'67 390/400HP IDLE SPEED RANGE

[Rob A.]

During the operational check phase of judging, what is the acceptable rpm range, choke off, warmed up, for 390/400 hp engines?

[Jack H.]

Re: '67 390/400HP IDLE SPEED RANGE

The Operations Check under Flight Judging isn't as strict as what's checked during Performance Verification. Basically, judges look for the car to go up on high idle when cold started and kick down to normal curb idle when warmed up and most judges cut slack with absolute RPM achieved in each position.

But, during PV, the pass/fail criteria are defined in the applicable Corvette Operations Manual & PV Test Guide (available at the STORE on this web site). One book covers all mid-year Corvettes and reading from the '66-67 table for a 427/390 HP car, the ranges are:

Cold: 1800-2000 RPM
Warm: 500-700 RPM

[Michael H.]

Re: '67 390/400HP IDLE SPEED RANGE

Jack,

I have a question on the cold idle speed check. Is this check supposed to be performed with the engine cold or at normal operating temp by manualy setting the choke fast idle cam to the last step? I'm a little curious as if it's supposed to start and run at 2000 when cold, that may be extremely difficult to do because "cold" would be much different depending on the air temp.

Michael

[Joe R.]

Re: '67 390/400HP IDLE SPEED RANGE

Mike,

It's a cold start! With both flight judging and PV it must be a cold start. The difference is the fact that during flight judging you live another day and with PV you die or have 15 minutes to fix the problem. Hard to have another cold start in 15 minutes. The more understanding PV judges usually take a long walk before returning for the choke check.

Regards,

JR

[Michael H.]

A Spec That Doesn't Exist

Thanks Joe. The reason I was curious was because the original fast idle RPM is supposed to be set to specs with the engine at operating temperature, not at cold start. There are too many variables to get one steady RPM with a cold motor so different conditions would result in different RPM. Example would be a cold engine at the Florida event when the temp is in the 70's or 80's. With temp like that, it's likely that the choke wouldn't even pull on completely and wind up on the next lower step of the fast idle cam. At that cam index, the engine wouldn't run near the 2000 RPM number that the JG calls for. The same would be true for engines that are started in cold climates. The choke would be full on but there are a few reasons why the engine would run quite a bit slower than it would in medium or warm climates. At those reduced temps, the engine oil viscosity would have quite an effect on RPM and also the air/fuel mixture would be disturbed because of cold intake runer temps.

This is the reason there is no such thing as a spec on cold engine fast idle RPM. There's absolutely no constant and all the variables would certainly make enough difference to dramatically change this figure. GM has always specified the RPM at fast idle be set with the engine fully warmed up to operating temp because it's the only constant. If this number is 2200 at full operating temp, it would probably run somewhere around 1400-1600 at cold startup temp, depending on the air temp and oil viscosity. Taken to extreme, an engine started at -18 deg would probably run at about 600 RPM for the first minute or two, then gradually increase to 2200 RPM as the oil temp came up to spec.

I don't think this is really a judgeable item, at least not the way it's currently being judged. I think the most accurate way to judge this would be to check for function and forget the RPM unless it were to be checked at normal operating temp and that's difficult on the show field. Any thoughts on this?

Michael

[John M.]

Re: A Spec That Doesn't Exist

Michael,
What is the GM recommendation for the fast idle RPM (warmed up engine) for a '67 427-400. I could set it at this warm and then see what the difference is for a cold NE day. Where can we find this spec?
Thanks,
John McRae

[John H.]

Re: A Spec That Doesn't Exist

The fast idle specs are shown on page 9 and 10 of the Specifications section at the rear of the 1967 Chassis Service Manual; for a 427/400, it's 2200 rpm. Those charts make no mention of hot/cold engine, but in the Engine Fuel section, page 6M-3, under "Fast Idle Adjustment", it indicates that the adjustment is to be done with a warm engine. There does not appear to be any published specification for "cold engine" fast idle, probably due to all the variables Michael mentioned. Probably time to re-think that portion of the P.V. requirement, as there is no spec for it.