So what exactly is the performance problem with the 2.5 and why do so many people say it's a dog? I've been thinking about that and researching the 2.5 for several years trying to wade through all of the misinformation that's out there.
I realized I had to stop thinking about how I would improve on the 2.5 and start thinking about how you make any engine in general make more horsepower. I started thinking about Chevy small blocks. What does it take to get more horsepower out of a Chevy 350? You need free flowing heads, exhaust, a cam and a good fuel induction system.
The problem with the 2.5 is that people can't simply pick up a Summit racing catalog and order the performance parts that are needed. But there are some good parts out there for cheap is you do your homework and know where to look.
I'm going to break this down into sections for each upgrade I made.
First off, the stock swirl port head is a worthless boat anchor. It was made for economy and low end torque. Basically half of the intake port is blocked. The 1980-83 Jeep 552 casting head is a relatively good flowing head and easy to find. I grabbed a rebuilt one off of Ebay for $79. It's a direct replacement for the Fiero 373 casting head. The only thing to note is that one head bolt will need to be replaced with a shorter one. It will drop the compression ratio from 9:1 to 8:3:1 because it has a larger chamber. I was worried about this at first but I will never go back to the original Fiero head.
Yes and no. It completely changed the characteristics of the engine. The stock 300 TBI wouldn't flow enough fuel during acceleration so it ran lean. I changed to a bigger TBI
I'll do a different segment each day and give as much detail as I can.
And I get that, that's why people ask over and over how to get power out of the duke. Not everyone wants or needs 300hp to enjoy a Fiero but the reason not to mess with a duke is the cost vs gain argument. I have been messing with my seized 4cyl fiero and have come to the conclusion it wouldn't be worth pulling the head off vs just buying another duke.
I'll stop now, I am curious so see what's been done and if you have any dyno numbers.
[This message has been edited by jmbishop (edited 02-11-2015).]
Keep the updates coming. I love to hear people working on the Duke. You can get custom pistons, to bump your compression back up, but that is some big bucks. (I know because of my SD4 project)
The 1980-83 Jeep 552 casting head is a relatively good flowing head and easy to find. It will drop the compression ratio from 9:1 to 8:3:1 because it has a larger chamber.
Can you shave the head, or deck the block, or use a different piston to get compression back?
I agree with others here that there is nothing wrong with playing with the Duke to see what you can get out of it, without spending much. My 88 started with the Duke and I found it was a blast to drive even with less than a 100hp.
My goal wasn't to build an SD4. I just wanted better performance without pushing the envelope on reliability.
I worked in the boating industry for many years so I'm familiar with the Mercruiser 2.5. There are no interchangeable parts between the Chevy 2.5 and Pontiac 2.5. It's like comparing a Chevy 350 and a Pontiac 350.
When I originally bought my '84 the engine was ruined. It had been run without oil or either the oil pump had failed. I rebuilt it using Pontiac 301 rods, stock pistons, a high volume oil pump, a rebuilt crankshaft which happened to be an '88 model and a very mild comp cam. If I had to do it all over again, I'd start with a roller camshaft block and have everything balanced.
Anyone who is tired of the valve train noise can upgrade to adjustable valves. You can use Mercruiser 2.5 or Chevy 250 inline 6 rocker arms and late model LS rocker arms studs and it works with the original pushrods. No machining required. The LS studs are 8mm on the head side and 3/8 on the rocker arm side. This is inexpensive and doesn't require much effort. One turn past zero lash will set the lifter plunger at .060.
My goal was to match the horsepower output of the Mercruiser 2.5 which is 120hp. It has the same displacement with a 2 barrel Dualjet carb.
I don't think anyone can break the 100hp mark using the stock Fiero swirl port head.
I couldn't find a reasonably priced Holley 500-1 TBI. But I did find NOS 500-3 TBIs for $100 a piece. But there's a problem. The linkage is different because it is for a GM 1.8L engine. I had to swap the bellcrank off of a Fiero tbi and use it on the 500-3. It fits perfectly.
I bored the stock Fiero manifold to 2" to match the new TBI. I also port matched the intake and head while the intake was off.
And now we come to the part where most people would get disgusted and give up because it runs awful. How many people have you heard complain about the Holley big bore TBIs?
And I get that, that's why people ask over and over how to get power out of the duke. Not everyone wants or needs 300hp to enjoy a Fiero but the reason not to mess with a duke is the cost vs gain argument. I have been messing with my seized 4cyl fiero and have come to the conclusion it wouldn't be worth pulling the head off vs just buying another duke.
I'll stop now, I am curious so see what's been done and if you have any dyno numbers.
THE PROBLEM here, is no one is honest about the TRUE cost of a swap.. I have yet to see a 3800 s/c swap with a listing of cost. from start to finish.. I do know you can't touch a fully dressed 3800 here for under 700.00 and that's not the s/c one.. now add mounts, ecu, harness and all the little stuff that nickles and dimes you to death.. Sure it's cheaper than a S/D4 swap.. but cheap it's not.. the quad 4 swap.. issue lack of engines, building a duke, sure you are not going to break records in power . The beauty of it, is no one wants them, that = cheap.. including parts to rebuild one. The short comings are easy to fix, a block main girdle will fix the block issue.. if that's even an issue, as I haven't seen any photo's of main webbing ripped out of a duke rods are an easy fix also.. How much of a sleeper would a non s/d duke be with 200+hp.. with todays small c.i.d. turbo engines, it be really easy to get past the head design short comings, to get 200+hp out of one sure it not be as refined as todays 4 banger.. but it also be much cheaper.. is it worth it to even bother, maybe, maybe not.. some would say, screwing with a fiero is a lost cause!!! as a c4 vette can do everything better and for the same money.. most times much less!!!!!!! , but that doesn't stop people..
I agree with you about the true cost of a swap, also factor in the wheel/tire and suspension upgrades that are needed to support the 300 hp. but I have absolutely no problem with someone doing one and I enjoy reading about them on here. But I am perplexed why many people ridicule anyone who posts about doing anything to the duke. I appreciate Ventura's input and plan on doing his adjustable valve conversion. I have both a V6 Fiero and a 2M4, they are both fun to drive. I just don't have the tools, the money, the shop space, or the know how to do a swap and I don't want to join the league of unfinished swaps or have a car sitting in my garage gathering dust for months on end.
Originally posted by E.Furgal: How much of a sleeper would a non s/d duke be with 200+hp.. with todays small c.i.d. turbo engines, it be really easy to get past the head design short comings, to get 200+hp out of one
I think I've read there are issues with the cylinder wall strength on the Duke. Some diehards have got about 20% increase in power but that is reported as being the limit before failure. Personally I would love to see if you could put 3 psi boost from a small turbo without a failure, but people always seem to say it can't be done. It's always so much fun the moment your turbo hits the boost, and even 3 psi would be a significant pull. Can the Duke handle this? Who knows...those that have done it always seem to try to put 15+ psi on so of course it doesn't have a hope of lasting.
I think you are on the right path for changes. Small inexpensive steps. It's a learning experience, and you don't need to spend a ton of money.
THE PROBLEM here, is no one is honest about the TRUE cost of a swap.. I have yet to see a 3800 s/c swap with a listing of cost. from start to finish.. I do know you can't touch a fully dressed 3800 here for under 700.00 and that's not the s/c one.. now add mounts, ecu, harness and all the little stuff that nickles and dimes you to death.. Sure it's cheaper than a S/D4 swap.. but cheap it's not.. the quad 4 swap.. issue lack of engines, building a duke, sure you are not going to break records in power . The beauty of it, is no one wants them, that = cheap.. including parts to rebuild one. The short comings are easy to fix, a block main girdle will fix the block issue.. if that's even an issue, as I haven't seen any photo's of main webbing ripped out of a duke rods are an easy fix also.. How much of a sleeper would a non s/d duke be with 200+hp.. with todays small c.i.d. turbo engines, it be really easy to get past the head design short comings, to get 200+hp out of one sure it not be as refined as todays 4 banger.. but it also be much cheaper.. is it worth it to even bother, maybe, maybe not.. some would say, screwing with a fiero is a lost cause!!! as a c4 vette can do everything better and for the same money.. most times much less!!!!!!! , but that doesn't stop people..
It's not hard to do a swap for cheap, especially if you can make your own harness and make your own mounts. A wrecked 3800 car is easily sub 700. Part out a few things then have it hauled off and you can be left with the engine, ecm and harness to hack up for little money.
It's g totally worth doing if you want, I'm excited to see dyno numbers.
The engine would barely run with the new Holley TBI. It wouldn't idle and it bogged and shut off when I tried to accelerate. I had read complaints from others that the Holley required tuning.
I knew from working on the Rochester 300 that the fuel pressure could be adjusted and the adjustment screw for the throttle plate had to be properly set or it would throw codes.
I can't find the exact numbers but I believe the flow rate for the stock Fiero injector is around 40lbs/hr. There's a calculator for fuel requirements and it shows a 100 hp engine requires 55lbs/hr. A 120 hp engine would require almost 70lbs/hr. These ratings are at 13 PSI fuel pressure.
The first thing I did was adjust the throttle plate setting and then I turned the fuel pressure up to 18 lbs. It started and idled perfect. A road test showed a huge improvement but it it would still bog and wouldn't pull past 4000rpms. Increasing the fuel pressure increased the flow to 48lbs/hr.
I believe the key was to match the injector flow rate to horsepower requirements because tuning the ECM isn't practical for me. The ECM is looking at vacuum (MAP) and the oxygen sensor to adjust the fuel. It really doesn't care what size injector is being used.
By changing to a Chevy 5.7 TJ7 injector, I can control the flow rate from 48-68lbs/hr by varying the fuel pressure from 9-18 psi. The holley makes this easy because it can easily be adjusted from the top.
It's not hard to do a swap for cheap, especially if you can make your own harness and make your own mounts. A wrecked 3800 car is easily sub 700. Part out a few things then have it hauled off and you can be left with the engine, ecm and harness to hack up for little money.
It's g totally worth doing if you want, I'm excited to see dyno numbers.
maybe where you are,, but most wrecks goto auction.. and yards around here know what they have.. a 3800 fully dressed is 700.oo s/c 3800 is more.. (I've looked..) a running car that's not wrecked isn't an option.. cars with 150k+ are 2500.oo+
I've looked around my area for the last 2 years.. short of sitting in from of a yard hoping a wrecked 3800s/c was rolling in and offer more than the scrap price..
I've been running the '84 with the TJ7 injector. I had the fuel pressure turned up the first time so it threw a code even though it ran great. I don't have any dyno numbers but it sure felt good. I turned the pressure back down to 13psi. It idles good and the code went away. No bogging. Pulls hard all the way to 4500 rpms. I'm hesitant to push it much harder. Did I hit the 120 horsepower mark? Maybe someone else will try the same thing and tell me what they think. Comparing it to my old Formula, In my opinion it's very close.
I've been running the '84 with the TJ7 injector. I had the fuel pressure turned up the first time so it threw a code even though it ran great. I don't have any dyno numbers but it sure felt good. I turned the pressure back down to 13psi. It idles good and the code went away. No bogging. Pulls hard all the way to 4500 rpms. I'm hesitant to push it much harder. Did I hit the 120 horsepower mark? Maybe someone else will try the same thing and tell me what they think. Comparing it to my old Formula, In my opinion it's very close.
Have you thought of getting this on the dyno? I would be great to have some hard evidence of the results.
I agree, seat of your pants is fine but far from good enough to make any claims. I'd also like to see you do some real tuning to the ecm. Have you looked into Megasquirt?
I just googled "dyno" for the area where I live. Two businesses with dynos showed up. Getting the horsepower results might be a possibility. I'll make some calls tomorrow and get some more info.
Looking forward to the results, after I am done rust repairs I am pulling the rear cradle and power train to clean up/improve.
As far as the motor I will port/polish, change the valve seals, gaskets... I was looking at cams from the fiero store is the increased lift/duration worth the cost?
I just googled "dyno" for the area where I live. Two businesses with dynos showed up. Getting the horsepower results might be a possibility. I'll make some calls tomorrow and get some more info.
missed these,, the last 2 are dead.. another is an 97 isn't that a series one,, the last one a 99 might be promising, but no listing on the miles,, 15 years old, I'm betting it's not low, as granny cars with low miles go for 3-4k here.. still it's 1000.oo not bad, but not cheap either.. I guess I have a different idea on what = cheap. I still have that problem with cutting up my fiero as it's an INDY.. I have an other 2.5 on the cradle waiting on me,, that I Think I'll try to build up cheaply, and add sequential efi.. and maybe some ole school tricks, just to see what can be done, I'd love to get a sd4 head,and epoxy the ports down to something more reasonable,,
[This message has been edited by E.Furgal (edited 02-24-2015).]
My goal wasn't to build an SD4. I just wanted better performance without pushing the envelope on reliability.
I don't want to discourage you, but it will help if you define "better performance" and "reliability." You need quantifiable goals to know if your swap is a success and worth the money. I think it will be an excellent learning experience regardless.
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Originally posted by Ventura: I worked in the boating industry for many years so I'm familiar with the Mercruiser 2.5.... My goal was to match the horsepower output of the Mercruiser 2.5 which is 120hp. It has the same displacement with a 2 barrel Dualjet carb.
As you stated, there are no interchangeable parts. Something else to consider is a Marine engine is much more heavy duty than a passenger car engine. They are designed for different duty cycles. The limiting factor on the Iron Duke, IMO, is reliability. It was extensively lightened to increase economy, which is one reason the factory rev limit is so low. Improving flow and gaining HP shouldn't be too difficult with the right parts - but you're going to quickly run into reliability issues.
I know you don't want to build a Super Duty, but look at the SD project and what parts were carried over and what parts were strengthened. That will give you an idea of the areas to look at to increase reliability.
As you stated, there are no interchangeable parts. Something else to consider is a Marine engine is much more heavy duty than a passenger car engine. They are designed for different duty cycles. The limiting factor on the Iron Duke, IMO, is reliability. It was extensively lightened to increase economy, which is one reason the factory rev limit is so low. Improving flow and gaining HP shouldn't be too difficult with the right parts - but you're going to quickly run into reliability issues.
I know you don't want to build a Super Duty, but look at the SD project and what parts were carried over and what parts were strengthened. That will give you an idea of the areas to look at to increase reliability.
Good luck and keep us posted!
He already stated in a previous post that his goal was 120 hp., I don't think that output increase would have much affect on reliability.
If you buy the holley 500-3 all you need to do is swap the linkage? If so I would definitely buy one.
Also, the injector from the the chevy 5.7, is it a direct swap in the holley?
And if i were to just port/polish would the stock holley supply enough fuel?
The 500-3 is the same as the 500-1 except the linkage bellcrank is different. The 5.7 injector is a direct swap. If you use the stock swirl port head then the stock injector is probably all you need.
I've been driving the car and playing with the fuel pressure. I turned the pressure up some and the check engine light eventually went away. I guess the ECM learned how to deal with it. I also added a few more degrees of timing.
By changing the head it changed the characteristics of the engine. It lost some bottom end torque but gained a lot at higher rpm's. It feels like it hits it's powerband at 3000 rpms. I'm running an automatic. With a four speed it would be a blast to drive.
I ordered an MSD coil and I'm waiting on it. I'm still looking for a Holley or Edelbrock intake manifold.
I still want to do a dyno run. It will probably be next week.
The 500-3 is the same as the 500-1 except the linkage bellcrank is different. The 5.7 injector is a direct swap. If you use the stock swirl port head then the stock injector is probably all you need.
Perfect, I ordered the 500-3 holley!
I am planning on staying with the stock head and just do a port polish job on the head and intake. I was looking just for fun for a 1981-83 duke head and cant seem to find any and the wreckers here in Canada do not keep stuff that old.
I have been doing a lot of reading on dukes and getting some great info from various sites, forums...
I have a 86 duke in my 84 SE and was looking into the fiero store mild performance cams, do you think its worth it?
I am planning on staying with the stock head and just do a port polish job on the head and intake. I was looking just for fun for a 1981-83 duke head and cant seem to find any and the wreckers here in Canada do not keep stuff that old.
I have been doing a lot of reading on dukes and getting some great info from various sites, forums...
I have a 86 duke in my 84 SE and was looking into the fiero store mild performance cams, do you think its worth it?
The nice thing about the duke is that it is so easy to work on.
That's true and I have a few dukes here so lots of spares
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Where did you order the 500-3 from?
Here is the link to the new auction, there NOS seems to come loaded with a new TPS and IAC, like Ventura said you need to swap the linkage bellcrank from your old throttle body. http://www.ebay.ca/itm/New-...em35e5d5114e&vxp=mtr
I have read that the s10 block was stronger due to webs in the casting, also that the starter can be moved to the other side. Also if you get a post 86 it will have the better heads.
So this brings me to could I put an post 86 s10 iron duke mate that to the old intake manifold to bolt the 500-3 Holley and go to town with a 85-88 performance cam and a port polish job.
And one day maybe a small t25 or t30 with a custom manifold with ~5 psi boost.
Opinions? Or am I loosing my mind lol. Its just to have a little fun will not cost to much.
