Requesting Guidance for 306 motor swap - Fuel? - SD? MAF?

AeroCoupe · Jul 28, 2025

So do not even hesitate and just move to five lug. The stock brakes off of a 94-98 V6 or GT is what our cars should have come with period. Putting four lug disc brakes on the rear does nothing for stopping power just brake fad under heavy (read track) use. You need larger rotors to get larger pads and larger calipers to increase braking. The larger diameter rotors cool better so there is less brake fade under heavy braking applications.

I can tell you from experience that I noticed a little bit of difference between the V6 / GT brakes and the full Cobra upgrade I did on both cars. The other huge bonus to running five lug is there is better wheel selection and I personally like how the five lugs look on a wheel vs four.

FastDriver · Jul 28, 2025

The brake upgrade is my personal favorite modification on a fox. It's not even close. Nothing makes as big a difference in confidence as upgrading ths stock brakes. On my high powered turbo cars, the brakes still make the biggest improvement.

So_Flo · Dec 19, 2025

long time no talk fellas, I know I haven’t updated the thread in a while, but long story short I ran into a friends mechanic who offered to work on the car, but on his spare time.

3 months later….

The foxbody is finally back from the mechanics and so far, the motor swap was successful.

I’m having issues uploading videos on my phone. Will attempt to upload again on a desktop computer later.

Good side,
Since the motor swap, the car has no more vibration & zero miles

However, I’ve encountered a new concern where the car is running rich on fuel. I’m going to try to upload a video later, demonstrating black smoke when giving throttle. (It’s good to note, the car is currently running on stock 19lbs fuel injectors and not 47lbs)

In addition there’s a separate concern with the idle, which might be a mix of the E303 cam clashing with speed destiny & no mass air sensor. .

I’m going to try to Band-Aid the idle problem tomorrow, in hopes it can be driven but I believe my next step will be to purchase a aftermarket tuner. (More than likely Holly.)

I’m also running into a problem where when I attempt to drive the car it wants to shut off on me…

Again for my visual learners I’m going to send a video later today bear with me, please.

Thank you

rednotch · Dec 19, 2025

Make sure he hooked up vacuum to the MAP on the firewall and there's no vacuum leaks, good idea to kind of treat them like old carb cars and check with a decent gauge or smoke test it... Lots off ppl forget about the MAP on the old Sd stuff, or leave it off because there used to mass air on these cars and it could cause some of the issues your having, but pretty easy to check. Might even be worth throwing fresh o2 sensors in it if they used the old stuff. SD should be able to idle that combo fairly well, timing and fueling wouldn't be perfect but enough to drive it until you switch to something better.

So_Flo · Dec 19, 2025

The uploading isnt working, but the link here should take you to a shared drive.

Let me know if you guys have issues opening the link. Videos of Mustang !!!

@rednotch - Thats the second time I see the O2 sensor. I'm going to pick one up at Advance tomorrow and check the Vac lines. In addition to playing with the timing. In hopes this banaids my problem for now.

If anyone has an RPM 2 manifold, can you guys tell me what hood are you using because my stock hood doesn't have clearnace now.... I'm thinking of swapping motor mounts, and maybe get away with a 2" Cowl hood..really trying to avoid the 4" -___-.

rednotch · Dec 19, 2025

Stock hood should fit, im guessing they used a spacer and taller valve covers though that raised it up. I like harwood for bolt on hoods but I'm biased and those things are priced stupid now + freight.

Mcmahst · Dec 19, 2025

You should put a vacuum gauge on it. Sounds like a vacuum leak. There are quite a few vacuum leak opportunities on these cars.

General karthief · Dec 20, 2025

Idle can be an issue with a E cam and SD. A vacuum leak makes matters worse.

AeroCoupe · Dec 20, 2025

Running an RPM II with a 1” spacer due to tall valve covers to clear the Jomar valve girdles. Have a 2” HO Fibertrends and have right at 1/2” of clearance.

GOvert · Dec 25, 2025

@So_Flo Have you had the time to check out you harmonic balancer? A friend of mine has been chasing down a vibration issue and when he finally decided to change the balancer, that ended up being the problem. He said the old one "looked" ok, but after further inspection (with it off of the engine) determined that it had drifted a bit.
Soon I have a job to do on a 1995 5.0 Mustang that the owner is complaining about an engine vibration. I bet it is the 30year old HB.

Jarhead67 · Jan 3, 2026

So_Flo said:
Hello Everybody,

Introduction
I've recently picked my project car back up. A 1988 LX 5.0 Mustang that’s been in my family for over 30 years! It was also my first car back in high school (2012), and it holds a lot of sentimental value. Now, I’m aiming to finally complete the build over the next few months.

I started this project over 10 years ago and picked up various parts along the way. Unfortunately, I didn’t keep the best records at the time, but I’ll list everything I know as clearly as possible below.

Let me know if anyone wants more info or has advice on next steps. Open to all feedback. Especially around prepping a long-stored motor or if transitioning from speed density to mass air flow is recommended or needed? Especially if I want to add an entry-level Vortech system. I also have no clue what kind of fuel injectors I should get.

Background on the Car:
This ’88 LX 5.0 has been in my family since day one, bought new by my dad in 1988, and only passed between him, my uncle, and me. It was my first car in high school back in 2012. It's been sitting for over 7 years, and I’ve recently started bringing it back to life.

Last month, I changed the fuel tank and pump, topped off all fluids, and was able to get it running. To my surprise, it fires up on the first or second crank!
Thanks for reading!

Car Details

Year/Model: 1988 Ford Mustang LX 5.0 (Hatchback)

Color: Light Spiral Gray

Mileage: 100,000+ miles

Computer: Speed Density (no mass air conversion)

Current Motor Build Status
My goal is to build the motor with an eventual plan to add an entry-level Vortech supercharger. Here’s what I have so far:
**Weekend cruiser

Block: Used Ford 302 block

Sent to the machine shop years ago for cleaning and boring

Now a 306 (bored over)

Fully assembled and torqued but it’s been sitting in the garage untouched for the past 7+ years

Parts Installed (As best as I can remember)

Cam: Mild E303 cam (by COMP)

Headers: BBK equal-length shorty headers

Intake manifold: Edelbrock Performer RPM II upper and lower

Heads: Edelbrock Performer heads (Part #: 6025, CARB #: D-215-33 MAK 95)

Pistons: Flat top pistons (brand/specs unknown)

Rockers: Blue roller rockers (brand/specs unknown - The heads of the bolts are stamped with TUZ, 12.9, but I can't find them online)

255 LPH Fuel Pump (Jegs)

Throttle Body w/ spacer 80mm (Stack Racing)

Suspension
I changed the shocks and struts years ago, but I cant recall the brand. Nothing to brag about. if anything i would love to get adjustables or go sliglhy lower. This can be done later down the road.
Gears
Rebuilt stock T5 tranny with stock gears. ( I would love to throw in 3.55 gears later down the road.)
Stock Clutch system - This will be swapped for a hydraulic clutch system. I hate the fact that the stock clutch feels like a workout for my left leg lol.

With that cam you’ll need to go to MAS air. The SD only likes a stock cam, but MAS air will compensate for it. It’s a very easy conversation.

So_Flo · Jan 4, 2026

Morning All.
Happy New Year, and thank you all for the help and suggestions.

Check out some recent pics.
Aside from the rough idle, I broke the E303 cam in, and the car has reacted good and drives well. Super excited for what's to come this year!

Currently working on the following:

Installing the adjustable fuel pressure regulator ( I'll check fuel pressure at the same time)
Installed the new intake spacer (still didn’t give me the clearance for the stock hood), waste of time lol
Purchased a timing gun to time it properly - Im hearing it's recommended to try 12 or 14??
Removing the fuel injectors to have them tested, if replacements are needed.
Checking TPS voltage since I recently swapped the TPS sensor and did not check the voltage after the fact.

@rednotch – Had I known the taller valve covers were going to cause this issue, I would’ve avoided them and gone with short covers . That said, I already bought a 0.5" spacer, but even with that, the tall covers won’t allow the throttle body to bolt on. Because of that, I’m hesitant to buy a 1" spacer since there’s still no guarantee I’ll have enough clearance

So for now, the hood is on the back burner. The stock hood won’t fit unless I swap valve covers, and honestly… F$%K that I'm loving the blue covers .

@Mcmahst – I have a friend who’s going to run a smoke test for me. I’ll update the thread once that’s done.

@General karthief – I'm hoping the smoke test shows me a leak, cause I can't find them. I'm also considering the Holly X once I save up. which should control the idling. I'm just trying to band-aid the issue without an MAF / nor a Computer for the time being.

@GOvert – I did purchase a new harmonic balancer for the motor. The machinist told me before the motor swap that he tested the original block and found a misfire cylinder. I’m not sure if that was actually the issue or if it was the HB. What I can say is that now that the new motor is in, the shaking/vibration is completely gone—THANK GOD.

@Jarhead67 – If I find an A9L for the low-low, I’ll jump on it for now. But my main goal is still to band-aid the car and save up for a Holley X.

Mcmahst · Jan 4, 2026

Hoods are over rated.

Requesting Guidance for 306 motor swap - Fuel? - SD? MAF?

AeroCoupe

lube between the nut and the face. I know my lubes

FastDriver

I was uncomfortably high & wearing a helmet

So_Flo

Member

Attachments

rednotch

I'd like to remain having one chocolate starfish

So_Flo

Member

rednotch

I'd like to remain having one chocolate starfish

Mcmahst

General karthief

wonder how much it would cost to ship you a pair

AeroCoupe

lube between the nut and the face. I know my lubes

GOvert

Jarhead67

So_Flo

Member

Mcmahst

Similar threads

Requesting Guidance for 306 motor swap - Fuel? - SD? MAF?

lube between the nut and the face. I know my lubes

I was uncomfortably high & wearing a helmet

Member

Attachments

I'd like to remain having one chocolate starfish

Member

I'd like to remain having one chocolate starfish

wonder how much it would cost to ship you a pair

lube between the nut and the face. I know my lubes

Member

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