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Bumpy cam causing low brake vacuum
http://forums.ctzcc.com/viewtopic.php?f=4&t=10006		 Page 1 of 2
Author:  Frank T [ Wed Jun 30, 2021 9:15 pm ]
Post subject:  Bumpy cam causing low brake vacuum
Gotta share this.
I'm about 100 years old and have been playing with and working on cars and motorcycles since I was a kid, but I've only *JUST* learned that a lumpy cam can cause you to lose brake vacuum at slow rpm. Nobody ever told me.

Background: My 240z has all new brakes and lines and all kinds of fancy stuff to upgrade it to modern traffic conditions. But occasionally, when I travel slowly for about a hundred yards and use the brakes a lot (such as driving slowly thru neighborhoods with stop signs at every corner, or crawling slowly along the highway in bumper to bumper traffic), my brakes just harden up and require a two-foot stomp to keep me from hitting the car ahead of me. My power brakes become manual brakes, with not much bite even at low speed.

I researched it and discovered this is a well-known problem for cars with vacuum power brakes and a big camshaft. I never knew that. My Z has a .490" lift and a long 290* duration grind Isky cam, which gives very happy performance but doesn't let the vacuum booster load up at low rpm. It gets downright scary in slow heavy traffic when I step on the brake and not much happens.

To combat this phenomenon they make electrical vacuum canisters which provide constant vacuum to the brake booster. Guess what I'll be installing on my Z this year?! :wink: Why didn't any of you guys TELL me about this??

Until that gets mounted, you're safer traveling behind me than in front of me.

Frank T

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Author:  jp260z [ Fri Jul 02, 2021 9:16 am ]
Post subject:  Re: Bumpy cam causing low brake vacuum
I'd have told you if I knew you didn't know about them.

Glad you are getting one, they work.

Jay
Author:  johnnyZ [ Fri Jul 02, 2021 9:54 am ]
Post subject:  Re: Bumpy cam causing low brake vacuum
:lol: Great Answer Jay! :thumbs_up:

Ah, that cam mod is just one of those mods that typically link to other required mods or cause more servicing issues than what was originally designed by Nissan. I have two stupid examples to share:

1) Installed MSA tuned header: Car runs better, but this mod now requires me to remove it if I am to pull the tranny, which also means I need to remove my intake manifold and install a special manifold to head gasket :shock:
2) Front air dam (spook): This does help with high speed handling but makes jacking the front of the Z up more difficult. Ditto with my Tokico springs and struts. And curse those parking curb stops! No longer have the spook. :evil:

Just thought I'd share the joy! :D

And Frank, to think I let you follow me on the last few fun runs?!! :roll:
Author:  Frank T [ Fri Jul 02, 2021 11:04 am ]
Post subject:  Re: Bumpy cam causing low brake vacuum
Yeah John, I came close to kissing your car a couple times. :oops: Even pulled alongside you once.

And you mention the curb stops! Back when our cars were new, a guy I knew in Hawaii mounted one of the first front airdam/spoilers to his car. It worked well at speed and we were all envious of it. It created drag which slowed him down a little but he said it have him hands-off dead-on steering at speed.

One day he hit a curbstop square-on and broke the center out of the spoiler. It was so clean it could have almost been a factory option. The result was stunning; the two ends covered most of the front wheels but the open center let air under the car, which actually let him go faster and still handle better than when it was intact. and it let him slide his floor jack under the front to lift the car. :lol: He was tempted to leave it that way.
Author:  jp260z [ Fri Jul 02, 2021 4:53 pm ]
Post subject:  Re: Bumpy cam causing low brake vacuum
Hey John! How's your summer going?

You bring up a good point about modifications, rarely does it not turn into a snowball rolling down a hill. And like you said, they usually improve whatever they are designed to improve, but there are unseen tradeoffs to it.

If we've learned nothing else from this thread, it's that from now on you let Frank lead. :shock: :lol:

Jay
Author:  Frank T [ Fri Jul 02, 2021 5:05 pm ]
Post subject:  Re: Bumpy cam causing low brake vacuum
:x Hey Jay ~ we have a new-join in New Member Intros who has a 260.
Why don't you go over there and tell him how well they roll backwards?! :evil:

BTW John, I note you have just passed your 13th anniversary with the Club. :thumbs_up: :mrgreen: :thumbs_up:
Author:  jp260z [ Fri Jul 02, 2021 6:15 pm ]
Post subject:  Re: Bumpy cam causing low brake vacuum
I was just having some fun with you Frank haha

With any luck the new member lives somewhee other than on the side of a mountain. :roll:

Jay
Author:  Frank T [ Fri Jul 02, 2021 8:29 pm ]
Post subject:  Re: Bumpy cam causing low brake vacuum
:lol:
Author:  jp260z [ Sun Jul 11, 2021 8:31 am ]
Post subject:  Re: Bumpy cam causing low brake vacuum
So did you get one of these yet Frank?
Author:  Frank T [ Sun Jul 11, 2021 1:12 pm ]
Post subject:  Re: Bumpy cam causing low brake vacuum
Not yet; things don't usually happen that fast in my life, and right now I have a houseful of relatives who are keeping me busy with other things. As soon as I come up for air I will order the one I think I need.

["I'd have told you if I knew you didn't know about them"].

Well, if I had KNOWN I didn't know about them, I would have asked!
I there anything else I don't know about which you should tell me? :lol:
Author:  jp260z [ Sun Jul 11, 2021 5:49 pm ]
Post subject:  Re: Bumpy cam causing low brake vacuum
If I think of any I'll let you know. :wink:

Enjoy the houseful.

Jay
Author:  jp260z [ Thu Sep 23, 2021 1:58 pm ]
Post subject:  Re: Bumpy cam causing low brake vacuum
So for the sake of updating this one, and because I'm curious, do you feel this issue has been fully resolved Frank?

Jay
Author:  Frank T [ Thu Sep 23, 2021 3:25 pm ]
Post subject:  Re: Bumpy cam causing low brake vacuum
Not fully, but the power brakes are back. I need to adjust the pedal a little higher to let me toe-and-heel now. The car is drivable but the lower pedal makes braking a bit dodgy.
Author:  HowardJ [ Thu Sep 23, 2021 5:37 pm ]
Post subject:  Re: Bumpy cam causing low brake vacuum
Frank T wrote:
Not fully, but the power brakes are back. I need to adjust the pedal a little higher to let me toe-and-heel now.


:mrgreen: :thumbs_up: Toe-and- heel and Dos-a-Dos! :lol: :thumbs_up:
Author:  Frank T [ Thu Sep 23, 2021 6:35 pm ]
Post subject:  Re: Bumpy cam causing low brake vacuum
I enjoy double-clutching when I downshift. It makes me sound like I know what I'm doing. It's hard to do that when the brake pedal is too low.
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