Fast idle speed

[William S.]

My 350/300 with quadrajet has a fast idle of 1500 rpm. The manual calls for 2400, which seems fast. Is that the right fast idle speed?

[Michael W.]

Re: Fast idle speed

Fast idle speeds are adjusted with an engine at normal operating temps, not 'cold' as contradictory as that seems.

2400 hot should give you around 1500 cold.

[Duke W.]

Re: Fast idle speed

Your 350/300 is an EMISSION CONTROLLED ENGINE, which is the reason for the high fast idle speed. Earlier non-emission contoled 300 HP engines typically specified a fast idle speed (set HOT) of 1750, and this resulted in an initial cold fast idle of about 1500 in mild temperature conditions. As a general rule most early emission controlled engines have a higher specified fast idle speed than their non-emission controlled antecedents. If you want to experiment, reduce the hot fast idle speed to the minimum that will allow good drive off characteristics immediately after a cold start, but for PV, set it to spec.

[Jeffrey S.]

Re: Fast idle speed

Your 350/300 is an EMISSION CONTROLLED ENGINE, which is the reason for the high fast idle speed. Earlier non-emission contoled 300 HP engines typically specified a fast idle speed (set HOT) of 1750, and this resulted in an initial cold fast idle of about 1500 in mild temperature conditions. As a general rule most early emission controlled engines have a higher specified fast idle speed than their non-emission controlled antecedents. If you want to experiment, reduce the hot fast idle speed to the minimum that will allow good drive off characteristics immediately after a cold start, but for PV, set it to spec.

I set my 69 350/300 auto w/AC at a cold RPM of about 1000 RPM which yields a nice smooth idle and excellent drivablilty when cold. It is also set to run at 600 RPM hot in drive.
Jeff

[Duke W.]

Re: Fast idle speed

Assuming it idles in Drive at 600 with the AC compressor not engaged, what does the idle speed drop to when the compressor engages?

[Jeffrey S.]

Re: Fast idle speed

Hi Duke,
The idle drops about 50 RPM to 550 hot, in D, with A/C on. Still smooth with no hint of a stall. This eliminates any run-on after ignition is shut (300 HP does not have solenoid on carb). I use 93 pump gas. I think the lower RPM is do-able with the cam you recommended and full time vacuum advance- not ported.
Jeff

[Tom L.]

Re: Fast idle speed

My '72 LS5 operates essentially the same. Idles at 600 and drops to 550, no indication of a stall at all. It does have the idle solenoid but is inoperable at this time.

[Duke W.]

Re: Fast idle speed

Hi Duke,
The idle drops about 50 RPM to 550 hot, in D, with A/C on. Still smooth with no hint of a stall. This eliminates any run-on after ignition is shut (300 HP does not have solenoid on carb). I use 93 pump gas. I think the lower RPM is do-able with the cam you recommended and full time vacuum advance- not ported.
Jeff

Thanks, Jeff. Since your McCagh Special camshaft has the same effective overlap as the original "300 HP cam", just phased later, it has the same idle behavior. I think you can get it to idle 50-100 RPM lower in Drive with the compressor engaged. That will result in less "creep", and less "clunk" when you shift from Park to D or R. Mike McCagh's '57 250 FI Powerglide idles very smooth and stable at 450 in Drive.

My rule of thumb for idle speed is "as low as possible commensurate with acceptable idle quality and stability". This criterion should be applied to automatics idling in Drive and Neutral for manuals, and if AC equipped the compressor should be engaged in both cases.

I'd still like to see a chassis dyno test on your car. BTW, I figured out how to keep it from downshifting when you punch the throttle above about 35-40 MPH. Just disconnect the electric kickdown switch on the throttle lever under the dash. That way you can get the full torque bandwidth from converter stall speed (1800-2000) to peak revs.

The combination of a large displacement, torquey engine and TH400 is a really nice driving experience... butter smooth shifts at light throttle and instant downshifts when you need them... much better IMO than modern six and seven speed automatics that take two seconds to figure out what gear to engage when you floor the throttle at 35 MPH.

Duke

[Jeffrey S.]

Re: Fast idle speed

Tom,
I may be wrong but I believe that the solenoid used on the 69 was only to drop the idle rpm down at shut off to prevent run-on and later ones were part of the emission system(?). I'm no expert on this so I stand to be corrected.
Duke,
I did try to lower the RPM to what you said and I wasn't happy with the results so I did what your "rule of thumb" indicates: I raised it slightly until I liked the result. I may just take it to a dyno shop here in Detroit this summer and do a few pulls.
Jeff

[Duke W.]

Re: Fast idle speed

Tom,
I may just take it to a dyno shop here in Detroit this summer and do a few pulls.
Jeff

That would be great! I really want to see what your engine will do. If you decide to do so please contact me well in advance, so I can help you develop a test plan. Dyno time is usually rented by the hour, and you can get a lot done if you have a good plan. Also, it's best to do the testing in cool weather, say no more than 50-60 degrees, which with the help of sufficient external cooling fans will keep the fan clutch from engaging that can eat 10 lb-ft of peak torque and 15 peak HP.

Duke