CR vs Octane Rating, thgouhts?

[jammin97jeep]

Sup guys, its my first post so dont flame me too hard. I got a 97 tj that Im swapping in a 95 block into. I have all these 4.2L parts so Im looking to run this set up but Im curious what gas id have to run with the 9.7 CR? What gas do you guys get away with and is there any knock/ping? Id prefer to run 87 gas but my 95 engine is in great shape so Id also like to avoid the .030 overbore pistons and machining costs. Im doing this on a poor ass kid college budget. Thanks for any help you can give me!

4.5L Low-buck simple stroker
Jeep 4.2L 3.895" stroke crank
Jeep 4.2L 5.875" rods
Jeep 4.0L 3.875" standard bore pistons
9.7:1 CR
CompCams #68-231-4 206/214 degree camshaft
Ported HO 1.91"/1.50" cylinder head
Stock 0.051" head gasket
0.081" quench height
Ford 24lb/hr injectors with adjustable FPR or MAP adjuster for '87-'95 engines, Ford 24lb/hr injectors with stock 49psi FPR for '96 and later engines
264hp @ 4900rpm, 322lbft @ 3500rpm

[Flash]

Your question is not a YES, NO, or Maybe Question.
Search on here(and the internet) for SC(Static Compression) and DC (Dynamic Compression)

Once you understand them 2 turms(or the differance between them) You will be well on you way to answering your Queston.

Then Click on the "Stroker Compression Ratio Calculater" (at th etop Right hand corner of this page) and play around the DC/SC button. Have some fun with it.

This will answer a lot of queston, and Create more

Theres a grate bunch of guys on here, willing to help. There is also a lot of thread, on here, that have lots of grate info as well.

O and welcome aboard

[jammin97jeep]

Thanks for the kick in the right direction. I read up on SCR and DCR here at http://www.empirenet.com/pkelley2/DynamicCR.html which I found very helpful in explaining this stuff. After putting my values into the CR calculator, I got these:

SCR 9.79:1
DCR of 8.54:1

The article I read said a DCR from 7.5 to 8.5:1 is desirable for a street engine. My build's DCR of 8.54:1 is pretty close, so what octane gas does this put me at? Thanks again for any help!

[Muad'Dib]

With 8.5 dcr id say your in premium range... but this also can be effected by your quench. If your quench is higher than .051, then you maybe want to drop down to about 8-8.2 dcr IMO. Altitude can also be a factor here.

[gradon]

I'm at 10:1SCR & 8.57DCR w/ .062" quench and have to run 93 octane. That article stated that 7.5-8.5 dcr is desirable for 91 octane(premium--some stations only have 87, 89, and 93). I agree with shooting for 8.25.

[Flash]

That link is one of the best ones i found on the net that i have found as well. that calculate at the bottom of the page is on my desk top.

There is another calculator out there that figures in "altitude" as well as DC. You don't have to download it like the one at the bottom of the page(the sight you posted up above) It on the sight and you add you info just like you do here at Jeep Strokers. Except that it has one more spot to add you altitude above sea level.
I live at almost 7000 feet above sea level and plan on having a vary tight quench height and 10 to 10.5:1(SC) compression with my stroker.
IF you live at sea level you my not be able to get a way with 8.5(SC) and then add Intake valve close time.................(DC) and you find one cam that will only except 93 octane and changing to another cam you can run 89 with the same SC ratio.

Now start doing search on "quench" hight and you start to realize that building a stroker is like climbing a tree,........the trunks is simple but as you start to climb there is many, many branch to choose from and one will be right one for you

there is plenty on the net about this but here on Jeep Strokers, is lot of grate thread that are rapped around are motors and the quench problems that can be addressed.

Our FAQ has some grate info as well, here is the link to make it easier.

http://www.jeepstrokers.com/forum/viewforum.php?f=34

[jammin97jeep]

Sweet guys thanks for the helpful hints. Ive been reading a ton but I needed to ask some stuff to direct my research. Thanks again! This is a great site

[Flash]

here yeah go, found the one with the "Target Altitude" Dynamic Calculator. Just more toys to play with

http://www.wallaceracing.com/dynamic-cr.php

[jammin97jeep]

hey guys, ive been doin a ton of research and playin in calculators and have a follow up question for my build. Im still going with the 4.5L stroker build listed above with either the CompCams 68-200-4 or 68-231-4 cam (they produce the same numbers in the calculator but Im going to base my final decision here on their hp and torque curves).

Again, im doing this on the tightest budget possible. Im in college so I dont drive much, and once I graduate and get a real job Ill be buying a car as my DD. So that being said, I did some cost analysis on machine work vs paying extra in gas money for 93 octane. By getting machine work now so I can run a lower octane gas, I wont even out for at least 10 years worth of me driving. If the block needs work, Ill get it done but if its good, im going with it.

Soooo, heres my question. Is this build with 9.71 DCR and .0645 quench height alright to run on 93 octane gas? Is anyone successfully running numbers like these?? Also, all my driving is at sea level. Thanks!!

[Muad'Dib]

With that high of a DCR and a loose quench, your in racing gas territory. Especially at sea level.

[jammin97jeep]

OOPS!! TYPO!!!

DCR 8.71 and quench .0645

Sorry! How about with these numbers?

[Muad'Dib]

Still pretty high. With your requirements to stay with pump gas at sea level you should shoot for a dcr of about 8. At least with that quench. If you tighten up your quench, you may be able to run 8.3-8.5dcr .. but its hard to say at sea level.

[jffffl]

I think you need to get your quench tighter, to me ideal is your gasket thickness.
Our stroker (sons DD) has a thousand miles on it, its a 4.0 long rod, K&B944 piston,
Comp ...231-4 build. The DCR is 9.45 : 1, SCR 8.29 : 1, and the quench is 0.525,
should have been a bit tighter. We are at sea level, started with 93 octane, now
running half 93 and half 89 octane, with no pinging. We will run the next 500 to 1000
miles on this mix, then try 89 straight. For more details on build search in the
Projects for the build.

Russ

[jammin97jeep]

Thanks for you imput guys. Im looking into more about pistons now to get the numbers you guys recommend and with my building constraints of little machine work and very little money.

So what is the max piston dish volume that the stock 4.0 pistons can be machined to? And would the cost of machining down stock 4.0 pistons be more money/hassle than if I just bought about $150 worth of pistons that come with my desired dish volume? Thanks.