Thumpr 279TH7 Cam issues

[OpiemusRex]

I have posted on here before about the problems I am having with Comp Cams Thumpr model 31-601-5, grind 279TH7 camshaft. After all that I have done the most vacuum that we an get is in the 8 to 10 inch range. My engine is a 302 bored .30 with Dart Pro 1 aluminum heads, JBA s/s ceramic coated headers, Edelbrock air gap intake and a Edelbrock Thunder series 650 cfm carb.

We have added a boost assist, a solenoid to increase the throttle idle speed when the a/c is turned on and modified the adjustable transmission modulator to shift correctly (we had to mill a longer rod).

The car has a very nasty idle but it wants to load up at idle and shut down. I took it back to my builder and he adjusted a few things and we went for a test ride Friday. He is trying to tell me that it is adjusted as good as it is going to get and wants me to drive it with two feet. Every time I come to a stop or slow down I have to feather the gas petal as I brake. I find this unacceptable.

I guess that I will have to give up on this cam, it is obviously too much for auto transmission, power steering, power brakes and air conditioning. With the low vacuum the brakes are very tricky too.

Can anyone recommend a good alternate cam that has a choppy idle that you don't have to drive with two feet? I think that we have done everything possible to make this cam work, it is just too much for all the accessories on my car. Oh, and when it does choke down at a stop light it takes forever to restart which is very embarrassing.

The specs on this one are gross intake .491 gross exhaust .476

Duration at .050 226 intake and 241 exhaust. :shrug2:

 

[wwhite72]

You need to speak wih the guys that made your heads and ask them what will work with their heads and still give you some vac.

 

[Don C]

How are your motor mounts? With that rough of an idle the fuel in the bowls might be getting sloshed around. Are the float levels correct?

 

[OpiemusRex]

How are your motor mounts? With that rough of an idle the fuel in the bowls might be getting sloshed around. Are the float levels correct?

I have new Prothane motor mounts and transmission mount, the motor does not bounce around too much.

You need to speak wih the guys that made your heads and ask them what will work with their heads and still give you some vac.

I is my understanding that the vacuum is created by the camshaft, I called Comp. Eng. and they advised me that 10 inches of vacuum was all I should expect. This would be fine on a manual shift car with no power options but too low for my set up.

 

[MotoArts]

That's not a "lot" of cam, so to speak.

First, it sounds like the carb is too rich at idle/just off idle. I agree with the possible float level issue too.

May simply require a carb tuning with the Edelbrock kit (rods, springs, etc).

What rear gear, trans and converter will you be running?

 

[Totalled]

Sounds like you need a stall converter. 2500 at least.

 

[johnwanzel]

What stall is your converter? Definitely sounds like a torque converter issue. That's not really a "big" cam. Although I would also recommend a vacuum canister if you're planning on running power brakes.

 

[OpiemusRex]

That's not a "lot" of cam, so to speak.

First, it sounds like the carb is too rich at idle/just off idle. I agree with the possible float level issue too.

May simply require a carb tuning with the Edelbrock kit (rods, springs, etc).

What rear gear, trans and converter will you be running?

We have already tuned the carb with the Edelbrock calibration kit and my builder says that the carb it tuned just right for the camshaft. My builder is not only a mechanic but a mechanical engineer and builds cars from Texas to Florida. He is well known for how meticulous his builds are and he says this engine is about the fourth most aggrevating one he has encountered.

I have a 3:55 tru-track rearend and it has a 2400 stall converter.

 

[Don C]

Could be a bad casting on the carburetor, with some porosity allowing excess fuel into the manifold at idle, wouldn't be so noticeable at higher RPM. I've seen this a couple of times.

 

[MotoArts]

Yep, try another (known to be good) carb.

Any carb/average CFM will do, doesn't have to be another Edel.

If nothing changes, look at other potential causes.

Only do ONE change at a time to isolate the engine's objection...

OR maybe try this:

Try to repeat the stalling maneuver (@ stop sign, whatever). Only this time, knock it into neutral before applying the brakes. If it gags and quits, it's not driveline related (converter, etc.). Also, if it gags, maybe lower the float level a little below its current setting and repeat the test. Again, you're just looking for change...

 

[woodsnake]

In my 69-351W car, I ran this comp cam

http://www.summitracing.com/parts/cca-cl35-246-3/overview/make/ford

And I had similar issues regarding the power brakes. I just learned to bump it up into neutral at a stop. As I always shifter the car manually, it just made sense. I also had a 3.89 gear, and 3500 stall TC in the car. Cam made good power, no doubt about it!

 

[VegasFastback]

I don't know how to get you more vacuum other than picking a cam with less overlap, but I have a idea about your carb. Is it possible that the butterflies are open enough to be into the transition circuits. The easy way to check is to fiddle with the adjustment needles. If they have no effect the transition circuits are feeding the engine. Unfortunately with your low vacuum the butterflies may need to be open that far just to get enough air.

The solution is to close the butterflies to where they need to be to shutoff the transition circuits and then open the secondary butterflies to let more air in. If you can't open the secondaries for some reason, for example if they have their own idle circuit, then the other option is to drill the primary butterflies to let more air through so the engine wont die.

 

[droptop73]

The specs on this one are gross intake .491 gross exhaust .476

Duration at .050 226 intake and 241 exhaust. :shrug2:

If you go with a cam with about .500" lift and 260 - 270 advertised duration and 110 to 112 centerline you should get a decent idle and drivability. The amount of overlap determines the idle quality to a point. With a wider lobe separation you get a smoother idle and better vacuum and drivability. The Thumper has a 107* lobe separation which provides the low vacuum and also the choppy idle.

This looks like a good cam to me. http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=851&sb=2 it has more lift and duration but a wider lobe center.

This one is a little more aggressive and could put you back where you are now. http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=852&sb=2

 

[OpiemusRex]

I appreciate everyones input and after consulting with Comp. Cams and my builder I have ordered this cam, it is supposed to cure all of my issues.

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=799&sb=2

 

[mustang7173]

OpiemusRex,

The new camshaft should work better for considering the Stock 5.0L HO cam advertised specs are 266°/266° duration, 0.444"/0.444" lift. I noticed that the first camshaft operating RPM 's were from 2,000-5,800.

I was going to say look at the Edelbrock performer camshafts. You are on a good path.

http://www.edelbrock.com/automotive_new/mc/camshafts/cams_ford_sb.shtml

mustang7173

 

[Jarturin]

Have you considered adding a vacuum pump and a vacuum canister? And if these were installed, did not work?

That cam has a great V8 sound but good for high rpms....

Hope you can get it to work. I was about to install a similar one, but finally decided not to because same problem was anticipated to me by Comp Cams people.

 

[marks73]

What is the cfm of the carb your running?

 

[will e]

I would have suggested some Rhodes lifters. i run them, they are a little noisey at idle but do help to tame a larger cam.

 

[uglydog56]

That isn't that much cam. Add an electric vacuum pump and a 3000 - 3500 converter. I had that problem with performer rpm cam in my car back when I had auto. Went to a 3k stall and stage 2 shift kit and got all my driveability back.