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Old 06-14-2011, 08:44 PM
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Fuel usage at different RPM's, same throttle opening

Let's say you have two identical engines with identical carburators and every other little component. Let's also say they are in a controlled situation with one turning 2100rpm and one turning 3100rpm both at the same exact same throttle opening (About 25% throttle). Obviously, the one pulled down to 2200rpm is under a greater load.


How would fuel usage vary between these engines? Would the engine turning a higher RPM consume more fuel due to the higher RPM, or would the engine under a higher load and lower RPM consume more fuel?


Are there any tests or anything out there or does anyone have some insight or knowledge on a situation such as this? Feel free to share any thoughts or opinions.

Hopefully this is the correct section.

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Old 06-14-2011, 09:59 PM
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IMO, same of fuel flow, same usage. You just wouldn't get as far in the vehicle with more load, like climbing a hill vs. flat road. You might not even get over the hill...
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Old 06-14-2011, 10:31 PM
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Quote:
Originally Posted by Sw1tchfoot
Let's say you have two identical engines with identical carburators and every other little component. Let's also say they are in a controlled situation with one turning 2100rpm and one turning 3100rpm both at the same exact same throttle opening (About 25% throttle). Obviously, the one pulled down to 2200rpm is under a greater load.


How would fuel usage vary between these engines? Would the engine turning a higher RPM consume more fuel due to the higher RPM, or would the engine under a higher load and lower RPM consume more fuel?


Are there any tests or anything out there or does anyone have some insight or knowledge on a situation such as this? Feel free to share any thoughts or opinions.

Hopefully this is the correct section.
Same throttle opening, one w/the higher RPM will be pulling more fuel from the booster, but the one under a greater load has the potential to pull additional fuel through the power enrichment circuit along w/what is being pulled from the booster- so IMHO there is no "correct" answer, w/o knowing the vacuum under which the engines were operating, and what the vacuum tip-in point is for the enrichment circuit along w/what the area is of the enrichment delivery circuit.
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Old 06-15-2011, 08:33 AM
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First of all, you can't have two otherwise identical engines at different RPMs at the same throttle opening with the same load at steady state conditions. If one is accelerating or decelerating and the throttles just happen to be at the same position, all bets are off since you'll be using transient fuel circuits like the accelerator pump. If the engines are truly identical and have the same load, they will be at the same throttle opening at the same RPM.

In any case, if we're talking about steady state fuel flow from the mains only, and ignoring any variables like vacuum secondaries, then the engine with more airflow will draw more fuel.
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Old 06-15-2011, 08:47 AM
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Quote:
Originally Posted by joe_padavano
First of all, you can't have two otherwise identical engines at different RPMs at the same throttle opening with the same load..
He specified one of the engines was loaded to pull it down to the lower RPM- so they aren't at the "same load":
Quote:
...they are in a controlled situation with one turning 2100rpm and one turning 3100rpm both at the same exact same throttle opening (About 25% throttle). Obviously, the one pulled down to 2200rpm is under a greater load.
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Old 06-15-2011, 10:25 AM
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It sounds to me like if they are both steady state, then the manifold vacuum must be the same for both states (since the throttle is the same). So I think they would both pull the same amount of fuel...the 2100 rpm will just have a richer AF than the 3100.

Oh well, I could also be completely full of it.
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Old 06-15-2011, 10:53 AM
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Quote:
Originally Posted by cobalt327
He specified one of the engines was loaded to pull it down to the lower RPM- so they aren't at the "same load":
DOH! Sorry, I missed that entirely.
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Old 06-15-2011, 11:35 AM
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You want to find out, are you man enough? Go to http://ecomodder.com/ and you will see guys talking about fuel usage who know more about the subject than you ever thought possible.

And believe what is discussed over there, I thought I had an idea about fuel economy, less drag, less rolling resistance, bla bla bla. They blew me away and I can by simple driving techniques change my fuel usage by 30% EASY, they REALLY know their stuff.

Post your question there.

Brian
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Old 06-15-2011, 01:22 PM
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Quote:
Originally Posted by MARTINSR
Post your question there.

Brian
And be sure to post the answer back here!
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Old 06-15-2011, 01:32 PM
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Quote:
Originally Posted by nofearengineer
It sounds to me like if they are both steady state, then the manifold vacuum must be the same for both states (since the throttle is the same). So I think they would both pull the same amount of fuel...the 2100 rpm will just have a richer AF than the 3100.

Oh well, I could also be completely full of it.
Could be, I suppose. But I would expect to see less vacuum for the engine w/the lower RPM than with a higher RPM, w/both at the same throttle opening. The slower engine w/less vacuum would be more likely to be into the power enrichment circuit to add the "missing" fuel that the engine would be pulling from the booster if it were able to run at a higher RPM w/more vacuum.

Then like you say, I could also be completely full of it.
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Old 06-15-2011, 02:27 PM
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Hmmm...I guess you're right, cobalt.

This is a great discussion...I really want to understand this stuff a lot better.

Perhaps we should just switch to electric cars.

Last edited by nofearengineer; 06-15-2011 at 02:52 PM.
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Old 06-15-2011, 06:35 PM
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Thanks for the insight guys. My thoughts were going both ways too..

I'll have to come up with some vacuum numbers to throw in here too and see if it helps with the discussion.
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Old 06-15-2011, 07:35 PM
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Quote:
Originally Posted by Sw1tchfoot
Thanks for the insight guys. My thoughts were going both ways too..

I'll have to come up with some vacuum numbers to throw in here too and see if it helps with the discussion.
Or you could just cut to the chase.
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Old 06-15-2011, 09:42 PM
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The vacuum gives a direct idea of fuel consumption. If your build can get 14" vacuum cruising , it will achieve 14 mpg. 15 for 15, 16 for 16, etc.
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Old 06-15-2011, 09:51 PM
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Quote:
Originally Posted by spinn
The vacuum gives a direct idea of fuel consumption. If your build can get 14" vacuum cruising , it will achieve 14 mpg. 15 for 15, 16 for 16, etc.
No ****? So how much vacuum will you say is made by an engine of a vehicle that gets 35 mpg?
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