What is Max safe C.R using 91 octane Calif gas?

Gregxster
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What is Max safe C.R using 91 octane Calif gas?

Postby Gregxster » Thu May 28, 2015 12:01 pm

I am building a 4AGE big port engine NA with Arias 10.5:1 pistons, Weber 40mm DCOEs, Poncams. I have a calculated (by OST) CR or 10.8:1
Is this a safe compression ratio using 91 octane Calif gas?

Yes... I searched! Alot about CRs but not talk of what octane is required for various builds.

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Re: What is Max safe C.R using 91 octane Calif gas?

Postby sirdeuce » Thu May 28, 2015 3:03 pm

Depends on your build/tune. Cams have a big affect an compression. 10.8 is easy enough though, just get your jets right and work with the timing. Could get away with as much as 11.5-1 if done right. Fuel injection is easier as it is capable of compensating for temp and air density. You'll have to keep an eye on things with change in weather and elevation.
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oldeskewltoy
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Re: What is Max safe C.R using 91 octane Calif gas?

Postby oldeskewltoy » Thu May 28, 2015 4:39 pm

Are you using Yoshi's "265 - 8.7mm lift cams"? What is his intake valve closing event? As Sirduece points out you can play a bit with ignition timing, and if you need too ( I doubt it) you can adjust cam timing to let a tiny bit of the compressing gasses go before closing the intake valve


Btw... Greg.... "Skippy" - OST-032, will be running 10.8 on stock cams ("240", 7.5mm lift) - viewtopic.php?f=7&t=15894
OST Cyl head porting, - viewtopic.php?f=22&t=300

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Re: What is Max safe C.R using 91 octane Calif gas?

Postby gotzoom? » Thu May 28, 2015 5:19 pm

I'm running 11:1 Supertech pistons with Poncams and 91 octane. No audible pinging with around 12 deg timing advance. FWIW, happily ran 89 octane on my previous engine that had smallport high comp pistons and Poncams with 12 deg ignition timing.

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Gregxster
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Re: What is Max safe C.R using 91 octane Calif gas?

Postby Gregxster » Thu May 28, 2015 7:22 pm

Thanks, all for the info. I know I will be at the edge for safe CR. I do have an AFR gauge installed to help tune the Webers and monitor the leaness. I also have programmable MSD ignition I can dial in some retard if I have a specific RPM issue.

OST - I decided to go with the Poncams and will be using Supertech single valve springs.

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Re: What is Max safe C.R using 91 octane Calif gas?

Postby yoshimitsuspeed » Thu May 28, 2015 8:04 pm

I'm curious what led you to the Poncams. I know in your last email you thought they would keep you non interference but as I said in my last email they won't with those pistons.
Also curious about the valve springs as the ST will cost about twice as much at TED or Toda due to needing to buy the whole kit.

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Re: What is Max safe C.R using 91 octane Calif gas?

Postby Gregxster » Thu May 28, 2015 9:36 pm

Yoshi,
I decided to go with Tomei Poncams because the MG 265 would be more efficient with a higher CR than I can/want to build. Also, they are more readily available and widely used.
I found a supplier that sells ST single springs for $7.06 a piece and free shipping...

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Re: What is Max safe C.R using 91 octane Calif gas?

Postby yoshimitsuspeed » Thu May 28, 2015 10:06 pm

Gregxster wrote:Yoshi,
I decided to go with Tomei Poncams because the MG 265 would be more efficient with a higher CR than I can/want to build. Also, they are more readily available and widely used.
I found a supplier that sells ST single springs for $7.06 a piece and free shipping...


The compression you can run is directly proportional to the cam you can run. I thought I had made that clear. This is part of why you were unable to find any direct answers to your question. If you can safely run 10.8:1 compression on the poncams then you could run a tiny bit more compression on my 265s. The difference is quite small though. On a 272 you could run more compression and on a 304 you could run a lot more compression. You need to choose your compression based off your cam.

Now if you are more comfortable running the Poncams that's fine and I can get you those but I want to make sure you understand the relationship and it still doesn't sound to me like you do.

I can get you single ST springs as well but Supertech has told me they will not work without their full kit. I have not tried to prove one way or the other but I take their word on this.

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oldeskewltoy
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Re: What is Max safe C.R using 91 octane Calif gas?

Postby oldeskewltoy » Fri May 29, 2015 7:35 am

yoshimitsuspeed wrote:
Gregxster wrote:Yoshi,
I decided to go with Tomei Poncams because the MG 265 would be more efficient with a higher CR than I can/want to build. Also, they are more readily available and widely used.
I found a supplier that sells ST single springs for $7.06 a piece and free shipping...


The compression you can run is directly proportional to the cam you can run. I thought I had made that clear. This is part of why you were unable to find any direct answers to your question. If you can safely run 10.8:1 compression on the poncams then you could run a tiny bit more compression on my 265s. The difference is quite small though. On a 272 you could run more compression and on a 304 you could run a lot more compression. You need to choose your compression based off your cam.

Now if you are more comfortable running the Poncams that's fine and I can get you those but I want to make sure you understand the relationship and it still doesn't sound to me like you do.

I can get you single ST springs as well but Supertech has told me they will not work without their full kit. I have not tried to prove one way or the other but I take their word on this.


you know....... you are a condescending SOB......



Greg.... concerning the ST springs... order one spring, and either check fit yourself, or send the spring to me and I can check fitment. From memory there may be an issue with retainer-spring fitment... but not remembering clearly... that may have been between ST springs and ST retainers... and not stock. ITs been a while but I vaguely remember some issue.

As to other spring options... the TODA springs are the "easy button".
OST Cyl head porting, - viewtopic.php?f=22&t=300

Building a great engine takes knowing the end... before you begin :ugeek:

Enjoy Life... its the only one you get!

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Re: What is Max safe C.R using 91 octane Calif gas?

Postby yoshimitsuspeed » Sat May 30, 2015 9:43 am

oldeskewltoy wrote:
you know....... you are a condescending SOB......


I cant' fathom how that post could have come across as the least bit condescending. I was just trying to share information.

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