what octane to run w/ 11:1 piston?

[Locknload82]

As you may have seen in another post we are having a reoccurring problem with pulling head studs in our 400ex engines, both running 11:1 pistons. When we had them installed people around here said we could run 93 octane gas in them.

After lots of head stud problems we are starting to think otherwise. We are installing GT Thunder heavy duty studs, but think we need to run higher octain gas as well

So this is a survey, what Octane level should we run in engines with 11:1 compression?

[Sjorge450R]

Originally posted by Locknload82
As you may have seen in another post we are having a reoccurring problem with pulling head studs in our 400ex engines, both running 11:1 pistons. When we had them installed people around here said we could run 93 octane gas in them.

After lots of head stud problems we are starting to think otherwise. We are installing GT Thunder heavy duty studs, but think we need to run higher octain gas as well

So this is a survey, what Octane level should we run in engines with 11:1 compression?

no lower than premium pump gas.

[DAVE14]

I run 11-1 on my 440 & have used as little as 91 octane without any problems I normally use 94 thats the highest around here & the head studs should be ugraded for sure or they will pull out that part has nothing due due with octane, theres just too much pressure on those 4 little studs the factory puts in there the gt thunder studs are much beefier & the threads have a much deeper bite into the cyl.

[Glamis400ex]

I used a 50/50 mix of pump 91 and VP C12. That came to about 100 leaded octane. Worked pretty good.

Glamis

[GPracer2500]

I always used a 50/50 mix with my 10.8.1 and HC stg 2. It knocked pretty badly on my local 91--way beyond what is acceptable. Even my earlier 10.5:1 with a WB Track would just start to knock under extreme conditions. That was easy to "ride around" though. On the 10.8:1, it would be so bad I couldn't ride the way I wanted and keep it from happening.

Detonation is fairly easy to hear on a 400EX. Being air cooled there's not a lot to muffle the tell-tale knocking/pinging sound that indicates detonation is taking place. If you can't here it in your engines it probably is not happening. However, if you're not sure what your listening for light deto can be easily overlooked. Deto that is so light that it cannot be heard isn't likely to cause damage.

Bear in mind there are a lot more factors than just static compression ratio and octane rating that go into creating conditions that produce detonation. A few important ones: camshaft selection, elevation, jetting specs, typical engine loads.

If you're not utilizing the stock auto-decompression system, then a simple compression check will tell you a lot more about the engine's octane requirement than piston specs.

[Locknload82]

I havent noticed a loud knocking, but at 42 my hearing isnt so good anymore.

Just trying to decide if 93 octane is good enough after we put the HD studs in. Hate to put them in and THEN have them pull the threads.

Just think its too much of a coincidence to have both quads do the same thing, one is a 400 with a stock cam, bored .020 over and a slip on, the other is a 426 with hotcam, port and polish. both have 11:1 pistons. Have checked the jetting and it should be ok, both show nice tan spark plugs. we are at 900ft elevation and dont ride at high elevations.

dont have a compression tester so i dont know what they are running. both were overhauled last summer so they are fairly fresh.

[brokenmike]

i have a 440 with a 11:1 piston and with 93 octane it pings on hard acceleration,so i just 50/50 mix 112 and 93 and i dont hear it anymore

[zrpilot]

Originally posted by GPracer2500


If you're not utilizing the stock auto-decompression system, then a simple compression check will tell you a lot more about the engine's octane requirement than piston specs.

Agreed, I'm pretty sure on my "quoted" 11:1 compression Wiseco piston using a base gasket and a multi layered head gasket (3 I think:confused: )I'm probably not getting 11:1 static compression. This makes a compression tester the best way to determine octane requirements.

[250X_project]

okay, so what is they PSI/octane requirements?

Also, how do you figure the octane of a specific mix, as Glamis400ex said? Just the average between the 2 octanes? EX. 110, and 93= 101.5 octane?

[GPracer2500]

Originally posted by 250X_project
okay, so what is they PSI/octane requirements?

Also, how do you figure the octane of a specific mix, as Glamis400ex said? Just the average between the 2 octanes? EX. 110, and 93= 101.5 octane?

1) On my guage, my 10.8:1 with a HC stg 2 pumped ~180psi cold at 1300ft. As I said, my local 91 pump was not adaquate. Would a conventional 93 or 94 octane pump fuel have been adaquate for me? I doubt it, but maybe.

2) No, you cannot just average the two figures. It's not quite that easy. Here's the deal: mixing leaded fuel and unleaded fuel can yeild unexpected results. Hydrocabons have a characteristic called lead response. Basically, some hydrocabons "mix" well with tetra-ethyl lead (TEL) and yield nice high octane ratings. Other hydrocabons do not react well to TEL. If you throw some unleaded fuel in there you can screw up that balance. It's possible that some regional boutique fuel blends could actually have a negative lead response--drastically reducing the effectiveness of a leaded race, unleaded pump mix.

If you're mixing a leaded fuel with an unleaded fuel you'll never really know what octane you're ending up with. It's best to just use a pass/fail evalutation. Either the mix meets the engines octane requirement or it does not. The actual octane rating is moot.

When mixing two leaded fuels together or two unleaded fuels together then a simple average of the two ratings is reasonably accurate.