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9.2:1 CR can I run 89 octane?

mjdriver

NAXJA Member #1376
Location
Myrtle Beach, SC
simple question.
 
don't know for sure but in my TA 9.5:1 i run only 93 plus 103 octane booster for the streets nothing above 5k if i run no booster it starts detonating at around 4k on the track 110 racing fuel 7.5k shifts.
so i'd say with very very light peddle nothing over 3k you should be ok
 
Transam something sounds screwed up if you need all that octane to keep it from knocking.

I have 9.5:1 on my stroker on 87 and NO knocks at all. I have 10.1:1 on my CJ on 87 and again...nothing knocking. Ran a 10.5:1 401 big block and still no knocking on 87. 11:1..yep it pinged a bit.

I'd say the margin has been about 10.5:1 and above, you want 93 or better octane.

Then again, I don't race much. Have just built about 35 motors for racing and such in my life.
 
Bouncy said:
Transam something sounds screwed up if you need all that octane to keep it from knocking.

x2 I can run my car on 91 (although I usually use 93) with no knocking and it's 11.5:1 CR
 
My Stroker runs about 9.25:1 CR, and the minimum I can run is 89 octane. I still get some pinging at low rpm's if I punch it. The 89 octane runs perfect during the winter when the temp is much colder. I usually run premium 91 octane, but the 89 is cheaper than 87 around Wisconsin. Damn ethanol is mixed into the 89 octane.:tear:

Oh yeah, my quench is quite wide at .095".
 
Slo-Sho said:
Here's another simple question; What is the quench height?

x2

If your quench height is where it should be, you should be able to get away with it. Mines a little more than it should be and I get slight pinging when I get on the gas real heavy from a dead stop.
 
altitude is a factor too, and I'm not sure what part of Oregon you're in - I run all the time on 85-87 octane in my 9.5 CR stroker, but I live at 6500 feet...
 
astoria is sea level.
 
streetpirate said:
astoria is sea level.
Then I don't think I would give 87 octane a try if I were you...:gee:
 
Slo-Sho said:
Here's another simple question; What is the quench height?


Basically Quench hight is the distance between the flat top of the piston and the flat deck suffice of the block(at TDC) +.....head gasket thickness.

Do a search on Quench on here and the net, theres several explanation much better then mine.

How much compression you can get away with, has more to do with then Quench.


Once you under stand SCR (STATIC COMPRESSION RATIO) DCR DYNAMIC COMPRESSION RATIO) QUENCH AND HOW ALTITUDE CHANGE THE CAPABILITY TO PING OR NOT TO PING THEN YOU CAN COME UP WITH THE MOST COMPRESSION RATIO............rearch these and then you can make a educated guess at what it will hold.

From all the reading and commenting on this from for the last year on this subject and the real world experienced of many here............9.1:1 to 9.5:1 is were you should have your goal.......If I was you.

Just my .02

Flash.
 
My stroker has a 9.25:1 CR with a 0.088" quench height and I have no problem running 91 octane in this 110+*F heat. We don't have 89 octane over here but 87 has recently been introduced, so I could try a 50/50 mix of 87 and 91 to see how the engine behaves.
 
I am just trying to figure out which stroker plan to build off of dinos page.

I like the 4.5 poor man simple stoker for the horsepower and that it uses the stock cam, cause I hear about all these cam failures but, the 4.6 creates less hp and more tq but at a lower rpm and it has an aftermarket cam.

I really wanted to do the 4.6 option 2 cause of the power output, but premium fuel, aftemaket cam and needing to mess with the fuel pressure kinda scare me a little( cause I don't want to get in over my head.
 
If I remember right, the 4.5 poor man stroker has 9.7:1......and thats if you don't shave you head or deck your block...........But it is the least expense way to get you more torque, and power to turn them 35's. With a stock cam you wouldn't need to worry about any of that, add on stuff. But you would have to run the higher grade fuel

With the added stuff(map adj,fuel pressure adj,......programmer........and that your Renix has a knock sensor to help with the timing when you are running poor grade fuel. you might......Might get away with it.

My self I'm leaning toward using a piston that can have the combustion chamber, of the piston,(Piston dish) enlarged to lower the compression down to 9.5:1(like a couple have done here now) w/ the thinner head gasket to help on Quench.

Maybe 9.7:1 was with the thinner gasket(.043/.045)....I'm sure Dr Dyno will straighten me out.

Flash.
 
Flash said:
If I remember right, the 4.5 poor man stroker has 9.7:1......and thats if you don't shave you head or deck your block...........

No. That's the 4.5L simple low buck stroker with the Compcams 68-231-4 cam and matching valve springs. The 4.5L "poor man's" simple stroker has a 9.2:1 CR with the stock cam and valve springs.
My stroker is a 0.020" overbored version of the "poor man's". It put down 198hp/254lbft to the rear wheels on a Dastek load-type chassis dyno, which is equivalent to 245hp/306lbft at the crank. Desktop Dyno 2000 predicted 249hp/303lbft at the crank so it was very close.
Flash, you can refresh your memory by going to this: :)

http://www.jeep4.0performance.4mg.com/stroker.html
 
Last edited:
Dr. Dyno said:
No. That's the 4.5L simple low buck stroker with the Compcams 68-231-4 cam and matching valve springs. The 4.5L "poor man's" simple stroker has a 9.2:1 CR with the stock cam and valve springs.
My stroker is a 0.020" overbored version of the "poor man's". It put down 198hp/254lbft to the rear wheels on a Dastek load-type chassis dyno, which is equivalent to 245hp/306lbft at the crank. Desktop Dyno 2000 predicted 249hp/303lbft at the crank so it was very close.
Flash, you can refresh your memory by going to this: :)

http://www.jeep4.0performance.4mg.com/stroker.html


What Rods are you Using in Your stroker?? 4.2L or 4.0L???
 
4.2 crank, 4.2 rods, and +0.020" 677P pistons. The rods were shotpeened and weight-matched (695g each), and I used ARP 112-6001 rod bolts. Here's my stroker build site entitled "From Junker to Stroker":

http://www.angelfire.com/my/fan/stroker.html
 
I can tell you that filling up with 91 in CA equals another 5 to 6 bucks a tank, so why bother, unless you are seriously strapped for cash?

In which case, why would you be buildnig a stroker? Just run the good stuff.
 
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