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Eaton M90 supercharger 258 low comp questions |
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71hornet6
AMC Nut Joined: Nov/18/2012 Location: Oakland, CA Status: Offline Points: 433 |
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Posted: Jan/13/2015 at 2:16pm |
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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 |
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'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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Ant
AMC Addicted Joined: Apr/03/2011 Location: Lapeer, MI Status: Offline Points: 1145 |
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What are u going to use to tune it?
I'm here if the need arises. |
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73 AMX project 401 stroker, t-56 mag
Full-time machinist |
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232jav3sp
AMC Addicted Joined: Jan/09/2013 Location: Texas Status: Offline Points: 2451 |
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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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imacarfan2
AMC Addicted Joined: Jul/01/2007 Location: Northeastern MA Status: Offline Points: 971 |
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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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67 Ambo conv
68 AMX 68 Ambo 4 door 70 Ambo 4 door (2) 71 Hornets (1 SC/360) 73 Hornet 2 door 77 Matador coupe 77 Sportabout 78 Concord 2 door 81 Concord wagon |
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gntim
AMC Apprentice Joined: Oct/30/2011 Location: northern VA Status: Offline Points: 80 |
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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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71hornet6
AMC Nut Joined: Nov/18/2012 Location: Oakland, CA Status: Offline Points: 433 |
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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.
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. |
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'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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71hornet6
AMC Nut Joined: Nov/18/2012 Location: Oakland, CA Status: Offline Points: 433 |
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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 |
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'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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71hornet6
AMC Nut Joined: Nov/18/2012 Location: Oakland, CA Status: Offline Points: 433 |
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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. |
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'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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Ant
AMC Addicted Joined: Apr/03/2011 Location: Lapeer, MI Status: Offline Points: 1145 |
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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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73 AMX project 401 stroker, t-56 mag
Full-time machinist |
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gntim
AMC Apprentice Joined: Oct/30/2011 Location: northern VA Status: Offline Points: 80 |
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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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