11.3:1 Compression ratio fuel requirement
 

Twin plug heads, PMO carbs, air fuel ratio gauge in the cockpit. Should I run 100 octane? Race car use.

Not enough information to answer your question with any certainty.

As posted before:
Detonation is a function of a plethora of conditions.
compression ratio (dynamic compression being key)
combustion chamber configuration
piston dome configuration
ignition timing
Octane
head temperature
altitude
fuel : air ratio
intake charge temperature.

I found some useful charts at: Daytona Sensors LLC - Tech FAQ Engine Tuning

It looks like you should be safe with 100 octane and twin plugs.

-Andy

Cam specs would help.

I should have known that. The motor is just going together, so some things are to be determined. I thought there might be some general rule of thumb.

Great link thank you.

The cams match the motor specs. They are a custom grind.

The general rule of thumb is that race engines exceed normal parameters.
They see higher loads, higher temperatures with missed shifts and sudden rpm changes. If your engine is tuned perfectly to run 100 octane on a cool day it is likely it will detonate on a hot day. If your gear box maximizes torque then changes in fuel / air ratio could easily create a problem.
Design for the unexpected. Leave yourself some room for variation. You will undoubtedly be surprised by the unexpected but perhaps you can still cross the finish line.
Good luck

I understand. I'm thinking 100 may be the minimum octane at this point. Thank you for your insight. Most of the tracks here in the SE are within a 500 ft in elevation, but we start this time of year with temps in the 40's F and go to low 100's in the summer. I'll definitely keep a good eye on the AF as we start to warm up. I've found I'm fully capable of doing unexpected things to an engine :eek:. Hopefully not real soon on this one.

I assume your class rules won't let you run a knock system? If you can, this takes a lot of risk out of the equation. This is how the 3.6 cars runs such high compression with low overlap cams without detriment. As evidence, have you ever seen a 3.6 on carbs (not using the DME)? I have seen more than a few holed pistons when the owner didn't realize you can't run these motors without knock detection and expect them to live.

Keith,

Like Henry wrote, more information is necessary to offer anything constructive.

For me, any engine without knock-sensing at 11.3:1 used for the track needs 108-110 octane fuel, especially in hot weather. One can run 100, but with reduced timing and that simply adds to the heat load in the heads.

...go efi and let it compensate. easier said than done, right? imho, staring at the afr gauge is dangerous and it'll probably be too late unless you just run rich.

Thank you. Octane is easy. Holes in a newly built engine are just no fun for anyone.

Thanks for that, the last thing I want to do is break this thing.

You are right Sir, it is tough enough doing it on an open track while tuning let alone during a race. Basically it would likely be over looked during a race and like you say to late to do anything about it. Kaboom :(

More important than static compression ratio is your dynamic compression ratio. You can run 11.3:1 on street fuel so long as your dynamic ratio is 8.6:1 and below ish. Race fuel is needed with ratios above that, but this is a good rough guideline. Your dynamic ratio is a calculation from your static ratio and when your intake valve closes thus creating your 'effective stroke'. This figure can be calculated if you tell me your rod length and at what point your intake valve closes ABDC @ .050 or whichever figure you have. A late closing intake valve lowers your effective stroke and raises peak bhp and torque to a higher rpm.

The motor is a short stroke 3.2 or a 3.0SC engine with 98mm pistons. I'll see if I can't get the cam information. Thanks

Here are some more engine details:

The rods are stock 3.0SC length 127.8mm I think.

The heads are stock 3.0SC that have been machined for twin plugs and mild porting of the intake/exhaust. Competion valve job. The basic shape is unchanged.

The pistons are JE. I believe they are 11.5 machined to 11.3.

The cams are built for the engine.

Seat duration (at .012" lift) intake 316
Seat duration (at .012" lift) exhaust 307
.050 duration intake 278
.050 duration exhaust 267
Gross valve lift intake .525
Gross valve lift exhaust .514
Full intake lift 101 ATDC
Full exhaust lift 103 BTDC

I don't know if this information helps clarify my octane requirements. Thanks for your help.

With that kind of intake lobe center and overall seat duration, your dynamic compression ratio will be very good for street fuel. Seat duration has your intake closing at 79 ABDC. Dynamic ratios are best calculated with the valve still open about .004, so 76 degrees is a safe figure to operate from for calculation effective stroke. I estimated what I didn't know (piston dome volume and deck height etc) to get to 11.3:1. I do not know your combustion quench or the specific efficiency of this engine, but I am calculating a dynamic ratio of 8.118. You should be in great shape. Power will be up there around 7400+. What a sound

Thank you for crunching the numbers. I'm looking forward to some good dyno work in a few weeks.