The Dreaded 3L Duke - Discuss In Detail

Blue Shift · Post by **Blue Shift** » Sat Jul 19, 2008 2:10 am

This topic has been beaten to death repeatedly, but I can't seem to find all the minute technical details on what's absolutely required to install a 181CI marine crankshaft into an iron duke. Or the minute technical details as to why it's not remotely feasible within reason.

Before we go further, everybody can probably agree that a stroker duke is an excercise in poor cost to benefit ratio. The reason I'm interested in doing it is to prove the concept just to satisfy my own curiosity. I think of it as a mechanical puzzle, a play toy. If it makes 110 HP, then that's ok as long as it moves the car around.

What I've read:

- Later 1 piece 181 crankshaft needs to have the snout machined ~.750" and the bolt hole pattern re-drilled for the normal flexplate/flywheel pattern. Not too hard to do if you work at a machine shop...

- Earlier 2 piece marine cranks can use a 2 piece to 1 piece RMS adaptor? From Old Europe: "place called Nu-Tech Machine in el cajon/lakeside, ca has the adapter in stock. the guys name is Britt. 411 will have number. area code is 619. i have it, but it is packed away in file in rafters. tell him that charley from Quality Truck had you call him." Though his whole thread sounds fishy to begin with...

- Gerry Dedonis from Kansas Racing Products claims that the rod journals are 1.029" wide, and that inline 6 Chevy rods will work. They're also longer (6.0"), which may or may not be the correct length. However Summit racing lists Chevy I6 rods as 1.050" wide across the big end, though.

- Pistons? 4.030" SBC Chevy pistons may work - various compression heights and dome/dish are available.

- Stock duke blocks are heaping piles of shit, though RWD blocks are supposedly decent. The problem seems to lie with the water pump?

- Whatever else I've missed?

Mach10 · Post by **Mach10** » Sat Jul 19, 2008 11:22 am

You can machine a rod with a wide journal down, from what I've heard.

I remember looking into this with a friend. Since he had a big box of duke performance parts, and another friend managed to split his marine 181 block (forgot to drain in fall, froze in winter), I had arranged a parts exchange

I don't remember the specifics, but from what I recall, it appeared that it would be a monstrous PITA to get the 181 marine crank working in an auto block, but that modifying the marine short-block was more viable. Obviously the heads aren't even close to the same, but it seemed that all that was missing off the Marine block were a few drilled/tapped accessory bracket holes, a strange water pump setup (reverse flow?) and a blockoff for the fuel pump.

All things being equal, I remember the marine block being much, MUCH heavier built than the duke. Bigger webbing, 4-bolt mains etc.

Thing is, I think I might remember someone saying that the 3.0L was a hybrid motor, made of different parts...

I'll talk to him and see if he remembers the details... The dude works graveyards, so he's a little hard to track down.

lucky · Post by **lucky** » Sat Jul 19, 2008 12:09 pm

Ya, marine water pumps are usually reverse flow.

Atilla the Fun · Post by **Atilla the Fun** » Sat Jul 19, 2008 6:14 pm

The trouble with 2.5s is that every 2.5 block has a weak spot. The torque specs for the head call for one bolt to not be torqued as much as the others. So you need to plan on tipping the block up and putting filler in that area, then bolting the head on per specs, with a new gasket, while the filler sets and dries.

Blue Shift · Post by **Blue Shift** » Sat Jul 19, 2008 6:52 pm

Atilla the Fun wrote:The trouble with 2.5s is that every 2.5 block has a weak spot. The torque specs for the head call for one bolt to not be torqued as much as the others. So you need to plan on tipping the block up and putting filler in that area, then bolting the head on per specs, with a new gasket, while the filler sets and dries.

I think the entire 2.5 (FWD) block is a weakspot in and of itself - I was amazed and amused when I pulled the pan to reseal it on my 86 duke - the main webs are seriously like 1/4" thin for most of it with it thickening up to main journal width just a bit short of the main bearing saddle. On a serious note though, the block fill idea ain't a bad idea at all. Got any more details on it?

However... if my stock 2.5 will run to 5 grand reliably, then it should do so with a much stiffer, massive crank that'll resist flex in the first place. Better rods would also be nice so it doesn't pitch one through the block.

Blue Shift · Post by **Blue Shift** » Sat Jul 19, 2008 7:04 pm

So here's my idea, anyways:

Head:
- Find a ~93 S-10 cylinder head and intake manifold, which has been shown to work with later year dukes in the Fiero application. More flow, less resistant to cracking. Dunno about earlier dukes though. Not going to win races, but it'll be a step forward.

Shortblock:
- Get a marine crank - they're quite cheap and plentiful nowdays. I still have questions about both using a later year marine crank and machining the snout down and redrilling the bolt pattern, and using an earlier crank and using a RMS adaptor. Indy, didn't you use one of these? What's the story here? I need to figure out the journal diameter and width - anybody know for sure?

- Determine if Chevy I-6 rods work as claimed, or if machining needs to be done, what, and how much. I've heard of people using Carillo 6" rods, but that'd be way overkill here.

- Determine if SBC or Duke pistons will give acceptable quench and compression ratio. Probably a very bad idea to exceed 9:1...

- Determine if a RWD/S-10 block is indeed stronger/thicker, and what, if anything can be done to use it in a FWD application. A little creative engineering won't be a problem. I've read more than one person who claims to have personally witnessed that there are starter bolt holes on BOTH sides of the S-10 block. People seem to mention problems with the water pump, but nobody has spelled it out in detail.

Tranny: Really steeply geared 125C should do the job ok. 4T440 even better.

Anybody have any thoughts, or have experience with these parts? I won't otherwise know till they're in hand.

Blue Shift · Post by **Blue Shift** » Sat Jul 19, 2008 11:20 pm

I've done some research on the piston and rod deal. My first question is why doesn't anybody seem to use the stock Mercruiser rods and pistons? The math seems to check out...

Stock deck height on the duke is 9.150"
Stock bore is 4.0"
Mercruiser rod: 5.7" long 1.029" wide, .927" pin
Chevy 250 I-6 rod 5.7" long 1.029" wide .927" pin (same as Mercruiser?)
Chevy 292 rod may possibly be 6"?

With the Mercruiser 181 crank (3.6" stroke), rods (5.7"), and pistons (1.625" CH) , you end up with 9.125", or ~ .025" below the deck.

Another idea I saw was Silvolite piston number 3473hc (LS truck engine?), with a bore of 4", and a CH of 1.328", with a 6" Chevy I-6 rod to give a total of 9.128", or ~.022" below the deck. The only problem is that the piston pin is shown as .9451", and the rod pin bore is .927". Hmm...

If it all works out, one could possibly just use the original rods and pistons...

The Dark Side of Will · Sun Jul 20, 2008 12:00 am

Blue Shift wrote:However... if my stock 2.5 will run to 5 grand reliably, then it should do so with a much stiffer, massive crank that'll resist flex in the first place. Better rods would also be nice so it doesn't pitch one through the block.

If you increase the stroke, you're going to decrease the rod/stroke ratio. That combo will increase bore side loading and contribute to putting a rod through the block.

Blue Shift wrote:Tranny: Really steeply geared 125C should do the job ok. 4T440 even better.

Why not a Muncie?

Blue Shift · Post by **Blue Shift** » Sun Jul 20, 2008 1:39 am

The Dark Side of Will wrote:
Blue Shift wrote:However... if my stock 2.5 will run to 5 grand reliably, then it should do so with a much stiffer, massive crank that'll resist flex in the first place. Better rods would also be nice so it doesn't pitch one through the block.
If you increase the stroke, you're going to decrease the rod/stroke ratio. That combo will increase bore side loading and contribute to putting a rod through the block.

Blue Shift wrote:Tranny: Really steeply geared 125C should do the job ok. 4T440 even better.
Why not a Muncie?

You can get 6.0" rods for the Chevy I6 that would probably work... Not sure about using the LS truck piston and what to do about the pin though. The stroke is part of the deal with the marine crank of the non suck, though less so with the 153 crank. Also my car's an auto, and I didn't intend on changing it due to future plans.

I'll probably eventually change it out for a 3.1TGP drivetrain I have in my garage (long life not all that necessary), but just wanted to fuck around with the 3L duke in the meanwhile just to see if I could figure it out.

Kohburn · Post by **Kohburn** » Mon Jul 21, 2008 9:20 am

is this going to cost more than buying the superduty parts from one of the circle track companies that still makes em?

The Dark Side of Will · Mon Jul 21, 2008 9:39 am

No. SD parts are $$$$$$$$$$$$$. Ask Indy.

Indy · Post by **Indy** » Mon Jul 21, 2008 10:01 am

^^^^^

Blue Shift, have you talked to Gerry Dedonis personally? I've posted this in several threads before, but I had KRP make me a 2 peice => 1 piece rear seal adapter. They didn't have any in stock and I had to wait for them to make it. If it wasn't already installed in my block I would take a look and see what it consists of, the best I have is pictures.

http://img.photobucket.com/albums/v87/F ... CF1062.jpg
http://img.photobucket.com/albums/v87/F ... CF0285.jpg

From what I've seen the RWD blocks are heaping piles of shit too. If it's any consolation I was using SBC flat-top pistons in my stock crank - stock rod Duke before the head cracked. There's no reason why SBC pistons won't work as long as the pin diameter is worked out.

Blue Shift · Post by **Blue Shift** » Mon Jul 21, 2008 10:18 pm

Indy, thanks for the pics. Yeah I'll probably just take the crank in to work and machine it down and redrill it. I'm unable to verify whether there was a Chevy Astro 2.5 block that had the side mounted water pump to begin with, so if not, I'll probably just toss the modified crank into a healthy early style block with marine rods and pistons (provided the deck height is the same between 181 and 151). Probably toss on a healthy stock head - I'll take it in to check it out, deck it, and then purely for the sake of humor, give it a valve job using the custom specified cutter we use on the Daytona Prototype LS6 engines. I can just set the cutter to cut the seat so the 45 comes just short of the edge of the valves, and it should work fine. Might even gain 1HP in the process. :la:

Blue Shift · Post by **Blue Shift** » Fri Aug 08, 2008 12:31 am

Anyways, Super Dookie aside, I've been prepping up to rebuild a 90' vin R duke to replace the dying one in my commuter in the meanwhile.

{repost from OE}
So I was at work a couple days ago, taking apart LS6 cylinder heads to do some machine work (I work for a race engine shop as a rookie machinist), when I got a crazy idea. So I asked what they do with the stock springs and it turns out they toss em in a bag and shelf em, never to be seen again. So today after work, I hit the engine build room and took some measurements on the spring machine:

2.5 Stock Spring, very tired after 18 years of use: (All measurements approximate - done with a beater caliper)
ID: .900"
OD: 1.237"
Length: 2.095"

2.5 Stock Retainer:
Intake: ~1.725" installed height
Exhaust: ~1.710" installed height

LS6 Beehive Spring, Never fired:
ID (Seat side): .860"
OD (Seat side): 1.275"
Length: 2.080"

LS6 Stock Retainer (on 2.5 valves with LS6 keepers):
Intake: ~1.745" Installed Height
Exhaust: ~1.725" Installed Height

SPRING PRESSURES VS COMPRESSION (As measured on a nice spring machine, zero already set for LS6 springs):

2.5L Valve Spring (from fully relaxed, includes shim/oil splash guard in place):
Intake, Exhaust
.050 18, 16 (lbs)
.100 19, 25
.150 40, 34
.200 49, 43
.250 59, 53
.300 69, 63
.350 79, 72
.400 89, 81
.450 98, 91
.500 108, 99

Coil bind at .950" total, approx ~.580" from installed height to bind. ~86 lbs on seat (intake), ~161 lbs @ .350" lift

LS6 Stock Valve Spring (from fully relaxed, bare springs, without valve seal/spring shim in place):

.050 24
.100 40
.150 55
.200 71
.250 87
.300 103
.350 119
.400 134
.450 151
.500 169

Coil bind @ .935" total, approx. ~.580" from installed height to bind. ~126lbs on seat, ~252lbs @ .350" lift

So yeah... This is what happens when I get bored after work. I also forgot to mention that the 2 center most stock exhaust valve springs on the cracked head had taken a .100" set! That cylinder head must have gotten very, very hot to make a thick layer of coked up oil and to flatten the valve springs, man. Anyways, LS6 springs and retainers look like they'll physically fit the iron duke head, and it's not like they'll ever come close to coil binding with the dukes wimpy cam lift. The question, is whether the cam (and its 3 bearings) and stock lifters will tolerate so much extra pressure. And unless you're spinning 7 grand, you won't need so much spring, but hey at least valvetrain float would be a non issue! On that note, I wonder what other GM cylinder heads I could install them in...

Atilla the Fun · Post by **Atilla the Fun** » Fri Aug 08, 2008 12:13 pm

so break in the cam and lifters with stock 2.5 springs, then swap in the LS springs. Problem solved.

CincinnatiFiero · Post by **CincinnatiFiero** » Fri Aug 08, 2008 12:34 pm

Come on where are some pics! I've only ever heard of one of these running and the car went up in flames 15 minutes after he started it up. Go for it, this could be really cool. It won't be TGP fast but you'll have a pansy looking Duke that will walk any 2.8L and probably have a lot of torque to play with.

Blue Shift · Post by **Blue Shift** » Sat Aug 09, 2008 10:36 pm

Hey Atilla... Do you gotta break in roller lifters lightly too? I've heard you can swap rollers around, put them from cam to cam, etc without consequence.

Cinci - I'm currently working on a 1990 VIN R 2.5 for my commuter car... Gotta have something utterly reliable for long range (almost 100 Mi/day) commuting. I have pics of the VIN R stuff and its improvements over any of the Fiero dukes (better head, intake, etc).

I figure what I'll do is get up close to a RWD truck block, or even a 2.5 Astro block and see if it can't be made to work reasonably - I'm going to the huge group of auto recyclers tomorrow. I can redrill starter holes (somebody said they even have them on the other side, even), and all that as long as the water pump will clear and can be hooked up to work - this is all assuming that it's worth a shit to begin with.

As for the rotating assembly, I've come down to two choices:

Typical Fiero Owner: Use the stock Mercruiser crank and rods and SBC pistons of 1.56" compression height (stock 350 SBC), or even stock 2.5 pistons if I'm in the mood to see how far I can scatter chunks. The marine pistons at ~1.6 compression height may even work - the stock pistons sit ~.045 in the hole so...? If compression height was accepable, the whole marine rotating assembly would = the ultimate cheapness.

Atypical Fiero Owner: Use the Mercruiser crank (or a SD or forged 151 crank if you can find one), 6.0" Eagle 4340 rods for the Chevy I-6 (should drop right in), and 1.265" Compression height forged pistons (6" 350 SBC application). Bump compression up to what you can run 91 on safely.

I bet that even a shitty 86 Fiero distributor block would handle power levels into the mid 100's, and/or the increased side loading of the stroker crank. I think it was the piece of shit crank and fragile rods that'd lead to parts departing the block when you'd push it.

CincinnatiFiero · Post by **CincinnatiFiero** » Sat Aug 09, 2008 10:48 pm

3L done up with bigger compression, I think more like ~150hp is possible. Trick is flow, vin R head, find a hooker header, and do something with the intake, hell convert it to carb cause that 1" throttle body is doing no one favors.

Blue Shift · Post by **Blue Shift** » Sun Aug 10, 2008 6:09 am

CincinnatiFiero wrote:3L done up with bigger compression, I think more like ~150hp is possible. Trick is flow, vin R head, find a hooker header, and do something with the intake, hell convert it to carb cause that 1" throttle body is doing no one favors.

If one was really bored and crazy, I bet you could adapt a double barrel TBI from a 350 or the like. The engine management couldn't be much more difficult, either.

CincinnatiFiero · Post by **CincinnatiFiero** » Sun Aug 10, 2008 6:51 pm

Go big or go home mate.