Mileage Went From 20 to 10 With New Motor...Why?!

Does this one have emissions compliance rating opposed to an 'off road use only' rating?

A different cam, cam timing, compression ratio, ignition curve, and the different carburetor fuel jetting required to work with those different ratios would greatly affect the fuel efficiency of the engine.

No, it's a gen I motor, virtually identical to the one it replaced.

What about timing, heads, intake, exhaust? Is that all off the old motor? What exactly did you reuse?

I'm leaning in the same direction as tcmtech, but 50% is a lot.

Nice ride, btw.

"What about timing, heads, intake, exhaust?"

What about the air filter? Did you spray it with clear lacuor? What about the transmission shifter? Did you repaint the D over the 2 position?

If its a newer one than the other one it will be tight and need bedding in

I would offer four suggestions.

1) It needs to seat the rings before it will free up.

2) The camshaft is not the same as the old one. Less bottom end power.

3) The ignition timing needs more careful checking. The mechanical advance may be frozen, or has soft springs, allowing the total advance setting to be same as initial setting.

4) A combination of all of the above.

Have you put a vacuum gauge on it at idle? Does it have as much low speed response as before? Do you want me to drive it for a day week few months just a little while just to try to help you resolve the mileage?

Rascal!

Wish I'da thought of that first.

Wait a minute, I'm closer. Hey SSCpal, I'll baby it.

BS. Like you babied my lovely Renault Fuego. The clutch has never been the same.

Well, at least it came back in one piece, mostly. And that mail box was not set back from the street as far as it should have been.!

I'm using the induction and ignition off the old motor. Timing curve is correct, vacuum and response are excellent, and it has several hundred miles on it now, so it should be broken in. In short it runs great, but is very thirsty. I'm stumped.

..and the plugs look great, so it isn't burning the fuel. Have I opened a portal into another universe and they're getting my gas?

It's lynlynch

He is siphoning your gas at night.

It seems illogical to have the same power as before, the same fuel metering system, and the same ignition system and settings, and still loose mileage.

One of my best questions for sorting out problems is this. What has changed since the vehicle was correct? Tires? Wheel alignment?

lynlynch stealing it at night, joyriding? (Don't let him touch it)

You mean her.

No, he means him.

No means no.

Whats the colour of the deposits on the plug? the bit that goes into the engine ?

is the cooling the same as old engine ie the fan

Plugs are light tan, no deposits. If it was burning twice the fuel the plugs would be fouled. Cooling system hasn't changed. Running temp is the same. Still turns 2000 RPM at 50 MPH. Everything seems normal, but it only gets half the mileage it used to. Very puzzling.

Did you remember to take the old motor out of the trunk?

How about aerodynamics? Are you running around with all the hoods bonnets and other such devices in place, or are you running with engine displayed. Other than what others suggested I'm stumped.

Your brakes arnt binding are they.

the spark plug should be a light brown in color.

if black micture to rich.

if white to lean or timing is advanced

Why are we assuming that 20mpg was normal or okay? What did the plugs look like on the old one? What made it expire? How big is the 4bbl cfm?

Have you checked the engine with a vacuum gauge? I've seen some issues with blower housings leaking vacuum after being taken off and reinstalled on new engines.

Plugs on old motor looked fine. Carb is 750 Holley. Vacuum reads great, steady 18" at idle, no fluctuations, etc. And as to why the old motor expired, let's just say pilot error and leave it at that.

Are you sure of your initial ignition advance? Sounds like you may be way retarded. You have such high power to weight ratio in that thing that you may not really notice the difference in any legal driving conditions. Are you using your old vibration dampener? A new one may have the timing mark wrong. One tooth off on the distributor? Or the cam timing? Are you getting the right amount of boost from the blower? Any chance of dragging brakes?

Ed Weldon

NO motor gives top fuel consump from new- all comps in motor need to bed in together- may take 20kmiles-assuming you know your stuff as how to run in etc- did you check mpg of old motor regularly-or now & then or only once- & how did you check mpg- actually- thinking about it- 10 mpg is more realistic anyway!.

Yeaaa, I also told Mrs Bushdriver my street-rod gives 20mpg.

Her reply was somthing like...."I might be stupid, but I'm not a bloody fool! I don't care if if gives 40mpg. You pay the fuel."

Wifey knows I am retarded, but my bucket isn't. I pulled a head, got TDC, calibrated the marker, and set the timing same as the old engine: 18 initial, plus 18 speed @ 2500 RPM. I don't have a vacuum advance.

Checked mileage on long trips of known distance. Dallas to OKC is 200 miles. I used to do it on one 10 gallon tank; now it takes two tanks.

I'm very aware this doesn't make any sense. If the motor was burning the fuel, the plugs should be fouled, but they are a lovely tan with no deposits. And I've set the car on cardboard to check for leaks, left the electric fuel pump running, ran my hand over every inch of the fuel system and it is dry.

This sounds like a case for, "Click and Clack, the Tappet Brothers" on NPR.

First we need to know how you are coming up with the MPG fugues. Fifty percent drop in fuel economy is not motor related. It would show up as a leak, or excessive heat...which does not appear to be happing.

Did you change your tires or the way you estimate the actual mileage? Hour meter or odometer?

Scratchin' my head...does the cam in the new short block have more valve overlap built in than the old cam? IE, could it be that a longer overlap interval is allowing the blower to move a lot more (pressurized) unburned mixture past the exhaust valve and out the tailpipe? Rayzer

Better lift, from a new cam, and better flow, (new clean heads, intake ports, gasket matching on the manifold gaskets) through the new motor would allow more fuel to flow from the carburetor. With HEI or some other high output CD ignition system, bad burn characteristics would not show on the plug, unless the carburetor calibration was grossly out of calibration on the rich side.

I can't see from the picture what method you use to either limit boost pressure or protect the motor from boost overpressure. Even if these controls are there, the temptation to lead foot is high and the possibility of engine or transmission damage is likely.

Probably some internal transmission damage occurred during the last fiasco. Since the clutches lock up fine at low torque low speed operation (50mph at 2500 rpm) you would not necessarily notice the slippage in normal operation.

Probably the best thing at this point is to find a local dyno shop and adjust the carburetion and timing to the best numbers. I don't think there are any quarter turn demon tweaks that will yield a 50% decrease in fuel consumption.

Bob

No limiting on boost, the little blower only puts out 5lb max and I hardly ever get into boost anyway.

Now the tranny...it's a TH400, 17 years in my car, and I can't swear to the history before I got it. I'll look into it. I'd like to try a new tranny anyway, one of the TH350C with a lockup TC.

The test of whether power is being lost in the tranny is by looking at oil temperature. I'm not sure what a normal oil temperature in a TH400 is supposed to be; but I'm sure that info is easy to find. For sure you are not pulling much of a load; so the tranny isn't working very hard. But if energy is being left in the tranny it has to be in the form of heat and the primary way that heat gets out of the tranny is by the oil going to the radiator and being cooled off there. (assuming you don't have a separate oil cooler). That amount of heat going into the radiator coolant would be pretty noticeable, even if you've got one of those high buck aluminum street rod radiators. So I don't think this scenario is very likely, but I thought I'd mention it.

Ed Weldon

Thanks, Ed. I do have a separate tranny fluid cooler, so it wouldn't show up in the water temp. Fluid looks OK and no bad smells or smoke. But as you say, it's not working very hard and power loss might not give any obvious clues.

I have a test that is used on Allison transmissions, that might shed some light on the transmission efficiency.

One method used to test for engine- transmission problems is a stall speed test. Lock up the brakes, and hold the throttle down till maximum engine speed is reached. If the speed is higher than before, the transmission has started to slip. If the engine speed is lower, the engine is low on power. Please note this procedure is going to raise the transmission temperature by 1 degree per second. If you need more than 10 seconds to do this, allow sufficient cooling time before continuing.

Do you know what the stall speed of the old engine was before LL put the valve grinding paste in your oil? Not the converter manufacturer's rated speed, the actual speed in your car.

If a tranny swap is in order, there is better mileage available with a 700R4, and they have made them live behind some strong Buick GNs.

What ignition are you using? The HEI units are common to eat coils up. This might not give a lot of indications if only breaking down under load. Have you looked at the plastic part of the coil next to the laminated steel part of it?

My next suggestion is not a cure for the MPG, more a test to see if there is improvement to be had after the mileage is back. Vacuum advance is a mileage booster on the street. If you are only making 5 PSI and only rarely at that, why don't you try the vacuum advance on a regular run? Watch the temp for signs of objections by the engine, and try it. Good luck.

Don't know the stall speed before, but that was a good idea.

Ignition is all MSD; 6BTM, etc. The distributor doesn't have vacuum advance; that's something I'm going to fix soon.

Hi,

I think the motor is not broken in yet. Have you turned it over by hand to see how much friction is in the rings? Was it bored with a substitute head in place? Did you change the carburetor? That could make a huge difference and still have plugs that look good.

Are you now using aviation fuel? Do you now have two 5 gallon gas tanks?

-S

New crate motor. Same induction, etc as old motor. I think this story belongs on Fringe.

Put taller tires on rear,then it will think you are driving downhill and get better milage.

Did you order a complete engine or just the long block assembly?The newer series(GenIII) engines have a different firing order, same as Ford.The old ones used the 18436572, and the intake manifold was optimized for this sequence.If you replaced only the long block, and used your old intake manifold, this may be your problem.The runners are different config for the new firing order.

Gen I long block, just like my old one.

If you have a Holley Carb, check your engine vacuum at idle.Holleys have a power valve that opens when the vacuum decreases below about 6 inches of H20 (normally when throttle is wide open).If your engine vacuum is lower than normal, this will cause excessive fuel consumption.Check for vacuum leaks.If no leaks, then changing power valve to a different value may help. A good rule of thumb is 1/2 of the idle vacuum for the power valve.Ie;12 inches at idle, then a 6 inch power valve.

Also, if the Holley is an older model, you will want to add a blowout preventer to the carb, a very simple mod, that prevents the power valve from being blown out when the engine backfires through the carb.Your power valve may be blown or sticking, might want to check this.

Vacuum is 18" at idle. And the power valve never opens when the carb is sitting on a supercharger; there's always plenty of vacuum under the carb. Holley sells a "supercharger" carb that is modified to read manifold vacuum so the power valve will work and avoid a leanout situation.

Has the mileage changed any since we started to chat?

At this point, I would have to try something, and timing is the easiest option. After looking at the plugs, do you think the engine will tolerate another 2 degrees?

One other question about before and after. Had the first engine ever been modified, Heads milled, valve job, advanced timing on the cam?

Hi Bob! Haven't had chance to test mileage lately. I'm going to check out the tranny this week. I'm sure the motor could take more timing, but doubt it would do much for MPG.

Now that you mention it, the first motor did have World heads and roller rockers that the new one doesn't. I can't tell any performance difference and it shouldn't affect mileage much.

Thanks to you and all the guys for the many ideas. I will post if I find anything. Or I'll send a postcard from the loony bin.

I think you should get those roller rockers cleaned up and ready for a swap. (Don't let LL touch it)

Hi Bob! Yep, the rollers are going on. They use 7/16" studs, so have to change those out too. I had forgotten about them; the original engine builder had apparently used parts he swept off the shop floor; the stamped rockers were hitting the 3/8" studs and finally snapped one off. It was one of many mistakes he made and I ended up fixing. Lesson learned: know your builder.

I can not believe that roller rockers would double your gas mileage. I do wonder about compression in the Dart heads though.

Are you still able to check compression on the old engine? Only need to do 3-4 cylinders. Compare to what new engine has.

Hi Bob! (do you know why I address you that way?) Nope, old motor is gone.

I'm beginning to doubt my memory. Wish I had kept a logbook.

1) I thought it had something to do with my name.

2) Too bad.

3) I'm beginning to doubt my logbook. Wish I had kept a memory.