DCR and Octane

[Antifreeze2]

I found this image when trying to research this stuff. Do you agree with it or not?

[Cheromaniac]

That chart makes something of a blanket statement so while it does make sense (92 octane for an 8.0:1 DCR at 180*F), there are many other factors that'll determine the engine's octane requirements.

[Muad'Dib]

x2... lots of factors to consider.. but the graph appears to be fairly accurate. Elevation plays a role also.

[Gorillaxj]

Posting this here as no need more multiple threads....

I was looking into thsi as I want a good all around motor for all elevations for when I drive to CA and do the rubicon. if I need premo there thats fine! just wnat the best ideal DCR... I was reading this and thought is was insteresting... anyone have any thoughts on it?

From "JPmagazine"

The bottom line is that the DCR is a major factor in whether you can run on regular, midgrade, premium, or race fuel, and not ping. Also, if the DCR is too low, you will lose bottom-end torque. The generally acknowledged safe "sweet spot" is an 8.0:1 DCR for an EFI engine, and it offers good low end and a decent high end, usually on 87-octane regular. Going below 7:1 usually costs too much low-end power. With fuel injection and a good quench dimension (read on) you can sometimes reap the benefits of going higher, perhaps up to an 8.7:1 DCR on regular or midgrade. Dynamic CR above 8.7:1 almost always requires 90- or 92-octane premium. Above 9.0:1 DCR, it's almost always ping city for pump fuel. For reference, the stock 4.0L has an 8.7:1 static ratio and a calculated 8.45:1 dynamic ratio. You can download a DCR calculator from the Internet, but the Keith Black Silvolite Piston site has a good one to play with (see sources).

Complete article
http://www.fourwheeler.com/techarticles ... z1oRaUPZoH

[Dezertxj88]

Wait a second..I thought the static compression ratio determined octane rating?

When I was going to build my storker, I was looking at 9.2:1 SCR and like 6.85DCR... .038 quench, stock cam, 199cc dish pistons.
Would it have been safe to run like 16-17cc dish which would give a 9.5:1 SCR and a 7.03DCR on 87 octane? Sea level, California desert.

[Antifreeze2]

SCR is important when it comes to what components you build your engine out of. I don't think the iron block w/ cast components will handle 10:1. DCR is for ping.

[Dezertxj88]

SCR is important when it comes to what components you build your engine out of. I don't think the iron block w/ cast components will handle 10:1. DCR is for ping.

So say I decided to not dish my pistons and run a SCR 9.78 and a DCR of 7.12..would that be ok for 87 octane? I was just always told stay under 9.2:1 SCR for 87 octane?

[SilverXJ]

A 7.12 DCR sounds nice for 87

[Dezertxj88]

A 7.12 DCR sounds nice for 87

Awsome, I mean if not its only a couple bucks difference per tank! Not too big of a deal!

[herbiehind]

i'm all for 87 to save some money . but.... at what point do the cost savings on fuel outway the reasons for stroking a 4.0l? where is the happy medium for everyone on this ? at what dcr are we getting good power yet reliable vacuum and a good dcr for lower octane? seems it's all personal preference but there must be some average. i spent upward of 1200.00 on machine work (boston area ) and i think i want some performance a bit more than i want 87 at this point . so i'm okay with 91 or 89 even . i'm even leaning toward decking to zero once i've done a dry build . why not? right? better now than find out it pings and needs 91 anyway . thoughts from the experienced? no hijack here intended .

[dwg86]

My stroker had 7.8 dcr with a polished combustion chamber, zero deck, and somewhere around .040 quench. I could run 87 on a 95 degree Georgia summer day, with NO sign of detonation. I would think you could run at least 8.0DCR with a tight quench.

[Dezertxj88]

My stroker had 7.8 dcr with a polished combustion chamber, zero deck, and somewhere around .040 quench. I could run 87 on a 95 degree Georgia summer day, with NO sign of detonation. I would think you could run at least 8.0DCR with a tight quench.

Whats your SCR and are you on a stock cam?
A higher over lap cam will bleed of cylinder pressure, thus less chance of detonation..so that also comes into play!

I want to run my mini stroker on 87 this summer cause I'm a tight wad lol..but honestly, the difference is litteraly a few bucks per fill up..I'd shoot to run on 91oct, better performance and no headache of crunching numbers trying to make the threshold! I have polished chambers, 9.1:1 currently, .038 quench..may take that to .025 which would put me above deck and give me a 9.5:1 compression ratio.
I'll be running 91 & possibly meth injection once the turbo hits my motor.

[604rail_king]

Wouldn't a .025 quench be extremely risky or likely not possible due to the metal expansion/stretch rates in the rods?

[Dezertxj88]

Wouldn't a .025 quench be extremely risky or likely not possible due to the metal expansion/stretch rates in the rods?

If you check my other posts..I asked that exact question!
Steel rods stretch slightly, yes. Steel rods stretch .02-.03 around 6800-7000rpm give or take..We are only running to 5500rpm, if that. So I expect around .2 rod/piston stretch MAX. So I'll be pushing the limits yes, but I've dealt with extremely tight tolerences before and feel safe doing so...used to build some bad B series hondas. And if you know them, you know the valves will come into contact with even small cams, so alot of adjustment's & measurements are needed!
I mean we were revving a couple of those things to 9800rpm..with aluminum rods..which stretch even more! So I'm used to mic'ing stuff out, used to claying motors.
Really it comes down to trusting your gut and going for it since theres no actual data to go off of for these 4.0s...I'm not sure exactly how much our rods will stretch, but my gut says that they aren't stretching much since we aren't revving to high..I think .025 quench is going to be the absolute tightest you can get the quench without hitting the head! These are non interface motors..meaning PVC isn't an issue..there will be no issue with piston to valve contact..just gotta worry about the piston hitting the head itself..

My plan to to dry assemble my motor with stock pistons..1.600 pin height. And see how far I'm out of the block. Then I'll decide if I want the 1.592 pin height pistons or a 1.581 pin height. Basically that'll judge my quench.
.038 with a 1.581, or a .027 with a 1.592...decking already accounted for, no more cutting will be done, the deciding factor is the pistons! I'll clay my motor just in case to check for PVC, but I'm 100% certain it won't be an issue!


Hopes that helps you understand what I'm doing lol.

[SilverXJ]

In considering this .025" quench, have you taken into account piston rock at TDC? The piston is quite a bit much larger than a Honda's and will have larger number for piston rock.