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Braking Improvements (vacuum upgrade)


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 its the mechanical design of the engine that determines the vacuum - However,  the lower state of mechanical tune of a turbo engine will develop less vacuum.   2 identical engines, with or without forced induction would create identical vacuum under normal braking conditions.  

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Jon - yes exactly, I also think compression ratio will play a part?

Anyway the results are in...

 9 inches at idle and around 16 inches at overrun from a modest 3000rpm blip   (Presumably this would produce another several inches on road conditions, where the engine would continue to be 'driven' by the wheels, rather than just having inertia helping in the garage.

vac1.jpg

vac2.jpg

Edited by 910Esprit
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2 minutes ago, 910Esprit said:

Jon - yes exactly, I also think compression ratio will play a part?

Anyway the results are in...

 9 inches at idle and around 16 inches at overrun from around 3000rpm blip   (Presumably this would produce another several inches on road conditions, where the engine would continue to be 'driven' by the wheels, rather than just having inertia helping in the garage.

OK, thanks for that all we need now is someone to do the same on a NA engine & see what vavuum that produces. :getmecoat:

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Cheers,

John W

http://jonwatkins.co.uk

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2 hours ago, 910Esprit said:

Jon - yes exactly, I also think compression ratio will play a part?

Anyway the results are in...

 9 inches at idle and around 16 inches at overrun from a modest 3000rpm blip   (Presumably this would produce another several inches on road conditions, where the engine would continue to be 'driven' by the wheels, rather than just having inertia helping in the garage.

vac1.jpg

vac2.jpg

Only 9"?

Pfffft.

  • Haha 1

Margate Exotics.

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My turbo sees 20-30 kpPa (~22"hg) on overrun and idles about 50kpa (~15"hg), but I'm running a single throttle body on a common plenum. I have a check valve going to a VW vacuum reservoir (~1.5 l), and have deleted my mechanical pump. Works OK for me with Hi-spec 4 pots up front, but I may add an electric vac pump & switch someday to see if it adds some assist in some situations. Or maybe a larger vac reservoir-

This with stock cams, head,  HC pistons, green dots, Alunox exhaust

 

IdleMAP1.png

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I have worked through quite a number of variations of tune/set-up on the 907 and 912 versions in a very well regarded engine sim application and 9" of vacuum is not at all consistent with any sort of tune, including semi-race AFAIK. Even with an SE tuned N/A motor I'd expect something like what Thomas has measured and so would advise anyone finding theirs to be significantly less to begin a search for vacuum leaks, because there most surely will be. Any of the pre-SE Turbo's will be running 107 cams at 104MOP if as stock, and this is quite a mild spec of valve timing.

As for brake upgrades, given that ALL is in order with the stock kit and still found wanting, I'd go for largest rotor that fits the front in a suitably well developed kit. Select the best pads for intended usage, braided s/s flex lines throughout, high spec NON-silicone fluid and go from there. Next phase if needed would be upsizing the rear rotors and, in turn, looking at the master and slave cylinder sizes for further refinement. Quite possibly a proportioning valve for the rear circuit at some point. Much is possible in braking tweaks and I'd dispense with the vacuum boost altogether in pursuit of the firmest pedal with minimal travel. Makes for best heel/toe function, greatest confidence diving into the bends Ricciardo style. Bear in mind that no set-up will deliver the goods to satisfaction if any relevant factors are amiss - eg: excessive pad knock-back due to loose wheel bearings or related excess deflections in calipers or axles.

The Lotus chassis is a sublime instrument, the finest aspects of which depend upon astute maintenance.

Cheers

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1 hour ago, drdoom said:

907 and 912 versions in a very well regarded engine sim application and 9" of vacuum is not at all consistent with any sort of tune, including semi-race AFAIK. Even with an SE tuned N/A moto

Yeah but this is a 910....   and its in rude hea;th.

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Prior to finalising my brake conversion, I measured the available vacuum without the pump. The engine generated enough to provide a good amount of servo assistance (approximately 23” hg from a high rpm overrun). This is likely to be when the assistance is required - off the throttle, and on the brake pedal. What it didn’t provide however, was a consistent, or reliable, level of vacuum in all cases. When pressing the brake pedal more than once or twice when driving slowly around town, inlet manifold vacuum was destroyed. At that point, it became very much more difficult to stop the car.

The conversion instructions clearly detail the connection of the servo vacuum system to the inlet manifold as well as the electric pump, and also the fittings necessary to do so. That’s how I designed it.

Margate Exotics.

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Yes, that can be the case, however not in this one.

The engine needs to build vacuum in order to provide assistance, and it doesn't produce as much vacuum when you're on the throttle, at low rpm, or even part throttle (not so bad this one), as it does on the overrun.  A couple of brake pedal pushes one after the other, and assistance falls away, unless the engine can build vacuum, and of course any turbo boost at all means no engine vacuum being produced.

Whenever vacuum is being produced, if it overcomes the differential in the servo and pipelines, the inlet manifold check valve will open to allow more vacuum in, and close when inlet manifold vacuum pressure is positive, effectively 'storing' vacuum for use. An accumulator increases this vacuum store, of course.

The point is, that it's enough to provide a reasonable level of braking asistance should pump failure occur.

Margate Exotics.

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I spoke to PNM some time ago about disappointing performance of their brake kit and they said there really isnt enough vacuum on the LC turbo engine. HC is better.

I have bought an electric pump from a VW Toureg and vac switch but not fitted it yet.

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I'd be interested to see what the vacuum readings are in that case. What diameter is the servo, do you know?

 

And for nerdy types like me, this is a very good read on the subject - http://what-when-how.com/automobile/engine-vacuum-test-automobile/

 

Margate Exotics.

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So just reading @Chillidoggy's post and I just had a penny drop! So my brakes had always been pretty good until I took her in for an MOT last week when I nearly went over a roundabout at 70....

 

Before the MOT I had put back in my standard idle jets and when I got home with the MOT I noticed this but didn't think much about it other than it would need to be re-fitted.

IMG_20180820_191149043.jpg

 

IMG_20180820_191118078.jpg

I'm guessing this would mean losing vacuum meaning no brakey!!!

 

Cheers all

 

Buddsy

 

 

 

"Belief is the enemy of knowing" - Crrow777

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On 20/08/2018 at 22:27, Buddsy said:

oh yes thats pressure isnt it not vacuum.

 

buddsy

Your vacuum is pulled through 2 small pipes connected on the intake manifold Buddsy. 

That connection is related to turbo boost. I think it’s to cut fuel if overboosting. 

Strange... just looked through the manual and the Dellorto bible and no information on this connection. There is a tube that connects to the fuel pressure regulator also therefor I would guess it’s cutting fuel. Anyone?

image.jpg

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