The Iron Duke Resource Site



Welcome to my newly redesigned Fiero 2M4 resource site.  The original site recorded over 12000 visits,  hopefully this one will also prove as useful.
Ira Crummey

Iron Duke Performance FAQ:
Here are some interesting incites into the Iron Duke's performance potential.

by: Gary Ohst

>> I was hoping anyone could tell me what the difference is between the SD valve cover and the stock 2.5. Specifically is the bolt pattern the same and is there room for needle bearing rockers?<<

The bolt pattern is the same for all years. There is plenty of room for valve train mods. That cover is too tall to fit under the stock air cleaner intake housing. There are no provisions for a PVC valve port. It would require some mods to fit on a production motor with all emission control hooked up.

>> Well I finally found the supplier of the Super Duty Block online. Check it out at:<<
Doug Thompson took over production of the SD block when GM dropped the ball. You should know that the cost is prohibitive at around $2,500 each. They are actually stronger than the original SD block and can accept a variety of cylinder heads. Buying a complete used SD race motor and de-tuning it for street use would be far more economical.  Aries pistons makes an aluminum SD block for around $4,000 if you think $2,500 is bad. By comparison, the last new SD iron block I bought from GM cost $850.

>>I wonder if the block, crank, cam and connecting rods will work with stock type pistons and a stock cylinder head?<<
Most parts interchange, but... The SD4 cranks have 2.10 rod journals (SBC). The stock crank is 2.00. If you run a SD crank you need to change the rods. The rod length and crank stroke influence the compression height spec of the piston. If you increase stroke or change rod length, new pistons are required. All SD heads use 1/2 head bolts.  All production heads use 11mm. You need to drill and re-tap production blocks if you run a SD head. SD blocks already have 1/2 head bolt holes. SD blocks also have all the stock mounting bosses, so you can bolt up all the stock accessories if you want.

>>I know this sounds like a "halfway" job but my reasoning is simple, use these components with about 9:1 or 10:1 compression, at about 2.7-3.0L, do a mild porting job on the stock head, install lighter valves and stronger valve springs with better rocker arms (rollers maybe).<<
The production "swirl-port" intake ports are restrictive. The first thing you should do is hog out the stock intake ports or use a SD head.  The next weak link is the crank. The S10 truck block and 1988 crank with center oil pump drive gear are the strongest stock parts. If you add displacement with a larger stroke SD crank, you are into a SD head to match increased flow requirements. An SD head on a production block is a far better match than a production head on an SD block.

>>I have looked into various options for my engine rebuild. A local machinist I visited, who builds racing motors said that the problem with the Duke is not necessarily the bottom end but the head. It is restricted, thin, and prone to cracking.<<
Yes. They are OK for performance street use if you open up the intake port and check for cracks.

>> So, maybe the way to go for a half and half is to leave the bottom end basically as it is and go for the upper.......<<
The bottom end is good for 150+- HP. Maybe 180 with the right components (88 crank/S10 block). The duty cycle goes down as power output goes up. Don't expected it to last like a 100 HP production motor.

>>I need facts here, not rumours or speculation. What is the problem with the Iron Duke regarding the 5000rpm ABSOLUTE rev limit I have heard mentioned.<<
It was a low-cost, entry-level power plant for a lot of GM cars, not just Pontiac Fieros. The crank was made as light as possible for throttle response and fuel economy. It was designed for low performance cars and low performance driving. The stock crank is cast and weighs 30-35 lbs. The Super Duty counterpart is forged 4340 and weighs 50 lbs.  The SD motors started to blow up at about 9,200 RPM according to Vanderly. These were under controlled dyno room conditions. Don't know of hard data on production crank limits, but sometimes they flex and cause rod bearing failure before they break. If GM "rated" it at 5,000, it can go higher. Just like the Getrag 5-speed is "rated" at 200 ft lbs torque, but handles more than that. You are cutting into life expectancy when these guidelines are exceeded. GM has warranty concerns in mind when they "rate" parts.

>>Is there an inherent problem with the crank and bearings? The block appears to have five well supported main bearings (even if the main caps are not exactly "beefy"). The connecting rods are reasonably long so they should not suffer from angularity problems. With a good forged crank and good quality connecting rods (tested so as not to be flawed as many of the early ones were) will this engine hold together.<<
There is nothing wrong with the design. It was just optimized for economy and not performance. Of the bazillion 2.5 liter motors made, how many saw performance driving?. If you were GM, how strong would you have made the production crank? A forged crank supports over 200 HP.  The problem is that while they are out there, they are near impossible to find these days. Only option is a billet crank from SCAT for $1,800.

>>Are people making to big a deal about this engine being weak because Pontiac ran the SD program to build a racing engine and made the stocker look inadequate or is there real cause for  concern.<<
Because it was designed as light as possible on purpose, it runs into performance limits sooner than other motors. The same rules apply to other motors, more power and higher rpm equals stronger components. The SD program came about because GM knew they could not get where   they wanted to go modifying production hardware. 150 hp is not a bad goal for four cylinders.  How many eight cylinder SBCs do you know of making 300+ HP without upgraded internal components? The difference is there is a sliding scale of performance hardware for the SBC. The Iron Duke is black or white, SD or not.

>> Thank you very much, you have cleared up many of the unanswered questions on  the 2.5. If I am reading you correctly, a good 2.5 block with no cracks, properly cleaned up but not overbored, combined with a good stock crank and good quality connecting rods. (I guess an 88 crank will not fit the earlier block, an 84)<<
Yes, the 88 crank will fit. The extra mass of the oil pump drive gear adds strength to the unit, but fits in all blocks. The major dimensions are all the same. Just be sure to use a 84-87 cam with the oil pump drive gear. The 1988 cam has no distributor or oil pump drive gears.

>> A cam designed for better mid range power (2000 to 4000 rpm)  a porting job on the stock head (check for cracks first, don't remove too much material), an exhaust header and a holley intake (if you can find one) or a stock intake with port matching. Combine with a free flow  exhaust system and the stock electronics would I be reaching 120hp (with a 5000rpm redline)?<<
Porting the head is most important. In the area under the intake valve in particular. Holly TBI & manifold will not add much. Stock intake manifold/TBI flows enough. You may need a higher pressure fuel pump and adjustable regulator. Use more fuel pressure to adjust for VE improvements in the engine. That saves messing with the ECM fuel maps.

>>Would I have a strong running motor capable of hauling the tall  3.32 ratio four speed at a relaxed highway speed with some power left for passing, and decent reliability?<<
It is harder to get more torque at the same rpm than it is to get more hp at higher rpms. If you want a lugger, then a stroked SD crank may be what you want. 125 HP will work on stock bottom end components in rebuilt condition, but peak HP will be at higher rpm than stock.

>>I was on the MSD ignition website today and found that they have a new super duty billet aluminum distributor.  The description mixes Iron Duke with SD. Are distributors interchageable?<<
Nope.  It's a "dumb" system that can't interface with the variable timing feature of the stock ECM.  It's just has a mag pick-up that matches the standard MSD modules.  Nice hardware, but only necessary for higher rpm use.  Just upgrade the coil on the stock ignition as the first step.  Unless you increase compression ratio or spin higher rpm, the stock system design is fine. Money is better spent elsewhere.

If they were to say it is "compatible", you should be suspect.  Since they say it only works with their module, you pretty much know for sure there is no way it will interface direct.

>>This distributor is designed to be used with their ignition controller (such as the #6).  I have seen other cars with aftermarket ignitions.  I understand the principle of multiple spark and cylinder effects.  Could not get a straight answer about emissions control/ECM/TBI etc......   How does installing one of these affect the computer control of the car?<<
It does not interface directly. See above comments.
I am not a big fan of MSD, but their marine grade stuff may be Ok.  My street driven beater car uses the stock system w/upgraded coil.  No reliability problems, plus it's emission legal that way.
I do run a simple Adrenaline CD system on another car rated at 180mJ (mil joules) per spark. If you look into it, that's a lot of spark energy for a single spark system.  The mJ spec is most important, but often never published by ignition peddlers... I wonder why??  Racer Wholesale used to sell these systems a few years ago.  The Adrenaline brand was nice stuff with mil-spec components, but I don't think they sell direct under their own name anymore.
The trend is to get rid of the distributor, get rid of the spark plug cables, and now to focus on a better spark event.  For a peek into the future of automotive ignition systems, check out the Adrenaline web page.  Probably a few more years before these new systems become cost effective and a mass marketer picks up the design.
And also:
Gary Ohst
Crank_Compare.jpg (141269 bytes) A 2.5 liter, 3 inch stroke 1988 production crank (left) next to a 3.0 liter, 3.625 stroke SD crank (right) so you can see the significant differences. (click on image for larger version)
Disclaimer:  These suggestions come from Fiero owners, I, however have not tried out all of the suggestions personally.  If it sounds feasible,  is not unsafe or damaging to the vehicle,  and does not involve any great expense I will include it in this section,  but remember, you are the ultimate judge of what you consider a reasonable modification.

Ira Crummey