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Old 04-12-2007, 04:28 PM
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How much HP required at 60mph?

I'm still tuning my Chev for max performance and economy (gas is up to $6,40/USgal now, and rising). I will be putting the car on a dyno soon. I was thinking that I would like to determine the perfect tune at 60 mph, to maximise mileage for long trips. To really do this right I need a fairly accurate estimate of the hp required to keep a 55 Bel Air rolling down a level highway at a constant 60 mph. Anyone know how much power is required?

Once I knew the hp requirement I could tune the engine's timing (dist. advance curve and vacuum advance) on the dyno to achieve the minimum fuel consumption at this given hp and rpm.

Currently car gets about 20mpg highway, I think it should be able to do better. Car has 283 engine with Vortec 305 heads, 9.1:1 CR, HEI, headers, duals, Crane 240H cam, 3.73 gears, 2004R with lockup, Qjet with OEM hot air air filter box - which really helped the cold weather mileage -.

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Old 04-12-2007, 04:45 PM
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I remember reading somewhere that it only takes 20hp to do highway speeds.
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Old 04-12-2007, 08:37 PM
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it is entierly dependant on each particular application. THe biggest factors being drivetrain loss, brake and bearing rub, ande especially wind resistance. It's more than 20 for a Bel Air though
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Old 04-12-2007, 11:17 PM
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http://yarchive.net/car/horsepower_measure.html
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Old 04-13-2007, 08:20 AM
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Quote:
Originally Posted by RippinRon
I remember reading somewhere that it only takes 20hp to do highway speeds.
anything over 50 MPH, it's 7-8 hp per 10 MPH, approx.
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Old 04-13-2007, 08:48 AM
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There's a difference between calculating the minimum level of power to get a car to maintain a speed and how much your car is actually making relative to its efficiency. For example, a fuel injected car will have a computer that says "ok, we're on a flat surface and up to running temp. Lock up the converter and lean out the injectors." the car will start turning fewer RPM and spend less fuel. So maybe its making about 120 horse when it only needs to make 60.

A carbureted engine only has one variable to determine how much fuel to use: air velocity. So the more RPM you spin the more fuel goes into the holes. The rate at which this begins is determined by your jets, so you're always making compromises which is inherently why carburetors are not as efficient as EFI. They simply cannot adjust for as many measures. So even if you have a 283 with a 200 R 4, you may be using excess gas even at lower RPM. But from a performance standpoint, you may be jetted correctly to avoid spark knock at part throttle with such a heavy car...

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Old 04-13-2007, 09:25 AM
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"A carbureted engine only has one variable to determine how much fuel to use: air velocity. So the more RPM you spin the more fuel goes into the holes."

Sorry, but this is not true. In a Holley carb you have a power valve (AKA "economizer" or "enrichment" valve) and a Qjet you have a power piston. What these devices do is add extra fuel (enrichment) only when needed (when the engine load increases.) At other times, they are closed and do not allow extra fuel into the engine.

These devices work on vacuum. Engine vacuum is inversely proportional to load. Low vacuum means heavy load/ high power and vice versa.

If the carb only sent fuel into the intake as a function of airflow- you'd flood the engine with fuel everytime you revved up in nuetral.
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Old 04-13-2007, 10:43 AM
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Quote:
Originally Posted by novajohnb
"A carbureted engine only has one variable to determine how much fuel to use: air velocity. So the more RPM you spin the more fuel goes into the holes."

Sorry, but this is not true. In a Holley carb you have a power valve (AKA "economizer" or "enrichment" valve) and a Qjet you have a power piston. What these devices do is add extra fuel (enrichment) only when needed (when the engine load increases.) At other times, they are closed and do not allow extra fuel into the engine.

These devices work on vacuum. Engine vacuum is inversely proportional to load. Low vacuum means heavy load/ high power and vice versa.

If the carb only sent fuel into the intake as a function of airflow- you'd flood the engine with fuel everytime you revved up in nuetral.
Well that's kind-of outside of my point. Four barrel carburetors can also add fuel with secondary jets at WOT as well. The similarity between the power valve and the secondary jets as opposed to the primary circuit is that the primary circuit responds as velocity changes continuously, the others do not. They enrich in an "on and off" mode preselected by demand at different points (secondaries by throttle position (sometimes vacuum) and power valve by vacuum). So what I mean is a 6.5 valve doesn't provide bits of fuel in an increasing curve as vacuum drops past 6.5 inches (although some say they start opening a bit about 2 inches before their setting, this is really not the point of the component), it basically is a "dumb" component.

You could also say that the idle circuit is another value if you want to add static, non-adjustable circuits to the equation.

As temperature, humidity, engine temperature, timing and altitude change, however, these "dumb" circuits continue to provide as they did before. An EFI system will take data from all these inputs and create a continuously adjustable output as a result.

I guess I should have been more clear about variable/non-variable circuits. All that said, however, I still prefer a carb myself!

K
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Old 04-13-2007, 11:16 AM
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A simple method to figure out the hp you are using when crusing would be to check the engine vacuum at a given load and speed. When you get on the dyno, assuming conditions are similar, just load the car to that same vacuum reading. Set the dyno to hold that load, and adjust away! This takes out any uncertainty of rolling resistance, drag, etc.
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Old 04-13-2007, 09:56 PM
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You can jet down for a part throttle fuel mixture in the 16-1 range at cruize speed but make sure you up the volume in the fuel enrichment circuit to compensate or there's going to be some serious power loss at wot.
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Old 04-13-2007, 10:15 PM
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shissa

do your self a favor get a vw bug and save your money and headache trying to figure out how much hp to get drunk with
go to a beer festival and make boat motor noise in your beer
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Old 04-14-2007, 02:57 AM
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Quote:
Originally Posted by orangef4
A simple method to figure out the hp you are using when crusing would be to check the engine vacuum at a given load and speed. When you get on the dyno, assuming conditions are similar, just load the car to that same vacuum reading. Set the dyno to hold that load, and adjust away! This takes out any uncertainty of rolling resistance, drag, etc.
That was going to be my exact suggestion. You would need to match engine load (either vacuum as you suggested or throttle position) and RPM. Get them at 60MPH, put engine on dyno, and find HP under those conditions.

Coast down tests however are fun to do and don't require much in terms of instrumentation. In fact you could do it with a camcorder trained on the speedometer with a stop watch also in the frame if you wanted.
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Old 04-14-2007, 06:35 AM
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Not to go off topic, but 20mpg in a 55 Bel Air is pretty damn good.
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Old 04-15-2007, 03:10 PM
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Great answer Orange4f!! That's just perfect, easy to do and very accurate to boot!! Why didn't I think of that myself??

20 mpg may sound alright, but I think I can do better. I had started a thread a while back asking why my car didn't get better mileage when I changed from a 3 speed to 4 speed tranny. Someone suggested that I try hooking up a vacuum guage while driving to see what happens. So 2 days ago I was driving around with the vacuum guage attached via a long hose to the engine. Very interesting! I discovered that when I drive 60 mph in 3rd gear the engine turns 3000 rpm and it pulls about 18 inches vacuum. This is what is was like when I had the TH350 tranny in it. When I dropped it into 4th the rpm would drop to 2200 and the vacuum would drop down to about 13-14 inches. Even the slightest grade (up) would drop the vacuum further to 10-11 inches. So back home I caused a huge vacuum leak to try to see if the power circuit was enriching or not at, say, 10 inches. It wasn't, it enriches at below 10 inches. Next test was to see what the vacuum can was doing. I hooked up the timing light and kept the rpms low, but the motor running (roughly with all the air leaks). I discovered that the vacuum advance is only functional down to 15 inches! In other words most of the time it was doing nothing when in 4th! So tomorrow I'll order a Crane adjustable can, install it and then I'll be ready for the dyno test. I'll post the results once I get a new mileage check.

Thanks again, I really appreciate the great answers you guys give, I've learned more about my car in the past 3 years due to this forum than the previous 20 years put together!

By the way, I don't like VW Bugs. I prefer 55 Chevies (and 55 Buick Roadmasters, bought one last month . But the gas price is a killer. Beer here, on the other hand, is way better than in the US. So you can't have everything...
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