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Eaton M90 supercharger 258 low comp questions

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71hornet6 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71hornet6 Quote  Post ReplyReply Direct Link To This Post Topic: Eaton M90 supercharger 258 low comp questions
    Posted: Jan/13/2015 at 2:16pm
Hi I'm going to be slowly building up a budget supercharged 258 engine with junkyard parts. I'll be following the basics from other 4.0L builds with the Eaton M90 from the GM 3.8L V6, but there are a few differences with this one. I want to add more boost by using a low compression engine and the engine will need to be converted to serpentine belts.

I have a low compression 258 from a '75 Jeep. I'm hoping it has 8:1 compression but I don't know a good way to know for sure without taking the head apart and measuring everything
-there are 8.0:1 and 8.5:1 Jeep 258s from this time and I don't know which it is
-it is a remanufactured engine and could have different pistons - sometimes they put in flat tops like Silv-O-Lite
-it has a '91 4.0L H.O. head on it which I think might raise the compression
Anyone know what compression ratio it might have? If it's 8:1 I can have roughly 10-12lbs boost with a 3.7" pulley

Second part is I need to convert the engine to serpentine belts. To adjust the boost, you need precise pulley sizing and the pulleys for the Eaton in .1" increments are in serpentine.
For this I want to get a '99 4.0L engine for the belt system and the larger volume intake manifold. The only problem with swapping the belts over to the 258 that I know of is the need for a crank snout spacer for the harmonic dampener, pictured at the bottom. Anything I'm missing? Will the 4.0L water pump swap over?
In the 4.0L picture, the supercharger will be where the alternator is and the alternator will take the place of the A/C pump.

To complete the package I think I'll get around 32lb injectors and a throttle body from a 4.7L V8 Grand Cherokee which is 67mm.




Belts from this guy




Edited by 71hornet6 - Jan/13/2015 at 4:30pm
'71 FrankenHornet:
'72 258 '95 4.0L Head Holley 390CFM
AX15 with R154 gears 5 speed Hurst shifter
AMC20 rear axle Detroit TrueTrac
Dual Exhaust
Concord Discs Spirit AMX sway bars
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ant Quote  Post ReplyReply Direct Link To This Post Posted: Jan/13/2015 at 6:00pm
What are u going to use to tune it?
I'm here if the need arises.
73 AMX project 401 stroker, t-56 mag
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 232jav3sp Quote  Post ReplyReply Direct Link To This Post Posted: Jan/13/2015 at 7:33pm
Cool project! I've thought about do forced induction on the old 232 from my car. I don't know if it will help, but, you can look into what these guys have done: http://momentumms.com/Momentum_Motorsports/MOMENTUM_MOTORSPORTS.html

You could always use a Megasquirt to tune it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote imacarfan2 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/13/2015 at 7:38pm
Just in case you didn't know, you could get a serp belt setup on the 258 in Concords in 1983.

Good luck with the project! Will be watching!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gntim Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2015 at 2:17pm
that blower and intake pipe are from a Ford. The Buick has the air inlet in the rear of the blower. I wouldn't push more than 7 pounds without an intercooler and especially with cast pistons.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71hornet6 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2015 at 2:22pm
Originally posted by Ant Ant wrote:

What are u going to use to tune it?
I'm here if the need arises.

Hey Ant! Now I'm trying to build an engine worthy of that transmission I'm building! It's a slippery slope

Jeep dudes say the cheapest is to get a 1995 Cherokee/Wrangler computer system because it's OBDII I think and the smog stuff is simpler.
Modern F.I. engine systems are supposed to be self-adjusting based on the reading from the first O2 sensor.
That would be the cheapest.
Of course, since it doesn't need to be smogged, I might want an aftermarket computer for better tuneability and also if I feel the need to run it a little rich etc.

Originally posted by imacarfan2 imacarfan2 wrote:

Just in case you didn't know, you could get a serp belt setup on the 258 in Concords in 1983.

Good luck with the project! Will be watching!


The reason I want to use the 4.0L setup is because I want that manifold pictured and the power steering pump is bolted to it which is pretty slick.
'71 FrankenHornet:
'72 258 '95 4.0L Head Holley 390CFM
AX15 with R154 gears 5 speed Hurst shifter
AMC20 rear axle Detroit TrueTrac
Dual Exhaust
Concord Discs Spirit AMX sway bars
Back to Top
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71hornet6 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2015 at 2:59pm
Originally posted by gntim gntim wrote:

that blower and intake pipe are from a Ford. The Buick has the air inlet in the rear of the blower. I wouldn't push more than 7 pounds without an intercooler and especially with cast pistons.


I'm trying to figure out a reasonable budget/performance compromise.
I got the engine for $250, the supercharger for $250 and the 4.0L would be $250.
It's hard to track down forged pistons for a 258 but they seem to be $500. I'm not sure if that's worth the additional 3 psi boost. I would have to do some calculations.
Do you think forged pistons and a cold air box intake would be enough for 10psi? I'm shooting for about 300 HP
'71 FrankenHornet:
'72 258 '95 4.0L Head Holley 390CFM
AX15 with R154 gears 5 speed Hurst shifter
AMC20 rear axle Detroit TrueTrac
Dual Exhaust
Concord Discs Spirit AMX sway bars
Back to Top
71hornet6 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71hornet6 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2015 at 3:28pm
Every time I look up boost versus compression ratio, there is a different formula and different numbers to be entered.
Also there are many varying opinions on the max final compression ratio with pump gas.
Here in CA we have 91 octane max. One chart I looked up said
8:1 with 10PSI boost is 13.4:1 which seems to be the absolute max that can be had with careful tuning as people have gotten away with that. Being that this is a junkyard/budget supercharged engine, I think 8 psi is much more reasonable for the scope of this project. These are rough numbers based on a simple chart
8:1 with 10PSI is 13.4:1 (MAX)
8:1 with 8PSI is 12.4:1 (BETTER)
8:1 with 6PSI is 11.3:1 (EASY)
But the engine could be 8.5 or 9:1 with the piston and head combo.
8.5:1 with 6,8,10PSI is 12.0, 13.1, 14.3
9:1 with 2,4,6,8PSI is 10.2, 11.4, 12.7, 13.9
I'm going to need to figure out the exact compression ratio of this engine. Jeep 4.0L guys with 9:1 compression (edit 8.8) run this supercharger with 6 PSI, stock pistons and no intercooler with no problems. With 8:1 I should be able to run 8 psi assuming the pistons can handle that.
'71 FrankenHornet:
'72 258 '95 4.0L Head Holley 390CFM
AX15 with R154 gears 5 speed Hurst shifter
AMC20 rear axle Detroit TrueTrac
Dual Exhaust
Concord Discs Spirit AMX sway bars
Back to Top
Ant View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ant Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2015 at 6:45pm
I think with an intercooler and proper tuning you should be able to get some good numbers with cast pistons.
I would look into megasquirt for tuning.
If I were you I'd invest in forged pistons so that you don't have to limit yourself nearly as much. Nice security blanket.

Edited by Ant - Jan/14/2015 at 8:39pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gntim Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2015 at 8:04pm
Your biggest enemy with forced induction is detonation. You can rattle a non-boosted engine for a long time before destruction, not so with a boosted motor. Tuning is the key. I admire your ambition, but you are in for a LOT of custom fabrication; adapting the blower, ductwork, belt drive, ect. Then, you have to figure out fuel management and timing. Basically-this is not an easy or cheap build...
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