Vacuum Pump Relay

David Murray · Post by **David Murray** » Mon May 01, 17 11:54 am

Sorry gents, quite a long post, but I felt compelled to give you all the context. if you want the short version then head straight for the BOLD text!

I'm planning ahead because I know my 512 stroker build won't produce enough vacuum for my brakes. Initially I was looking at a hydroboost system, but I'm not sure I can make one of the off the shelf systems easily fit a c-body...plus the custom vacuum pump setup I've put together in my head works out to about 1/3rd the price!

The main complaint with the off-the-shelf type street vacuum pumps is the noise, followed closely by reliability. So, I want to copy what I've seen a few others do and that is use the Hella UP30, which is a quieter rotary style pump and is the OEM pump in several electric and hybrid cars where they run constantly. Basically, they should be both quiet and reliable.

But, I want my setup to include a vacuum switch (like the off-the-shelf solutions), so that it doesn't run all the time, and this is where I need your advice and is why I posted in the Electrics and not Brakes section....

I intend to power the pump with 12v from the battery, via a 30A fused relay, with the trigger coming from a switched live. I'll probably re-use the live going to my electric choke, as my new carb won't need that. So, is it best to have the vacuum switch control the relay (i.e. throw a ground signal for the relay earth), or should I have it wired in between the relay output feed and the pump so that it directly controls the pump?

I'm thinking if it controls the relay then the relay will be opening and closing every single time the vacuum switch wants to cycle the pump on and off and most relays seem to be rated as having a finite life of about 100k operations before they burn out. But if controlling the pump then the relay will only switch on/off with the ignition key and so should last much, much longer. But then the switch will take more of a hammering I guess? By the way, I know that some of these vacuum switches are only designed for operating relays and not acting as switched grounds for the pumps themselves, but Summit sell a replacement switch for their brand vacuum pump that I could use and that (I believe) would actually screw into the boss in my vacuum canister that is designed to accommodate a gauge. So quite a neat solution.

So, after all that rambling....what you think?!?

Mick70RR · Post by **Mick70RR** » Mon May 01, 17 12:49 pm

If this is a street car why don't you fit a camshaft that will give you enough vacuum for the brakes? It will also be a far nicer car to drive.

David Murray · Post by **David Murray** » Mon May 01, 17 1:23 pm

Mick70RR wrote:If this is a street car why don't you fit a camshaft that will give you enough vacuum for the brakes? It will also be a far nicer car to drive.

Well, the aim is street and strip and I don't think I'm going too crazy with the cam, but I still suspect the vacuum will be on the low side. My current cam is a flat tappet Lunati Voodoo...

http://www.lunatipower.com/Product.aspx?id=1477&gid=287

and I'm going to fit this Crower 'street' roller cam (along with the stroker kit)....because I have one (and lifters) already...

https://www.crower.com/camshafts/mopar- ... illet.html

If it doesn't work then I'll downsize a bit, but I want to give it a go first.

adrianp · Post by **adrianp** » Mon May 01, 17 1:28 pm

Hi David,

Don't know if you've made your mind up on the power assisted brakes but I'd ask if you really need them ? I had the same issue as you after installing the 493 stroker and again (like you) looked at Hydroboost, vacuum etc. in the end I ditched the lot and went to manual brakes. To be honest I don't miss the power assist at all - just something to think about.

If you've still got your heart set on it I'd give the Hydratech guys a call - i did when looking round as they didn't have parts for my car listed on their site and when i contacted them they were able to recommend something that'd work.

adrian.

Mick70RR · Post by **Mick70RR** » Mon May 01, 17 2:03 pm

You may well have enough vacuum with that solid roller. I use a solid flat tappet cam that has more duration than that and I have 16" vacuum at idle.

David Murray · Post by **David Murray** » Mon May 01, 17 3:27 pm

@Adrian - yes I have thought about going manual brakes, but haven't explored that option any further than just realising it is indeed an option. I'm thinking it might be more involved than running the vacuum pump though....don't you have to find a different pedal that will give the correct ratio and then about with the linkage? Regarding the hydroboost, a mate of mine got a system from Tallon for his 67 Charger (so assume would also for your 66), but they don't even mention c-bodies on the Tallon or Hydratech sites.

@Mick - you might be right.....possibly vacuum won't actually be an issue for me. Out of interest, what idle rpm are you seeing that vacuum at and is that in gear or in neutral?

Appreciate the comments/suggestions...thanks.

Mick70RR · Post by **Mick70RR** » Mon May 01, 17 7:38 pm

My car idles at about 800rpm, its a 4 speed so that's in neutral. It helps to use plenty of ignition advance at idle. I run 18 degrees and have vacuum advance connected to manifold vacuum so that bumps it up to 30 degrees.

Dave999 · Post by **Dave999** » Tue May 02, 17 10:28 am

i would use an expensive specialist switch to switch a relay

and let the relay handle the power side to the pump

relays are cheap

you could go manual brakes

if you do however be prepared to spend time drilling new holes in the brake peddle arm to get travel vs leverage sorted out.

boosted car has the master cylinder rod in the wrong place compared with where it would be on manual car because the peddle ratio needs to be exact. The limit to travel is the internal workings of the booster it needs precise travel to work properly.

manual brakes don't as much, you are acting on the master cylinder direct.

therefore moving the rod pivot up and forward a little gives more leverage + more travel = better feel
go too far and peddle gets too close to the floor

go too up or down and the rod binds.

Dave

David Murray · Post by **David Murray** » Tue May 02, 17 11:50 am

Thanks for the info Dave....the manual brake conversion sounds like a fiddly job but at least one you can do by modifying existing parts. But, I'll leave the vacuum thing until after the new engine is fitted....as Mick says, it might produce enough vacuum anyway. if not then I now have a plan A and a plan B!!

Matt · Post by **Matt** » Tue May 02, 17 6:48 pm

There's no proper way to answer this question without knowing

1) How much current does the vacuum pump draw
2) What current is the vacuum switch rated at

And the doing some maths, whilst making a few assumptions.

In any case you're probably best just using the vacuum switch to trigger the relay.

I wouldn't worry about the relay's service life. If the relay's rated at 100k operations and you trigger it as often as once a minute (unlikely), it'll probably outlast most other parts of the car....!