CarForum.net - Car Forums, Automotive News and Community
Home Gallery Register Calendar Members List

Go Back   CarForum.net - Car Forums, Automotive News and Community > General Car Discussion > General Automotive Discussion

Reply
LinkBack Thread Tools
Unread 10-10-2012, 01:54 PM   #1
S D
Veteran Member
 
S D's Avatar
 
Join Date: Mar 2007
Location: WI
Posts: 1,988
Rep: 64 S D will become famous soon enough
Default Running some numbers on the GM 4.3 and 5.7

I was bored so I ran some simple numbers relating to the most efficient way to accelerate with a 4.3. I've also had enough caffeine to kill a horse..

Quote:
Originally Posted by MechEngVT
I have read that BMW instructs drivers to accelerate at 75% engine load and shift when the engine's mean piston speed reaches 1200-1500 feet per minute. Mean piston speed is a function of engine stroke and RPM.

Vp=RPM*S/6

Vp= mean piston speed
RPM= engine speed
S= engine stroke length, inches

This keeps you in closed-loop operation (for EFI) at high volumetric efficiency with low frictional horsepower loss for as short a time as possible before you can achieve steady-state cruise and/or EOC. It is what I like to say is accelerating "smartly."
Vp=RPM*S/6

Vp= mean piston speed
RPM= engine speed
S= engine stroke length, inches (3.48 for the 4.3 and 5.7)

I came up with:

1200=3.48x/6
1200*6=3.48x/6 *6
7200=3.48x
7200/3.48=3.48x /3.48

x=2068 RPM (Low end, 1200FPM in the 4.3)

1500=3.48x/6
1500*6=3.48x/6 *6
9000=3.48x
9000/3.48=3.48x/3.48

x=2586 RPM (High end, 1500FPM in the 4.3)

Accelerate at 75% load on a warm engine (Not full throttle) and shift between 2100 and 2600 RPM for best FE in a GM 4.3 V6, according to the equation above.

Incidentally this is also just short of peak torque production, where you are in the meaty part of the power band while accelerating.

According to the info available online, In the 4.3 the BMEP is around 150 PSI (10.3bar) at 2800RPM (Peak torque); Meaning that we're making nearly 1lb-ft per 1 cubic inch.

BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

(150.8*260)/262=149.6 psi Divide by 14.5 to get bar
149.6/14.5=10.3 Bar

Now we'll compare the 4.3 to it's bigger brother, the 5.7.

The numbers for a typical Vortec 350 in a Silverado are:

(150.8*330)/350=142 psi
142/14.5=9.8 Bar

What in the heck does this mean?

The 4.3 is more efficient at making power than the 5.7.


The 4.3 makes 260lb-ft of peak torque, and the 5.7 makes 330lb-ft of peak torque.

We can also take 330*.75 to get 247.5. Knowing that the 4.3 is 75% of the 5.7, we can take 260-247.5 and get a difference in output of 12.5lb-ft at 2800rpm.

That means if the stock 5.7 were to have two cylinders directly cut off it would make 12.5lb-ft less than a stock 4.3. Interesting, eh?

At 80% DC on our 19lb injectors we're looking at 19*.8=15.2
And 15.2 per cylinder *6 cylinders is 15.2*6=91.2

BSFC = 91.2/190 (HP) = .48 Not bad at all for stock.

Source - http://www.epi-eng.com/piston_engine..._yardstick.htm
http://www.epi-eng.com/piston_engine...efficiency.htm
__________________
Simplify, then add lightness

Last edited by S D; 10-10-2012 at 01:57 PM.

S D is offline S D's Gallery  
Reply With Quote
Unread 10-10-2012, 05:03 PM   #2
S D
Veteran Member
 
S D's Avatar
 
Join Date: Mar 2007
Location: WI
Posts: 1,988
Rep: 64 S D will become famous soon enough
Default

Quote:
At 80% DC on our 19lb injectors we're looking at 19*.8=15.2
And 15.2 per cylinder *6 cylinders is 15.2*6=91.2

BSFC = 91.2/190 (HP) = .48 Not bad at all for stock.
This also assumes that we are maxing out the poppet's flow at peak power, which I don't believe is true. Injectors are run at a max duty cycle of 80% to avoid going static (Wide open, not good!).

If the 19lb injectors have more headroom and are running at, say 70% DC at peak power the 4.3 is even more efficient at turning fuel into power.

19*.7=13.3
13.3*6=79.8
BSFC = 79.8/190 = .42, even better! IIRC that's in DOHC territory.

I need more info to determine anything accurate. These are just theoreticals based on max flow, so there is a max fuel consumption limit available. Actual FC may be very different.
__________________
Simplify, then add lightness

S D is offline S D's Gallery  
Reply With Quote
Unread 10-10-2012, 08:31 PM   #3
Community Director
 
Mike's Avatar
 
Join Date: Jun 2008
Location: Minneapolis, MN
Posts: 7,602
Rep: 153 Mike has a spectacular aura aboutMike has a spectacular aura about
Default

Everything I've heard says the 4.3 is quite literally a 350 with two cylinders lopped off.
__________________
"He shouted: 'You've won! Didn't you know?' I didn't, and it's about the nicest thing I've ever been told" -Bruce McLaren, 1968 Belgian Grand Prix
Read the rules! | DO NOT REPLY TO SPAM
2009 Pontiac G8 GT - 6.0L, 6AT - 95k miles | 1994 Chevy Blazer K5 - 350, 5MT - 140k miles | 2006 Mazda 3 S sedan - 2.3L, 5MT - 130k miles
Wife's Car: 2013 Chevy Volt - 70k miles
Bikes: 1979 Yamaha XS 750 | 1985 Honda Sabre 700


Mike is offline Mike's Gallery  
Reply With Quote
Unread 10-10-2012, 09:11 PM   #4
Now accepting bribes.
 
lautinjr's Avatar
 
Join Date: Oct 2007
Location: Currently Las Vegas
Posts: 4,311
Rep: 193 lautinjr has a spectacular aura aboutlautinjr has a spectacular aura about
Default

Good info!
__________________
Please help reduce our mini van population and wear condoms!

lautinjr is offline lautinjr's Gallery  
Reply With Quote
Unread 10-11-2012, 12:33 AM   #5
S D
Veteran Member
 
S D's Avatar
 
Join Date: Mar 2007
Location: WI
Posts: 1,988
Rep: 64 S D will become famous soon enough
Default

Quote:
Originally Posted by Mike View Post
Everything I've heard says the 4.3 is quite literally a 350 with two cylinders lopped off.
Physically it is. The 4.3 is a 350 missing two opposing central cylinders. They even share the same pistons, compression ratios, head design, valves, etc.

What we can do is take 260/6=43.3
This tells us that the 4.3 makes 43.3 lb-ft per cylinder in stock form. If we add two more cylinder the difference should be 43.3x2, for 86.6 more lb-ft, follow me?

Now add the 260 to 86.6 and we get 346.6 lb-ft of torque, which is 14.6 more than the stock 350's 330lb-ft at peak. A little more accurate than taking 75% of 330.

If power scales fairly linearly, then the stock Vortec 350 is being choked off even at low revs!

If anyone has fuelling data on the LS engines I can run some numbers for those engines too. Injector size and DC @ WOT would be a good start.
__________________
Simplify, then add lightness

S D is offline S D's Gallery  
Reply With Quote
Unread 10-11-2012, 11:38 AM   #6
Community Director
 
Mike's Avatar
 
Join Date: Jun 2008
Location: Minneapolis, MN
Posts: 7,602
Rep: 153 Mike has a spectacular aura aboutMike has a spectacular aura about
Default



I believe I would have that info. I'll have a look!
__________________
"He shouted: 'You've won! Didn't you know?' I didn't, and it's about the nicest thing I've ever been told" -Bruce McLaren, 1968 Belgian Grand Prix
Read the rules! | DO NOT REPLY TO SPAM
2009 Pontiac G8 GT - 6.0L, 6AT - 95k miles | 1994 Chevy Blazer K5 - 350, 5MT - 140k miles | 2006 Mazda 3 S sedan - 2.3L, 5MT - 130k miles
Wife's Car: 2013 Chevy Volt - 70k miles
Bikes: 1979 Yamaha XS 750 | 1985 Honda Sabre 700


Mike is offline Mike's Gallery  
Reply With Quote
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not 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



All times are GMT -5. The time now is 07:36 AM.

Shogun Interactive Development Copyright 2015 Shogun Interactive Development. All rights reserved.