Fuel octane for comp ratio. - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum

Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
lost password?   |   register now

LinkBack Thread Tools
  #1 (permalink)  
Old 02-11-2012, 03:16 AM
Registered User
Last photo:
Join Date: May 2008
Location: New Zealand
Age: 41
Posts: 165
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Fuel octane for comp ratio.

Hello fellow petrolheads. Ive got a 408ci sbc with 10.4/1 comp ratio and running a set of 195 AFR eliminator heads on it. I've recently discovered that it seems to run perfectly fine on 91 octane, even with a good thrashng. Our fuel here in New Zealand is crap and everybody tells me there is no way it should run without detonation on that octane. My cam is reasonably agressive, a lunati 60105 flat tappet hydraulic cam with .525 and .546 lift - 241 and 249 degrees at 50 thou. So I am wondering whether the cam is letting me get away with a little more compression by leaking a bit past the valves. Would I be right in thinking this? My engine has so many 'so called' experts bewildered because they dont think a cam like that should'nt be streetable, yet it idles at 600rpm in gear with a 2900 stall and pulls from way down low. A real nice behaved performance street engine :-) I know that for best performance you should run as low an octane you can without detonation, so should I stick with the 91 or be on the safe side and run the more expensive 98 octane?

Reply With Quote Quick reply to this message
Sponsored Links
  #2 (permalink)  
Old 02-11-2012, 06:06 AM
Registered User
Last photo:
Join Date: Mar 2007
Location: Corinth, MS
Posts: 607
Wiki Edits: 0

Thanks: 0
Thanked 30 Times in 29 Posts
Here's a link that might help. It's got a dynamic compression ratio calculator at the end that I find very helpful. Can't say that I have verified the results personally, but I am building my current engine on some of the data I've got from using it.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 02-11-2012, 06:45 AM
Registered User

Last journal entry: update
Last photo:
Join Date: Dec 2011
Location: over here
Posts: 216
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Should be fine on 91 octane, especially if you're up in the mountains. Those heads have really efficient combustion chambers, and good swirl, both of which helps resist detonation.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 02-11-2012, 06:58 AM
Last photo:
Join Date: Dec 2004
Location: San Antonio, Texas
Age: 50
Posts: 4,023
Wiki Edits: 0

Thanks: 0
Thanked 23 Times in 23 Posts
yes, aluminum heads and the big cam are lowering the octane requirement.

Other things that effect octane requirement is load on the engine (weight, gear), elevation, engine and air temperature.

I run a 11:1 362 cid sbc with Dart iron heads with 93 octane with a 244/244 cam at 0.050 in a 2800 lb car with 3.90 gears and have no issues.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 02-11-2012, 12:41 PM
techinspector1's Avatar
Senior Curmudgeon
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 12,982
Wiki Edits: 326

Thanks: 768
Thanked 1,024 Times in 856 Posts
On the KB calculator, your Dynamic Compression Ratio is 8.55:1, so that's why you can run pump gas without detonation. I'm guessin' that you built a good, tight squish into the motor also (0.889-1.143 mm).
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 02-11-2012, 12:42 PM
F-BIRD'88's Avatar
Yada Yada Yada
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,153
Wiki Edits: 0

Thanks: 5
Thanked 380 Times in 374 Posts
10.4:1 is a workable compression ratio for good modern unleaded premium pump gas. Its up there, but workable. If you get any hint of detonation use the better fuel available. If you plan to run it hard ( racing, lots of WOT hard running) use the best fuel available.

You should not use the lowest octane pump fuel thinking you will make more power. This is false and a myth born by comparing modern unleaded gas that has a relativly fast burn speed, reguardless of its octane, to old school race gas for extremely high compression ratios that does have a slower burn speed. Compareing them in a low cr motor that does not benefit form the fuel octane.

Most all modern unleaded fuels (even the 114 octane unleaded) have a fast burn speed and tend to make a bit more power than the old school stuff.

In a 14:1 ++ cr race motor both type race fuels burn fast and make good power.

Its all what you compare it in.

Use the best fuel you can buy.

This is what we use here when called for. www.profuels.ca Blends great with pump gas.

when you get to 11:1+ you need a bit.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 02-21-2012, 03:02 AM
Registered User
Last photo:
Join Date: May 2008
Location: New Zealand
Age: 41
Posts: 165
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Well that was a learning experiance guys. As it turns out running 91 in my engine wasn't the right thing to do after all. It fouled some plugs recently, I had a feeling it may have been getting some crankcase oil in past the intake manifold gasket, so I removed the intake manifold. Most ports were stained a dark colour but not all the same. I showed my local engine builder and he said it was from pre-ignition. Fuel has been igniting early before the intake valve has closed! Yuk!
Something else I learned is that the advice I recieved from you lot is probably bang on, what you people dont realise is that our quality of fuel over here in little old NZ is absolute crap. So what works for you doesn't always work for us. Buga Buga Buga.
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Please enter a password for your user account. Note that passwords are case-sensitive.
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Fuel mileage, compression ratio, and octane? BlackAndChrome Engine 3 03-14-2010 05:22 PM
Compression ratio/87 octane gas Sinister Engine 7 04-21-2009 09:11 AM
Comp Ratio @ sea level on 87 Octane w/no ping ralph1090 Engine 29 04-02-2006 05:02 PM
max compression ratio on 89 octane gemini86 Engine 31 07-01-2005 03:35 PM
compression ratio vs octane pitsen3 Engine 6 11-01-2002 05:17 AM

All times are GMT -6. The time now is 06:19 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.