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Using vacuum leak for performance 

 

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 Post subject: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 9:59 am 
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Hi, just a quick question. Can I use a vacuum leak as an extra way of getting more air into the engine.
I know it'll prob be rough at idle, but at revs won't it just suck extra air in, therefore gaining power as long as there's enough fuel goin in also?
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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 10:10 am 
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:roll: ummm no,
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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 12:29 pm 
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The throttle blade is the biggest vac leak you can get.

A small pipe disconnected is going to do SFA at WOT.
It will only stuff up your idle and increase your cruising fuel usage.

 

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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 3:27 pm 
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I've heard road race guys angle machine half the inlet port manifold, so that only half the manifold face seals against the head, therefore leaving a gap at the bottom of the manifold.
Apparently at high revs on carby engines, the engine sucks extra air through this gap, basically doubling the amount of air goin into the engine.
My question is, would this work successfully?
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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 5:36 pm 
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This also bypasses your air cleaner/filter and lets crap into your combustion chamber. I wouldn't do it to a daily driver myself as unlike a race engine I wouldn't want to pull it down every few months to recondition it. This practice also leans out the air/fuel mixture which comes with it's own problems
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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 5:52 pm 
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not to mention if you introduce a vac leak to each port in the manifold, it would probably idle around 2500rpm...lol
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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 6:06 pm 
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Does your motor have under 100000km?
I think the vegetable might be back.

 

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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 6:21 pm 
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Matt_jew wrote:
Does your motor have under 100000km?
I think the vegetable might be back.




does sounderr...... suss :shock:
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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 7:07 pm 
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It is for a race motor, and each inlet runner has a ball valve and little air filter on each one, would it run fine at high revs if tuned like that?
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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 8:00 pm 
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dave7040 wrote:
I've heard road race guys angle machine half the inlet port manifold, so that only half the manifold face seals against the head, therefore leaving a gap at the bottom of the manifold.
Apparently at high revs on carby engines, the engine sucks extra air through this gap, basically doubling the amount of air goin into the engine.
My question is, would this work successfully?



dave7040 wrote:
It is for a race motor, and each inlet runner has a ball valve and little air filter on each one, would it run fine at high revs if tuned like that?


hang on....
which is it??
did you forget what you posted first?

why bother when you can use larger throttle body?
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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 8:55 pm 
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That's the thing, can't use a bigger throttle body, it's actually a carby engine, and our rules don't allow us to go any bigger.
So I've heard of guys angle machining the inlet manifold face, but I'd prefer to weld in breathers on each inlet runner with air filters
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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 9:00 pm 
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So, your looking for an idle speed of about 2,500 rpm then ?????

 

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 Post subject: Re: Using vacuum leak for performance
Posted: Wed Dec 17, 2014 10:05 pm 
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What class of speedway are you in and what engine? Crossflow??
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 Post subject: Re: Using vacuum leak for performance
Posted: Thu Dec 18, 2014 8:53 am 
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Its an EA 4ltr with a 350 Holley
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 Post subject: Re: Using vacuum leak for performance
Posted: Thu Dec 18, 2014 12:39 pm 
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productions or modifieds? the biggest thing I found during flow testing for speedway engines with there restrictions is to make sure there is no restriction before the carb itself, you need at least a 14in x 3in filter to give enough surface area of filter, you need the 3in minimum height as to not choke to top air entry into the carb.
I will assume that your running the cpi manifold with a holley adaptor, porting the radius into the runners from the plenum area gives good gains, the manifold itself is not a bad unit. DO NOT have air pulled in from the runnuers after the carb, it will lead to lean outs and dead engines, you can sneak a bit of unmetered air in through the pcv fitting from the base of the carb, just plumb this into the "clean" side of the aircleaner, being from a central point you can tune the carb for this extra air.

Removing the choke horn from the carb showed no airflow improvement on the flow bench, using the 500 base blended properly into the 350 body is the go, also slimming down the boosters gain flow. Machining the throttle shafts is another good gain in flow through the carb, making a carb top to smooth the entry of air into the carb is a worthwhile exercise as well.
Here are some pics of a webber carb with some of the mods which can be done to the holley.

Holley 500 base on 350 body

Image

Doing the choke removal did nothing for flow

Image

Webber carb top and machining throttle shafts

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Image
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