afr change from flat base to drop base filter

[mbrooks]

changed from a flat based to drop base air filter and the cruise afr went from 14.5 to the neighborhood of 16-1. didn't know if the straight pipes changed this or something happened at the track so i put the flat base back on and it dropped back to previous readings. air filter is 14x4 and didn't think the top was too close to the carb inlet although the air flow must have been disrupted somewhat i'm thinking. the wot afr didn't change much from what i could tell.

here's a pic of the flat base, the drop base prob dropped around 1.5" or 2. wonder how many people end up trying to tune this out.

[Rick1999]

changed from a flat based to drop base air filter and the cruise afr went from 14.5 to the neighborhood of 16-1. didn't know if the straight pipes changed this or something happened at the track so i put the flat base back on and it dropped back to previous readings. air filter is 14x4 and didn't think the top was too close to the carb inlet although the air flow must have been disrupted somewhat i'm thinking. the wot afr didn't change much from what i could tell.

here's a pic of the flat base, the drop base prob dropped around 1.5" or 2. wonder how many people end up trying to tune this out.

You probably should post this in engine tech, but different air filters do different things, tune for what you have, if you have more than one option, use whatever works best.

BTW, most customers I deal with would rather poke a hole through their head than through their hood of a Cobra.

[mbrooks]

i use this for the track, i have a smaller more appropriate you would say scoop that goes over the hole but can only get a 2 x 12 filter on then. i've read lots better info on the tech portion of the forum, didn't think this is worthy.

one guy suggested since the drop base leaned the afr at cruise that more air would be getting into the engine, i thought it was disruption of the airflow, which would be correct assumption?

[Doug Schriefer]

Depending on how much the drop base actually drops it may have actually decreased your air flow through the filter. If you look at this picture I’ve attached you can see the red line straight up and down to show where a standard filter would be. This shows how restricted the bottom of the filter is. Using a drop base helps with air flow, but to make it work optimally you need to either use a taller element or a canted element like show in the picture (larger at the top than at the base).

[/img]

[mbrooks]

thanks doug

that is exactly what my top and bottom look like, without the flared filter, and my top is more rounded. i thought the part where the stud exits the top at the flat portion would mess up the air flow, but if i understand you correctly the drop base helps the flow, just have to use a taller and/or flared filter.

i might try the drop base and use a taller filter and see what happens or give you a ring and check into the flared filter, and i guess top to go with it. geez, i hate to mess with the carb again.

thanks again.

mark

[MadBill]

In my experience, the worst flowing drop bases all resemble the cutaway above: Good looking spun pieces that effectively block much of the filter and cause a 4" tall element to flow like a 2" one! (and as far as using a taller filter to compensate, why use a drop base at all, if there's room for one?)

The only Holley drop bases that are worth a bean are stamped, not spun, and are moulded to the shape of the bowls but scalloped away for the remaining ~270° of circumference to provide a smooth flow path and full access for the element. The GM part for the old L-88 big block Corvette is as good as any I've tested.

Speaking of same, I'm constantly amazed at how many guys tune their engines (street or race, to a fare-thee-well on the dyno, but never make even a single pull with the aircleaner/scoop they are actually going to run!

[Doug Schriefer]

Speaking of same, I'm constantly amazed at how many guys tune their engines (street or race, to a fare-thee-well on the dyno, but never make even a single pull with the aircleaner/scoop they are actually going to run!

I can understand that and agree 100% with running air cleaners on the dyno, but running a hood scoop is pretty much worthless on most dynos. Most scoops have negative pressure for the first 60’ or so, and then build pressure as you go down track. Not many dyno cells in the country can duplicate that.

[MadBill]

Re scoops, I was thinking more of catching basic design issues early, but to your point, ultimately nothing replaces the short black (or long white) dyno...

[Doug Schriefer]

Re scoops, I was thinking more of catching basic design issues early, but to your point, ultimately nothing replaces the short black (or long white) dyno...

Exactly....

We don't race Dyno's or Flow Benches!!!

[stealth]

changed from a flat based to drop base air filter and the cruise afr went from 14.5 to the neighborhood of 16-1.

If your a/f ratio went from 14.5 to 16 your pulling more air now then previously correct? Which means less fuel is being pulled through the boosters or some combination of the two.

To my way of thinking if you put a restriction in place (ie filter) the system would pull MORE from the booster or fuel side as this has become less restrictive in relation to the added air restriction. You should go richer if there is an air restriction not? (think choke system...)

The filer base may very well help guide the airflow into the carb better, increasing flow (at cruise) Now at WOT, there may be a different set of air fuel ratios to contend with, as this is where you'll see the restriction actually show up if in fact the filter base is causing the usable area of the filter to be blocked.