Throttle bodies and brake servo.

[steve-V8s]

What is the normal way of generating a vacuum source for the brake servo when replacing a plenum with throttle bodies ? Is a feed off one of the headers normally adequate or is it more usual to combine a feed from every one of them (eight in my case) in some sort of manifold.

[fatjon]

My V8 cerbera takes it off 1 throttle body, works fine.

[steve-V8s]

Thanks for that.
Roughly what bore pipe does it use and is there a collector tank, or does it go directly to the servo ? currently I have 4mm pipe linking each header and feeding to a collector. The pipe to the servo from the collector is more like 8 or 10mm.

[rev-erend]

I have tapped 4 of the 8 TB's on my Rv8. and feed via 10mm pipe to a billet collector then single pipe
(10MM) to a non return valve then on to the servo.

Valve like this ..

http://www.ebay.co.uk/itm/METAL-ONE-WAY-NON-RETURN...

[stevieturbo]

Linking them all would be best. But 4 would likely do too. Put a non return valve on the line so the servo can retain vacuum.

[sparkybean]

Wouldn't taking a vacuum off of only one throttle body change how that cylinder performs? Id take it off all 8 and into a 'collector' box with a one way valve. Should do the job

[fatjon]

steve-V8s said:

Thanks for that.
Roughly what bore pipe does it use and is there a collector tank, or does it go directly to the servo ? currently I have 4mm pipe linking each header and feeding to a collector. The pipe to the servo from the collector is more like 8 or 10mm.

about 10mm, no collector.

[steve-V8s]

sparkybean said:

Wouldn't taking a vacuum off of only one throttle body change how that cylinder performs? Id take it off all 8 and into a 'collector' box with a one way valve. Should do the job

Yes that was my thought as well but once it has filled the pipe with suck it should remain at the same pressure as the header provided there is no leak in the system so should be neutral.

You would think that linking all the hearers together where they are doing different things at different times would be more of a problem. When one is on the induction cycle the one next door won’t be so it seems to me they are bound to breathe between headers rather than produce a nice vacuum where they are combined. There must be some transfer of pressure between them unless there is a separate non return valve for each take off.

10mm seems to be the norm for the servo feed but it feels like a very big hole to drill in my nicely ported manifold.

[sparkybean]

Just posting to see what route you eventually decided on taking. I can see what you mean about linking all of the throttle bodies together, but am not entirely convinced about taking a vacuum from one throttle body. If that cylinder has to suck air from the brake booster, doesnt that mean it has less 'suck; for air past the throttle butterfly? Very little difference i guess.

[Mr2Mike]

sparkybean said:

Just posting to see what route you eventually decided on taking. I can see what you mean about linking all of the throttle bodies together, but am not entirely convinced about taking a vacuum from one throttle body. If that cylinder has to suck air from the brake booster, doesnt that mean it has less 'suck; for air past the throttle butterfly? Very little difference i guess.

It doesn't draw air from the servo all the time, only when the vacuum at the throttle body is higher than in the servo reservoir, e.g. when the start the car after it's been left standing for a while and after you apply the brakes. Even then it's only a small amount of air.

[plasticman]

If you take the vaccuum from four throtle bidies then you will need a one way valve for each or it will mess up your idle.