Just about to change the old Rochester carbs on my 75 Formula 400 for a new set of Hollys. Apparently I'm going to need an adapter plate which will add 3/4inch meaning there may be a clearance issue with the top of the air filter and the inside of the hood. The filter insert is 2" deep and the whole air cleaner assembly is 3" at its highest point in the center. Need to know if its possible to get a shallower air cleaner or a flat air cleaner lid to reduce the overall height.
air filter clearance dilemma
- Thread starter ralmow
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what Holley are you going to install and why do you think it will perform better than the Quadrajet? The fact that you cannt bolt a square bore carburetor to a spreadbore intake without an adaptor is already going to cost you power. If you are going from a 3" tall filter to a 2" tall filter will produce an intake restriction. Other than that place some modeling clay on top of the air cleaner with the stock setup to measure your hood clearance. A Quad is 750 CFM which is more than enough for a 400.
I've got a 75 Formula 400 and recently had a total engine rebuild. It still has the original Rochester carb and was advised to replace it as its 40years old and pretty worn. I've bought a Holley 4160 750 CFM with adapter plate. I didthe putty trick and I've only got about 1/4" clearence however, my air cleaner top is an inch higher in the centre. If I can swap the lid with a flat one that'll solve it. Just having a job finding one. The whole assembly is 3" tall with a 2" element and has a drop base.
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are you having problems with the engine? It stalls, hard starting, bogs, what is the reason for changing it, they can be rebuilt. Is the 4160 using center hung float bowls or side hung the center will not work with the factory air cleaner. Any reason you didnt get a 4175 Holley which does not need an adaptor between the carburetor and intake. The 4175 is a spreadbore pattern with vacuum secondaries. Did the original air cleaner use a 3" element? You need a minimum of 3/8" above the float bowl vent tubes for the Holley to work so a lid to low will cause other issues.
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I'm not sure about the original air filter, I imported the car from the states and it had the 2" filter on. The choke was iffy on the carb and was running rich, also hated starting from hot which made calling at petrol stations a bit of a gamble. The 4175 was over £200 more which is why I opted for the 4160 with an adapter.
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I assume by the reference to £200 you are somewhere in the United Kingdom. Did you actually take any measurements with the existing Quad by using clay on the front and rear edges of the air cleaner. Is the air cleaner base a drop down base, a picture would help. The stock set up used a 3" air filter element. This link shows the original factory system a closed air cleaner with a single snorkel.
http://www.gentrylane.com/s/domestic-inventory/sold-domestic/1975-pontiac-firebird-formula-400
You can try sites such as www.summitracing.com and www.jegs.com for aftermarket air cleaners
http://www.gentrylane.com/s/domestic-inventory/sold-domestic/1975-pontiac-firebird-formula-400
You can try sites such as www.summitracing.com and www.jegs.com for aftermarket air cleaners
Thats great, really appreciate the advice. I've just checked again and I don't have a drop base so I think I'll go for that which will solve the hood clearance issue. If there isn't enough clearance above the carb jets I've been told to then replace the top with one that has a filter element built in