Why a Cold Engine Runs Poorly - 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 04-08-2006, 07:22 PM
72NOVA454
 
Last photo:
Join Date: Nov 2004
Location: chicago area
Age: 51
Posts: 922
Wiki Edits: 0

Thanks: 0
Thanked 2 Times in 2 Posts
Why a Cold Engine Runs Poorly

Ok

this is crazy but I will ask it anyway. why is it that a cold engine, especially a rebuilt modified hi performance V8 engine like my BB Chevy runs poorly until it warms up. Yes I know this is "basic engine 101" type of discussion but I am asking it anyway.

I can think of 2 or 3 things that would make it run poorly when cold. First is the air/fuel mixture when it is cold doesn't vaporize well and results is poor combustion. Second, is the motor oil is cold and has a higher viscocity when cold. However, I though colder air is better for combustion because I believe it is denser and contains more nitrogen and oxygen............?, Third, perhaps the cylinder walls and pistons have more friction when they are cold and don't move as easily.

All this came to my mind just a few minutes ago when I took my car for a ride. It's a 72 Nova with a recently rebuilt 468 Chevy. It has a huge hydraulic cam (comp cams extreme magnum), iron oval port closed chamber heads (ported with valve job), 10.5 to 1 comp ratio, roller rockers, dual plane intake, Holley 4BBL carb - single pumper - vacuum secondaries - no operational choke, MSD Ignition and distributor, 2" headers, Holley high flow mechanical fuel pump @ 8 psi, no emissions or warmup controls whatsoever.

The first 5 minutes or so that the engine warms up it runs rough. I don't care because it's just a warmup period. Once it's warmed up the engine runs very well and strong - I'm guessing near 550 HP and really smokes the tires. but why is it that those first 5 minutes or so the thing runs like crap.

thanks.

Lee

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 04-08-2006, 07:55 PM
docvette's Avatar
Hotrodders.com Moderator
 
Last wiki edit: Rebuild an alternator Last photo:
Join Date: Mar 2004
Location: Lafayette, california
Age: 62
Posts: 7,362
Wiki Edits: 12

Thanks: 0
Thanked 6 Times in 3 Posts
Doc here,

I think you've answered your own question..

"No operational Choke"

The choke allows extra fuel to the engine during that warmup period to help it run better during warmup.after things warm a bit, it begins to open..(about 3 minutes on a standard electric choke.)

So If you want it to run better During warmup, connect and adjust the Choke properly, It should run fine after.

Doc
__________________
Aftermarket Solutions
Electronic & Electrical
Innovations
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 04-09-2006, 12:21 AM
curtis73's Avatar
Hates: Liver. Loves: Diesel
 
Last wiki edit: How to find cheap parts
Last journal entry: 1999-2001: Getting it on the road
Last photo:
Join Date: Dec 2005
Location: Pittsburgh, PA
Age: 40
Posts: 5,128
Wiki Edits: 16

Thanks: 0
Thanked 2 Times in 2 Posts
The main reason for a choke, and the reason why it run so poorly when cold is from a phenomenon called fuel puddling or fuel shear. The carburetor vaporizes fuel and mixes it with air in a proper ratio. All things being perfect, the fuel stays in suspension. When the intake and heads are cold, often times the fuel comes out of suspension and wets the walls of the runners. That leans the mixture that the cylinders actually see, so the choke richens it up to compensate. Since the choke richens the mixture (which would normally cause the engine to die) the choke mechanism also includes a little cam or pin that holds the throttles open a little for fast idle.

During this cold period, the wetted walls will from time to time give up a drop or two which causes the rough performance. When you go to accelerate when its cold, more of the same happens. An unpredictable amount of the fuel that is metered through the carb will actually make it to the cylinder and stumbling and stalling might happen.

Once the intake is hot, any fuel that comes out of suspension and touches the runner walls is quickly evaporated. Intakes often have an exhaust crossover that speeds the heating process.

I don't think the cold oil or friction has anything to do with it. Those two factors alone might reduce performance, but not cause poor running properties. You could tune the carb to run better when cold, but then you would give up the proper tune when its hot. Since engines spend 90% of their time at operating temps, it makes sense to tune them for hot running.

EFI mostly cures these issues since they inject the fuel directly at the valve. There is almost no chance for fuel to puddle or come out of suspension. Cold enrichment can be much less drastic and high cold idle speeds much less of an issue.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 04-09-2006, 07:58 AM
powerrodsmike's Avatar
Hotrodders.com Moderator
 
Last wiki edit: Make a fiberglass fan shroud
Last journal entry: Next.. ..Bagging the king B (barge)
Last photo:
Join Date: Jan 2005
Location: gilroy, california
Age: 53
Posts: 4,108
Wiki Edits: 161

Thanks: 0
Thanked 3 Times in 3 Posts
Quote:
Originally Posted by curtis73
The main reason for a choke, and the reason why it run so poorly when cold is from a phenomenon called fuel puddling or fuel shear............................................. .......... Cold enrichment can be much less drastic and high cold idle speeds much less of an issue.
That was beautiful, Curtis.
__________________
my signature lines...not really directed at anyone in particular..

BE different....ACT normal.

No one is completely useless..They can always be used as a bad example
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 04-09-2006, 08:26 AM
Member
 
Last photo:
Join Date: Nov 2004
Location: Lakeland FL
Age: 65
Posts: 4,110
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
great explanation Curtis!

can only add in this case, the "big" cam = lower Hg (12hg?) at idle = much less velocity to keep it suspended....1200rpms idle when cold will increase the Hg/velocity (18+hg?) and that helps reduce wetting

a stock motor will pull approx 18hg/600rpm, 22hg/1200rpms choked for more velocity/less wetting cold just for reference

and to help appreciate (?) what the Hg values and velocity translate to: a 2HP wet/dry shop vac can only pull about 4Hg max!
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 04-09-2006, 08:46 AM
72NOVA454
 
Last photo:
Join Date: Nov 2004
Location: chicago area
Age: 51
Posts: 922
Wiki Edits: 0

Thanks: 0
Thanked 2 Times in 2 Posts
Quote:
Originally Posted by docvette
Doc here,

I think you've answered your own question..

"No operational Choke"

The choke allows extra fuel to the engine during that warmup period to help it run better during warmup.after things warm a bit, it begins to open..(about 3 minutes on a standard electric choke.)

So If you want it to run better During warmup, connect and adjust the Choke properly, It should run fine after.

Doc
thanks buddy
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 04-09-2006, 08:47 AM
72NOVA454
 
Last photo:
Join Date: Nov 2004
Location: chicago area
Age: 51
Posts: 922
Wiki Edits: 0

Thanks: 0
Thanked 2 Times in 2 Posts
Quote:
Originally Posted by curtis73
The main reason for a choke, and the reason why it run so poorly when cold is from a phenomenon called fuel puddling or fuel shear. The carburetor vaporizes fuel and mixes it with air in a proper ratio. All things being perfect, the fuel stays in suspension. When the intake and heads are cold, often times the fuel comes out of suspension and wets the walls of the runners. That leans the mixture that the cylinders actually see, so the choke richens it up to compensate. Since the choke richens the mixture (which would normally cause the engine to die) the choke mechanism also includes a little cam or pin that holds the throttles open a little for fast idle.

During this cold period, the wetted walls will from time to time give up a drop or two which causes the rough performance. When you go to accelerate when its cold, more of the same happens. An unpredictable amount of the fuel that is metered through the carb will actually make it to the cylinder and stumbling and stalling might happen.

Once the intake is hot, any fuel that comes out of suspension and touches the runner walls is quickly evaporated. Intakes often have an exhaust crossover that speeds the heating process.

I don't think the cold oil or friction has anything to do with it. Those two factors alone might reduce performance, but not cause poor running properties. You could tune the carb to run better when cold, but then you would give up the proper tune when its hot. Since engines spend 90% of their time at operating temps, it makes sense to tune them for hot running.

EFI mostly cures these issues since they inject the fuel directly at the valve. There is almost no chance for fuel to puddle or come out of suspension. Cold enrichment can be much less drastic and high cold idle speeds much less of an issue.
thanks curtis

it's funny sometimes how we can build race cars and often forget some of the basic "engine 101" theory.

Lee
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 04-09-2006, 08:48 AM
72NOVA454
 
Last photo:
Join Date: Nov 2004
Location: chicago area
Age: 51
Posts: 922
Wiki Edits: 0

Thanks: 0
Thanked 2 Times in 2 Posts
Quote:
Originally Posted by red65mustang
great explanation Curtis!

can only add in this case, the "big" cam = lower Hg (12hg?) at idle = much less velocity to keep it suspended....1200rpms idle when cold will increase the Hg/velocity (18+hg?) and that helps reduce wetting

a stock motor will pull approx 18hg/600rpm, 22hg/1200rpms choked for more velocity/less wetting cold just for reference

and to help appreciate (?) what the Hg values and velocity translate to: a 2HP wet/dry shop vac can only pull about 4Hg max!
thanks mustang

you are right about the manifold vacuum. It's only about 10" at idle if that.

Lee
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 04-09-2006, 07:10 PM
automotive breath's Avatar
Registered User
 
Last photo:
Join Date: Mar 2006
Location: New Orleans
Posts: 891
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
Quote:
Originally Posted by curtis73
…During this cold period, the wetted walls will from time to time give up a drop or two which causes the rough performance…..
Curtis, this is a great explanation. After years of dealing with this problem and how it becomes more pronounced with a camshaft with a long overlap period, it’s easy to overlook the reason this happens. I’m not disagreeing with what you are saying but hope to take the discussion a little further.

It’s important to think about what happens during combustion in a cold engine.

You point out how EFI helps to cure this problem; in addition modern ignition systems have also done much to improve the warm up issues. I remember my first engine with a camshaft with a long overlap period, no choke and a point’s distributor. MSD and the like have done there part to light the lean mixtures encountered during cold start.

Another important point is when the air/fuel mix is compressed during the compression stroke and even further during the early phase of combustion the fuel has a tendency to drop out of suspension. This further leans the mixture adding to the problem; it also increases hydrocarbons in the exhaust to a point that it can irritate the eyes in a confined area.

I’m somewhat hesitant to say this but I have found a cure for this problem. The 67 Camaro in my avatar idles cold at 800 RPM with no choke.

Here are the engine specs:
AFR 190 heads with 67CC combustion chambers
Mini dome pistons - steel rods
Isky solid flat tappet cam 263/268 0.050” - 0.555” lift.
11:1 compression - 0.037” squish clearance
750 Holley - 83 jets square – Victor Jr. intake
RPM range 5200 – 7200
10.80 ET @ 121 MPH at 3200 Lbs.

The cylinder heads have been modified with grooves that aid mixture motion during combustion. The improved idle is one of the benefits that were realized with the modification. I realize this is hard to believe to most people, but for me seeing is believing.
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 04-10-2006, 08:13 AM
Member
 
Last photo:
Join Date: Nov 2004
Location: Lakeland FL
Age: 65
Posts: 4,110
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
auto breath,
not really a fair comparison....your at sea level with humidity and much higher early morning ambient "cold" temps....leejoy is in Chicago!

due to temp and humidity, it's the same here in FL, "most" motors do OK without a choke, if you do need one it's usually turn the cap full left for minumum time and (?) 1000rpm

leejoy,
there's your answer...move to New Orleans!

Last edited by red65mustang; 04-10-2006 at 08:18 AM.
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 04-10-2006, 11:29 AM
automotive breath's Avatar
Registered User
 
Last photo:
Join Date: Mar 2006
Location: New Orleans
Posts: 891
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
Quote:
Originally Posted by red65mustang
auto breath,
not really a fair comparison....your at sea level with humidity and much higher early morning ambient "cold" temps....leejoy is in Chicago!...
I agree you bring up a good point. I did not try to compare my engine to his or my location to his. I am very familiar with cold natured engines in moderate cold weather; we race in December and January!

The problem is even more pronounced with the huge BBC combustion chamber, flame time is increased making it even more cold natured than the SBC.
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
Rebuilt Engine Squeaking, Driving me Nuts! BH69Camaro Engine 24 04-22-2005 08:51 PM
Too much engine? frankenvette Engine 39 05-18-2004 03:26 AM
olds 455: engine idles rough when started cold. 98rocket Engine 5 05-09-2003 05:01 PM
engine nocks when cold 85irocss Engine 4 11-28-2002 04:48 PM
hot side, 350 tpi engine runs hot on one bank? Ellisb Engine 16 10-27-2002 06:28 AM


All times are GMT -6. The time now is 11:41 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.