specs in the GM manuals for fast idle

As an example the '63 shop manual says to set the cold idle speed at 1750 with the engine fully warmed up and the fast idle screw on the fast idle cam index mark, which is where it should be with the choke valve fully closed. If the cam index mark doesn't link up with the screw, bend the link as required so it does.

This will result in a cold start idle speed of about 1500 immediately after startup.

I don't know the specifics for setting the cold idle speed on later 300 HP engines with Holleys, but it's in the applicable CSM. There are a lot of little "bend link" type adjustments on carbs, and you should be sure that everything is set corrrectly and the choke mechanism is clean and doesn't bind.

Duke

[Chuck Sangerhausen (20817)]

Re: the specs in the GM manuals for fast idle

I have always wondered about the very fast idle on startup and what auto engineer thinking was on this subject.

Up north, with temps down to low teens and below, the fast idle at 1500-1600 used to concern me a little. It guarantees the engine is going to start and run, but man, it seems like it would be really hard on the bearings, pistons/bores, etc. with the oil starting up in an almost semi-solid state. I suppose they know that people are going to want to go regardless of the temp, and would rather take the chance with a lube problem than having them crank it until the battery is dead.

[Clem Zahrobsky (45134)]

that is why they sell multi vis oil

if i lived where it was very cold and my car sat outside i would have a block heater and use 0W-30 oil. GM had trouble with the ZR-1 engines with the oil not reaching the cam bearings and scuffing the bearing journals in the heads.

[Duke Williams (22045)]

Re: the specs in the GM manuals for fast idle

Modern cars usually have lower cold idle speed than vintage cars. In fact, most modern cars control cold idle speed based on actual coolant temperature, and idle speed usually goes up a little with decreasing temperature, but is still usually below 1500. With the even fuel distribution of FI the engine doesn't have to run as fast or as rich after a cold start as with a carburetor.

On a vintage carbureted engine that has a fixed throttle postion for cold starts from room temp down to sub-zero, the initial fast idle in extremely cold weather will usually be lower due to increased internal friction including more power required to pump cold, thick oil.

Back then the carb and everything else had to be tuned to cold start reliably at any ambient temperature, and no on the fly adjustment was available as with today's electronic engine management systems, so everything was a "best compromise" for any cold start condition from tropical to arctic, and this required a lot of testing and tweaking.

Given how crude vintage engine management was back in the carburetor days compared to today, it's a wonder they started as reliably as they did, but a decent battery and proper SAE viscosity was usually all it took, and using engine heaters in sub-zero weather was a real boon.

Duke

[Michael Ward (29001)]

Here in the GWN

we typically use 5W-30 in the winter. I'm not so sure that high idle/cold starts have that much effect on engine longevity, ours seem to last just as long as yours.

I haven't used a block heater since the late 70s, always forgot to unplug them!