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more free help requested ( engines this time)


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#1 mariner

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Posted 23 January 2012 - 20:35

I am again hoping to get some free expert advice for my "toy car" om the experts here- at least I am open about it!

My car has 477 cu. ins Chevy V-8 running a 850cfm Holley carb., max revs are 6,200.

When I originally built it I made a simple open front air scoop for the carb. air intake. This OK but there is no space for a proper air filter and I have to have gauze over the Holley carb to guard against stones. So I am now making a longer air intake which goes forward to the area under the bonnet/hood which is the plenum chamber feeding the radiators.

A K and N filter sized for 625bhp will fit under the bonnet in the plenum space and this has a curved 150mm duct feeding the air up into the new, longer , air duct to the carb.

the cross section of the duct is 10cm by 23 cm and it is about 75 cm long for airflow purposes.

My question is simple but as I am about to start the glass fibre moulding process rather important to me!

Is there enough cross section in the ducts to allow the engine to suck in enough air without restriction please.

The main air intake to the plenum is 72cm by 23 cm so that should not be restriction.

Any advice gratefully received as i don't want to spend hours mould making the wrong size!

thank you

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#2 Engineguy

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Posted 23 January 2012 - 21:21

... 850cfm Holley carb...
... the cross section of the duct is 10cm by 23 cm...


Four x 1.75" throttle bores = 9.6 squares inches
10cm x 23cm duct= 36 square inches... = 3.7 times the throttle bore area

The length is not excessive... should be fine.

#3 NeilR

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Posted 23 January 2012 - 21:50

no need to do a mould. Do a foam plug, wrap it in glass and epoxy and dissolve the foam and test it. If it works then do a pretty one.

#4 Anthem

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Posted 23 January 2012 - 23:43

I have used Kinser filters (off-set air cleaner housings also) on and off road when using DP holley type carbs. They are more compact then the norm of paper base units. Check your air temps when feeding from the engine cooling ducting/plenum at low speeds (starts, yellow flags, traffic...).

Edited by Anthem, 23 January 2012 - 23:47.


#5 Lee Nicolle

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Posted 25 January 2012 - 07:50

I am again hoping to get some free expert advice for my "toy car" om the experts here- at least I am open about it!

My car has 477 cu. ins Chevy V-8 running a 850cfm Holley carb., max revs are 6,200.

When I originally built it I made a simple open front air scoop for the carb. air intake. This OK but there is no space for a proper air filter and I have to have gauze over the Holley carb to guard against stones. So I am now making a longer air intake which goes forward to the area under the bonnet/hood which is the plenum chamber feeding the radiators.

A K and N filter sized for 625bhp will fit under the bonnet in the plenum space and this has a curved 150mm duct feeding the air up into the new, longer , air duct to the carb.

the cross section of the duct is 10cm by 23 cm and it is about 75 cm long for airflow purposes.

My question is simple but as I am about to start the glass fibre moulding process rather important to me!

Is there enough cross section in the ducts to allow the engine to suck in enough air without restriction please.

The main air intake to the plenum is 72cm by 23 cm so that should not be restriction.

Any advice gratefully received as i don't want to spend hours mould making the wrong size!

thank you

And use a smaller carb, it will drive and run smoother and still rev to 6200. Depending on the application a good modern 750 vacc or a modern double pump of which there is several types around. They are all more driveable and economical than the old 4779 Holley. In fact almost as good as efi and a lot easier to maintain.But unles you drive it hard all the time the vacc will be better.
With your intake/filter try and get the intake about a 100mm forward of the radiator and try and insulate the tray from the radiator. It does seem to make a little difference to intake temp.