Underdrive Pulley

Jae 'Bird

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Dec 11, 2023
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45
Age
19
Location
Edmonton, Canada
Vehicle Details
1990 Ford Thunderbird Base with PEP 151A, 3.8L NA V-6
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Question, would this underdrive pulley kit work on my 1990 T-bird? It says I would need a shorter belt but It doesn't say what size, what would I need then?I know there's some debate about underdrive pulleys but I figure any bump in power will benefit the 3.8. Thanks
 
I'm not sure of your answer unfortunately but I do feel it would be useful to other members to know is this a 90sc or is it a n/a 3.8? The belt routing is very different so I'm sure that's important
 
They really don't make any difference, you'll notice your headlights dim at idle more than any change in performance
 
@Jae 'Bird , my fellow V6 owner:

While I always claim that I'm completely happy with the V6's power, I admit that I've done my share of reading about possible upgrades.

All my research ultimately led to the conclusion that any such small mods would yield minuscule gains at best.

The only mod I ever read about that apparently does produce results is that Mustang/Windstar intake. Someone correct me, but I think that started in 1999? It would require a tune I suppose. Mustang V6 horsepower made a big jump that year.

Meanwhile, the one easy thing you can do to feel power is turning overdrive off. Driving in the 30-45 mph range, the car is dramatically more responsive.
 
Yeah, underdrives are only worth 1-2 HP or so; pretty hard to quantify because there are so many other variables that can lead to differences in power output (density altitude, for example).

The splitport top end upgrade is definitely worthwhile. @racecougar might be able to share some details on this swap, as he had done this fairly early on.
 
Yeah and if you junkyard the parts (99-04 V6 Mustangs are abundant in yards right now!) the cost of the heads and intake might even come in below that of a new set of underdrives, just gotta set aside a weekend or whatever afternoon get your hands dirty.
 
Counterpoint to this admittedly effective mod would be personal driving style, at least in my case.

Looking at the specs before and after split port introduction:

1998 Ford Mustang Automatic Coupe
150 hp @ 4,000 rpm
215 lb-ft @ 2,750 rpm

1999 Ford Mustang Automatic Coupe
190 hp @ 5,250 rpm
220 lb-ft @ 2,750 rpm

What this tells me is that you really have to rev it to redline to generate those horsepower gains. Meanwhile torque gains are negligible.

That's just not how I drive. The most my car ever sees when accelerating onto a highway is a bit over 4,000 rpm. And that feels plenty quick to me.
 
Ok thank you all for the amazing info, thats why I asked. Side note for the splitport, how would I tune my car considering its a 90'? theres no obd 2 so would the parts just bolt on and thats it?
 
Counterpoint to this admittedly effective mod would be personal driving style, at least in my case.

Looking at the specs before and after split port introduction:

1998 Ford Mustang Automatic Coupe
150 hp @ 4,000 rpm
215 lb-ft @ 2,750 rpm

1999 Ford Mustang Automatic Coupe
190 hp @ 5,250 rpm
220 lb-ft @ 2,750 rpm

What this tells me is that you really have to rev it to redline to generate those horsepower gains. Meanwhile torque gains are negligible.

That's just not how I drive. The most my car ever sees when accelerating onto a highway is a bit over 4,000 rpm. And that feels plenty quick to me.

No tradeoff though, still about the same low end but there's an extra 1000rpm to keep pulling. If it were a radical cam/timing or intake change that trades low end for top end it's a valid counterpoint, but in this case you can have your cake and eat it too
 
Ok so its def the best mod to do with little downside. would a tune be required?
 
Normally I'd say a top end swap is just added airflow and the PCM would automatically adjust for it, so no tune needed (though added manifold volume does impact transient fueling, a shifted power band would also mean shift points could be moved - so a tune would help).

HOWEVER....

The '90 still used a speed/density system, not a mass air system. You would need a tune to adjust for the swap if you found a way to keep the system as speed/density. I'd say though that you are probably best off converting to a MA system, which would require some other donor parts.
 
Since I'm somewhat guilty of diverting this thread from its original subject, here's another one I picked up from reading Mustang forums:

Taking slack out of the throttle cable.

I haven't done this myself, but it's on the to-do list. It makes a lot of sense. My cable has at least 2" of play in it.

While it doesn't give you more power, it takes some initial travel out of the throttle pedal, making the throttle subjectively more responsive.
 
No no this is all amazing info, thank you, I do like the idea of taking the slack out of the cable
 
Looking at the specs before and after split port introduction:

1998 Ford Mustang Automatic Coupe
150 hp @ 4,000 rpm
215 lb-ft @ 2,750 rpm

1999 Ford Mustang Automatic Coupe
190 hp @ 5,250 rpm
220 lb-ft @ 2,750 rpm

The SC made 210 or 230 HP @ 4000rpm and 315 ft lbs torque @ 2500 rpm in 1989-1995. Those numbers are weak. 🤣
 
Yeah, underdrives are only worth 1-2 HP or so; pretty hard to quantify because there are so many other variables that can lead to differences in power output (density altitude, for example).

The splitport top end upgrade is definitely worthwhile. @racecougar might be able to share some details on this swap, as he had done this fairly early on.
I did the splitport heads/intake upgrade followed by the windstar upgrade and after all of that work, if I really wanted to stick with a V6 I should have bought an F150s 4.2L V6 and rebuilt the block starting from that point. You'd continue driving the V6 around while you took your time building the 4.2L V6 on your stand.

Here was my DIY from 12 years or so ago https://forum.birdcats.com/threads/gunns-3-8l-single-port-to-splitport-v6-diy.142/
 
I did the elimination of throttle cable slack today. Apparently a very popular mod in the Mustang forums.

It actually makes a huge difference subjectively. The pedal sits slightly closer to your foot. As always, proceed at your own risk.

In my case, this worked great:
I took a half inch (I think) nylon spacer, drilled through it, and made a cut on one side. Then all I had to do was to slide the spacer over the cable slack aft of the throttle pedal. Finally, I put a zip tie through the holes I had drilled, which prevents the cable from sliding back out through the cut.

VideoCapture_20240329-174342.jpg
 
@gunn
Wow, that's a fantastic write-up! But what I took from it is that it's a lot of work. Definitely not something I would tackle.

Side note: I always thought it was funny that the Windstar got the most power out of this engine. I mean the fwd minivan, that's where Ford got serious about performance.
 
Side note: I always thought it was funny that the Windstar got the most power out of this engine. I mean the fwd minivan, that's where Ford got serious about performance.

Too bad the engine department forgot to tell the transaxle department about that seriousness
 
Side note: I always thought it was funny that the Windstar got the most power out of this engine. I mean the fwd minivan, that's where Ford got serious about performance.

Neither of these statements are factual.
 
Neither of these statements are factual.

Well, I was being sarcastic about the minivan performance focus. But didn't Windstar (or Freestar) make over 200 hp at some point?! And isn't that more than this engine made in other models? Talking naturally aspirated of course.
 
Well, I was being sarcastic about the minivan performance focus. But didn't Windstar (or Freestar) make over 200 hp at some point?! And isn't that more than this engine made in other models? Talking naturally aspirated of course.

No Winstar ever made over 200 HP from the factory. The additional 10 HP over the Mustang variant is due to the use of IMRCs, so yes that statement is correct if we are talking about 1999+ model year vehicles. Ford stopped making the Tbird in 1997 and even then, they were looking into the 4.2L as a potential replacement for the 1998 model year; so any 3.8L comparisons become irrelevant if we are talking about NA variants from the factory.
 
If I came across a clean V6 car, I would consider a 4.2 swap. The 4.2 in my F150 was rated at 202hp, not sure of the torque, but even in a 4400lb truck, it was adequate power with the 5-speed. I think a split-port 4.2 with the M5R2 in an MN12 would make for a decent daily driver.
 
I did the elimination of throttle cable slack today. Apparently a very popular mod in the Mustang forums.

It actually makes a huge difference subjectively. The pedal sits slightly closer to your foot. As always, proceed at your own risk.

In my case, this worked great:
I took a half inch (I think) nylon spacer, drilled through it, and made a cut on one side. Then all I had to do was to slide the spacer over the cable slack aft of the throttle pedal. Finally, I put a zip tie through the holes I had drilled, which prevents the cable from sliding back out through the cut.

View attachment 4820

Quick update a few days later. I really love this.

I thought the effect would wear off as your foot muscle memory adapts to the higher position of the pedal; but that doesn't seem to be the case.
 
Quick update a few days later. I really love this.

I thought the effect would wear off as your foot muscle memory adapts to the higher position of the pedal; but that doesn't seem to be the case.
Funny that you mention muscle memory not adapting.... One of the primary features of my Gen 3 Prius eco mode is that it deadens the throttle response for the first 50% of the physical travel so if you just coming off the line you aren't giving it as much gas. It achieves stated goal, I guess, by retraining the pedal versus the user
 
Funny that you mention muscle memory not adapting.... One of the primary features of my Gen 3 Prius eco mode is that it deadens the throttle response for the first 50% of the physical travel so if you just coming off the line you aren't giving it as much gas. It achieves stated goal, I guess, by retraining the pedal versus the user

That explains so much about my negative Prius driving experiences. I need to remember to turn eco off next time I get a chance to drive my mom's '14 Prius V, she hates the random feeling pedal response almost as much as me.
 
Quick update a few days later. I really love this.

I thought the effect would wear off as your foot muscle memory adapts to the higher position of the pedal; but that doesn't seem to be the case.
Where did you get the nylon spacer? I'm starting to get sold on this modification, def one of the first things i'm doing once I remove my car from storage.
 
Perfect, thanks. I figured that's where i would look first, and this might be a stupid question, but you said it was a half inch, was that a half inch long?
 
Perfect, thanks. I figured that's where i would look first, and this might be a stupid question, but you said it was a half inch, was that a half inch long?

Yes. There's at least double that in slack though. But I wouldn't do more than a half inch, because that raises the pedal plenty.
 
Yeah, underdrives are only worth 1-2 HP or so; pretty hard to quantify because there are so many other variables that can lead to differences in power output (density altitude, for example).

The splitport top end upgrade is definitely worthwhile. @racecougar might be able to share some details on this swap, as he had done this fairly early on.
The Splitport swap definitely wakes up the NA V6 cars. I did mine in '02 with a '00 Mustang engine/trans. It made 165 rwhp/198 rwtq; right up there with a stock 4.6L NPI.

splitty 2.jpgMy20splitport20beating20a20vette.JPG
 

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