Idle & Fast Idle RPM

[Gene M.]

I can not find in NCRS judging manual nor reference manual the engine RPM at Idle & Fast Idle for any engine.
I may be missing something, but can't locate it.

The owners manual has them listed but with the vacuum advanced disconnected and plugged. I want the specification for the RPM with the vacuum connected as on the operations judging field. As you know idle is higher with vacuum advance connected.

I would require this information for all judged years and engine HP combinations.

[Mike M.]

Re: Idle & Fast Idle RPM

check out the PV manual.mike

[Gene M.]

Re: Idle & Fast Idle RPM

Thanks Mike, got it now.........

[Timothy B.]

Re: Idle & Fast Idle RPM

Gene, Mike,

In my 63 shop manual it calls out 500 rpm idle for 300hp manual, are you saying this is with vacuum advance disconnected? I don't recall reading this...

[Gene M.]

Re: Idle & Fast Idle RPM

Gene, Mike,

In my 63 shop manual it calls out 500 rpm idle for 300hp manual, are you saying this is with vacuum advance disconnected? I don't recall reading this...

Tim,
The 65 owner's manual spec the idle RPM with the vacuum advance disconnected and plugged.

[John H.]

Re: Idle & Fast Idle RPM

Tim,
The 65 owner's manual spec the idle RPM with the vacuum advance disconnected and plugged.

Gene -

The idle speeds shown in the owner's manual are ONLY for setting initial timing (with the vacuum advance disconnected and plugged, as noted in the shaded comment below the chart).

The normal idle speeds (vacuum advance connected and operating) aren't shown in the owner's manuals - they're shown in the Shop Manuals and Chassis Service Manuals, in the "Engine Tune-Up" chart in section 6, Engine.

[Gene M.]

Re: Idle & Fast Idle RPM

John, that is all fine and I have that information in my shop manuals too. But I was interested in NCRS published specifications (that Mike answered) which members can refer to while doing operations check on the show field.

My plan is to make copies of the RPM's at curb and fast idle for all judged Corvettes to have at my finger tips to have when asked at our chapter meet. As this question (what should the RPM be?) comes up a lot.

So now knowing where it is, all is good. thanks

[Jack H.]

Re: Idle & Fast Idle RPM

Understand, there really is NO fast idle spec. The tuneup chart gives you instructions on how to properly set the fast idle, but it PRESUMES the engine is fully warmed up, you've FORCED the fast idle follower onto the Nth step of the fast idle cam, and adjusted the fast idle screw to obtain the designated RPM.

That's DIFFERENT from actual choke fast idle operation. Why?

Well, a truly COLD engine will exhibit high friction and it's actual 'initial' idle RPM will depend on the exact ambient temperature, the viscosity range of the oil, whether or not the carb's pull-off has engaged, how long the engine has been running after its initial fire, Etc. The tuneup spec for setting fast idle is intended to cover a RANGE of operating conditions and be high enough to 'mask' a stone cold engine that's been first fired and needs to idle without stalling.

If it's REALLY cold outside (like the conditions we had at the Rocky Mtn Chapter Regional in Denver), you can expect the engine to fire and idle without stalling. Then, as the engine warms, the idle RPM will climb all by itself and EVENTUALLY it will reach the speed indicated by the tuneup chart when FULLY warmed up.

Savvy PV judges understand this and don't RUSH reading the tach after the initial engine start. They let the engine warm up (driver's foot OFF the gas pedal) while they're checking other aspects of operation and when they see the temp gauge is running in the 'normal' range, read the tach to verify idle RPM. Then they ask the owner to kick the accelerator to release the fast idle and verify the carb releases its high idle setting and assumes normal curb idle.

Unless you're doing a PV, it can be 'dangerous' to publish a table of fast idle RPM figures for typical meets where often novice judges are doing Operations Checks vs. actual PV tests. Depending on the actual ambient temperature and exactly WHEN the judge reads the tach to verify fast idle you can expect variable results. In fact, you could well be 'flunking' cars that are operating properly if the judge doesn't really understand the warm-up process!

That might be why most Judging Guides don't publish the fast idle RPM data... An Operations Check is NOT a Performance Verification test and shouldn't be as stringent due to inconsistency of judges' knowledge. Most judges CAN verify the 'gross' high idle function and that should be good enough for an Ops Check under Flight Judging.

[Gene M.]

Re: Idle & Fast Idle RPM

Jack

The NCRS operations and PV guides does indicate the fast idle and curb idle for all year engines and HP's.

Good information for veteran as well as novice judges alike. Great for owners to verify they are in the ball park.

[Wayne M.]

Re: Idle & Fast Idle RPM

Jack

The NCRS operations and PV guides does indicate the fast idle and curb idle for all year engines and HP's.

Good information for veteran as well as novice judges alike. Great for owners to verify they are in the ball park.

For most on this board who have no intention of advancing to the PV stage (and therefore have not purchased the manuals), here's what's in the '65 Corvette Shop Manuals:

... with engine normalized, lift fast idle cam and let throttle close on high step of cam; adjust fast idle screw (contacts step of cam) to obtain specified RPM. Spec tables show 1750 RPM hot on car for both base and 300hp versions; 2200-2300 RPM for L76 and L79, 2600 RPM for L84, and 2000 for the 396.

[Michael H.]

Re: Idle & Fast Idle RPM

... with engine normalized, lift fast idle cam and let throttle close on high step of cam; adjust fast idle screw (contacts step of cam) to obtain specified RPM. Spec tables show 1750 RPM hot on car for both base and 300hp versions; 2200-2300 RPM for L76 and L79, 2600 RPM for L84, and 2000 for the 396.

Hi Wayne,

This issue was first brought up here about five years ago. I think most/all of the newer JG's have reworded the section about a specific RPM range with choke on/fast idle.
Here's the link to that discussion.

https://www.forums.ncrs.org/showthre...speed&uid=3162

[Jack H.]

Re: Idle & Fast Idle RPM

Yep, that's what I was alluding to. The data published in various NCRS books (principally the PV guides) comes from the shop manuals that tell you how to SET the high idle. The numbers are NOT what the tach will read under any/all cold start situations.

Plus, note the advice to 'force' the cam follower onto the nth cam step...

My concern is novice judges wanting 'correct' high idle RPM specs for traditional Operations Check use in Flight Judging can mis-apply. We do NOT get out, remove the air cleaner (that'll probably bump the fast idle cam) and verify which cam step the carb is on when the judge 'verifies' the car's high idle operation! Plus, the judge needs to KNOW that the actual RPM exhibited will vary with specific engine temp, ambient temp and the viscosity of the oil used in that specific engine.

I once experienced such scrutiny at a Chapter Meet. It was COLD outside (ice and patches of snow). One Ops judge was inside the car with me when I cold fired my '71 SB. The equivalent shop manual instructs to set for 1350 RPM with the engine HOT and the cam follower on the SECOND step of the cam.

The engine fired and the car idled sluggishly at 550 RPM. The judge 'failed' the car. I objected and without touching the accelerator, explained the process. The judge was unconvinced because he'd READ the PV manual and there was a hard number there...

The Meet Chairman came walking by and I mentioned our 'dispute'. He stopped and watched, along with the Team Leader while we went about doing other Ops Check items. As I'd said, the car continued to idle without stalling and engine RPM climbed over time as the car warmed up.

Ultimately, it was purring at 1350 RPM just as the shop manual calls for. Begrudgingly, the Ops Check judge reversed his decision. That's where I'm coming from when I say there is 'no fast idle' spec...there's only a spec to SET fast idle operation.

While the difference is seemingly gossamer, it's REAL! And, publishing spec tables for use in Flight Judging under Operations, can be dangerous UNLESS one goes further and instructs judges on exactly HOW they should interpret the RPM numbers.

The big picture is:

(1) Does the car start and run without stalling?

(2) After a 'reasonable' warm-up period, can you kick down the carb, disengate the fast idle mechanism and see a second & lower curb idle RPM?

That's really all one needs to do in an Operations Check under Flight Judging. Let the qualified PV judges assess the absolute RPM obtained performance...