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Vacuum blues

JamesG

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I have a 1970 Gran Coupe, 383 4bbl setup Auto with factory original power steering and I am having a squishy pedal problem. I have a cam in it that I think is killing the vacuum. The power brakes are ok, but if I really needed to do a panic stop I don't think they will lock up the wheels. It has the original power disc brakes on the front, I just went through the system... replaced the master, wheel cylinder rebuild, etc. but still not where I want them to be.

Question I have, the car is currently running a 750 Holley double pumper on a single plane Edelbrock Torker intake. It has a Mopar Street/Strip cam in it (can't remember the specs, it was a while ago). Before I go with a vacuum pump, should I try a 650 Holley first? The 750 seems like too much carburetor on the 383 especially with the stock heads on it. I like the idea of better throttle response with the 650, just curious if it will help with the vacuum problem also.

Not crazy about the idea of spending the money on a carb if the problem not being solved and still having to go with the vacuum pump... though I hate the idea of adding a vacuum pump also under the hood.

I need to get a gauge to see what the vacuum pressure is right now, probably the first thing I should do.
 

moparleo

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Carb has nothing to do with vacuum. In the first place, you need a vacuum/fuel pressure gauge to adjust the carb. This is how you tell how much vacuum you are pulling. You know that you don't want to lock up your brakes, that doesn't stop the car quicker it just ruins your tires by flat spotting the tread. Why do you think all current cars have ABS, Anti-lock brakes ? You carb and manifold choices are not conducive to good street performance. Everything that you do has an affect on something else. You want that choppy idle ? The reason you get it with your cam is because it is not firing properly at low rpm's. The longer duration camshafts are designed to operate efficiently at a higher rpm. Not where many people normally drive their cars. Double pumpers are really not for the street, again more of a strip design. You just use more fuel everytime you put your foot down. New cars produce much more torque and horsepower as well as better fuel economy because of the sensor feedback, computer controlled system. These alter valve timing, air/fuel ratios, etc... Vacuum is also a "lack" of pressure...it is a vacuum.
 

Chryco Psycho

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I don't believe the carb swap will help you at all in any respect the 750 CFM is the right size for a 383 , tuning can help a lot though , more initial advance & a shorter timing curve can help a lot , make sure the vacuum check valve in the booster is good , the booster should hold the highest vacuum achieved between brake applications so if you let off the throttle at higher rpm it is possible to see high teens for vacuum & hold it until needed .
 

JamesG

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I don't believe the carb swap will help you at all in any respect the 750 CFM is the right size for a 383 , tuning can help a lot though , more initial advance & a shorter timing curve can help a lot , make sure the vacuum check valve in the booster is good , the booster should hold the highest vacuum achieved between brake applications so if you let off the throttle at higher rpm it is possible to see high teens for vacuum & hold it until needed .

Thank you. Yeah, that is why I asked about the Carb, didn't think it will make a difference. My first thought is there is still air in the system somewhere since it isn't really a hard pedal I am dealing with, it is a soft pedal. When I get a chance over the next few days I will put a Vacuum gauge to get an idea what the numbers are. If the Vacuum looks good, then just try to find the air in the system. If it is the vacuum, try going with a vacuum reserve canister before putting an actual vacuum pump on it.
 

Chryco Psycho

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A Reserve canister should help .

Seriously start tuning it , make sure all of the throttle plates are open evenly , you can open or close the rear to balance them , , get the timing optomized , get it tuned to peak & you may be surprised how much you can improve the vacuum .

Another option if you have power steering is Hydroboost , probably cheaper than a vacuum pump & then vacuum doesn't matter ay all & you will have better stopping power .
 
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JamesG

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So a couple of things, I did test the vacuum, and it was very low, 11". I am not ready to concede that is the point yet but if it is, I will likely go with a reservoir before a pump.

Second, I do want to pull the rear wheels and see if the rear drums are adjusted properly. I also understand that if the drums have been cut, the pads should probably be arced to match them, but that seems to be a lost art and finding someone to do it with the right machine might take a bit.
 
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