Octane rating vs CR

[plovett]

The criteria here is 2V wedge and heart shaped chambers, both aluminum and cast iron. Old school, if you will, not direct injection.

A safe guild line, in a loaded engine, with a .040 - .060 squish.

80F deg 800 ft elev.

Randy and Paul. Temp and Elev are as above. Loaded is loaded, no matter the weight. Consider it a steady dyno pull at 160 deg engine temp. The 80 deg is assuming air intake, out side the engine compartment.

I know by application, there are things that work. Such as 6.8 DC but, I would like a upper end number to build a engine to.

Will 7.5 do it, in those exacting numbers, supplied above?

I had a version of my 428 with a fairly short duration cam and 10.4:1 compression. The cam was a 270/276 degree advertised (@0.020") solid flat tappet. Installed on a 107 degree intake centerline the DCR was about 8.47:1. It ran fine on high octane pump gas. That is usually 91 octane and 93 when I could get it. I didn't run vacuum advance.

I still think the "load" matters, if only because it increases the time in the danger zone. However on the dyno the rate is steady anyway, as you noted.

For me personally a DCR of 8.0-8.25:1 is safe on good pump gas with a sane combination.

paul

edit: I see you are stipulating 87 octane. I was assuming around 91 octane in my response above. I think your 7.5:1 would do okay given your stated conditions, maybe even a little more.

[rfoll]

I have a flat top 350 with Vortec heads. Static compression is 9.6. The cam is a Comp X4254H, 210/218/111. Iron manifolds with a 2.5" single pipe. The online calculator says 8.8 dynamic. I have 15 degrees initial, 31 total, and I have yet to hear it ping on regular 87 fuel.

[Geoff2]

An easier question to answer would be: how long is a piece of string....

[frnkeore]

Is the conscious that no one can build a high performance engine, that will run on the the 87 octane gas, at the pump, closest to where they live?

Or does it mean that dynamic compression ratio means nothing?

Or does it mean that DCR is only useful if certain other criteria are met and if so, what are they, in the order of how important they are, to retard detention?

It's pretty, universally thought, that ~8/1 DCR is good for 91-93 oct in a good street application but, I've never heard what it might be for 87 oct.

I've specified squish, chamber, air temp, eng temp and altitude but, I will add a carb and 12.5/1 A/F and unequal length, short headers.

[rfoll]

The Vortec head on my engine allows me to run compression and timing on regular fuel that would clatter badly with a 76 cc smog head. If there is a consensus, it's that there is no rule outside of variations on a given combination. If I boosted my static to 10:1 It would probably require premium fuel. The OE applications get away with the high ratios by employing sophisticated computer monitoring. A friend had a 2008 Z06 Vette with monitoring equipment on board, and noticed the ECM retarded timing more often than not under normal driving conditions. It still got 30 mpg on long trips.

[F-BIRD'88]

the ECM retarded timing more often than not under normal driving conditions.

Does a old school carbed engine have that?

[mt-engines]

the ECM retarded timing more often than not under normal driving conditions.

Does a old school carbed engine have that?

Yes.. ever heard of a vacuum retard?.. seriously dude.. what is your passionate hate against compression. And pump gas?

[David Redszus]

Engine knock control
Detonation effects of selected engine variables

Factors that increase knock intensity
Compression ratio increase DCR
Spark advance increase
Spark plug heat range increase
Inlet temperature increase
Coolant temperature increase
Inlet air pressure increase

Factors that decrease knock intensity
Engine rpm increase
Fuel MON increase
Lead concentration in fuel
Fuel components
Humidity increase

Factors that can do either
Fuel ratio

[F-BIRD'88]

When you run the high CR Corvette motors on GT98 unleaded they do not need to retard the timing and MAKE FULL RATED HORSEPOWER and go faster..

[F-BIRD'88]

I don't have a hate against high compression nor premimun pump gas... I do have an issue with ignorance and incompanence reguarding combining excessive high CR with fuel that lacks the octane performance to protect that expensive engine from knock damage..

Running around with retarded timing is a crutch..

I had a 350 with shaved ported 305 heads CR was 12.65:1.
Ran fine on 94 unleaded as long as timing did not exceed 27 deg BTDC... But when you gave it the correct fuel for the cr, allowed optimum spark timing (36deg btdc) oh what a difference in power and performance..

I don't have a problem with cr nore pump gas.. Just incompitance and those that advise others that this engine works (with ECM spark timing control) , or (clearly retarded timing well short of optimum as a tuning crutch) thus your engine will too.

[mt-engines]

When you run the high CR Corvette motors on GT98 unleaded they do not need to retard the timing and MAKE FULL RATED HORSEPOWER and go faster..

You tune alot of EFI engines? Have the dyno data to back it up?

[F-BIRD'88]

When you run the high CR Corvette motors on GT98 unleaded they do not need to retard the timing and MAKE FULL RATED HORSEPOWER and go faster..

You tune alot of EFI engines? Have the dyno data to back it up?

The pro teams that do race these cars with 11:1 and north of fill the tank with GT98 or GT100 unleaded.
Cause they KNOW BETTER $$$..

I have a dyno and ran an engine with cold water, cold air inlet and ran it at wot FOR LESS THAN 10 SECONDS at a time then cooled.. Therfore All that related to real world driving conditions.. I have a dyno therefore I'm the smartest person in the room... ,--- OR Not...

[mt-engines]

When you run the high CR Corvette motors on GT98 unleaded they do not need to retard the timing and MAKE FULL RATED HORSEPOWER and go faster..

You tune alot of EFI engines? Have the dyno data to back it up?

The pro teams that do race these cars with 11:1 and north of fill the tank with GT98 or GT100 unleaded.
Cause they KNOW BETTER $$$..

I have a dyno and ran an engine with cold water, cold air inlet and ran it at wot FOR LESS THAN 10 SECONDS at a time then cooled.. Therfore All that related to real world driving conditions.. I have a dyno therefore I'm the smartest person in the room... ,--- OR Not...

Never answered my question... DO YOU TUNE OR NOT?

WHAT IS YOUR CURRENT OCCUPATION?

DO YOU REALLY THINK MORE TIMING BY DEFAULT EQUATES TO MORE HP?

[F-BIRD'88]

No, correct timing makes more power..
Rerarded crutch tuning timing "De-Tuning" makes less power.. Give it the fuel it needs Give it yhe timing it wants and you'll go faster and for a lot longer.

[F-BIRD'88]

Are you a Tuner or are you a professional "DE-TUNER" ?