compression ratio and fuel confusion - 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
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 03-14-2010, 02:12 PM
55effie's Avatar
Registered User
 
Last photo:
Join Date: Mar 2010
Location: Colorado Springs
Posts: 45
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
compression ratio and fuel confusion

Alright....I am confused as hell here. I just bought a 1955 F100 with a 383 stroker in it. It has a compression ratio of 10.5:1 with flat top pistons with six inch rods and 64CC heads with 200CC intake runners. On top is a 650 CFM Demon 4 barrel carb, and hydraulic lifters. I am not sure what the numbers on the cam are, and the previous owner cannot remember either, but it is rather steep.
I just had the Carb rejetted to match my elevation here in Colorado Springs of 6200 feet. THe new primaries are 65PPM. Here is where I am confused. It ran fine on 91-93 (pump gas) octane when I drove out from Cali. However, when I went to a local hotrod shop, the owner scared the hell out of me by telling me that I might have injured the engine because of the low octane fuel. So, now I am running a mixture of 110 and 91 octane to keep it safe for now. However, friends have told me that 91 octane is fine. Which is it.

BTW, these simple factors might have saved me: It was jetted to 70PPM jets, which was very rich, it was sucking some oil in through the PCV into the carb through the PCV hose, and the cam might have been steep enough to bleed off pressure fast enough to keep from damaging it. In any case, it is not damaged and I am grateful.

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 03-14-2010, 03:53 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 12,564
Wiki Edits: 326

Thanks: 698
Thanked 878 Times in 746 Posts
Iron-headed motors have been run on pump gas at 11.0:1 static compression ratios or even higher without detonating. You just have to run enough intake valve closing point on the cam to bleed off enough compression to prevent detonation. This is a balancing act, to close the intake early enough to trap enough mixture to make a good bang, but not so early that the motor detonates from the excessive cylinder pressure.

If you drove the car up from the coast where you had sea level air density without it detonating, I think the chances are pretty slim that it will detonate on the rarified air at altitude so long as you keep the air/fuel ratio handled properly.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 03-14-2010, 04:07 PM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,539
Wiki Edits: 0

Thanks: 3
Thanked 331 Times in 328 Posts
You didn't drive it from Calf. at wide open throttle.

The ignition timing many be dialed back a bit from the point that makes max power. If say the engine wants 36deg @WOT for best power output and the timing is set at 32-33deg, doing that that will induce a engine knock threshold cushion to a point, (But reduces engine power)

The measured engine cr may not actually be 10.5:1

Reguardless of popular over simplified internet theory about compression ratio, intake valve closing pont and fuel octane if you are going to run it hard and expect max performance out of it without enigne damage, feed it the fuel octane required to prevent detonation.

At least the best available pump gas. If mixing in some race gas is required to spike up the octane a bit, so be it.
It's your motor. It's up to you to take care of it.
Good fuel is cheaper than new pistons and a rebuild.

You want to avoid running on the edge and avoid lean fuel air mixture.

You needn't be scared. You do need to use common sense.

There have been many posts here from other people that $$$$have not had such good luck$$$$ with their local fuel quality specificly in the Mid West states. No one that promotes this that or the other as to what works or don't work ever showed up at their house with a wheel barrel full of money to help when things went bad.

As always, your mileage may vary.

11:1 is too much for a real world conditions street driven vehicle on todays pump gas. Real world conditions are always less than optimum.

Last edited by F-BIRD'88; 03-14-2010 at 04:44 PM.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 03-14-2010, 05:10 PM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 59
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
Quote:
Originally Posted by 55effie
THe new primaries are 65PPM.
When I see "ppm" I think "parts per million" like when talking about pollutants in exhaust gas.

Jets are measured by the orifice size- this can coincide w/the diameter, like a 65 jet being 0.065", though there are small variations in the numbering systems used by the various manufacturers. So a "65" jet is just that, a 65 jet, not a 65ppm jet. Just so you know.

Adding oil through the PVC- or from anywhere if it's getting into the combustion chamber- can induce detonation. It shouldn't be considered a "safety" factor, but one to be avoided.

If you have Al heads, that will also help to offset some of the octane requirement as long as your cooling system is operating as it should. That, along w/the RPM being below the torque peak at cruise speed helps the engine to not have had a detonation problem.

Unless you have loud exhaust, you should be able to hear the ping of detonation and if that wasn't present- and your system quiet enough to have heard it if it was present- would be enough for me to relax.

It sounds like the speed shop counter help may be a 'flat lander'.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 03-14-2010, 07:04 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 12,564
Wiki Edits: 326

Thanks: 698
Thanked 878 Times in 746 Posts
Quote:
Originally Posted by F-BIRD'88
over simplified internet theory
It's not internet theory. It's just a fact that you need to coordinate the static compression ratio with the intake closing point of the camshaft. Call up any cam grinder and ask for a recommendation. The first thing they want to know is the static compression ratio so they can match the opening and closing events to your motor so they can provide you with a cam that will operate at its max potential on the fuel you have available to you.

Last edited by techinspector1; 03-14-2010 at 09:04 PM.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 03-14-2010, 08:14 PM
55effie's Avatar
Registered User
 
Last photo:
Join Date: Mar 2010
Location: Colorado Springs
Posts: 45
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thanks for correcting me on the jetting issue. I am rather new to the technical side of hot rods. SInce I cannot get any info on the cam from the previous owner, I kinda have play it safe with the fuel I guess. I do have rather loud exhuast (dual 40 series flowmasters) and cannot hear the pinging, if it happened at all. There are no signs of engine damage yet, and I have corrected the PVC/oil issues already. Since I now have a 65 jet in the primaries, I think I will keep mixing the race fuel in to play it safe until I figure out some other options.
I want to eventually find out what my actual compression ratio is, since that race fuel is three times as much as pump gas. All I know how to check is the cranking pressure of the cylinder. How would I find the actual CR?
Also, I am gonna have to do some more research on the intake/cam timing issue. All that was a bit over my head at this point.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 03-14-2010, 08:49 PM
Blazin72's Avatar
You got a leaky spark tube...
 
Last wiki edit: Rearend removal
Last journal entry: General Lee
Last photo:
Join Date: Jul 2005
Location: Union, WA
Age: 32
Posts: 2,868
Wiki Edits: 19

Thanks: 0
Thanked 2 Times in 2 Posts
If you didn't hot rod it all the way back you may have been able to get away with driving it home on 89 octane without too many issues. The only time the engine makes its max cylinder pressures is at WOT and the engine is at max load. At part/light throttle the cylinder pressures will be lower and you should be ok.

I had a 9.7:1 350 that I supercharged and had to mix 110 and 92 like you are doing. There were a few times I had to fill it up on 92 only and I just drove it lightly and never gave it a chance to build a lot of pressure and it was fine. I didn't do it all the time but there were a few times I didn't have much of a choice.

I'm also inclined to agree with Techinspector about your altitude saving you too. The air isn't as dense the higher you go which means your engine isn't bringing in as much and that will cause your cylinder pressures to drop as well. Of course, you will have a drop in power output as well.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 03-14-2010, 09:10 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 12,564
Wiki Edits: 326

Thanks: 698
Thanked 878 Times in 746 Posts
Quote:
Originally Posted by 55effie
All I know how to check is the cranking pressure of the cylinder.
So, what is the cranking pressure?

Detonation can be seen on the plug insulators and will look like someone sprinkled pepper on them. The little black specks are molten aluminum from the piston crown that have been deposited on the porcelain.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 03-14-2010, 09:33 PM
55effie's Avatar
Registered User
 
Last photo:
Join Date: Mar 2010
Location: Colorado Springs
Posts: 45
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
I have not actually measured the cranking pressure yet. I will get to it. I just got my Demon carb back on and fixed a few other little things this weekend. That 383 sure loves the race fuel.
Thanks, techinspector for the tip about the speckled spark plugs. Now I will know what to look for when I change the plugs next weekend.
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 03-14-2010, 10:08 PM
Registered User
 
Last photo:
Join Date: May 2006
Location: Kamloops B.C.
Posts: 231
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
ping

Someone (like a young engineer) has to be able to come up with a simple way to detect pinging on a non computor engine so the driver can crank back the timing with a m.s.d timimg control until suitable fuel can calm the savage beast.


ps I,ve got the timing control and it,s great if I could just hear the pinging ...
Clint
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 03-14-2010, 10:38 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 12,564
Wiki Edits: 326

Thanks: 698
Thanked 878 Times in 746 Posts
Scroll down on this page to see aftermarket knock sensors all the way back to '61 models....
http://www.racepages.com/products/?N...+Sensor&N=1588
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 03-15-2010, 07:23 AM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 8,678
Wiki Edits: 0

Thanks: 17
Thanked 281 Times in 261 Posts
Yep, I was just going to say you can actually build your own knock sensor. If you're that worried about it I'd go that route. Get a cheap sending unit from a junkyard and find the directions to make the box on the internet. My buddy who's an EE did it in a weekend and granted he's an EE but he's still just an average over educated redneck like myself- it may take you two weekends but still isn't too bad.

Another thing to consider is cranking compression. Cranking compression and vacuum at idle (along with idle speed) can tell you a lot about how radical an engine is. Many "10.5:1" engines are more like 10:1 or even slightly less.
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 03-15-2010, 03:47 PM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,539
Wiki Edits: 0

Thanks: 3
Thanked 331 Times in 328 Posts
Quote:
Originally Posted by price
Someone (like a young engineer) has to be able to come up with a simple way to detect pinging on a non computor engine so the driver can crank back the timing with a m.s.d timimg control until suitable fuel can calm the savage beast.


ps I,ve got the timing control and it,s great if I could just hear the pinging ...
Clint
Makeing you own "engine ears" is not that hard. A mechanics stethoscope connected to the side of the engine block will allow you to hear the engine detonating quite clearly. I build a simple electronic version using a common "listener" amplifier+ head phones + alligator clamp from radio shack http://autospeed.com.au/cms/title_DI...8/article.html
based on this article .


it works

When the engine detonates you hear a unmistakeable sound.
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 03-15-2010, 04:00 PM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,539
Wiki Edits: 0

Thanks: 3
Thanked 331 Times in 328 Posts
Quote:
Originally Posted by techinspector1
It's not internet theory. It's just a fact that you need to coordinate the static compression ratio with the intake closing point of the camshaft. Call up any cam grinder and ask for a recommendation. The first thing they want to know is the static compression ratio so they can match the opening and closing events to your motor so they can provide you with a cam that will operate at its max potential on the fuel you have available to you.

Yes they do ask these questions when specing a cam. But that won't tell you what the octane requirement of that engine will be.

There are definate real world limits to how far delaying intake valve opening will get you on pump gas.

Mechanical compression much higher than 10.5:1 is ( at minimum) problematic on premium pump gas (even real good stuff), reguardless of how big the cam is.
Potential power gains of trying to push the limit are more than offset by risk of inevitable engine damage when things are not optimum.

Engines do not heal themselves. You'll soon ende up with a prematurly tired, worn out less powerfull engine if it does not come unglued altogether.

I base my real world practical recomendations on 30 years of building and tuning engines, not oversimplified theory.

If you want to go fast for more than a few 1/4 mile blasts and like most of us pay our own bills, avoid the mindset of getting the last bit of engine cr reguardless of what others claim to do or claim theory will predict.

$ Its just not that simple $$$ After you have build a few that went over the edge, you figure that out.

Last edited by F-BIRD'88; 03-15-2010 at 04:07 PM.
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 03-16-2010, 06:29 AM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 8,678
Wiki Edits: 0

Thanks: 17
Thanked 281 Times in 261 Posts
Quote:
Originally Posted by F-BIRD'88
If you want to go fast for more than a few 1/4 mile blasts and like most of us pay our own bills, avoid the mindset of getting the last bit of engine cr reguardless of what others claim to do or claim theory will predict.

$ Its just not that simple $$$ After you have build a few that went over the edge, you figure that out.
VERY SOUND ADVICE!

Besides, what few people consider is that by going this route you lose low end tq, and on the street you will feel 15 ftlb at 2500RPM a lot sooner than you will feel 15hp at 6500 RPM. If you're running pump gas odds are you're planning to run on the street, which means you need to fit your powerband to street levels. I can't think of too many people that make every shift at 7,000 RPM when they go out on a Tuesday evening to get the groceries...
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

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.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

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, aluminum heads, forced induction, Gladis Engine 3 02-27-2008 08:10 AM
Octane Booster Info SLR_65 Engine 46 09-30-2007 09:01 AM
Compression Ratio ron eckman Engine 4 01-13-2004 06:43 PM
High compression ratio pistons invincible Engine 6 09-22-2003 10:16 PM


All times are GMT -6. The time now is 08:25 AM.


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.