Just a heads up on vacuum

I have had my 71 vert at the builders for 14 months and have replaced everything mechanically on it. We kept having issues with the carburation and have been through 4 carbs trying to get one to idle properly so that we could iron out everything else.

Well, it seems that my Comp. Cams Thumpr is almost too much for a automatic car with power steering, factory a/c and power brakes. It is only making 8-10 inches of vacuum. The builder was just about to pull the motor to re-index the camshaft to ensure that it was right but I called the techs at Comp Eng. and they stated that 8-10 was about all I should expect with 9.5 to 1 compression. The tech says that it just has too much overlap and was not designed for all of the vacuum robbing options on my car. I was just about ready to have him pull the auto and put in a 5 speed.

Lesson learned, we have added a vacuum booster, added a relay to up the idle rpm if the a/c kicks on at idle and we settled on an Edelbrock Thunder series 650 cfm but he had to buy the kit and re-calibrate it. The latest mod has been to mill a longer rod for the adjustable modulator at the transmission so that the trans shifts correctly. Previously it has not been allowing the engine to rev up to optimal rpms before shifting and it shifted too soon. The builder has had to run the car then mill the rod down after each trial run until he got it just right.

I haven't been very active on here as of late due to all the heartache this has caused but it is ready now and I will pick it up next week. He is ironing out the last few minor adjustments. I'll be back posting like crazy as soon as I get it. This has been a long, strange, and expensive trip!......and I still plan to re-paint it and install the new custom TMI sport seat upholstery and foam in the garage.

Wow.

At what point did you find out the vacuum was only pulling 9#'s???

I think that is in the "harley" range.

I am curious about this also. I am going to be running a big mutha thumper cam in my 408 stroker and was never told about how the vacuum would be??? The builder has all my specs for my car. No AC or brake booster. Just an auto trans so hopefully I will be good. But I am planning on swapping to power brakes in the near future. We will see when I pick up the motor this week. Thanks for the heads up.

Hi Folks,

This is my first post to this forum. I've been lurking for months, but never had a reason to post.

I've recently had my 351c rebuilt with a Mutha Thumpa cam, along with every other part in and on the motor.

It was run in and tuned on a dyno - [my son works in Australia's top race motor shop] It only pulled 7mmhg of vacuum at idle, and even after tweeking the timing and playing with the Holley Ultra S/A we could only get 10mmhg.

Clearly, nowhere near enough to power the vacuum booster on the brakes. I had to install a vacuum cannister and a billet 12V vacuum pump to feed the brakes.

My 73 Mach 1 has most of the optional vacuum sucking add-ons - so I had to go this route.

I have a beautifully torquey motor 420lbs at 3500rpm with a 3000 hi-stall t/converter and a 3.50 diff ratio. Brakes are critical.

regards,

Rod A

Interesting. My 4v (see sig for specs) has 11 inches at idle in gear and the brakes work fine in it's current tune. I am in the middle of tuning it (trying different things). I noticed at cruize speed that vacuum is closer to 14 inches.

OP - what is your initial timing set to? Are you running vacuum advance on the distributor? Do you have it hooked up to port or manifold?

FYI - don't overlook Hydroboost conversions either. Easy to come by in junkyards.

-Kurt

FYI - don't overlook Hydroboost conversions either. Easy to come by in junkyards.

-Kurt

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What he said I recently did a Hydroboost Conversion on another vehicle and it is a night and day difference. I found a article from one of the major magazines that measured brake pressure before and after the conversion. On the vehicle they tested it went from 800psi or so to almost 2000psi of pressure at the calipers with the Hydroboost conversion. They used a Astrovan Booster I think which seems to be a favorite for many conversions. A company called Talon Hydraulics makes conversion parts. They sold me the parts I needed for my conversion. Personally after the switch I don't know why I waited so long. It was alot easier and alot cheaper than I would have ever thought. The brakes are a hell of alot less mushy now also.

Vacuum sucks! (It had to be said)

When I was deciding on the cam for mine, I was interested in the thumpr line... I read a good deal of negative reviews on line about people having big trouble getting them tuned or making their motor idle.

Some good info I came across this morning:

http://tru-442.tripod.com/camselect.htm

Wow.

At what point did you find out the vacuum was only pulling 9#'s???

I think that is in the "harley" range.

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When the builder told me, so that is why I called Comp. Engineering to see what it should be pulling. The sad news is that they said it was right on and I was achieving about maximum vacuum.

***Update, I picked up the car Monday, it has a very lumpy idle which is what I wanted. It is shifting just about right but may need a bit more adjustment. It carb does seem to flood rather quickly especially after trying to re-start the car after having just turned it off. I also have a wierd vibrtation that may be the two piece driveshaft (basiclly one inside another). Last thing is I will definately be looking for an overdrive transmission. The C-4 is turning about 2500 at 45mph. It's going back to the builder today to work on some of the problems listed above. I hope to submit a video as soon as I get it back becuase it does sound awesome.

carb may not be flooding, might be fuel boiling. you may need a thin phonic spacer under the carb to prevent heat transfer that boils the fuel after the engine is shutdown for a while you get heat soak when that happens.

What rear end gearing do you have?

What rear end gearing do you have?

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I have a 3:55 gear in the back and changed it from the stock 3:00. Latest update is that I took the car back to the builder because it would take several attempts to start after stopping, acted as if it was loading up. It stalls out at the stoplight sometimes and it is just not right. I am waiting for his response. :huh: We also figured out that the weird vibration is the stupid two piece driveshaft that came on the car, like a driveshaft within a drive shaft. He is changing that out as well to a single heavy duty drive shaft.