383, 11.6 compression, pump gas advice

[JoePorting]

I've run 11:1 SBC's on 91 octane California pump gas in the 650 hp range and all I had to do was run total timing around 28 degrees locked.

[skinny z]

That last post raises a question that's been the subject of more than one discussion.
At what point does compression trump timing?
I understand it's combination and application specific but there's a point where the timing is "optimum" isn't there? With too much compression, that optimum value has to sacrificed but to what gain?

This is of particular interest to me as my iron headed SBC has given me grief with 10.5:1 and a IVC of 63. Cranking compression was north of 200 PSI. I wanted a full timing curve for all the benefits not only in performance but also things like improved drivability and some decent highway MPG but it was a little problematic.
To be fair, I was aggressive with the lean cruise AFR's and timing via vacuum advance but still, it makes me wonder how that compression vs timing relationship plays out.

[RAS]

Don't care what your mechanical compression is. Check the cranking PSI.

[Chris_Hamilton]

Don't care what your mechanical compression is. Check the cranking PSI.

What would be the limits with cranking PSI?

[bob460]

Hehehehe... Aussies know how to run high comps on pump gas.

Yep........We love high compression and pump gas.......

[Tom68]

Hehehehe... Aussies know how to run high comps on pump gas.

You wanna see the price for race gas here.

[F-BIRD'88]

Solid roller 249/255 @ .050, .670/.640 on 106.

How do you calculate the duration at .006" off the seat?
That's obviously after lash and the valve has started to move.

You cannot , without more information.
You do not know the length. incline, intensity or height of the opening side or closing side lash ramp. They may not be the same either.. You analyze that by measuring the camshaft on a cam doctor.
The graphing feature helps you visualize it.
You still end up testing out valve lash setting variation for running effect...on the engine.
You can calc till the cows come home but testing for effect on your engine in your car is where the rubber meets the road.
Not all camshaft lobe lash ramps allow the same amount of +/- valve lash variation from cam card hot lash spec. Your cam grinder can advise.

[F-BIRD'88]

I've run 11:1 SBC's on 91 octane California pump gas in the 650 hp range and all I had to do was run total timing around 28 degrees locked.

Have you tested A-B compared to race gas and optimun
best spark WOT timing to see the on track difference in car performance VS your "pump gas tuneup"?
The difference is....?

[skinny z]

Solid roller 249/255 @ .050, .670/.640 on 106.

How do you calculate the duration at .006" off the seat?
That's obviously after lash and the valve has started to move.

You cannot , without more information.
You do not know the length. incline, intensity or height of the opening side or closing side lash ramp. They may not be the same either.. You analyze that by measuring the camshaft on a cam doctor.
The graphing feature helps you visualize it.
You still end up testing out valve lash setting variation for running effect...on the engine.
You can calc till the cows come home but testing for effect on your engine in your car is where the rubber meets the road.
Not all camshaft lobe lash ramps allow the same amount of +/- valve lash variation from cam card hot lash spec. Your cam grinder can advise.

Considering that this is a cam that already exists, it may be that .006" information is on the record somewhere.
This isn't a matter of testing to see what's what, it's a "till the cows come home" calculation and trying to compare the OP's 11.6:1 solid roller with my 11.5:1 Aluminum headed hydraulic roller 355 that's quite happy on 94 octane. Mostly street, some drag strip for validation.

[F-BIRD'88]

There is a BIG difference between our 94 Ultra unleaded premimum and all the other 92 and 93 octane unleaded gasolines. I have tested this. Apples to oranges comparo. You are wasting your life on all this internet theory and endless calc- ing.

Just try out the valve lash variation and test for effect.
Its either going to help or its not. You can tighten up the lash setting from cam card spec by quite a bit to add seat to seat duration for effect.. There is a limit to how much looser you can go on valve lash setting.
In a very limited way you can do this with hyd cams too.
Running with a slight small running valve lash setting for effect. (brief short term testing). ( Its a bit hard on valve seat life if over time)
On hyd cams/lifters the oil vescosity and oil temp and lifter clearances can effect this quite a bit too.

Its easier just to just dial back the spark timing as required to let it live. Try both... You do need some valid way of detecting high speed WOT detonation. or it can be a short ride. Hard to hear from the drivers seat at 6000-7000 rpm.. Bad things happen fast and the engine does not self heal from the detonation.

[F-BIRD'88]

A simple easy way to lower the compression ratio on this engine is to add a extra head gasket. A typicall sbc 4.166" x .041" compressed head gasket is 9.1cc which will drop the CR to mid 10's . Try it... Then you can decide if modifying the piston domes and or combustion chamber volume in the heads is right for you..
Brodix may have a CNC program for this, depending on which chamber size you have.

Just eyeballing the IK200 as cast chambers there is a lot of room for playing with the chambers shape and net volumes.
Solid dome pistons can be reduced. The change in balance is not that critical... VS the effects of detonation.

[ClassicComp]

What elevation do you live in?
Higher the elevation the higher the comp ratio can be.

[ChopperScott]

Punch your numbers into this dynamic compression calculator…

https://rbracing-rsr.com/comprAdvHD.htm

[HQM383]

Punch your numbers into this dynamic compression calculator…

https://rbracing-rsr.com/comprAdvHD.htm

A very good place to start. Then what needs to be considered is:

*Combustion chamber shape and influence on turbulence
*Location of spark plug relative to cylinder center and exhaust valve
*Spark timing
*Piston shape
*Cylinder head material
*Quench clearance and area
*Air fuel ratio
*Efficiency of intake port
*Coolant temps
*Cylinder size (flame travel distance)
*Supercharged/turbocharged

[skinny z]

I find it odd than any calculator used to estimate dynamic compression would use a value for IVC that's .050" off the seat. Clearly not closed.

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