# Thumpr cam in '66 gto taking vacuum from power brakes



## take5 (Aug 9, 2009)

I've just rebuilt the original 389 cid and placed a THUMPR CAM with Valve Lift of .501 intake and .486 exhaust. Complete valve job for heads, etc.
I have also rebuilt the original 4 barrel carter carburetor to factory specifications.
Is it common for power brake system to be starved and lack sufficient vacuum with running this larger cam?
Power brakes are reduced to manuel with very hard brake pedal.
Has anyone experienced or familiar with this situation, and if so remedy to correct with the exception of removing THUMPR CAM.
Thanks as the restoration project keeps moving forward.


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## jetstang (Nov 5, 2008)

Yep, thats a huge cam for a Pontiac and you wont have vacuum to run the brakes. You can put a vacuum can in the car to help the vac systems. Bet the cam sounds great!!:cheers


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## Rukee (Feb 8, 2007)

You could add a vacuum pump like what the diesels run, or a hydro boost system, or convert to manual brakes and take the booster off and install a manual master cyl.


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## take5 (Aug 9, 2009)

Thanks guy's - I was finally able to drive the "little gto" yesterday & posted "the issue".
Yes, the cam sounds fantastic, but a reliable braking system is obviously a must.
My next question is about changing to manuel brakes - I also retrofitted new disc brakes for the front of the car - will a manuel system work with front disc, rear drum brakes?
I will keep you posted with all the progress and many thanks for your timely response 
I'll be seeing the restoration shop later today , Friday 9/3/10.
Also, I will certainly get plenty of pictures posted here soon.


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## mikes06goat (Jan 13, 2009)

Much easier to just add a vacuum pump. Fairly cheap off Summit or Jegs too.


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## 05GTO (Oct 6, 2004)

take5,

Your vacuum problem is not a result of the cam having a lift of .501 for the intake and .486 for the exhaust. The problem comes from the lobe separation angle of the cam. A lower LSA (110 for example) will decrease the low-end power, increase the high-end power, produce a rough idle and decrease the idle vacuum.

I would test the brake booster on the "PCV valve" manifold vacuum by temporarily plugging the current brake booster vacuum line and running a temp vacuum hose from the booster to the "T" in front of the Carter AFB which connects the manifold to the PVC valve. 

The manifold vacuum is increased during idle when you need the brakes, if the results are satisfactory swap the 2 lines.


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## Eric Animal (Oct 28, 2007)

:agree OR see e-bay #270627359494


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## take5 (Aug 9, 2009)

Randy & Eric,
I'll give it a try re-routing the vacuum hose and testing the brake booster as you have detailed & described Randy.
Probably won't be able to tinker with it until next Tuesday, but I'll keep you posted with results experienced.
Thanks


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## LEGUESSCY (Dec 1, 2021)

take5 said:


> I've just rebuilt the original 389 cid and placed a THUMPR CAM with Valve Lift of .501 intake and .486 exhaust. Complete valve job for heads, etc.
> I have also rebuilt the original 4 barrel carter carburetor to factory specifications.
> Is it common for power brake system to be starved and lack sufficient vacuum with running this larger cam?
> Power brakes are reduced to manuel with very hard brake pedal.
> ...


Wow I just installed a muthathumpr cam and man was that 1st drive scary my breaks are hard. Can you share what you did to correct the issue. I too have a 389 engine in my Lemans front fisc breaks rear drums


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## BearGFR (Aug 25, 2008)

Your brakes are "gone" because that cam has severely affected engine vacuum, especially at idle/low RPM. That cam has a ton of duration for a flat tappet, and also a very tight LSA. Both of these things combine to create a TON of overlap. Overlap is what makes the cam "sound" so nasty at idle, and is also what absolutely KILLS your idle vacuum - which is why you have no brakes. 
This cam is a high RPM, almost "race only", cam. Comp's own web site states "needs 9:1 compression, 2500+ stall, intake, gears and headers". You should read that as, AT LEAST 9:1 compression (true, not just advertised), AT LEAST a 2500 rpm stall converter (if an automatic), (free flowing, high capacity) intake (probably a single plane), (much "lower - numerically higher" ratio rear) gears, (4-tube) headers. Do you have all that? They put that information in the notes for this cam for a reason. 

On paper this cam with that tight LSA looks to be a lot more aggressive than the one in the factory Ram Air IV, and those cars all came with a MINIMUM 3.90:1 rear gear as standard, with 4.30:1 as optional, plus A/C was not available in those cars - all for good reason. 

This cam is going to sound VERY mean and rowdy at idle, but "on the street" it's probably going to be a dog because it will rarely get up into its RPM sweet spot where it can work well and start making torque.

The top solutions to your problem, in no particular order, are:

Add a supplemental vacuum pump to make up for the vacuum you've lost. There are several available. Some people have complained that they are noisy, but I have no personal experience with them.
Replace your vacuum-actuated power brake booster with a hydroboost system. Hydroboost uses hydraulic pressure from your power steering pump to provide the assist. I'm running hydroboost on my car. You can either purchase a ready-to-install kit that has all the components, hoses, fittings, etc. or you can create your own system by spec-ing out and acquiring the individual parts. The latter is quite a bit less expensive, but you have to "know what you're doing". That's what I did - I created my own.
Replace that cam with one that's not so aggressive and doesn't kill vacuum so badly. For power brakes, you should have at MINIMUM a solid 15"-18" of manifold vacuum at idle. More is better.
Convert to 100% manual brakes and "just deal with it". 
Bear


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## Baaad65 (Aug 29, 2019)

My cam is 230/236 with .510/.521 then I added 1.65 rockers but it has a 114 LSA, I have 13-14 inches of vacuum and I'm running an 8" dual booster, front disc and it stops good but I did notice a little difference after I changed the motor from a 1970 stock 455. What is your vacuum reading?


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