compression ratio discussion

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brewmenn
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compression ratio discussion

Post by brewmenn »

compression ratio discussion

It seems that most people where they build their stroker try to get the CR close to stock. Why?

Is it just for ease of getting it to run right and run on 87 octane? Or is there another reason that I'm missing?

If I wasn't worried about either, and was willing to have to run 93 octane how high do you think I can go on compression ratio?

Has anyone tried this?
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Re: compression ratio discussion

Post by Muad'Dib »

Good questions .. i would also like to know.
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Shark
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Re: compression ratio discussion

Post by Shark »

i have been pondering these questions myself. in the beginning i was wanting my CP close to stock to run 87 octane mostly because i was planning on putting my stroker in my daily driver. with gas prices the way they are running higher octane in a daily driver is just not worth it economically for me. lately, i have been planning on putting the stroker im building in my trail/weekend rig, so im willing to run a higher octane/CP. obviously, you can run as high a CP as you want as long as you have a fuel with a high enough octane rating. anyone ever run a 4.0 on alcohol?
maybe someone can come up with a CP/octane FAQ to give us an idea of what compression ratios are practical with a given octane rating at standard atmospheric pressure.
'91 MJ 4.0 ax15 Resto-mod street truck project, stroker candidate
'93 XJ 2door 4.0 aw4 np231 7" lift 33's
'95 XJ 4door 4.0 aw4 2" 31's
'95 XJ 4door 4.0 aw4 np231 4" lift 31's RIP
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Re: compression ratio discussion

Post by 1bolt »

I'm also interested in this topic. It seems like no one out there is doing intentionally high CR strokers meant for running on high octane. Probably because even a average low 9's CR stroker will tend to want 93 octain or get part throttle high load ping.

Another side to the subject (a topic all by itself) is E85... you could build a budget 4.5L stroker with the 4.0 rods deck the block and heads end up with some serious compression with nearly ideal quench and the fuel would love it... In fact you'd want to add a blower to up the compression even more... but I guess that's another subject.
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Re: compression ratio discussion

Post by whitey2001 »

I am building my stroker with a compression ratio of between 10.0:1 & 10.5:1 so that I can run e85 or any combination of ethanol and gasoline. I am a member of another group that designs & builds alcohol stills based on the Charles 803 still. I have the plans to build it myself, but local laws prevent me from distilling my own ethanol. I do, however, have a local source for ethanol that I can mix myself with gas. I can get it for ~$1-1.50 per gallon. If that source is not consistent, I can go to the pump for e85 (~$2-2.50/gallon). I intend to keep my CR closer to 10.0:1 so that if I have to use just gas, I still can.
-Jason

'88 YJ 2.5, AX-5, NP231, 4" SUA, 33" MTRs
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Flash
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Re: compression ratio discussion

Post by Flash »

Our 4.0s/ tend to ping at a lower compression then other.........Do to our quench hight.....

Thats the short answer :)
There is a lot more to it then that, and you need to know the rest to make a stroker that you will be happy with.

No time now, maybe later tonight, if some one don't bet me to it.

Normaly i would say look up quench to start with............But this is the section that the compression depth should be dug into deep................................................



Flash
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Re: compression ratio discussion

Post by 1bolt »

There's the thing, lots of drag racers running around in the world with 10 to 1 compression normally aspirated. Is the nominal Quench of the 4.0/Stroker so bad at .043 head gasket thickness? Or is it more to do with the combustion chamber shape?

And please no one say it's aluminum heads, plenty of high CR drag race engines are all iron. Car Craft once did a test of identical Iron and Aluminum Dart heads (identical except for material) they dyno'd within the engine dyno's error tolerances of one another... neither one allowed more timing than the other, and neither pinged... Interesting read.

Incidentally that article is 5% of the reason I will never buy a HESCO's Aluminum head... the other 95% being price :roll:

I'm guessing chamber design has to have something to do with it, that and factory default timing curve.
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Looking for a 232 crankshaft see my want ad: http://www.jeepstrokers.com/forum/viewt ... =17&t=1292
http://www.jeepstrokers.com 94 XJ Stroked lifted locked. 89 MJ restored Work truck, 88 YJ going on third build up and second Stroker.
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Re: compression ratio discussion

Post by John »

Read your spark plugs and you will put even more weight on the factory timing curve.
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Re: compression ratio discussion

Post by Shark »

how does dynamic compression and the type of cam you run factor into this?
'91 MJ 4.0 ax15 Resto-mod street truck project, stroker candidate
'93 XJ 2door 4.0 aw4 np231 7" lift 33's
'95 XJ 4door 4.0 aw4 2" 31's
'95 XJ 4door 4.0 aw4 np231 4" lift 31's RIP
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Flash
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Re: compression ratio discussion

Post by Flash »

1bolt wrote:There's the thing, lots of drag racers running around in the world with 10 to 1 compression normally aspirated. Is the nominal Quench of the 4.0/Stroker so bad at .043 head gasket thickness? Or is it more to do with the combustion chamber shape?

And please no one say it's aluminum heads, plenty of high CR drag race engines are all iron. Car Craft once did a test of identical Iron and Aluminum Dart heads (identical except for material) they dyno'd within the engine dyno's error tolerances of one another... neither one allowed more timing than the other, and neither pinged... Interesting read.

Incidentally that article is 5% of the reason I will never buy a HESCO's Aluminum head... the other 95% being price :roll:

I'm guessing chamber design has to have something to do with it, that and factory default timing curve.
I have run out of time tonight..........but i will get to it tomarrow night......... The tighter the quench the better the cooling of the air, as compressing is heating it up!!! all the way up, untill the piston hits the head :o :mrgreen:


Flash
89 XJ with 300,000 on the original eng

"I've also never completed a motor, yet. My mouth (fingers) is also writing checks my ass can't cash."
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Re: compression ratio discussion

Post by John »

Most excellent explanation, well put, also should be stickied.
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Re: compression ratio discussion

Post by brewmenn »

Flash wrote:Now you have the facts, all you have to do is decide how much compression you can live with
Thats what I'm trying to figure out.

I'm not afraid to spend the money necessary to build it right, and I don't care if I have to run premium gas.
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Re: compression ratio discussion

Post by Shark »

thanks flash
'91 MJ 4.0 ax15 Resto-mod street truck project, stroker candidate
'93 XJ 2door 4.0 aw4 np231 7" lift 33's
'95 XJ 4door 4.0 aw4 2" 31's
'95 XJ 4door 4.0 aw4 np231 4" lift 31's RIP
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Re: compression ratio discussion

Post by seanyb505 »

Flash wrote: Just one more thing to consider if you live at sea level you will not be happy with a simple stroker as it will not run on anything but premium fuel.........and maybe not even then.
Define "simple stroker." Is this basically a crank and rod swap and no machining of the head and block? I live at about 30 ft so this kinda concerns me. I am planning on decking block and head btw.
Now I can be like all those other awesome people with more than one Jeep in their sig, but now I have to say one of them is sold:(
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Flash
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Re: compression ratio discussion

Post by Flash »

My Opinion of a simple stroker is,.....Having a running 258 as a donor and a low mileage 4.0(so you can re-use the pistons and stock cam.)
This is the simplest way, and for those that live close to sea level will make the most power out of the pkg.............BUT will have the most frustration with knock or ping. At sea level ,the air is thicker....If you will......and as you go up in altitude the air becomes thinner(Allowing you to get away with more compression with out ping)

Take a good look at the piston section here, especially the one's that can have the piston dish enlarged. You need to have a larger CC piston bowl(to lower the compression) and then you can deck the block( which will raise the compression back up........But with a much better quench Number)

Option #2 Buy a custom built piston that give you the .045 Quench with the longer 4.0 rods and a SCR around 9-9.5:1

There is many option out there to combat the eng ping or fuel knock........but before you build it........A little extra cash will save you a lot in the long run.

I don't thing any body that live below 4,000 Feet, will be happy with a stock 4.0 piston(with out having the bowl enlarged to lower compression) 258 Rods and Crank.
NOTE: I have not built a stroker as of yet, all of my opinion is from doing my homework and asking lots of question on several jeep board such as this. and learning from other people success stories and not so successful stories......

I encourage you to go look at the thread in here called "Available pistons FAQ"
John has done a bang up job of piston facts..........The most facts on piston and there spec's that i have seen in one place.


Flash
89 XJ with 300,000 on the original eng

"I've also never completed a motor, yet. My mouth (fingers) is also writing checks my ass can't cash."
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