is 11.2:1 compression too much for 91 Octane

-
67cuda360

67cuda360

Well-Known Member
Joined
Nov 11, 2009
Messages
462
Reaction score
23
Location
St.John's NL, Canada
Hi I'm wondering if 11.2:1 compression is too much for 91 Octane (highest we have available here) to run properly without engine knock (pinging).

Engine is a stoked 440 (500 CUI) with edelbrock aluminum heads, roller rockers, RPM Intake custom ground hydraulic cam (540 lift), TTI headers and a 3" exhaust. Car has a 727 with 3800 stall and 3.73 gears, engine is currently in a dart.

That's pretty much all the info I have on 'r. I've read somewhere that Aluminum heads will take about a point more compression than steel heads, so just wondering if it would be fine or if it might be borderline running on 91 octane :)
 
I run that in my small block. 200 psi cranking psi. I would not race it without mixing
 
What's duration @ .050 and LSA?
 
Wow. My 273 Commando, nominally 10.5:1 compression (probably less at .030 over), at factory timing is right at the edge of pinging on 91. It's iron, of course. I'd say it's a maybe -- you'll just have to try it.
 
mvh said:
Wow. My 273 Commando, nominally 10.5:1 compression (probably less at .030 over), at factory timing is right at the edge of pinging on 91. It's iron, of course. I'd say it's a maybe -- you'll just have to try it.
Click to expand...

Over bore adds compression. It does not take it away.
 
A 458 Italia runs 13.5 to 1 compression and does it just fine on pump gas. Just sayin
 
1970Duster said:
A 458 Italia runs 13.5 to 1 compression and does it just fine on pump gas. Just sayin
Click to expand...

I have no idea what that is or how modern.

Off the cuff I'd say no except the cam is a unknown. If the cam is really large in duration, hou may, may get away with it. You may, may have to find yourself a very thick head gasket to run at a slight less compression ratio.
 
The simple answer is no. I have an 11.4 to 1. I use 87.

To be fair, it has little in common with a 440.
 
i have an eddy headed stroker at 10.03 to 1 with 250@50 cam and to be able to run 32 degrees timeing on 91 oct i need to run the the coldest plug i can get RN9YC if i run a 10 it will detonate . this is just my experience nothing written in stone .
 
Its all about the Dc/r, and the al heads suck some heat out, and timing, and loading, and engine temps, and driving style, and traction
FYI, I ran 11.2 with a 240*cam@ 195*F in my 360M/T and 3.55s @ 3650#with 325/50-15DRs. No issues.Never had a BB. Currently running 10.5 with a 230*cam @ 205*F.Not sure if it helps.Ran them both on 87E-10
 
rumblefish360 said:
I have no idea what that is or how modern.

Off the cuff I'd say no except the cam is a unknown. If the cam is really large in duration, hou may, may get away with it. You may, may have to find yourself a very thick head gasket to run at a slight less compression ratio.
Click to expand...

Ferrari 458 Italia

http://en.wikipedia.org/wiki/Ferrari_458


Super sorry I had that wrong, it doesn't have 13.5 to 1 compression, it has 14 to 1 compression! It's a very amazing motor that is used in the 458 Italia Speciale.
 
Would really like to know "all" the cam numbers, but regardless, the converter & some gear helps. With the coolant temps low, the total timing at a reasonable point, and the carb rich enough, maybe?

I know I didn't answer your question, but most of us can't. It's gonna take a little experimenting with pushing that limit to know for sure.
 
Thanks for all the feedback, I'll see if I can dig up all the numbers on the custom ground cam. I feel it might squeek in at 91 octane, but it seems it would be pushing it. Any advantages or drawbacks to being at sea level?

Is there any way to easily up the octane of 91? haven't heard many good things about octane boosters... but haven't really looked into it either.
 
My daily driver has 14:1 compression ratio and runs great on 87 octane. It has a piston pockets, dual variable valve timing, header exhaust, electronic ignition, and direct fuel injection. It is loud for the first 5 seconds after starting due to fuel enrichment, then it sounds and runs great.
 
Chamber shape and quench come into play. Conservative timing and colder plugs will be needed.
 
I wouldn't be too worried about 11.2:1 static. I'd be more concerned with what your dynamic CR is. That will tell you more about what fuel you can run than static will. For example, my setup is a stroked SB 11:1 static, iron heads, 91 octane. What makes it work on 91 is the big cam that brings the dynamic down to 8:1, efficient combustion chambers, quench, etc. I don't run it down the track with 91 though. Will always swap in race fuel for that. Although I did open it up and ran it to about 115 mph with 91 octane on a back road and never heard a ping out of it. See if you can find that cam card though. That will help a lot.
 
Okay, I have the cam specs:
Grind: 250/263H13
Lobe Lift: Intake: 0.360 (intake and exhaust)
Rocker ratio: 1.5 (intake and exhaust)
Valve lift: 0.540 (intake and exhaust)
Adv Duration: 307 (intake) 318 (exhaust)
Dur@.050: 250 (intake) 263 (exhaust)
Lobe centreline: 110 (intake) 116 (exhaust)
Lobe separation: 113
Degree intake lobe to: 110
Spring pressure: 135 seat 325 open

Does this look like it would be able to run 11.2:1 on 91 octane?
 
67cuda360 said:
Okay, I have the cam specs:
Grind: 250/263H13
Lobe Lift: Intake: 0.360 (intake and exhaust)
Rocker ratio: 1.5 (intake and exhaust)
Valve lift: 0.540 (intake and exhaust)
Adv Duration: 307 (intake) 318 (exhaust)
Dur@.050: 250 (intake) 263 (exhaust)
Lobe centreline: 110 (intake) 116 (exhaust)
Lobe separation: 113
Degree intake lobe to: 110
Spring pressure: 135 seat 325 open

Does this look like it would be able to run 11.2:1 on 97 octane?
Click to expand...

The card doesn't happen to have the valve opening and closing events does it? Need that for the dynamic CR. 97 octane shouldn't be an issue, unless 97 is something different than in the states? I haven't heard of 97 before....
 
Sorry, I meant 91 octane, we can't get anything higher than that here :(

Valve timing @ .050
Open - Int 15 BTDC Exh 67.5 BBDC
Close - Int 55 ABDC Exh 15.5 ATDC
 
Ok with the info you gave, I made some assumptions and just used an online dynamic CR calculator (Wallace Racing).

My inputs (which may need corrected) were:
Bore: 4.32
Stroke: 4.15
Rod Length: 6.76
Static CR: 11.2:1
Inlet valve closes ABDC: 55
Boost pressure: 0
Target altitude: 1000 ft

"Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 11.2:1.
Effective stroke is 3.48 inches.
Your dynamic compression ratio is 9.35:1 .
Your dynamic cranking pressure is 194.30 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 9.35 :1.
V/P (Volume to Pressure Index) is

Use this calculator to see what the effect of bore, stroke, rod length, cam timing, compression ratio, boost pressure and altitude is on your dynamic compression ratio. Of the variables, the most important is cam timing which has a dramatic effect on your "dynamic" as opposed to your static compression ratio. The more "overlap" your cam has, the lower your "actual" as opposed to your static compression ratio will be. This is one area you can easily change.

Another consideration is rod length and it's effect on the engine's output. Changing the rod length will affect the piston location in the bore relative to cam timing everywhere except at TDC and BDC thus changing the dynamics of the engine."

With that information, the Dynamic CR seems a little high for 91, but I'm far from any sort of expert on this. Let's let some others chime in.....
 
Thanks Clinteg,
so the bore and stroke you mentioned there are based on a stroked 440 (500cui) correct?

I'm just trying to wrap my head around if the car that I'm looking at (that has this engine) will run on the fuel I have available here. Like you mention 91 octane might not be enough (not that we'd know for sure until we try it LOL).

The current owner mentions that the car seems to have some rattling under load, so I assume this is pre-ignition and I'm trying to figure out if It would be possible to tune this out (timing, curve and carb setup) or if something more drastic is needed to make it run on 91 octane.

How much difference would a thicker head gasket make you think, or maybe porting and polishing the heads would make a difference (not sure if this is already done).

I guess the only other way (and probably best way) to reduce compression would be to go with different pistons, but I'm not sure if the current cam would work with anything lower than 11:1.

ha ha, questions, questions, questions LOL.
 
polishing the chambers and making sure there are no sharp edges will help a little. run a very cold plug
 
67cuda360 said:
Thanks Clinteg,
so the bore and stroke you mentioned there are based on a stroked 440 (500cui) correct?

I'm just trying to wrap my head around if the car that I'm looking at (that has this engine) will run on the fuel I have available here. Like you mention 91 octane might not be enough (not that we'd know for sure until we try it LOL).

The current owner mentions that the car seems to have some rattling under load, so I assume this is pre-ignition and I'm trying to figure out if It would be possible to tune this out (timing, curve and carb setup) or if something more drastic is needed to make it run on 91 octane.

How much difference would a thicker head gasket make you think, or maybe porting and polishing the heads would make a difference (not sure if this is already done).

I guess the only other way (and probably best way) to reduce compression would be to go with different pistons, but I'm not sure if the current cam would work with anything lower than 11:1.

ha ha, questions, questions, questions LOL.
Click to expand...

Yes I looked up what a 500 CI stroked 440 was (I wasn't familiar), as in what stroker kit that would be with rod length, stroke, etc. That's why I was saying I'm "assuming" those numbers are correct on the stroker setup because that's what I came across. If there's rattling under load, it might be safe to say that there's some pre-ignition/detonation going on. From reading, it sounds like 8.5:1 Dynamic is about where you want to be for 91 octane (with Al heads). Does he run 91 in it? Has he ran race fuel in it to see if the rattle goes away under load? That would be a good indicator of knowing what fuel it needs (for the timing he has it set at). And doing what EL5DEMON340 said along with pulling the timing out of it would probably help. If the guy has the timing fully advanced, with hot plugs, then yeah it's probably going to ping under load with 91. For example, I pull my timing down to 28 degrees on 91 octane and advance to 34-36 on 110. I also run NGK-7 plugs which are a colder plug. It could be as simple of a fix as pulling the timing out and running a thick head gasket. If that doesn't work, then either a cam change would be in order or piston change. Here's a long detailed read about Dynamic CR: http://cochise.uia.net/pkelley2/DynamicCR.html.
 
You must log in or register to reply here.
-

Back
Top