My engine sucks

Posted By: Kern Dog

My engine sucks - 03/18/11 07:35 AM

I was tuning the 440/493 in the Charger and part of this includes checking idle vacuum to determine the proper size Power Valve for the Speed Demon 850 carburetor. I already have a 3.5 PV in the carb now. The motor is a '74 440 with a 4.15 stroke crank, 10.8 to one squeeze, Edelbrock heads and RPM intake. 2" TTI headers and the '509 Mopar cam.
Holleys website suggests to take a reading of the idle vacuum while IN GEAR and install a power valve with a number that is HALF of the vacuum. When I checked mine, I got a number of 7 inches of vacuum! Granted the engine wasnt fully warmed up. I pulled the choke plate some time ago since the car will idle dead cold even when its 40 degrees outside. It does idle better once it is warmed up, but my question is.......
Whats your combo and idle vacuum numbers? I never checked this on other cars before, so I dont know what to think. I'm going to take another reading with the motor at operating temperature and I'm guessing that it will be a higher number than 7 inches. If not, I guess the PV is right as is.

Posted By: ireland383

Re: My engine sucks - 03/18/11 10:53 AM

Yes, make sure it is fully warmed up. I'm at 10 in gear and run 4.5 PV.

Posted By: RapidRobert

Re: My engine sucks - 03/18/11 01:29 PM

Dont have actual #'s to offer but bottom line does it have a flat spot and on the other end you dont want it open at a higher in hg than needed. EDIT yes the 1/2 rule is good but when the vac levels are low enough on a particular build it can change that general rule

Posted By: Scamp408

Re: My engine sucks - 03/18/11 01:35 PM

I was told and use the 1.5-2.0 below idle vaccum.

Posted By: bigblock340power

Re: My engine sucks - 03/18/11 02:40 PM

How do I tell what size power valve I need?

To properly size a power valve, take a vacuum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vacuum reading in gear at idle and if the vacuum is below 12" divide that in half for proper size. Example 9" of vacuum in gear at idle will require a 4.5" power valve.

What's not to get? Don't over think it!

Posted By: Twostick

Re: My engine sucks - 03/18/11 02:49 PM

Quote:

I was tuning the 440/493 in the Charger and part of this includes checking idle vacuum to determine the proper size Power Valve for the Speed Demon 850 carburetor. I already have a 3.5 PV in the carb now. The motor is a '74 440 with a 4.15 stroke crank, 10.8 to one squeeze, Edelbrock heads and RPM intake. 2" TTI headers and the '509 Mopar cam.
Holleys website suggests to take a reading of the idle vacuum while IN GEAR and install a power valve with a number that is HALF of the vacuum. When I checked mine, I got a number of 7 inches of vacuum! Granted the engine wasnt fully warmed up. I pulled the choke plate some time ago since the car will idle dead cold even when its 40 degrees outside. It does idle better once it is warmed up, but my question is.......
Whats your combo and idle vacuum numbers? I never checked this on other cars before, so I dont know what to think. I'm going to take another reading with the motor at operating temperature and I'm guessing that it will be a higher number than 7 inches. If not, I guess the PV is right as is.

My combo is almost identical to yours only 8.97:1 and EFI. Same cam too if yours is the 108 lsa. Mine IIRC pulls about 8-9" in gear at 900-1000 rpm. I run 18 deg initial timing and I run a 2800 T/A converter. That cam has 76 deg of overlap so it's not big on vacuum.

If the weather stays nice today I'm going to bring it out of hibernation and I can verify my sketchy memory

.

Kevin

Posted By: Twostick

Re: My engine sucks - 03/18/11 05:33 PM

Just went out and it fired up like I just parked it yesterday. Been in a snowbank since December. 11" at 1100 RPM in Park. 8ish in gear. I have EFI so the idle/air control compensates for the drop when you put it in gear and it comes back up to 1100.

Kevin

Posted By: Dean_Kuzluzski

Re: My engine sucks - 03/18/11 08:19 PM

Quote:

I pulled the choke plate some time ago since the car will idle dead cold even when its 40 degrees outside. It does idle better once it is warmed up

Typically, that mean you're too rich if no choke is required when cold.

What is your initial timing?? Depending on where it's set, more will typically create a stronger vacuum signal. The narrow LSA (108) has tons of overlap, which means more initial timing. Modifying the mech. advance slots (welding/brazing) in the distributor is what many do.

Posted By: Kern Dog

Re: My engine sucks - 03/18/11 08:25 PM

I run mine at 17 degrees initial, limited to 31 degrees total to avoid detonation with the 91 0ctane fuel. Regarding the idling while cold, I should have wrote that I do have to feather it for a few seconds.....

Posted By: Anonymous

Post deleted by moparts - 03/20/11 08:26 AM

Posted By: Kern Dog

Re: My engine sucks - 03/20/11 09:30 AM

Thats okay. I trust Mike.

Posted By: Dodgem

Re: My engine sucks - 03/20/11 05:31 PM

Quote:

I was told and use the 1.5-2.0 below idle vaccum.

With stockish cams with 17" in gear vacuum a 8.5 to 10.5 is good on performance cams 1.5 to 2.5 above so typically I would use 5.5' on a 7 to 7.5 in gear bigger cam motor.

Make sure the power valve is good or you peeing in the wind!