compression ratios and pump gas??? - 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 11-09-2008, 04:31 PM
this hobby is expensive!
 
Join Date: Aug 2008
Location: south haven
Posts: 105
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
compression ratios and pump gas???

on a chevy small block - 383 what is the limit on compression when i have to stop using pump gas and go with the high dollar juice? i know some people might have different opinions im just looking for a rough number so i know my limit on this build.

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 11-09-2008, 04:43 PM
Cyberats's Avatar
Hydrogen Burning Engine, YES !
 
Last photo:
Join Date: Nov 2008
Location: California, USA
Age: 46
Posts: 48
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Compression

I haven't seen any cars sold with higher than 11:1 so I would say it's the limit
baring improved engine materials, which are costly. Beyond that I doubt you get better performance out of higher octane even. But I don't know anything about race builds, there maybe different setups in those engines.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 11-09-2008, 04:53 PM
this hobby is expensive!
 
Join Date: Aug 2008
Location: south haven
Posts: 105
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
see, im building a street car and with the setup i have on mind ill have about 11:1 maybe a little less. eagle rotating assembly all forged 6" rods flat top pistons and a set of dart iron eagle platinums for the head 2.02 valve 64cc combustion chamber. that makes about 11:1 right there. i just dont want to run that octane fuel in a street truck. but who knows i might.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 11-09-2008, 05:46 PM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 9,476
Wiki Edits: 0

Thanks: 20
Thanked 386 Times in 362 Posts
13:1, however that is in a superfinely tuned and built engine.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 11-09-2008, 06:20 PM
Registered User
 
Join Date: Nov 2008
Location: South Jersey
Posts: 66
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
I agree with F-Bird, 10.5 comp. any higher and you will not always be satisfied. Also, he hit it right with the larger chamber heads... this actually reduces compression ratio. There are lots of articles out there on low compression pumpgas motors that are HOT.
Rick
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 11-09-2008, 06:46 PM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 9,476
Wiki Edits: 0

Thanks: 20
Thanked 386 Times in 362 Posts
If you're going with anything over 10:1 make sure you have cnc'd chambers. you can't afford any hot spots due to casting irregularities.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 11-09-2008, 06:52 PM
Registered User
 
Last photo:
Join Date: Aug 2008
Location: Santa Rosa,Ca
Posts: 55
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
it all depends on the build. compression relate to not just pistons, but stroke and cc of heads. if you'r motor is 11 to 1 total with stroke, cc in head, and pistons you will be "ok" with 91 octane.but no higher. thats the max. it will run better with higher octane if its 11 to 1. so better to go with 10 3/4 to 1 total.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 11-09-2008, 06:54 PM
this hobby is expensive!
 
Join Date: Aug 2008
Location: south haven
Posts: 105
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
yeah i know about the larger chamber makes lower comp. and all that stuff i have racked my brain reading and asking questions and thinking. i just wasnt sure on the limit. thanks for the advice i appreciate it



adam
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 11-09-2008, 08:20 PM
coldknock's Avatar
Member
 
Last photo:
Join Date: Feb 2004
Location: Social Circle, Ga.
Age: 43
Posts: 1,531
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
Mechanical compression isn't the most important factor. Cylinder pressure is. Contrary to popular thinking, the two aren't connected in a linear fashion.

A larger camshaft with higher compression is better can be just as happy with 93 octane as an engine with a smaller cam and lower compression.

Dynamic compression, look it up.



Larry
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 11-09-2008, 08:34 PM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 9,476
Wiki Edits: 0

Thanks: 20
Thanked 386 Times in 362 Posts
Quote:
Originally Posted by coldknock
Mechanical compression isn't the most important factor. Cylinder pressure is. Contrary to popular thinking, the two aren't connected in a linear fashion.

A larger camshaft with higher compression is better can be just as happy with 93 octane as an engine with a smaller cam and lower compression.

Dynamic compression, look it up.



Larry
Static compression is STILL an important factor. most people that learn about dynamic compression think that its the end all when in reality static is just as important if not more so. Sure dynamic compression give you an idea of how you can squeeze a little more out of an engine, but it has its limits.
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 11-09-2008, 09:00 PM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 9,476
Wiki Edits: 0

Thanks: 20
Thanked 386 Times in 362 Posts
Quote:
Originally Posted by F-BIRD'88
If that was actually true then racing motors with long durations cams would not require racing gas with a high octane. They do...so....

Its just not that simple. You're ignoring the effect of the "heat of compression".
If you compress a volume of gas x amount, it is heated up. Compress it more, it gets hotter. Once it exceeds the temp that allows self ignition all hell breaks loose. (detonation, preignition and piston holes isn't far behind) Its the net temperature of the charge during combustion that determines the auto ignition (spark knock limit) threshold more than the (cylinder) pressure. "Gas Laws" "heat of compression" Look it up.

You're right about compression causing heat, and its simply stated in the gas law PV=nRT, or PV/T(1)=PV/T(2), but that kind of proves the dynamic compression theory. the actaul reason the dynamic theory doesn't work is actaully much simpler. dynamic compression is only effective where there is no intake or exhaust scavenging- at REALLY low engine speeds (idle or lower). As soon as you put these effects into the mix the cylinder pressure go up tremendously inspite of the extended cam timing. What extending the cam timing does do is push their effects higher in the RPM range where fuel atomization and distribution is much better. There i still a limit as to the cylinder pressure a fuel can take, but when you improve the delivery of the fuel you delay detonation, to a certain point.

A race engine with a high compression ratio and large cam can run to 2,000 RPM on 87 octane all day- because the cylinder pressure is still low- but as soon as the "cam comes on" (scavenging begins to take effect) it will rattle like a can of beans.
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 11-09-2008, 09:12 PM
coldknock's Avatar
Member
 
Last photo:
Join Date: Feb 2004
Location: Social Circle, Ga.
Age: 43
Posts: 1,531
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
You're not paying attention to what I said.

When you start playing with cam timing to lower cylinder pressure you have to pay attention to volumetric efficiency so that you can control the point at which detonation would theoretically occur. It's best to have this happen at a higher rpm than it would've otherwise. There's less real time for it to happen and therefore not happen. It's a careful balancing act.

Combustion efficiency is a factor, the longer it takes to complete a burn cycle the greater the chance of detonation occurring. Better efficiency = lower timing = more tolerance for higher cylinder pressures using a given fuel. This is especially important at lower engine speeds.

As for the comment alluding to competition engines......any competitive engine is running on the ragged edge of detonation, for maximum combustion efficiency. That almost always amounts to more power. So the argument is a moot point.

The maximum mechanical compression ratio for a given combination is directly tied to where maximum cylinder pressure occurs in the rpm range, combustion efficiency, volumetric efficiency, the type of fuel used and the application in which the engine will be used. It's not simply a number arbitrarily given from a text book based on another person's experience.

It's a lot more complicated that my simplified explanation, and your rebuttal. My point was that cylinder pressure can be manipulated so that higher static ratios can be used with lower than normal octane ratings. You just have to do your homework to make it happen.


Larry
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 11-09-2008, 09:28 PM
coldknock's Avatar
Member
 
Last photo:
Join Date: Feb 2004
Location: Social Circle, Ga.
Age: 43
Posts: 1,531
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
Ok, AP72's got it and I'm getting more of it.

These are precisely the kinds of discussions that need to take place here. I learn more, others learn more and we all go faster!



Larry
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 11-10-2008, 07:32 AM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 9,476
Wiki Edits: 0

Thanks: 20
Thanked 386 Times in 362 Posts
Quote:
Originally Posted by F-BIRD'88
What extending the cam timing does do is push their effects higher in the RPM range where fuel atomization and distribution is much better.

But always at the expense of low end and mid range torque. Think of how you actually use a "street motor". You end up with a motor that is lazy thru most of the rpm band you want to use and then at high rpm (where cylinder pressure is now high) you cannot run optimum spark timing to avoid detonation.

You end up with a motor that has no torque, doesn;t accelerate the car or truck as well and is very heavy on fuel consumption and does not go as fast as it could.
You do not end up ahead of the game or even equal. You end up with a poor, inefficient engine design. It just doesn't work well in the real world.

The most common mistake that people do when designing a "performance motor" is over camming it.

but that kind of proves the dynamic compression theory.

No, it proves the real world limitation and over emphasis that people put on "dynamic compression theory and calc", in fooling them selves that they can compenstate for excessive mechanical compression ratio (with X fuel octane) with (over) extended valve timing. by picking a bigger cam than the motor actually needs.

Higher compression engines (that actually make real power) need higher fuel octane, than lower compression engines do.
It does lower your effective displacement in the lower RPM, which does cause you to loose some torque, but it may not be "lazy" at all. You just have to limit yourself. I agree that you cannot go overboard on the cam just to lower cranking compression. You do need to limit yourself, HOWEVER you can run 11:1 on the street in a properly built engine matched to the right drivetrain in the right car.

Another thing to consider is that to take advantage of the power upstairs you'll run steeper gearing, which often time reduces the amount of low end laziness that you feel. Also, increasing the stall helps this. Sure you're getting away from a freeway cruiser and closer to a track bruiser, but the whole intention of building a higher compression street engine is to make it more "race car-esque"

I'm not trying to disprove you, I agree with you that there are limitations- but the limitations are not set at 9.5:1 compression and 220 @.050" that so many people assume. If you set your car up right you can go all the way to 11:1 and 250 and still be very street friendly. And no those numbers are not the exact limit, its a range. I'm sure you understand that F'bird but I don't want someone else taking it as the gospel.
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 11-10-2008, 04:25 PM
coldknock's Avatar
Member
 
Last photo:
Join Date: Feb 2004
Location: Social Circle, Ga.
Age: 43
Posts: 1,531
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
Not really, GM stuffed 10.8-1 compression into the LT4 engine, 10.5 in the LT1, with a short cam. They tamed the combustion with reverse cooling and a wide lsa, it was a nice engine. It really woke up with a better cam and ported heads too.

Like I said, nothing's written in stone. Many engine builders prove it on a regular basis. I've done it twice myself. Neither of the engines were what you would call docile though. Not even a daily bruiser kinda engine. In fact, they both ran better with 110 gas and a bit more timing. The point is that they can, and do, run on 93 octane on a regular basis.

I understand your point of view though. Once upon a time I would told me that I was a nutjob, but I learned.



Larry
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:
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
Octane Booster Info SLR_65 Engine 46 09-30-2007 10:01 AM
Need help on my engine. evil.350@hotmail.com Hotrodders' Lounge 4 08-18-2007 08:06 PM
Compression question for the Pro engine builders.. Axelrod Engine 14 01-16-2007 11:29 AM
cams, heads, and compression ratios Litdave Engine 3 05-22-2004 08:47 AM
Compression 4x4vandale Engine 5 10-09-2003 12:59 AM


All times are GMT -6. The time now is 05:49 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.