CR on the street and your octane requirement...

Carry on.

My EQ headed 408 Magnum came out at 10.3, runs fine on the pump 91 we get around here. I run a dual pattern Hughes 230/234, .52/.57" @.050" (as near as I can recall at the moment anyway). 750DP, air gap, 1 3/4 Super Comps, 2.5" exhaust, 904,2500 stall, 3.55 gears. I'm running 18 initial/36 total mech with a vac advance. Mech advance curve is slow (all in at 3200). I'm still fine tuning the whole package for bullet proof street driving. And I'm at 5K above sea level.

When I get to the track, I'll spike my fuel with race gas. My 9.0:1 Brand X equine with over head cam and aluminum heads will start to knock on your 2nd or third run down the 1/4 on pump 91, even pure gas 91 (or 93 at sea level for that matter). Not that I would do this, but were I to wring out the 408 in all three gears (on a closed course with a professional driver), I am quite confident, it would show no signs of detonation in any gear. Were I to repeat that exercise, I'm pretty sure it would be in my best interest to spike the fuel with race gas or octane boost like I would at the drag strip.

Normal street driving is a traction limited blast.

You can run a higher-than-you-think static compression ratio for a given fuel octane, provided your dynamic compression remains within acceptable limits. Your local density altitude and the altitude range you operate the vehicle in will have an impact on dynamic compression. Cam selection is critical.

http://www.enginebasics.com/Advanced%20Engine%20Tuning/Static%20vs%20Dynamic.html

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

We'll see when I get to it, but I'll for sure I'll have to re-jet the carb and probably have to run octane boost or cut with race gas when I get this near sea level.

Now, how much power is that extra 0.8:1 compression getting me vs. if I had kept it around 9.5?? Not much with my setup currently. But I'm a head gasket away from an acceptable forced induction static CR (would still have to open up the rings), or real compression requiring real gas. Durability and flexibility were where I was going here.

There are so many variables,stay SAFE and read your plugs!

https://www.google.com/search?q=octane+required+for+compression+ratio+charts&tbm=isch&source=iu&ictx=1&fir=I5vBMm4xO5y2BM%253A%252C19J5ko9OrKSMqM%252C_&vet=1&usg=AI4_-kSJNUY5iVTnNmn9DMOptHP38VXfTA&sa=X&ved=2ahUKEwj02ryc1YXkAhUshOAKHV1VCs4Q9QEwEXoECAkQFQ#imgrc=lKy3u_EFLFr_SM:&vet=1

508 cuin. 4.25" arm/7.100 rod. Closest piston (quench) is 0.038" rest are 0.040, 0.041. Mopar/Eddy head. Flat top pistons w/valve relief.

I live in western PA. I'm at about 1000ft, and probably see 500ft on occasion while driving. My highest CR cylinder is 10.85. My cranking cylinder pressure is 190-ish with my preferred lash setting. Intake valve touches down on the set at a measured 77° abdc. No vacuum advance, and mechanical timing all in about 3000. Initial at 23° total at about 35°. I set it and leave it. 160° thermostat. Some -8 NGK plugs.

The car w/ driver is about 4150 lbs, has 3.23 gear, and tight converter (1900 rpm) and no part throttle kick down.

Its on the edge of detonation using 92 octane. Winter grades of fuel are worse.

Worked at giving a thoughtful response. Wish I had that 15 minutes of my life back.

No kidding, huh?

Why would I do that? The post doesn't violate Tom's rules, it's not a duplicate, and it's in the correct forum.

I'll change the title and y'all can keep it if you want. The intent wasn't to waste people's time, so my apology for that.

I saw something brought up in one of the earlier replies and thought to myself: Nope, not going down that rat hole again.

I went with 9.7 cr. on my 512, I have made the mistake more than once trying to run more CR on the street,pump gas, here in HOT, almost sea level Southeast Tx, all is good..038ish quench

Dynamic compression ratio occurs at (and somewhat above) cranking speed, and disappears long before your torque range. The only factor in DCR calculation is reduced charge mass due to reversion ABDC - when that stops, DCR is not relevant.
The 12.0:1 engine that only has 9.0:1 DCR due to late intake closure has 12.0:1 actual ratio by 3,000 RPM.
Above critical RPM (when reversion is nearly stopped) you have the static ratio, and more than that, when your exhaust tuned length is working.

Running a later IVC than your engine wants (by too much intake duration, wider LSA, or retarded) to reduce knocking is not a good idea, it's not reliable or predictable, too many variables.

I'm just a tick over 10.8 with my 493 Indy head engine. I cant hear any detonation with the pump 93 here in NE Ohio and it runs very well. I haven't checked my cranking compression with my new cam but it seems to run good on pump 93 I limit my total advance to 34degrees mechanical with no vacuum advance.
Gus

My 518 C.I. pump gas motor in my old Duster ended up having 10.78 to 1 mechanical compression ratio with the set of Indy SR with 76.0 chambers, it never acted like or showed signs of detonation(on the plugs) on Oregon 91 octane pump swill with no ethanol in it back then
I ran the same timing as Gus (34 degrees)with no vacuum advance also

11.5:1 Hemi running on 93 with 10% Ethanol cause thats all I can get.....timing at 34* or less. Runs awesome. I run 34* on the street and turn it down to 31* at the track.....ran 9.90

512/440 running 38* degrees on the street or track. Indy EZ1 heads...Also 93 with E10%.....No issues. Ran 9.70....

It is looking like the 93 around here is quickly going away, and 91 will be all I can get.....I hate NY.

I have an idea which one that might be, and we can just carry on and pretend it isn't there if that'll help.

Being that potentially useful content is somewhat lacking these days, in my humble opinion I'd like to see some discussions kept alive if possible.

TBH I forgot my intake closing point long ago, but on my 451 which
has aluminum heads and .045 quench, 10.75:1 measured, I'm at about 175lbs cranking compression and don't hear or see any detonation
on local E10/93. I have not noticed a huge difference in performance or tendency to detonate between 34, 36, and 38 total timing.

Of every grade of normal street gasoline available, I feel 93 is the most likely to be discontinued/obsoleted in the future and this
made me comfortable building my future 572 with 10.2:1.

505
10.95 compression
440-1 cnc 325
.585/.600
260/268 @ 50
Indy 2D
Holley 1000HP
28* idle, 34* total in by about 2,200
93 Sunoco with ethonal
-9 projected tip NGK

No detenaton

11.0's @ 120
With tailpipes
8" Dynamic
3.54 gear...3,950plus

There are a lot variables, but in my area I stick to Sunoco Ultra 93(tested at 5% eth) and Marathon 93( tested at 7% eth).
I'm running a 10.6:1 static ratio in my 440 with no ill effects to date.

Just a cautionary note.

A friend of mine in the R&D biz put pressure transducers in the combustion chambers of a GM Ecotech 2.2 some years ago and was able to demonstrate that just because you don't hear any detonation, doesn't mean it isn't happening.

Kevin

Thanks.
I suspect that factory knock detection by frequency may be compromised in a race engine by changing torque of fasteners, adding/subtracting attached accessories, raising the RPM range etc. and both give false positives and fail to warn on new noises not within the planned range.

I hate to be a broken record, BUT they make engine programs that will predict what octane fuel an engine will need, and for example if you have to run pump gas, it will calculate an advance curve to safely do that as well. Program I use is less than $200 and I have been using it for many years. It may not be perfect but it gets you a lot closer than asking suggestions. It also predicts cranking compression, idle vacuum, and a bunch of other stuff. If you take the time to enter flow bench numbers, and calculate the REAL static compression, etc. it has hit every engine on a real dyno within just a few horsepower. I set up a pump gas procharged 410 LA engine, and simply ran the numbers in the program without boost and then with boost, plugged those numbers into the timing maps on the Fitech, and it ran perfect, just one example. I have a distributor machine, and when I build a street engine I calculate the advanced curve, set the distributor to that curve on the machine, and have yet to have an issue. I still look at the heat mark on the electrode, but they are always right where it needs to be.

It doesn't matter; I'm transitioning into more of an observer here anyway.

I'm curious.
How do you know the RPM where reversion stops and the cylinder volume captured approaches 100%? It would be different in 2 engines with the same cam, same head, and same CR if the rod ratio were different.
The hot spot that is frequently the cause of knocking is the closest to the plug and/or exhaust valve. How do you know how hot that is since it follows load as well as water jacket temperature?
I'm certain that these are relevant factor, but do their ranges contribute so little to the calculation that the results are still useful?

I'm at 10.63:1 and my software tells me 89 octane is safe. I usually run 91.

I wouldn't, and don't ,trust any software formulas on predicting detonation on any Hi Po pump gas motor To many variables like fuel quality, current air density, spark plug heat range and so on
I do start off on the safe side on total ignition timing and spark plug heat range, colder is far better than a tad to hot on the spark plug heat ranges IMO. I'll drive and tune the carb. on the light part throttle cruise and idle mixtures first and then step up to the higher RPM on the primary only cruise to get it where I like it (I like lean, 14.8+ AFR) and then tune for WOT throttle last.
I will find out where the edge of detonation is on both the AFR, timing, gasoline and spark plug heat range and record the weather and all the tune up information

I run 87 on the street in my Rampage street rod and put in 91 if I take it to the track

My 511 has 11.3:1 static. I use Kroger 93 octane fuel. No sign of any detonation on the plugs yet. Just this year a have been mixing in a gallon or 2 of 110 race fuel to a tank of 15gal just for the smell. 3 or more gallons and it will run richer and I see it on the wide band. Exhaust note changes from crisp to a dull note and the car feels slower and less responsive.

My 11:1 hemi loses about a tenth and about 1.5 mph on straight VP 110 versus pump 93.
Not very scientific. I ran low on fuel at the track one day and all they had was race fuel, so I threw some 110 in it. Didn't change timing or jetting.

My 408 did the same on straight 110vp. Tried jacking the timing up to compensate the lose to no avail. Non of my engines liked straight race fuel. I say 11:1 of less will be fastest on straight pump gas.

There may be significant differences between A and B besides octane that color power and mileage: how well does it atomize (minimum droplet size), what's the vapor pressure.
Chemists: more?

Fuel density also. So it can change the mixture (optimum jetting).

More info: https://www.sunocoracefuels.com/tech-article/specific-gravity-matter
The specific gravity of pump gas will typically range from about 0.720 to 0.770.
https://www.sunocoracefuels.com/fuels/compare-fuels

Can cranking compression give any indicator as to what you can get away with? My motor is a 10:1 W2 headed 416. The pistons have a nice smooth dish and no quench. Solid roller, 270ish @.050 off the top of my head, last cranking compression was 165. I just got it running again after a fresh valve job and replacing an incredibly worn timing chain so I need to check it again. It makes about 575-600HP now and has no trouble on 91 octane with 40* of timing WOT.

I had a 340 that was 10:1, quench motor, 243@.050 hydraulic flat tappet with rhoads clickety clack lifters. It cranked like 210psi and ran on 93 octane.

I would like to bump the compression up on the 416 on the next refresh but I do not have the budget or desire to run race gas since It is a street car. I do have E70 available in town and the fuel system to support it but I get mixed reviews about ethanol and limited use street cars(I don't drive it near as much as would like to).

Yes it can.. I use to have a chart on one of my other computers that was pretty good but its long gone

My previous motor was 10.8:1 compression (410 small block), ported eddys, small solid street roller (248/254 @0.05 with about 0.575 lift), Victor, 750dp, 727, 4:10 gear, 8" converter in my 3250# Barracuda. This ran fine on 93 octane (34° timing, locked out). It ran 10.8's...then I switched to e85 and it ran 10.7's. Blew that one up about 4 years back....

Fast forward to now...current motor is a 408, 13:1 compression, same heads/carb/transmission/ same everything except a larger solid cam (264/268 @ 0.05 with about 0.620 lift). Running e85 in the heat of summer it runs low 10.4's to high 10.3's...should really boogie in the cool fall temps (hoping for 10.1's) AND I can drive it all over town.

My old 446" motor w/ Indy heads is over 14:1 compression (Ross domes and the heads have been shaved a lot). The old roller cam in it was around 262/268 at fifty on a 106 LSA and it cranked at 275 psi. Changed to a custom roller from Dwayne Porter w/ 274 at fifty duration on a 108 LSA and it cranks 185 psi now. I don't think I'll be trying pump gas in it, lol. My hemi at 11:1 w/ 275/284 at fifty on a 112 LSA also cranks 185 psi.