Custom airbox base


Well-known member
Mar 24, 2008
I'm going to make an airbox base to take in air from the back of the hood

Heres what the base is going to look like. I'm trying to get it cnc plasma'd but it's not that easy finding someone to make just one. And the one place that did make a quote wanted 80 dollars

I'll add some more pics on what I'm going to have to cut out later

Before you go to much trouble, have you seen the box from Allstar Performance Products that Jason/White76 modded for his C3? Think I may follow his path when I finally isolate my Dom/L88 hood setup.
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Here's the air pan I modified from Allstar. I had to notch it and change the step to clear the HEI and wiper motor. Allstar does sell an aluminum one too.

I went with their fiberglass version just because I have more experience working with fibergalss than aluminum, and I knew I was going to have to modify it. The unit was a good 1-2" taller before I fitted it to my hood so it should fit high rise hoods too.



Crap...I just ordered all the L88 stuff and I am going to "make" this fit on my 69 L46 & Qjet setup which hopefully should not be too hard. Cost was $300 for all the parts from Zip including the fresh air chamber which runs $130. I did not buy the L88 drop base as I think my current stock airbase will work fine with that big foam seal ring from the base to my hood.
I think that the allstar box is way too wide

I'm trying to use the stock hood still

My box will be 18" at the widest spot, and I'm going to have to cut out some of firewall behind the windshield wiper motor. I'll have to take the motor out also.
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I thought you had one of those nascar boxes??? Just use the bottom part, seal it up against the hood and you're almost done????
When I get to that stage, I'm going to make use of the stock 82 hood where the air comes over the radiator then goes between the hood and the liner to where the TB air filter used to be. Gonna cut a hole large enough for my current air filter to fit up into that gap then use glass or rubber attached to the hood to come down and enclose the air filter then cut a hole in the hood above the air filter and put a nice little low profile hood scoop on - I have the type of air filter lid that is also a filter. Should be good for sucking colder air :thumbs:
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I think that the allstar box is way too wide

I'm trying to use the stock hood still

My box will be 18" at the widest spot, and I'm going to have to cut out some of firewall behind the windshield wiper motor. I'll have to take the motor out also.

Yellow, I'm not sure how a 73's hood is made, but I sealed the rear of the box against the bottom of my 76's stock hood. The air intake is actually through the stock hood at the cowl area. I cut two oval shaped holes in the hood where its hard to see them when the hood is closed.


That doesn't seem like it flows enough

I was going to cut it all out

The area of the two openings is 18.875 in^2. The carburator has an area of 8.94 in^2 at the butterflies. Even if you assume a pretty bad flow coef. for the opening (say 0.5) that equates to a flow area of 9+ in^2. Therefore, it shouldn't be a significant flow restriction.
Here is my L46-88 combo with the L88 CAI installed. It is working great and I have noticed a very nice increase in power (honestly). It took a little fabrication as my long L88 hood for some reason did not match up well with the L88 airbox around the wiper door area such that I had to trim about 4" off the back of the CAI box. I don't know if what I did was wrong but I decided to glass/resin the SOB to the hood. Just mixed up some homedepot resin and glued/resin the sucker to the hood. Trust ain't going anywhere. I had to lay some fiberglass strips down by the wiper door area but nothing major and the rest fit pretty good. The true L88's but have sit a lot higher than my whimpy small block as I added both a 1" carb spacer & 1" air base to carb spacer. I am now within 1/8" of the hood. I finished this on Sunday morning without the carb spacer and there was a very noticeable improvement in acceleration at 60mph while in 6th gear...just seemed like I did not have to push the pedal as hard. I found a really sweet Qjet dual plane/divided wall 1" carb spacer that I added tonight. So tomorrow should be some fun seeing how she does with the carb spacer which I am hoping gives me some more top end since my car literally dies after 5,500rpms which from what I understand to be is the dual plane intake design giving more low end torque but not enough air on the top end. total cost excluding car spacer was at $300 from Zip which ain't too bad for about 10rwhp having now cool air into the carb...and now that I got the carb spacer I am hoping for a little extra bonus HP too.

btw..I re-used my L46 stock air base as I understand the L88 airbase would be about 1" further lower can cause problems clearing the throttle cable bracket so I don't see a need for the extra air base. I did have my old stock air base valve cover breather pipe cut and re-welded downward at a 45* angle to keep the rubber hose aligned more correctly due to the carb spacer & air base spacer.

Here are the pics...


Here is a pic of the 1" carb spacer (btw...Qjet dual plane carb spacers are not cheap!!!)

Here is a pic of how it seals to the hood. As you can see the hood is of course angled where it seals tighter in the front vs. rear but I think I got her pretty darn good & tight. Thanks to 69 fender vents I was able to get this cool shot.

the only thing I noticed is that she was a little more noisey with the hood closed but its a hot rod...not a caddy, LOL!!!

took her for a long drive today...1.5 hours. She is very happy with the new setup. The dual plane specific carb spacer for my dual plane intake definitely added some upper end HP as my engine use to die instantly starting at 5,500rpms such that it was totally pointless while drag racing to even keep her in gear past 5,500rpms. Now she pulls strong and smooth all the way too 6,000rpms. I will have to get my G-tech back from my buddy so I can see the actual difference in HP curves compared to before.

Also noticed under hood temps increased a small notch..nothing major and she still holds steady at 180* but I can hear the electric fans working a lot more with the DeWitts radiator. I already had the cowl induction holes cut in the hood and have been driving that way for a while so she has been getting cool air to the engine compartment from the cowl induction but now that it is totally sealed off from the engine all she gets is air from the front radiator.

I will post in a couple of weeks the new dyno curve hoping that my butt dyno is not playing tricks on me as she really does now pull strong till 6,000rpms.
Nice post, LUCFR...

But, I wouldn't hold my breath expecting better top-end from installing the divided spacer in place of an open one, as it's lessening the division (to a point) which typically improves a dual-plane's higher rpm performance. If pulling above 5,500 is a priority, you might well be a candidate for a single plane manifold. My $.02.

BTW, great pic with the hood down there. :beer:
thanks....yeah I do argree with ya about going with a single plane manifold but then I am getting into concerns about engine longevity since I have no real idea how healthy my engine is since I don't have the complete rebuild history. I do know has 24K on it since the rebuild and I put 16K of those miles in the last 2.5 years. I will be getting a compression check done later this year to find out more but long/short term plans are for a LS swap.

I needed another 1" of spacer to make it seal better to the hood vs. making a custom foam donut so i went the harder and more $$$ way of doing things with that dual plane carb spacer. Honestly though...she does pull a lot better past 5,500rpms, but I know butt dynos can be decieving.
Nice job LUCFR. If I ever get around to changing my hood, I probably will do something similar to yours.
Several years ago at Sebring I heard Hans Stuck make the case that a driver sits on the most sensitive instrument in his car, so if your butt dyno is sufficiently well calibrated there's little reason to doubt it. :D

BTW, do a leak down too.