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351C with high RPM-trop between P and D and a high consuption (other cam?)


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I have a 71' Mustang with 351C and C6-Automatic and problems with the idle (high  difference about 400-500 rpm between P/N and D/R/1/2).
The Cleveland has 4V heads, the orignal 4V iron intake and a 600 cfm Holley (80457 with vac. secondarys).

Engine is bored out to 4.02

Pistons are keith black KB177-020

4V iron-headers with 2¼ H-Pipe

The heads are from 73 with small valves and open chamber but I want back to my 2V-Heads from 71 (open chamber) with 4V Edelbrock intake (2750).
The cam ist a Howard Street Force 2 (230021-12)

Ignition (Pertronix 3): 16° by idle and 34° totaltiming, vaccum at timed/ported.

Which cam will be better for my setup?
(Howard Street Force 1, Melling CL-MTF-2, Summit SUM-5200? ...or some other?)
I want better idle and fuel economy.

Or is the reason for the trop in the RPM the C6 (to big?) or the combination from cam and C6?

Some other ideals to optimise?


Greetings from Germany
Christian
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A big drop in RPM going into drive, and/or stalling is usually a lean mixture issue. Could be a vacuum leak or a the carb idle mixture needs to be richened. I ran the Melling version of your cam many years ago and had it hooked to a C-6 first with a 2500rpm stall converter and then a stock 1500-ish unit, and it never exhibited that big of a drop. You should be able to idle in gear around 700.

As far as fuel consumption, you should be able to pull down 15mpg easily. My best with that cam was 20mpg on a long hiway trip with 3.00 gears.

I'd start by spraying around the carburetor base with carb cleaner to check for vacuum leaks. You'll also want to disconnect the vacuum advance and set your idle mixture to highest vacuum reading, then richen 1/8 turn. 

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I had besides the original iron intake, an Edelbrock intake (2665) on it before. And I already had 3-4 different carburetors (600 cfm, Summit and Holleys) and these are adjust by Vaccum and AFR-Gauge. Most of the time I have 20-26 l / 100 km (9-12 MPG).
In terms of the drop down ... I have around 1200 RPM in N/P and 700 RPM in D/2/1/R The pressure with the Howards Street Force 2 is 14 HG.
(My gears are 4.11, if I remeber correctly ...but it is no reason for the drop)

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The 4.11:1 gears don't contribute to your idle speed drop, but do contribute to your fuel consumption problem. The only way you can enjoy the acceleration of the low gears and have decent fuel economy is with a transmission that has overdrive.

Follow Hemikiller's advice. Finding a vacuum leak may take some time. Check every hose and every gasket and every device that is driven by vacuum, such as vacuum modulator, vacuum advance, power brake booster, etc. Even though you think they are good, check them again.

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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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3 hours ago, Don C said:

Follow Hemikiller's advice. Finding a vacuum leak may take some time. Check every hose and every gasket and every device that is driven by vacuum, such as vacuum modulator, vacuum advance, power brake booster, etc. Even though you think they are good, check them again.

Smoke puffers are good for finding vacuum leaks in hoses and joints.  Check with a local auto parts store and see if they have any they can loan or rent to you.

---

Mike

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That drop is not that horrible, but seems a little bit higher than it should. What is your timing curve on your distributor? Do you have a stock stall speed on your converter? That cam is not that big, so you should be able to use it with a stock converter. What is the actual RPM on neutral and what is the RPM in gear?

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As others have said, the cam is not aggressive enough to cause that large a drop in RPM when in gear. Checking for vacuum leaks as Hemi suggested is a good idea. If none are found, check initial timing advance. It should be about 16 degrees advance without any centrifugal advance. If that does not help very much then it is likely a carb adjustment or calibration issue. How much manifold vacuum does it have in Park and Drive? What do the spark plugs look like? With everything properly adjusted, with that cam, you should see a 150-200 RPM drop when placed in Drive. Chuck

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