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Bullitt Intake Tract Mathematics.....Mods?

818 Views 19 Replies 8 Participants Last post by  JP DEMOLET
Hi folks,

I've been doing a little investigation into our intake setup and have learned the following:

1. Dual 57mm-plate throttle body:
- Total Area: 5103.5mm^2
- Effective Flow Diameter: 80.6mm

2. MAF Sensor Assy:
- Intake Diameter: 92mm
- Output Diameter: 85mm
- Diameter at vane: 79mm
- Area blocked by vane: 1501mm^2
- Total *unobstructed* Area: 3401mm^2
- Effective Diameter: 65.8mm
- Sampling tube Diameter: 9.9mm
- Sampling tube Area: 77mm^2
- Ratio of Total Area/Sampling Area: 44:1

3. Rubber Airbox Silencer
- Input Diameter: 74.5mm
- Input Cross-sectional area: 4365mm^2

OK, So what am I getting at here? Basically, I've just convinced myself that the bottleneck in our intake tract is the MAF sensor body and then the silencer. In my case, I have a 100mm Densecharger so I have no silencer restriction.

Interestingly enough, a standard GT has a 65mm diameter throttle body and it's MAF sensor has an effective diameter of 65.8mm.... therefore they are matched! Now, Ford gave us the Cobra throttle body which, if it were a single plate TB, it's effective diameter would be 80.6mm. However, we have the same MAF with an effective flow diameter of 65.8mm.

I'm starting to think that an aftermarket flow meter like C&L or Pro-M would be a good next mod for me. What I'd really like to do is build my own new MAF body and reuse the stock sensor. I'd basically use an 80mm I.D. tube with NO post inside and then add another sampling tube inside that has the same ratio of cross-sectional areas therefore the original calibration should still be good. This new sampling tube diameter would be 12.8mm.

What do you guys think? Do any of you have the Pro-M or C&L meters and how much gain (or loss) did you get on the dyno?



Mods: Steeda Triax, Steeda Underdrive Pullies, JBA Silver-Ceramic Shorty Headers, Magnaflow X-pipe w/cats, Magnaflow Cat-back Exhaust, FRRP Aluminum Driveshaft, 100mm Densecharger

<font size=-1>[ This Message was edited by: mjchip on 2001-11-25 17:42 ]</font>
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I have a 87mm Pro-M MAF with a 100mm Densecharger. I haven't had it dyno'd yet becasue I am waiting for my long tube headers and cat'd h-pipe to go on this Tues. However, I can tell you that the Pro-M matches up with the flange of the Densecharger better. If you take your Densecharger out and leave the stock MAF attached you can look down the tube and see a HUGE overhang where the two pieces meet. If they don't line up correctly then it's actually more restrictive that the stock airbox/MAF setup.(w/o the intake silencer) After I installed the Pro-M I let the computer reset overnight and took it out the next day. It definitely felt and sounded like it was breathing easier. As far as power increase, I'm not sure I have enough miles on it yet to allow the computer to adjust. It idles a little more irregularly with the Pro-M also. But, I only have 1800 miles on my car and I have a long way to go before it's broken in, so all this could change once that happens. It's not a cheap mod. If I were you I would add the Steeda timing adjuster first. It's more bang for the buck.

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Hi Brad,

Thanks for getting back to me. I've been holding off on the timing adjuster because I'd rather have the timing changed electronically (I may end up just buying the T/A though).

I know what you mean about the densecharger. I just got back from the dyno and was a little disappointed with the results. I expected to get a bit more gains then I got. See my sig for the mods and results. Then I took apart my intake from the MAF and saw the mismatch between the pipe and my MAF body. This probably killed my high end. In fact, above 4500rpm my A/F goes really rich and this mismatch would explain that.

Basically, I was kind of surprised that the MAF sensor ended up being the big restriction in our intake setup. I think that there is HP/Torque to be gained here! Do you agree?


First, IMHO the T/A is a better choice because it's continuosly adjustable, whereas a chip sets it at one spot. I was able to dial my timing in right where I wanted it. However, I don't know how it would work with an off the shelf chip, since those all advance the timing anyway. Remember that when you change the timing the stock computer should do the math and adjust the A/F ratio. Pretty much what a chip does at half the price. No doubt that more can be done with a chip, but I'm just jaded. I bought a DiabloSport chip when I first got my car and after sending it back twice, once with my ECU, it still has never started with the chip installed.

Second, I was looking for the bottle neck also. Yes, part of it is the stock MAF with the huge sampling tube molded in the way, but it's also the tube that attaches to the throttle body. It really squishes down and becomes flat after the MAF. I would be interested to know what the flow area is at it's smallest in that tube. Something to remember, if the Bullitt engine uses the Mustang GT camshaft, then the profile is not set up for all the air our engine sucks in. So after a certain point the bottleneck is going to be at the valves. I'm going to attack the exhaust side with the headers and h-pipe and see what happens. I've seen much talk recently about Extude Honing the intake and that my help smooth things out a bit, but again were back at only so much air will go in without a blower.
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I am going with a cold air system 150 buck not to bad
Before doing that,try and read that article,in the old MM&FF issue,I posted in here couple days back. The Shepard GT,was running a virtual duplicate of what we have.
I wouldn't worry too much about the flattened tube, as it tends to not be a restriction even on heavily modded cobras. Though I think you are on to something w/ the larger MAF thing, but DO NOT just cut the stock MAF housing, definately go w/ an aftermarket MAF rather than cutting out the post in the stock MAF. (I know, nobody brought it up, just thought I would bring it up, just in case :grin:)
Well, I decided to go with an 80mm C&L MAF body and my stock electronics. We'll see how my theory tests out when I go back to the dyno with the new MAF housing and *without* the Densecharger. I'll post the results.
You've got to remember that these are "draw through" meters. The most basic analysis is that the meter draws as much air as the computer is programmed for for the proper mixture with a given injector size.
If the overhang with the pipe affected the flow negatively then why would C&L and PRO-M have the filter attached directly to the meter with the filter diameter at the face of the meter extending outward which in effect creates an overhang. Overhang would only have a negative effect with a blow through meter where the incoming, forced air, would hit a lip, causing turbulance.
It's the incoming air temperature that's the most important thing. The C&L and other underhood setups are going to be breathing hotter air than if their pickup point were in the fender. Fan wash negatively affects a draw through meter because more air is being blown at the meter than it is set up to "draw", which is why these companies have added the plastic shields to the underhood setups. Unless there is something physically blocking the flow path the computer is going to draw as much air as is needed. Ram air systems have the same effect as fan wash since these systems are "draw through".
When I went to the K&N engineer for the best filter for the 100mm system he told me the most important performance consideration is diameter. The largest diameter filter which will fit in a given space will deliver the best performance. This given, as filter diameter increases, whether attached to the meter or not, you are in effect creating more "overhang" which has the opposite positive effect to going too small.
With all of this in mind I wanted a system which would accomodate the larger meters yet not be restrictive to the factory meters.

<font size=-1>[ This Message was edited by: JP DEMOLET on 2001-11-27 07:02 ]</font>
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With ratiometric, hot-wire anemometer type mass air flow meters, like those found on our cars, the two most important things necessary for *accurate* measurements are: 1) equal flow velocities across the cross-sectional area of the meter body and 2) laminar air flow. If the airflow into the sample tube is nonrepresentative of the flow through the unmetered section of the MAF body, due to either flow gradients or turbulent flow then the measured results will be inaccurate.

During my last dyno session, I acquired data which leads me to the hypothesis that my PCM is getting less air than it thinks. The data shows that my A/F ratio drops from 12.5:1 at 4000rpm to around 11.6:1 at 5800rpm (as shown on the dyno). This over-richening of the mixture is abnormal (as far as I can tell) and may be caused by a flow velocity gradient across the meter diameter due to the 90-deg bend in front. If the average air flow into the sampling tube was faster than the average flow through the entire meter, then the PCM would *think* that the car was getting more air than it actually was and would richen up the mixture in an effort to remain safe at higher rpm loads (it probably does this anyway to keep the combustion chamber temperatures lower).

By switching to the C&L MAF body, I hope to achieve two things. First, remove the MAF restriction (i.e. *flow match* the MAF body to the throttle body). Second, remove the bend directly in front of the meter body. With these two modifications, I hypothesize that my A/F ratio goes up (gets leaner) and my hp/torque figures also go up as a result. I will dyno this combination.

John, with regards to intake air temperature, I understand your point and I totally agree with you. Since intake air temp. is directly related to hp output, with the C&L setup I may be drawing in warmer air, so my hp may not increase BUT the A/F mixture should stabilize and this is the main thing that I'll be looking for.

Anyway, the dyno will show all.

Thanks for your replies.

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Hi folks,

I just finished removing my Densecharger and installing the C&L 80mm MAF kit. My observations are that the MAF itself is very nice quality but the filter shield is a P.O.S. Anyway, at the same time I installed the Steeda Timing Adjuster but it's now set to stock timing of 10-degrees BTDC.

As you know, I dynoed on Saturday and was a little disappointed in my numbers given the mods that I currently have and based on what I've seen of regular GTs over on and My mixture was pretty rich and got way rich after 4500rpm.

The over-richening above 4500rpm I believe is due to the bend in the Densecharger ducting right before the MAF (see previous post). However, up until tonight I had no theory on the generally low torque numbers (HP too). Well, I think that I found the source of that problem today while removing the Densecharger. I'm pretty sure that I pushed the K&N on too far thereby reducing the effective filtering area of the K&N. My calculations show that the required airflow for our cars is approximately 450cfm minimum at 6000rpm. The filter that comes with the 100mm Densecharger can flow approximately 600cfm so there is plenty of margin. However, I pushed the filter on so far that I reduced the effective filter area to 66in^2 which, according to K&N engineering, can flow only........398cfm. This may explain my low HP/torque numbers. Opinions?


Mods: Steeda Triax, Steeda Underdrive Pullies, JBA Silver-Ceramic Shorty Headers, Magnaflow X-pipe w/cats, Magnaflow Cat-back Exhaust, FRRP Aluminum Driveshaft, 100mm Densecharger

<font size=-1>[ This Message was edited by: mjchip on 2001-11-27 20:10 ]</font>
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Man what ever happened to re-jetting the carb, reading the plugs advancing the timeing....
You all lost me.....I need a class in fuel injection 101
Are you breaking it down to the surface of the pleats or just outside surface area?
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On 2001-11-27 20:16, Mid Life Crisis #388 wrote:
Are you breaking it down to the surface of the pleats or just outside surface area?


On K&N's website, they state that 1in^2 of pleated, oiled-gauze filter media flows 6.03cfm. Based on their examples, they use the outside area. I believe that the 6.03cfm number takes into account the pleats. Check out their website and post your opinions.

If you had the filter pushed onto the short pipe too far this would block flow right at the worst spot. This is a 614 CFM K&N but with the shortened length because of the bigger diameter it would block more airflow than say on a 9" K&N if it was even an inch too far pushed in. Try putting the filter on the down pipe as I send the system, which only allows for just enough pipe to mount it. Add some extra airflow when you dyno dowm in this area and see what results. This "motion active" variable is the variable that's tough to simulate while dynoing.
I'm sending everyone who bought the 100 system a new, much better, full faced gasket which will be the meter lip diameter instead of the screen diameter. This will smooth the entryway more. I went with the original gasket to enable customers to retain the screen which in the meantime has been shown to cost hp and is just there for debris. I'm having these gaskets made now and will begin sending them out in the next few days.

<font size=-1>[ This Message was edited by: JP DEMOLET on 2001-11-27 21:25 ]</font>
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Exactly my words Jimmy! Not to be smart or anything,but if I took some of these guys into my world of carb tuning,they would be lost. From what I am seeing,with these new Mustangs,we tend to spend a lot of money,for little if any gains. With my old 85,I can pickup 10-20 HP with a screwdriver!
I agree with you too, but at 140,000 miles on my 5.0 with the MAF system still working perfect, none of my "old engineered" Mustangs would have made it to this mileage without any problems, oil burning, or carb overhauls. Hell my 390 tunnelport wouldn't match the 13.8 Bullitts in the quarter.
I had more trouble with dual point distributors and dual quads. I miss the simplicity too but it's tough to argue with the efficiency of todays' "brain equipped" Mustangs.
I would like to see though how my 5.0 would run with a Victor intake topped off with a 650 double pumper, fed via a big hole in the hood like the old days!

When I referred to the overhang between the stock MAF and the 100mm Densecharger it was the included gasket that caused the problem. I just made a new one when I put the 87mm MAF on. I'm glad that you looked into it and are sending new gaskets. Thanks.
I'm sorry, I misunderstood you.
I try to incorporate customer suggestions into system improvements with the gasket and eliminating having to use the permatex mentioned numerous times with the original system. I knew the screen ring diameter would encroach on the meter face a little but you would still loose the blockage caused by the screen itself. I should have gone with a full faced gasket to begin with with the gasket lip at the meter edge. The new gasket fits better and enables a flush flange fit without having to go back to the permatex.
I'm anxious to see how the 87 meter/new TB pipe, eliminating the bend behind the meter works. I'll have a pipe which will be adjustable to fit the factory meter and then allow the longer 87 meter also.
The GT TB pipe is also being R&Dd now in the 77mm size.
I was hoping to have the TB pipes ready as you guys could tell your wifes " Honey, what a great stocking stuffer that would be"!
But they've got to be R&Dd with both filter positions for 250 miles each and then I'll get some dyno results.
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I forgot to mention that there would still be less restriction with the current gasket as compared to factory airbox being that you have to run the screen with the factory airbox which will soak up 2-3 hp.
I'd like to see someone try a burnt chip compared to a mail order chip as the Cobras' factory programming dumps them rich right at 6000 rpm, the max. hp spot, and I'm wondering if the Bullitts have a similar fail-safe enrichment point.
Mus 408, how did the A/F ratio look after you had the Pauls chip dynoed at the 5000 rpm spot?
Interestingly, a Cobra customer who had run the 77mm system, had a chip burnt, and then switched to the 100mm system, related to me that the 100 leaned the chip programming too much. He moved the filter up, reset the computer, and now it's been fine. It was fine over the summer and he thinks the filter in the down position with the colder weather was actually too cold with the original chip programming.
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