I finally got to test my home-made cowl induction set up last night and was pretty disappointed. It slowed the car from 11.96 to 12.22. MPH dropped from 112 all the way to 108. I did back to back to back passes and am 100% certain the cowl piece is causing the problem.

The engine really fell on its face when I shifted from 2nd to 3rd. It actually bogged when I shifted then picked back up --- but the engine really struggled through 3rd gear.

The carb (Speed Demon 750) was jetted a hair on the fat side just in case the extra cold air leaned out my mix. Plug color was pretty close to ideal, just a little rich IMO.

My setup is real simple. I used a Quick Fuel aluminum air bell as the base and cut down a 16" diameter aluminum cake pan then added a rubber gasket to seal it to the cowl opening. No air filter yet because my current K&N is too tall.

Any ideas what could be wrong?

Anyone with experience using a RAM AIR system on a carburated first gen? I've seen quite a few prefabbed systems for imports, but nothing recently for a 1st gen.

thanks

I think we need a little more info....was that an angel food cake pan or a bundt cake pan? Angel food is so light and airy it most like likely leaned out your mixture. Bundt, on the other hand, is notably heavy and probably made you run too rich.

graemlins/waving.gif

Eric - just an educated guess, even though you ran rich to compensate for the colder denser air I think your mixture may have been more ideal without the cold air induction. My other thought is your setup may have created a disruptive flow to the throttle bores. You may recall I tried a K&N filter lid on my 14x3 filter and could get my car to rev past 4000 rpm. Put the chrome lid back on and was able to pull 6000 rpm and it was still pulling strong...

I was gonna ask if your wife knows that yer cuttin' up her good baking stuff, but BPOS beat me to it.

Eric, can you take pictures of the setup so we get a better idea how it all comes together? I'm wondering how airflow would be affected with the carb adjustment you made and no air filter in the cake pan.
Dennis, would putting a filter in his new setup change the flow charactoristics positively or even more negatively, if that is the case? Could it be the hood is pressing down too much on the seal/induction setup and affecting the carb that way?

Originally posted by BPOS:
I think we need a little more info....was that an angel food cake pan or a bundt cake pan? Angel food is so light and airy it most like likely leaned out your mixture. Bundt, on the other hand, is notably heavy and probably made you run too rich.

graemlins/waving.gif It was a FRUITCAKE, FRUITCAKE :eek:

It is amazing how much a cowl induction system will change your fuel mixture. I have to run 72's in the primary side of my 3310 Holley in the winter time and 70's in the summer time. Before I installed my own cowl induction system, I ran 70's all year round and that was a little on the fat side. Right after I first installed my cowl induction, I noticed that my engine ran a little hotter and actually didn't pull quite as well up top. That was on a 65 degree afternoon. The next day was quite a bit hotter and the problem wasn't as pronounced as it was the previous day. Cold air can make more of a change in fuel mixture than you might have thought. I believe that CarlC had a similar experience when he added his homemade cowl induction. It might be worth your time to go up a jet size or two and take another run down the track just to see what happens. You can see pics of mine by clicking the link in my signature, I just love telling people that I made it myself.

OK, here's a pic if it works


http://community.webshots.com/sym/image4/6/56/2/70765602jPuSmT_ph.jpg

[ 04-25-2003, 04:50 AM: Message edited by: Eric68 ]

That looks pretty cool Eric. How are you going to run a filter with that setup though?

Ok, guys no laughing. A long time ago there was such a thing as a velocity stack. I made a set for my 2x4 Vega/350. Made a huge difference with a cowl induction scoop.
K&N, I think makes a “stub stack” that basically straightens out the column of air before it enters the carb/air bleeds etc. I think you are experiencing a tremendous amount of turbulence resulting in mixture problems given the high pressure area between the windshield and L-88 hood.
I used a chunk of tubing same size as the Hollys and tacked it to the air cleaner base. Left it about 1 ½’ short of the lid

Did you forget and leave the cake in the pan? tongue.gif

-Mark.

I've heard of guys running w/o a hood and filter, which resulted in a loss of ET/mph. You would think that w/o a hood, there would be no restrictions for the engine to breath. But in fact it causes turbulence over the top of the carb and not a lot of air actually goes into it. Car Craft experienced this with their BB vs. SB article.

On the other hand a friend of mine use to run a dyno at a local shop, and he said that running w/o a filter results in a hp gain. That might be true in a dyno cell, but in an environment where you're moving I don't think that holds true.

IMO you were getting too much turbulence over the top of your carb, and not a lot of air was going in. You either need to put a filter on it, or something to force the air down and into the carb. Just my thoughts and opinions. ;)

Originally posted by HwyStarJoe:
Dennis, would putting a filter in his new setup change the flow charactoristics positively or even more negatively, if that is the case?I think it would help with the right filter. Eric's design isn't that far off a factory unit, just a lot more shallow. Anyone with a functioning cowl hood care to pull the element and make a test run?

DJD
did you say that WITH a k&n filter LID that you LOST RPMS???? and you put the chrome lid back on and you could rev to 6000 +??????
what kind of a chrome lid was it??? stock GM or an after market cheapo???

is a taller filter [as tall as you can get within reason] better?
i have a k&n 14 by 3 in tall filter in a stock GM cowl induction set up with the stock GM lid. will a 4 in tall filter help do you think?

Thanks for the ideas. Maybe I need to find a way to get an air filter of some type in there to help with the possibility of turbulence. Maybe one of those cheapo triangle shaped foam ones would be enought to help a little.

Can anyone explain why the carb COULD be going lean? I mean guys run blow-through supercharger setups with carbs and they don't go lean, you would think that once the mix is right it would stay that way.

Could the cowl opening be acting like a restriction because my base is so shallow?

When I first saw the picture, the first thing to stand out was how shallow it is. How are ya gonna get a filter on there? Won't a 14x3" K&N fit on top without a cover and still fit inside the hood cowl? Give it a shot.

I don't have cowl induction, but I do have cold air induction picking up air from the front of the grill.

Initiall, I also lost about 2 tenths because it caused a secondary bog. After I replaced the VS spring, the car picked up 1.5 tenths over my fastest time, along with 2MPH. I'm using a 4" tall filter, and see my webpage for photos.

Several theories with respect to turbulence and air flow. The bowl vent tubes may be acting like venturies and a vacuum could be formed in them, sucking on the fuel in the bowls instead of venting the fuel. Eric you could try extending your vent tubes with rubber hose just for grins. As air flow increases (higher rpm and in Erics case at higher speed) it could be bouncing around as it enters the throttle bores. This could have a negative effect on the vacuum created in the secondary venturies causing lazy or no secondary.

Traditionally ram air cars wake up at speed not lay down and the effect is kind of like adding boost. I don't know how effective a sealed cowl hood pulling colder air off the base of the windshield is compared to a ram air system with a forward facing scoop though. The benifit is colder denser air with a functioning cowl but is it also being forced air like a modern 4th gen SS?

An interesting observation with my filter lid and I need to experiment with is, I was on my way to a gathering of gearhead friends and picked up the filter lid and installed it. Drove for 2 hrs on the freeway to get to my friends house. We were in a rural area and after socializing and a BBQ some of us started to show off on a untraveled section of road. I spun the tires and at about 4 grand it just started crapping out. Shifted to 2nd and it ran again until 4000... I looked like this :eek: and parked the car. checked timing and the basics and didn't find anything wrong. I still had a 2 hr drive home still and it was getting late so my buddy and I headed out. Stopped for gas and to get a jacket out of the trunk and getting back on the freeway ran it up a bit and it still crapped out but not as bad. Key here is it's getting colder. Dropped Alan off and now it's real cold by comparison to the sunny part of the day. I jumpped on it and it went right to 6000 without a hint of problem. :confused:

The next day when it got good and hot out I went for another ride and sure enough it crapped out at 4000 again. Swapped the lids back and the problem went away!! graemlins/hurray.gif So now until I get off my butt and play around I have this real expensive 14" frisbee...

graemlins/waving.gif [/qb]It was a FRUITCAKE, FRUITCAKE :eek: [/QB][/QUOTE]

Isn't there only one fruitcake in the whole world and it keeps getting recycled as a gift each year? I'm glad it got put to good use!

After swapping to cold-air induction the engine seemed to like 2-3 sizes larger jets depending on temperature. If the car was driven in cold weather with a summer tune it ran like crap. It was also difficult to tune and more tempermental to drive with a 160* thermostat.

My experience with the air filter lids has been the opposite of DjD's. With a drop base air filter and full choke horn the engine ran significantly better above 3500 RPM. Now with the cowl induction it may be different since there is no drop base but I have not changed the lid. The engine bounced the 6000 RPM limiter with no problems. Why it works in one car and not another.....

That's very interesting Dennis. you know I once had a 355 with 700 double pumper on it and the thing ran strongest when the engine was cold, it ran REALLY strong when the air temp AND engine temp were cold. But once it warmed up the power was noticably off everywhere in the RPM band especially on a hot day. Well, I finally wised up and jetted it leaner (2 sizes down both front and back) and the problem was cured. It sounds like maybe your engine might be a hair on the rich side based on your description of the cold vs warm tempertature thing, but the weird thing is that you would think that the filter top would provide more air making it leaner. Very Weird. When I put the filter top onto my air cleaner setup it made very little if any difference at all. Maybe a very slight HP increase.

Also, I think I just learned something about reading plugs to determine A/F mix. I learned that a hotter plug will read leaner and a colder plug will read rich with the exact same A/F mix. Even different brands that are supposedly the same heat range read differently in the same engine. I think that I may abondon reading plugs to prompt jet changes and instead go strictly by MPH at the track. Once I have the best MPH I will find a plug with a heat range that reads correctly and stick to it. Then reading plugs might actually trell me something once I KNOW I have the correct heat range plug.

I'll probably do a little s.o.t.p. testing this weekend to see if I can change anything to make this cowl setup work. Will let you guys know what I find.

EDIT: Dennis. I think I know what the deal is with your "frisbe". With the extra air flowing straight through the center of your carb the signal across the venturi increased (normally a good thing) which made your carb run rich (stronger signal will pull more fuel regardless of jet size). I bet if you leaned out the primaries one step you would pick up a little power and throttle response with the filter top installed. This may be the key with my setup, only possibly the exact opposite is occuring WITHOUT my usuall filter top air cleaner in place. I think you're onto something.

Silver69 --- I REALLY like your ram air setup. Where did you get it (or parts for it)? Especially like the filter box. Very nice.

This might be the route I take if I cannot make my cowl setup working.

Two things to consider (based upon my track experience)...

1. DjD had a very good point with tying the vent tubes together with a vent in the middle. When I was running my SBC combo with an old style snorkle scoop I had no problems until one day a built a base plate to seal the carb to the hood/scoop. Same thing as you described happened, car fell on its face. Ties the vents togather to stabilize the carb and the problem went away. Carbs are not designed for true ram air with no air filters or lids.

2. What is your clearance from the carb to the top of your ram air set up? BG says that he wants at least 3" to avoid reversion. When I got this 528 Hemi car dropped of at my shop, it had a 1" drop base air cleaner and a 3" tall (paper filter) with a flat top. If you stood on the car it would pull like a banshee but when you got our of it it would suck fuel out of the vent tubes and flood the carb. Taller air cleaner and it went away. Look for signs of fuel spatter or raw fuel itself. Carbs need room to breath (especially at the levels you are building to) and need space to develop and take advantage of a strong and natural signal.

Hope this helps.

James
www.adivanman.com (http://www.adivanman.com)

I thought of air turbulance messing up metering as well. I have heard that the triangular filters are restrictive. What about one of those wire mesh L-88 "filters" used on Corvettes. They don't really filter dirt, but I understand the purpose was to straighten air flow - fine for you at the track.

The reason blow thru carbs don't go lean is that they are tuned rich. My carb provides a 12.5:1 A/F ratio across the board. A little too rich sometimes, but the MSD keeps the plugs clean. Also, the air entering the carb is very turbulent, but it is under pressure, so it is forced thru the venturis more or less equally.

Hey Carl - I think in my case I have multi issues with the lid. I think it actually hits the vent tubes or is at best too close. I jetted up from 68 to 72 on the primary side and 76 to 78 on the secondary but haven't run the lid again due to clearence above the carb. The over all effect though was a slight "seat-of-pants" improvement over the stock jetting. I liken this to my RD350 days and removing the air box and using UNI filter elements ment carb tuning. In your case the element seems to sit way higher than mine and I bet you had a better performance tune prior to the filter lid addition. I've never really gone past ironing out drivability issues with mine and though it is well manored I think there is a lot of performance left to be found. My plugs look ok after cruises. Haven't checked them since the jet change. Also I've never pulled a WOT run and pulled over and checked them.

That may explain the differences we've seen. When I posted about it, I got a lot of "too lean" jet up recomendations.

James67RS & Dennis,

I tied the two vents together with some surgical tubing to a Tee, then ran a long piece off the Tee through the cowl and back into my engine compartment. I went for a quick test drive and bingo, no more bog going into 3rd gear. I did have to short shift into third because of traffic and got out of it quick but it felt good. Repeated 2 more times and parked. Must be related to turbulence and the bowl vents. Will have to try it at the track to be 100% sure, but I think we got it.

Also, the cowl is 2" deep and my pan is another 1 to 1/2" deep for a 3-1/2" clearance between the filter base and the top of the cowl. From the actual carb (where the air filter sits) to the top of the cowl I have 5". I don't think clearance is a problem in this case.

Originally posted by Eric68:
Could the cowl opening be acting like a restriction because my base is so shallow? The cowl opening would not be the restriction, but the reduced volume of the base would be. The thing is, there is a pressure to volume issue you are dealing with here. Meaning that if you change the overall volume of the plenum itself, the size of the duct opening has to be changed to compensate.

In either a 'ram-air' or 'cowl-induction' intake system, there really isnt a substantial working pressure like what most people would think. The actual working surface pressure area is (of a cowl-induction system); measured from the base of the windshield 'center-out' is 13"L by 8"H. In this 'pressure window' you actually only see about 0.7 PSI. Outside of the 26" x 8" window the pressure drops dramatically.

A suttle change in the overall system volume will disrupt the pressure to volume ratio enough to create some problems. One of the most common problems is, the change creates reversion, which would also be seen as turbulence across the vent tubes. Therefore, any attempt at proper metering is a lost cause, unless there is a change in the system to compensate for either the volume or pressure change.

GM spent a fair amount of time figuring the ratio out. It all started in 62' with the Impala and was grafted into the F-Body in 67'. When they changed to the 'Hood' inlet, it was basically for ease of maintenance and the ease of implimenting other fabricated induction systems.

The shape of the base itself also plays a hugh role in this. To sum this up, I will simply refer to this link:

http://www.moroso.com/catalog/categorydisplay.asp?catcode=34003

Which has been further refined by K&N in the development of their 'Flow Control' systems. It's all about the shape and how the shape turns air.

http://www.knfilters.com/Racing/flowcontrol.htm

Just something to think about.........

DJD
im confused....is the K&N filter lid you used one of the filter element lids? and not a solid metal lid?

i have the stock GM cowl induction set up with a stock vacum sec 780 holly carb.its jetted 69 primarys and 76 sec.and a 37 shooter.
from the factory they are jetted with 68 prim and 76 sec.
we played around a lttle with the jetting and the shooters and went up a few sizes but it seems to run real good with that jetting...we did have to go up to one spring higher than the purple spring on the vac sec.
the point im trying to make here i think is what my car has for jetting with the stock cowl induction and a resonably hipo motor.

the other thing that i was wondering about with your problem eric is the flat air cleaner base you have as the stock GM is a drop bottom base, it actually drops quite deep,the top of the 3 inch filter is not very high off the top of the carb but the stock air cleaner lid has a fairly high dome to it per say.....no doubt to smooth out the air flow and turbulance. a stub stack might help????

In an effort to learn something, I'm going to stick my neck out and ask a question. How can cooler air require rejetting of the carb? The denser air should result in a greater pressure drop through the venturis and pull more fuel in to the motor. I don't see why rejetting is required?

-dnult

Originally posted by z/27 camaro:
DJD
im confused....is the K&N filter lid you used one of the filter element lids? and not a solid metal lid?Yes, it is the element lid.


i have the stock GM cowl induction set up with a stock vacum sec 780 holly carb.its jetted 69 primarys and 76 sec.and a 37 shooter.
from the factory they are jetted with 68 prim and 76 sec.Is this a 302 with a List 4053 carb? Factory primary discharge nozzle is .025, what lead to so much shooter? And what pump cam are you using?


we played around a lttle with the jetting and the shooters and went up a few sizes but it seems to run real good with that jetting...we did have to go up to one spring higher than the purple spring on the vac sec.
the point im trying to make here i think is what my car has for jetting with the stock cowl induction and a resonably hipo motor. You can't compare my 383 to a 302 just because they have close to the same carb. (My carbs a List 4346, it's almost identical to the 4053) The cubic inches, head flow, cam profile and exhaust all effect how you set up a carb. The 302 winds up and really needs the 780 cfm in the upper rpm and my 383 stroker is all torque from 2000 - 5500 rpm.

Originally posted by dnult:
In an effort to learn something, I'm going to stick my neck out and ask a question. How can cooler air require rejetting of the carb? The denser air should result in a greater pressure drop through the venturis and pull more fuel in to the motor. I don't see why rejetting is required?

-dnult I think the answer to you question has to do with the fact that you can only flow so much fuel through a given diameter orifice. In other words the jet actually works by limiting the flow no matter how much pull is applied only so much can squeeze through. So when the air is cooler and thicker there will be more of it to mix with the same amount of fuel. I'm no expert so speak up if I have this wrong...

Dennis - Actually, you CAN pull more fuel through the same size orifice. Flow is a function of velocity and size, so the faster you move fuel through a jet the more it flows. Jet size just determines the RATE fuel flows through for a given velocity.

Since the venturi signal is actually pulling fuel through the jets (not the fuel pump pumping it through as some might think) the stronger the venturi signal the more (faster) fuel flows through the jet. This changes with engine RPM too --- if it didn't, the engine would be rich down low and lean up top.

In my case I think that turbulence (due to no air filter) is messing with the signal maybe a couple different ways. I think I've already proven it is presurizing the vent tubes (since the temporary tubing on the bowl vents helped) but it also might be creating an unsteady signal across the venturi.

One other question is, how are air bleeds affected by pressure spikes? Could an erratic pressure at the air bleed opening cause improper reduced/improper pre-mixing of fuel and air in the emulsion tubes?

I think the bottom line is that I need to get a filter setup to work with my cold air induction. A filter should smooth out the spikes and help create a steady signal at the same time. I think I might try buying a short 2"x14" paper filter and run my extreme filter top on it. a shorter filter should clear the top of the cowl.

ps. a stub stack wouldn't help because the base of my cowl setup is actually an air flow bell.

DJD
my motor is a 1969 350 with a 1969 z/28 top end on it.heads, alum intake and carb. [an LT 1]
it does have a list 4053 780 vac sec. z/28 carb on it
i have an orange pump cam on it [19cc] in the 1st slot. its got the short yellow spring in the vac sec.dont know why we have so much shooter....but it works real good.
ya your right about not being able to compare the motors and how the carbs are set up....sorry.

from what i have read about cowl induction [a lot of good stuff in the book "camaro untold secrets" about the 67 to 69 trans am series] its the prefered method over ram air......ram air can cause you lots of grieve getting it set up right. the denser charge of cold air will require you to jet up, as it leans the motor out.its not the velocity and amount of air being forced into the motor [obviously there is some of that]its the COLDER DENSER air that makes cowl induction such a good method of easy HP gain.

Originally posted by Eric68:
I finally got to test my home-made cowl induction set up last night and was pretty disappointed. It slowed the car from 11.96 to 12.22. MPH dropped from 112 all the way to 108. I did back to back to back passes and am 100% certain the cowl piece is causing the problem.

The engine really fell on its face when I shifted from 2nd to 3rd. It actually bogged when I shifted then picked back up --- but the engine really struggled through 3rd gear.

The carb (Speed Demon 750) was jetted a hair on the fat side just in case the extra cold air leaned out my mix. Plug color was pretty close to ideal, just a little rich IMO.

My setup is real simple. I used a Quick Fuel aluminum air bell as the base and cut down a 16" diameter aluminum cake pan then added a rubber gasket to seal it to the cowl opening. No air filter yet because my current K&N is too tall.

Any ideas what could be wrong?

Anyone with experience using a RAM AIR system on a carburated first gen? I've seen quite a few prefabbed systems for imports, but nothing recently for a 1st gen.

thanks http://www.spiralmax.com/images/SMX_photo.jpg

Maybe you need the Spiral Max Turbo Air Twister, you could be the first one on your block....

This is a very interesting post. Those bowl vent tubes are cut at an angle so that the pressure they measure is closer to "total" pressure. That is the static pressure at the vent, plus the additional pressure caused by intercepting a portion of the air flow. This gives an indication of the total energy of the airflow, and is also the reason that the air bleeds are set where they are. This is also how a Pitot tube is used to indirectly measure air velocity, since it measures total pressure on the nose (inner tube) and static pressure on the side (outer tube). The difference is related to the kinetic energy of the air flow.

When air comes through the filter (with solid lid), it is at low velocity and at a pressure that is hopefully not much less than the cowl pressure. As the engine sucks it toward the carburetor, especially with a low lid, the air is forced into a smaller and smaller area. This causes a velocity increase near the carburetor, and a resultant pressure drop to maintain the same total energy of the flow. This is exactly how the carburetor venturi works, too. The bowl vent compensates for this pressure drop by picking up total pressure. This way the carb wont lean out from low fuel bowl pressure. If the filter is restrictive, the bowl vent will compensate for the static pressure drop too.

A tall air cleaner minimizes the velocity difference between the air filter and the bowl vent. I guess adding the filter lid had enough influence on the fuel bowl pressure to require a jet change. And the flow through an orifice like a jet goes with the pressure drop across it. The pressure drop is the difference between the bowl vent pressure, and the pressure at the booster venturi.

Originally posted by onovakind67:
[QUOTE]


Maybe you need the Spiral Max Turbo Air Twister, you could be the first one on your block.... Now that`s just too funny!! :D :D

I thought somebody would appreciate that... :D

Hey, sorry to dig up an old thread here, but I have a question for Eric, or anyone else for that matter... Has anyone tried using a large panel filter (in a fabbed plate) mounted to the hood in the opening above the carb? I have issues with my new setup not getting enough air, so I'm researching what others have done in the past to get around these problems.

Sorry, never tried a flat filter. I abandoned the cowl setup in exchange for a sealed filter that has two cold air intakes one on each side of the radiator.

I did the same setup back in Feb. I recently took it down the track and have noticed that around 3500-4000 rpm it lays down a little and then picks back up. I wonder if this is the reason?
Here is a pic:
http://www.camaroclubkc.com/images/wow/2003/World%20of%20Wheels%20032.jpg

rs68ss. sweet looking engine compartment. Mine was behaving the same way yours was --- it would lay down in 3rd.

I did manage to temporarily cure the problem though by using surgical tubing to tie the vent tubes together and vent them to the engine compartment using a plastic Tee. Never ran it at the track like that though, I can only say that it didn't "lay down" any more.

I will take the K&N lid off and replace it with the solid one to test it. I'll leave the baking pan on however. :D