headers for the 3.4 swap (Page 2/3)
Dennis LaGrua JUL 17, 03:26 PM
There is a restriction in the exhaust which can be overcome by porting the manifolds. However the big restriction is the Fiero 2.8 liter plenum. It just doesn't flow enough to make a big difference on a 3.4 liter P/R engine. My solution was to just add a turbo on my 3.4L so that a higher volume of air under pressure can reach the engine. It worked and provided a massive difference in power. Haven't taken the car to the track but around town it feels like my 3800SC

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

82-T/A [At Work] JUL 17, 04:46 PM
Atlanta1, just to add to what Dennis LaGrua said...

Years ago, I rebuilt a 2.8 into a 3.1 w/ 3.1 crank and rods. I went with a "Phase 2" camshaft (forget the name... maybe Fireball or something?). The cam was supposed to provide a LOT more mid-range power, specifically up through 5,200 rpms. Basically, a slightly enhanced version of the stock Fiero's cam.

One of the things that hit me immediately when the engine was put back together and I installed it in the car, was that it basically "hit a wall" at ~4,400 rpms. It definitely had more low-end torque, and got off the line more quickly, but as soon as it hit 4,400 rpms, it felt like the car just ran out of breath. It speaks to what Dennis is saying, because in a stock 2.8 V6/60... you don't even notice it. That's because the power curve correctly matches the size of the plenum... as in, the plenum is ALREADY maxed out so the results of the restriction is more gradual. With larger displacement and a hotter cam, it's absolutely more abrupt.


I am also building a "stock looking" 3.4 V6/60 and have everything I need to make it as powerful as it's ever going to get while looking totally stock. The only thing I haven't done, is resolve the restriction in the neck of the intake plenum. There's the DAWG mod, which you can search for... but basically, that'll more or less open it up. I'm planning on doing an "extrude hone" process after I've modified it slightly. I already have a bored throttle body with port-matched neck. I'm not going for insane horsepower, just want to max it out while looking stock, and having fun.
La fiera JUL 17, 10:16 PM

quote
Originally posted by 82-T/A [At Work]:

Atlanta1, just to add to what Dennis LaGrua said...

Years ago, I rebuilt a 2.8 into a 3.1 w/ 3.1 crank and rods. I went with a "Phase 2" camshaft (forget the name... maybe Fireball or something?). The cam was supposed to provide a LOT more mid-range power, specifically up through 5,200 rpms. Basically, a slightly enhanced version of the stock Fiero's cam.

One of the things that hit me immediately when the engine was put back together and I installed it in the car, was that it basically "hit a wall" at ~4,400 rpms. It definitely had more low-end torque, and got off the line more quickly, but as soon as it hit 4,400 rpms, it felt like the car just ran out of breath. It speaks to what Dennis is saying, because in a stock 2.8 V6/60... you don't even notice it. That's because the power curve correctly matches the size of the plenum... as in, the plenum is ALREADY maxed out so the results of the restriction is more gradual. With larger displacement and a hotter cam, it's absolutely more abrupt.


I am also building a "stock looking" 3.4 V6/60 and have everything I need to make it as powerful as it's ever going to get while looking totally stock. The only thing I haven't done, is resolve the restriction in the neck of the intake plenum. There's the DAWG mod, which you can search for... but basically, that'll more or less open it up. I'm planning on doing an "extrude hone" process after I've modified it slightly. I already have a bored throttle body with port-matched neck. I'm not going for insane horsepower, just want to max it out while looking stock, and having fun.



Absolutely right T/A. Like Dennis said the intake is a major restriction for the 3.4L with the 2.8L hardware. The restriction is not so much on the neck, its is most on the "straw" looking runners of the upper and lower intake manifolds. The fact that your car hit a wall at 4400rpms it was due to the combined runner length and narrow inside diameter. With the 3.4L the intake system of the 2.8L goes into vacuum at WOT as the RPMs climb up pass 4400rpms and that explain the drop in power. So what is happening is that the sucking ability of the larger 3.4L short block empties out the 2.8L manifolds much faster than the manifolds and TB can supply causing a significant drop in power as the engine revs up.
MarkS JUL 18, 08:52 AM

quote
Originally posted by 82-T/A [At Work]:

Atlanta1, just to add to what Dennis LaGrua said...

Years ago, I rebuilt a 2.8 into a 3.1 w/ 3.1 crank and rods. I went with a "Phase 2" camshaft (forget the name... maybe Fireball or something?). The cam was supposed to provide a LOT more mid-range power, specifically up through 5,200 rpms. Basically, a slightly enhanced version of the stock Fiero's cam.

One of the things that hit me immediately when the engine was put back together and I installed it in the car, was that it basically "hit a wall" at ~4,400 rpms. It definitely had more low-end torque, and got off the line more quickly, but as soon as it hit 4,400 rpms, it felt like the car just ran out of breath. It speaks to what Dennis is saying, because in a stock 2.8 V6/60... you don't even notice it. That's because the power curve correctly matches the size of the plenum... as in, the plenum is ALREADY maxed out so the results of the restriction is more gradual. With larger displacement and a hotter cam, it's absolutely more abrupt.


I am also building a "stock looking" 3.4 V6/60 and have everything I need to make it as powerful as it's ever going to get while looking totally stock. The only thing I haven't done, is resolve the restriction in the neck of the intake plenum. There's the DAWG mod, which you can search for... but basically, that'll more or less open it up. I'm planning on doing an "extrude hone" process after I've modified it slightly. I already have a bored throttle body with port-matched neck. I'm not going for insane horsepower, just want to max it out while looking stock, and having fun.



I made contact with DAWG a while back to see if he would still consider making more modified intakes. Basically no- I guess some problems arose and doesn't want to continue with the mods. Which is a shame.

[This message has been edited by MarkS (edited 07-18-2025).]

82-T/A [At Work] JUL 18, 10:46 AM

quote
Originally posted by MarkS:

I made contact with DAWG a while back to see if he would still consider making more modified intakes. Basically no- I guess some problems arose and doesn't want to continue with the mods. Which is a shame.





If you can find a good "in town welder," though... the DAWG mod is not overly complicated. I think the best version of it that I've seen is one that CarterCarbs on here did. I think he found some aluminum piping, basically cut a big ole chunk out of the bottom underside of the neck, and then mated a cut out section of half the pipe and welded it in. When installed, you couldn't even tell the difference.

What La Fiera says also makes me wonder if just increasing the neck is even enough, as I know he's done a TON of work to 3.4s in the past, and as he says above, the runners are just too narrow. I know port-matching will help of course, but I'm also hoping that a good extrude-hone will make at least a perceivable difference. I've heard you don't want to port or smooth out intake pieces when it involves fuel (because it helps atomize it), but I think that's mostly TBI / carburetors. The fuel is injected just before the valves, so a perfectly polished intake should be desired.
Dennis LaGrua JUL 18, 07:10 PM

quote
Originally posted by 82-T/A [At Work]:
If you can find a good "in town welder," though... the DAWG mod is not overly complicated. I think the best version of it that I've seen is one that CarterCarbs on here did. I think he found some aluminum piping, basically cut a big ole chunk out of the bottom underside of the neck, and then mated a cut out section of half the pipe and welded it in. When installed, you couldn't even tell the difference.

What La Fiera says also makes me wonder if just increasing the neck is even enough, as I know he's done a TON of work to 3.4s in the past, and as he says above, the runners are just too narrow. I know port-matching will help of course, but I'm also hoping that a good extrude-hone will make at least a perceivable difference. I've heard you don't want to port or smooth out intake pieces when it involves fuel (because it helps atomize it), but I think that's mostly TBI / carburetors. The fuel is injected just before the valves, so a perfectly polished intake should be desired.


West Coast Fiero made or still sells a highly modified Fiero upper plenum. Years back there was a guy by the name of Francis who was from Virgina who fabricated an intake from scratch and reportedly they ran well. They didn't look anything like the stock plenum

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

Patrick JUL 18, 07:28 PM

quote
Originally posted by Dennis LaGrua:

Years back there was a guy by the name of Francis who was from Virginia who fabricated an intake from scratch and reportedly they ran well. They didn't look anything like the stock plenum.





quote
Originally posted by Francis T Here:

Here's my attempt at 220+ HP @ 6 - 7+K rpm with a norm asp 2.8 engine...
La fiera JUL 18, 09:34 PM
They were better than stock but but not by much.
82-T/A [At Work] JUL 18, 10:47 PM
I'm sorry that I'm kind of hijacking this with the intake stuff, but we basically already answered his question... haha...

Extrude-Hone e-mailed me back and asked for more pictures... so I sent them pictures of the three pieces, bottom and top of each. They're going to give me a quote by Tuesday. I expect it'll be in the range of $1,000 to $1,500. They can typically increase the overall port size by a couple of mm (ID), and polish it smooth as well. I will likely do this with the parts from my parts motor, and then do the DAWG mod afterwards.

I'll let you guys know what the quote ends up being...
lou_dias JUL 19, 10:55 AM
I got 187 rwhp out of the stock(ported) intake and I didn't have the DAWG mod and I has a stock upper intake gasket that wasn't cut to match my bigger ports. It peaked at 4500 but made 249 ft*lbs of torque.
I then gimped my future dynos by running wheels that were way heavier than stock because I was racing. See my 22-page thread in the construction zone, much of which was attacked by trolls in the middle pages, but there is still a lot of good info there.

Anyway, the point is - don't believe the bs about the iron-head 3.4...! It can make plenty of power if you rebuild it correctly by simply applying the same advances 'newer' motors now have.
Newer motors use more efficient alternators that come with a clutched pulley that slips at higher rpm. Smaller STOCK diameter crank pulleys. Everything adds up.

New motors use a lighter rotating assembly, roller cams, more compression and bigger valves and they come with bigger intakes and exhausts. That's all I did to make > 200 rwhp. It's not rocket-science and doesn't require some magical heads made from Vibranium like the trolls swear by.

Again, personally, I like using the 3400 block as a base for the roller-cam. It also already has the starter holes in the correct location.

Some people like slapping a big turbo on a junk yard motor to produce 1 big dyno pull and a week later the car is down for another junk-yard motor swap. What is the point of that? It's actually insane.

With a rebuild, you get to enjoy your car for years. In 20 years, I've done 3 rebuilds and raced the car season after season. Meanwhile, some trolls take that long to do a swap. It's pathetic actually.