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Augie,
You might not need the tall dual plane with EFI. I would not worry much about low RPM torque impact. This is a real small plenum single plane. Big inch SBC has plenty of low speed torque. You probably wont notice any real difference there. EFI will fix that. Healthy top end improvement. I really like your cold air intake and would do whatever you need to keep it. A Weiand IMCA single plane would be worth considering. You can get the hood closed and make great power. Check out the paragraph 2d in the EFI Sticky in the Tech section.
Popular IMCA racing single plane. Easy to find and very low profile.
This Weiand Team G Street Ram intake below would also be good but they are very hard to find. It's a baby single plane that works great for a hot street car. Weiand Team G 7525 EGR style intake. Long out of production. 2000 - 6000 RPM range. Very low height to fit under the hood of any C3. If I did not have it, I would just buy the IMCA intake above.
i have been looking at low rise single planes myself. I have already ported the polished air gap clone I have so I will wait tillI get the 406 swapped in and remove the 1" spacer im running and modify the CAI baseplate and see what I have to work with.but yeah I'm on the same page that I want to keep the CAI it works very well.
I love the data from the 19 dyno tests. I analyzed it to make it relevant to our C3s.
I measured the hood clearance on a 100% stock 70 LS5 with a GM open element air cleaner, it had 1.25" clearance, using clay.
Here are the dyno results, pro-rated down from a huge 750 HP engine, down to a more normal 400 HP engine.
And looking only at A-B comparisons, with only 1 variable, I came up with these conclusions:
Just like Lars said, anything you can do to raise the base, or the lid of the air cleaner, helps HP.
Air likes to go straight!
There is not a whole lot you can do to improve on a GM drop base, and still get it under the hood!
I spent 35 years analyzing results like this, I found it fun!
I found and measured an original 2" drop base air cleaner today.
Believed to be an original GM. The drop from carb flange to base was 2.0"
Stock 3" filter measures 3-1/4" high.
From the carb base to the top of the lid measures 2.0" Maybe 2-1/8".
Lars says you need 3.0" at this location, above the carb, not 2.0", for max power.
Engine Masters says this style costs 12 HP. Over none. And if you do not have a 752 HP engine, it is likely less.
But it might be as big as you can get and still fit under the hood!
But on some cars maybe a 3.5" high air filter would fit?
And that would raise the lid some more!
A 4" high filter on Engine Masters cost almost no HP. Maybe 1.5 HP. So raising the lid really works.
"raising the lid really works".....4" tall filter vs 3" is what makes the power come back completely. Again it's worth saying again. The drop base isn't the lion's share of the problem on a 750HP Big Block's air flow needs. It's really all about the extra area you get from a an extra inch of filter height on that 4" tall filter vs the 3" that fits so much better.
The 750HP engine in the test lost 12hp when running a 3x14" drop base and it also lost 10HP when running the flat base with the same 3x14" filter.
500HP engine or less and I'm betting there is no difference to be found in the drop base vs standard flat base.
You want to be absolutely sure you're giving up NOTHING in the quest for reduced engine wear vs sucking up every bug and bit of dust and dirt through an open intake with no filter?
You need that minimum 4x14" filter. Yet, if fitting a 3x14" under the hood without a drop base is already really tough it won't get any easier with a 4" tall element. A drop base is probably going to be part of the best way to do it even with a taller hood. L88 hoods are cool but to my they are the limit in height before tipping over the edge into pretty horrible looking with something taller still. For the 1-2HP loss with the drop base vs a flat base can't imagine messing up the appearance of a good looking car.
'With you' on all said......especially the appearance of some extra height L88 style hoods.......but that 12hp loss recorded on na 750hp engine most likely not even be noticed by the driver 'out on the street', especially considering the even larger potential losses we can experience from differing external ambient temperatures, humidity levels and perhaps the occasional 'poor' tank of gas
You got the basics right Walter, but not the numbers
A 752 HP engine lost huge HP with almost every air cleaner, except for none or a velocity stack.
They used it's 1100 CFM of airflow to really show big numbers.
But that is not real world.
Who in their right mind would put a single snorkel air filter on that engine?
Very few of us would even put a 752 HP engine in a C3, and it won't fit under the hood without severely choking it off anyway.
Those that do (Rothamn et al) run a huge 4" extended height L88 hood, that looks almost like a pro-stock setup. And they still have height troubles.
My race buddies 800-1000HP BBCs run dyno optimized 8-9" tall intakes! 5 inches taller than mine.
Those of us with more moderate engines, and desiring to keep stock hoods, are much more height limited.
400 to 500 HP is really pushing that "space" envelope.
On a 400 HP engine, and it's much more moderate airflow needs of around 700 cfm, the stock 2.0" drop open element air cleaner still costs 12 HP over nothing at all.
But it fits and it looks great!
Going with a 1.5" lesser drop base adds the most HP possible back to that system, it gives back 5 of those HP, and likely still fits under the hood. With the same 3.0" high paper element. It also raises the lid, which helps straighten the air flow. And gives more room over the choke horn. There are only 7 HP left to be recovered. It should have 3/4" clearance. This may be the strongest combination we can fit under our hood!
Adding a K&N filter really does nothing of consequence, they proved in back to back dyno tests, it only added 1 HP back. And according to other independent filtration test, it filters much worse, maybe by 30% or so. I do not like that trade-off. K&Ns marketing results are much stronger, but our 14x3 air cleaner system is pretty good to start with, so airflow gains are tough to come by.
Going to a 4.0" tall filter, on the same base, also did not help as much as I expected. it only gained 3 more HP back. Yes it raised the lid, but without raising the base also and consequently having two changes at once, and straightening the air flow out some more, it had a minimal impact. It is a huge 33% increase in surface area. Yet it shows less than a 1% increase in HP.!?!? This is why we run dyno tests. Common sense failed us here. That lack of response means our 14x3.0" high filter is big enough. BTW that will not fit under a C3 hood anyway now with the 1.5" drop base, and the taller base is worth more anyway, like 5 HP. The 4.0" looks cool, and satisfies common sense, but the dyno proved that a "taller" drop base was worth more. It would look good, and it may fit under a stock hood, with a 2.0" drop base, but hood clearance will be real tight. Like 1/4" or less. I know of one guy who even dropped his motor mounts 1/4" -3/8" to make this combo work.
Also contrary to popular belief, the dyno showed adding a K&N top filter to a system that already has a round filter, actually costs HP. Only 1 HP, but it does not help. When the side filters were tapped off, and only the top filter was used, that system did have strong results. It added 10 of the 12 lost HP back. What this proved was that straightening out the airflow path is very important to HP, and much more important than other factors like air cleaner height, or filter media.
The one extra thing that can help even more is moving the base up to a flat base, which actually has a 1/4" rise lip. That does add another 5 HP over the 1.5 drop, or 10 HP over the 2.0 drop. You are now within 2 HP of no air cleaner. But it will not fit under a C3 hood, with a 3" air cleaner, by a lot.
One combination that they did not test, would the flat base with a much shorter air filter, which may fit under the hood if you go with a 2.0" high filter. The lid would only be 1/2" to 1.0" higher than the stock drop base. I would love to see a dyno test on this combo. It may fit under the hood. The advantage of the higher flat base towards straightening the airflow may just help more than the disadvantage of less filter area. But with out a dyno test, or a A-B 1 variable comparison, we will never know. I guess I'll just stick with a 1.5" or 2.0" drop base.
Another test they did not make was a 3.5" high filter on a 2.0" drop base. It should fit, there should be enough room. Common sense would say the extra filter area should help, and raising the lid over the choke shorn should help. But based on the 3.0 vs 4.0" comparison, and only a 3 HP gain there, I would not expect very much help. Maybe 1 to 2 HP.
I am certainly not changing my BB hood, not for 12 HP!!!
Last edited by leigh1322; Feb 11, 2025 at 11:12 AM.
The one thing I don't understand about the really tall L88 hoods, is that you don't need the extra height in close to the windshield at all. You only need it farther out. I don't have a good picture of my hood, but we just cut it and lifted the front to the height we needed it and left the windshield side alone as much as possible. Still taller than the stock L88, but without the visibility issues of the height at the windshield. I deemed it necessary after I found what a difference it would make getting that 4 inch air cleaner under there. My 502 is making 653hp so didn't want to choke it at all. Also cannot underestimate the effect the cold air induction has on power, keeping that hot under hood air out of the intake.
I could not agree with you more Bill!
Your monster of an engine needs all that airflow, and so does AJRothman's and OldCarBum's.
But some of us have more "modest" desires. LOL
I built my BB to a given goal of a stock look & hood. And wound up with "only" 500HP.
I optimized it as much as possible, under that "stock look" limit.
I know I left HP on the table, but the BB hood, low rise intake, 074 Winters heads, and stock 454 crank, are all restricting more HP.
Based on these dyno tests, a 14x3 air cleaner is the least of my problems.
A 496 stroker crank would have been the "easiest" change, and I did not even do that, wanting to be true to the original 454 size.
I'm very happy with it.
Last edited by leigh1322; Feb 11, 2025 at 01:47 PM.
FYI - In near future I will be listing some original dropped bases that have vent tubes cut off , bent etc if some one wants to build a custom cold air induction system utilizing an OEM dropped base these might be perfect
If I remember correctly, the one oem style air filter that scored high on the Engine Masters dyno test was the Chrysler dual snorkel with the lid turned upside down (this was really popular to do back in the 60’s and 70’s and we all did it).
I wonder how it would have performed with the K&N filter lid since the K&N lid did so well with the open element sides taped off?
I run the shortest team g intake they make and still make and they work very well my 434 with a short intake makes over 650hp and fits under stock hood on my 69 vette .
Some images with drop base filters on my standard deck big block with Performer RPM Air Gap - also underhood clearance with 1" drop base and 2" filter + K & N top.
I looked for a long time for an aftermarket 2” dropped base like yours that would fit with my carb.
Where did you find that 2” aftermarket drop base?
Does it measure a true 2”?
I looked for a long time for an aftermarket 2” dropped base like yours that would fit with my carb.
Where did you find that 2” aftermarket drop base?
Does it measure a true 2”?
2" drop from where filter sits on base to mounting point where base sits on carb.........sorry, I don't know what make it is. Something I bought years ago when experimenting with various carb/filter/base combinations. If I recall correctly it was sourced from one of the retailers selling what were perhaps 'no-name shiny' chrome accessories?
Thanks!
My 1.5” drop base is designed like your 2” and clears everything on my carb.
I’ve looked but can’t find a 2” of that design.
I tried an aftermarket L88 2” but it interfered with the double pumper carb.
I’ll try a couple different ones when my hood is permanently installed so I can try and squeeze a 4” filter element on which would give me an additional 1/2” between the base and the lid.
Chris - I'm using the 1" drop base with the 2" filter. Its the only drop base that fits over the carb/cable bracketing. Its interesting reading the comments about 'taping-up' the filter and just using the K & N 'top' filter. Kinda brings it all home (when you put the 2" filter element on the GM 3" drop base) what a compromise the OEM drop base could be if not applied carefully. I checked the underhood clearance - about 1" at front - perhaps 2" at rear.
I had planned to fabricate a central base to the underside of the hood with a circular cut-out over the carb with a rubber seal and encapsulate the inlet tract from the carb inlet to rear hood opening. Similar to the L88 arrangement and ultilising cooler air from the pressure point at the base of the front screen.
It would be interesting to understand how high that positive pressure might be at the base of the screen?
FYI - I have listed a GM drop base that has been altered in the C3 parts section.
If someone needs a base to modify or alter for their custom air induction / filter system this could be a good candidate
