RPM intake and drop base aircleaners?
 

What air filter/drop base are you guys using on 65-67s GTOs? I've got a 14", 3.5" tall element with a drop base. RPM intake, 4150 carb and 1/4" spacer. The QF vent tubes are about 3/4" inch from the lid. I'm using a 1/4" phenolic spacer to insulate the carb heat. The vents are already pretty short and can't be trimmed much more.

Carb is going excessively rich at WOT, and I'm thinking its vent tube clearance.. Is there a domed lid that clears the early GTO hood?

On the factory intake with a 3/4 spacer/adapter to put a Holley on it , a generic 1.5" drop I have plenty of clearance to the hood with a 4" element. A good 2" with foil to check for clearance. I know one of my old NMCA buddies had an RPM and the factory Ram Air pan he grafted a drop base on and ran a 3" air cleaner. Pretty sure 3/4" to vents is OK.

I run this air cleaner on my '65 with a QJet. I'm going to switch to an RPM so it'll be interesting to see how much hood clearance I'll have.

https://www.pro-touringf-body.com/pr...se-air-cleaner

Chip, question for ya,

What’s the ball park hp you think your motor is making?

Likely 370-405 at 5300 RPM. It seems about as strong as my previous 67 that used to run about 103mph in the quarter. Except this car has less gear.

To me for only making that level of HP there’s no need to fight with anything more than a stock or mildly modded iron intake.

Northwind, holley carb, 66 GTO. I run a 16x4 filter from Keyser Manufacturing.

That's what I'm looking for. I found a very similar 14" design on Summit. I bought the black lid. It has a 1-1/8" dome that should give additional vent clearance and fit under the GTO scoop. I'll try it on my 1" drop base and 3.5" filter and report back.

https://allstarperformance.com/air-c...lack-all26088/

Got it in and test fit it last night.

The dome gives 1" of vent clearance above the top of the filter. That gives me 1-3/4" of effective vent clearance.

With a RPM intake, 1/4" spacer, 4150 carb, 1" drop base, 3.5" filter, I've got just over an inch between the lid and the underside of the hood.

The closest points to the hood are the front of the dome directly under the scoop, and the 3 o'clock position on the driver's side. Both of them have about 5/8-3/4" of clearance.

Those dome lids pictured are what the roundy guys always run or anyone serious about making power, and Ive had them worth a little power on the dyno as they help direct air into the carb, that's the theory anyway.
Doubt you had a vent clearance problem as 3/4" is plenty, but any gain you might see is likely just from the lid design itself. The flat lids with sharp angles suck.

Same goes for the drop bases. The flat sharp angles suck. You'll want the smooth round drop base and they come in all depths. I typically like to use the OEM drop bases as they are round and have the most drop of any. They are repopped for the various cars they were equipped on like 67-69 Z/28's, L78's, etc...
Even the 67 GTO with the optional HO 400 that came with an open element likely had the same smooth round drop base from the factory.

Just throwing out food for thought

Update.... Got the Allstar domed lid and curved bottom to fit with a 4" air filter. 1/4" open spacer below 4150 double pumper.

WOT AFR is now 12.1-12.5. With the 3.5" filter and sharp flat drop base, the AFRs were in the low 11s. All I did was swap the base and 4" filter for 3.5". So even though I already had 3/4" between vents and lid, the base shape and total filter height makes a difference in the heavy throttle AFRs.

Now the mains are slower to start. Pullover on the mains was 2200 RPM, now its gone up to 2700 or so. It's got a slight surge at 2200 RPM, AFRs are in the high 15s. I'm going to have to play with fuel level and HSABs to dial it in, but it's getting better!

I am curious if you have a Holley Stubstack you could try both ways.

Wow, the stubstack. Haven't heard that in many years. I'm sure it would have some influence, as this combo seems to be very sensitive to aircleaner/airfilter shape.

To be clear in my earlier post, the WOT AFRs were previously in the high 10s with the typical pointed type lid that gave 3/4" of vent clearance with a 3.5" filter.

With the dome only, the WOT AFRs were in the mid 11s. That's with 1.75" of vent clearance with same 3.5" filter. The dome adds about 1 inch.

With the domed lid, curved bottom and 4" filter I had 2.25" between the carb vents and the lid. Now the WOT AFRs are in the 12s. This is with the carb set up the old school way, with .027 HSAB and 2 emulsions, fuel just showing on the sight glass. This setup makes it slow to pull over, which is what I needed with the older aircleaner setup. Now with this new air filter, its too slow to start the mains (RPM at pullover is higher) and it's showing leaner on the highway.

Just going into detail, to show the minimum vent clearance rule of 3/4"has some caveats.

A couple of pictures from a similar thread recently I posted. Factory intake with 3/4" adapter for the Holley 3310, 4" air cleaner element and a shot through the hood.
got a stub stack on a shelf!