Compression Ratio / Boost

[Coffee Commando]

I have the Avenger Supercharger and currently have a 4.7 Stroker with a compression ratio of about 9.6.

The pulley with the avenger produces about 5.5 lbs of boost.

Will this require 91 octane fuel at sea level (I live in Florida) ?

At what boost + compression do you need 91 octane?

I actually intend to run higher octane fuel regardless just because it's less likely polluted with Ethanol, or so I am lead to speculate. If I didn't mind running 91 octane, what would be a good amount of boost higher than 5.5 to run?

If I rebuilt an engine and wanted to run 8-10 PSI of boost would I need higher than 91 Octane on a 9.5:1 Compression Ratio?

Is there a calculator for this?

[Muad'Dib]

We need more information... Whats your DCR? What about other engine specs? Quench etc. I take it your currently running regular?

[Coffee Commando]

We need more information... Whats your DCR? What about other engine specs? Quench etc. I take it your currently running regular?

I found this: http://www.kb-silvolite.com/article.php ... ad&A_id=57

I am currently debating whether or not to rebuild another engine and sell off my current 4.7 so I can run a stroker with my Avenger.
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What I intend to run on my new stroker (judging by the math on that site I'm going to need to in order to run 6 lbs of boost) is this:

CC: 251.51 (4.6)
4.12 Litre
SCR: 8.82
DCR: 7.58
QH: .043
Intake Valve Closing: 53.95 (stock 4.0 Cam)
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I just input my stroker build from AJeepThing.Com and it gave me a 10.0 compression ratio.... am I supposed to change the combustion chamber volume?

This is the build (supposed to be a 9.6:1 CR)

4.5L Low-buck simple stroker

~ Jeep 4.2L 3.895" stroke crank
~ Jeep 4.2L 5.875" rods
~ Speed Pro H825CP +0.030" bore pistons
9.6:1 CR
~ CompCams #68-231-4 206/214 degree camshaft
~ Stock 0.051" head gasket
~ 0.081" quench height
~ Ford 24lb/hr injectors with stock 49psi FPR for '96 and later engines
~ 260hp @ 4850rpm, 321lbft @ 3500rpm

[Coffee Commando]

It also gave me an 8.8 DCR.

I run 87 octane fuel since I built it last January and have put a few thousand miles on it... although it does sound like a diesel when it first cranks up and idles.

[Cheromaniac]

I have the Avenger Supercharger and currently have a 4.7 Stroker with a compression ratio of about 9.6.

The pulley with the avenger produces about 5.5 lbs of boost.

Will this require 91 octane fuel at sea level (I live in Florida) ?

Anyone who runs a forced induction engine on low octane gas is asking for trouble so yes, you'd be wise to run at least 91 octane. Premium gas would give you an even bigger safety margin against the engine detonating and it would allow you to run a slightly higher boost.

[Coffee Commando]

I have the Avenger Supercharger and currently have a 4.7 Stroker with a compression ratio of about 9.6.

The pulley with the avenger produces about 5.5 lbs of boost.

Will this require 91 octane fuel at sea level (I live in Florida) ?

Anyone who runs a forced induction engine on low octane gas is asking for trouble so yes, you'd be wise to run at least 91 octane. Premium gas would give you an even bigger safety margin against the engine detonating and it would allow you to run a slightly higher boost.

... I'm looking for trouble. . .. .

[Dezertxj88]

I'm currently deciding the same....sort of.
9.2:1 4.2L mini stroker, .045 quench and a 6-8psi on 91oct vs around 10:1 4.2 mini stroker, .038 quench on 91.
My issue is I already bought my 2229 pistons..which is for the second NA setup, although it'd be easy to dish them or switch to another set. And yes even though many haven't had issues with 6-8psi without engine management..idk if I wanna risk it. I have a safc II in and working already..what were your plans for engine management?
And maybe get some opinions on 6-8psi turbo or supercharged...many are ok without anything but adjusting fuel trims...what say you?


Don't be scared of a small compression bump over stock...If I do the forced induction route, it'd be 9.2:1 for me..with a .042-.045-ish quench height. Idk if yo stated what your is..but the small bump you get from a tighter quench isn't an issue.. What I would do however, is polish the combustion chambers to a mirror finish...super smooth, and also the exhaust runners...get them super smooth!! Even N/a jeeps will benefit from theis...notice polished, not port..unless your comfortable in your abilities..I just cleaned up alot and matched & polished, unshrouded ect..didn't really mess with the ports to much other than that.
This will help prevent detenation, & help get the nasty hot exhaust gasses out quicker. Also since your supercharging..even only 6psi, look at the aftermarket headers, and find one with the bigger primaries..I forget which it is, but stock I believe & most aftermarket are 1.5" OD..

[Coffee Commando]

I also have an SAFC II.

I'm going .043 quench. A website I found listed 8.8 SCR with 6 lbs of boost the perfect (safest) match for 6 PSI of boost. With an engine that already is making 241 HP I'll be happy with the 6 PSI and bottom end torque the supercharger is going to make.

I'm also going to use the stock cam. I know it'll make enough power to make me happy, I'm curious as to how gas-efficient I can make it. I would love to still have 17-20 MPG on over 300 HP with a platuea shaped torque curve.

I was thinking about using the FTC-01B with a 2 bar MAP sensor for a piggyback tuner. But I've been reading about this AEM FIC unit that controls the injectors directly and I'm still not sure what the benefit is over the FTC.

I found a web site that sells the FTC with a supercharger tune (I imagine it's preset for the Avenger, which I have) for $500. Sure I get send in an ECU and have it tuned for less, but wouldn't that all go out the window if I ever reset the ECU with the battery disconnect/ECU resetting procedures with the headlights? I'm not sure. Hence I want the piggyback unit so I never have to worry about it.

I also have an Innovate wideband 02 sensor with the LC-1 programming unit so I can log my readings onto my laptop and make changes accordingly.

I will also be installing the 180 degree thermostat and using Dino's 4K ohm resistor setup to fool the engine into cooling faster so I can maintain a nice static 180 (cooler engine, less room for detonation and better performance). I also have electric fans to run in tandem with it as well.

Given that I would feel safe running 8PSi later on after I had the engine broke in and running well.

[Cheromaniac]

... I'm looking for trouble. . .. .

1. Why buy trouble when you can get it for free?
2. Even if you don't find trouble yourself, trouble will always come looking for you so it's best to make sure you're well-camouflaged.

[SilverXJ]

You don't want the SAFC II for boost.

[Coffee Commando]

You don't want the SAFC II for boost.

It's not for boost. It's simply for getting perfect closed-loop AFR in open-loop mode. That's all it's for. It doesn't do anything until you hit the defined WOT % you set for it, then it alters the MAP sensor reading according to your adjustments. You use it with a wideband 02 sensor reading so you can get better gas mileage.

With the way gas prices are it would pay itself off in a year.

[Coffee Commando]

... I'm looking for trouble. . .. .

1. Why buy trouble when you can get it for free?
2. Even if you don't find trouble yourself, trouble will always come looking for you so it's best to make sure you're well-camouflaged.

Trouble doesn't find me. I find it. Frequently. When I want to how I want to, when trouble tells me to and how it tells me to.

[SilverXJ]

It's not for boost. It's simply for getting perfect closed-loop AFR in open-loop mode. That's all it's for.

I know how it works, but you don't want closed loop AFR in Open loop.

[Coffee Commando]

I see.

12.4-12.7 is what you want for a typical boost AFR at all times correct? Even with only 6 PSI?

[SilverXJ]

I would go between 12-12.5, depending what the engine likes