EXTREMELY poor fuel economy modified fc with 179 t700 auto

[Skuta]

I hate to sound naive and I have avoided asking the question because I like to trouble shoot without relying heavily on others but this has me stumped.

Here's my relatively consistant fuel economy 26 L/100km or 9 MPG around town

My station sedan has a 179 with a 4 speed t700 auto and (I think) 3.55 diff ratio.

It has electronic ignition and a 32/36 weber carb (tried to improve economy by installing a smaller carb)

I have fiddled with the kickdown cable so it changes pretty early in the rev range and I drive like a Nana 95% of the time. At 100km it is at 2500 rpm in 4th.

Now like I said I don't want to sound naive but the economy on this thing is ridiculous, I don't mind it being a little thirsty but 26L/100KM is crazy. I'll have to confirm with Fatboy (the guy that originally done the motor) what lift and duration is in the cam (it does sound pretty lumpy) and what head work was performed but even a completely bombed 179 shouldn't even come anywhere near that should it? Apparently an XU1 had economy around 17 MPG (14 L/100km)

It idles well, runs like a dream, doesn't cough and splutter and it doesn't bog down unless I flatten it at really low revs (and I mean really flatten it and really low revs)

I don't have a hole in the fuel tank or any of the lines, I bought a ridiculously massive air filter for it, I've changed the coil and as mentioned earlier I have a 32/36 weber installed as I originally thought the 350 holley was causing half the problems. I even had it dyno tuned (for economy) and that made stuff all difference.

I'm lost, I love this wagon and I love driving it but the fuel costs are killing me and killing any spare $$ I would rather spend on parts etc.

Any advice would be greatly appreciated.

[FCRB26]

What convertor is it running ?

There a big box for the 179 and with the 3.55 ratio i thought most overdrive autos would be running a 3.08 ish centre so maybe change the kickdown to change later and put a 3.08 centre into it ?

Get it onto a dyno if your that stumped see what the fuel mixtures are doing and see when the cams coming into play ?


I think for the sake of a few hundred bucks a reputable dyno shop may lead you in the right direction. Good luck sounds like you need it

Pete

[FATBOY]

The motor came out of a Eh ute it had a 500 4 barrell on it so i would reckon its had a fair bit of work done to it .As for rear ens ratio not to sure but doesnt take long to pull the axles and drop the diff out.I just swapped some banjo centres so  cant help you there re ratio change .Maybe putting 14 or 15 inch tyres on the wagon will get u some better mileage.But would start with a dyno for sure .Mark (mini) put a new 350 holley on it and electronic ignition after he got it of me
regards fatboy

[hsv-001]

I'm not a mechanic , over the years I've had plenty of worked engines and in the near future will be rebuilding my FC van red motor . I originally had XU1 cam, 5stage, 350 holley ,headers and a sports exhaust etc.. Changed jets and power valve without improvement then changed to a webber . Without going into too much detail , the modified cam creates overlap in valve duration and below the cams rev range this overlap reduces intake manifold vacuum [airflow] and this reduces the ability of the carby to atomize the fuel . You are driving an engine that is built to run between 3000 and 6000 rpm and that's where it will be most efficient  [relatively] . Got tired of the lack of economy and pulled out the cam and now run a varajet 11 on the holley manifold and its nice to drive . Haydn 

[ACE]

Get a locking petrol cap
ACE 

[Skuta]

Thanks all for the replies,

I'll change the kickdown cable to let it rev longer/higher in each gear and hunt for a 3.08 diff centre, I might also just leave it in 3rd around town for now to see if that helps. So I suppose driving it like a Nana is doing more harm than good economy wise in the case of this engine.

Pete, you mention it's a big box for the 179, do you think it would be robbing a lot more horsepower than say a trimatic?

Thanks Fatboy, I would swap the wheels but man those indy's look awesome, I couldn't bring myself to do it. I did get it dynoed but he didn't record any of the details or give me a printout, I should have been more specific about what I wanted when I asked him to do it. He did say that the cam kicks in just below 2900.

Thanks Haydn, that all makes sense.

[FCRB26]

After thinking about it first thing would be a dyno run see what that cam is doing maybe order a replacement cam tell them the specs.
I would have a diff centre im at newcastle.


Pete

[john253a]

deff dyne the car
I've seen a few 700's behind 202's but never a 179
what the ecomeny like if you drive it in 3rd and not 4th on the fwy

the 179 came out with rear ratio or 3.55 manual 3.36 auto and 3.86 commercial

if you have the std 23.9" tyres
an eh man is 3000rpm @60mph (same as you in 3rd)
auto 2840@60
3.88(3.9) 3300@60
your car in 4th should be around 2100rmp@60mph

your engine is more then likely labouring at the rpm (something you'll be able to find out on dyno and prove or elimate)

if your still running the banjo rear end 3.9 or 4.44 are all that on offer in a taller ratio
the 3.9 will be 3300@60 in 3rd and 2300@60 in 4th
but idd go the 4.44 3750@60 in 3rd 2625@60 in 4th (this will still offer good economy on fwy and a very good city package as well)

4.11  (3470@60mph in 3rd) 2430rmp@60mph in 4th
or 4.3 (3630@60mph in 3rd) 2540rmp@60mph in 4th

[Skuta]

Thanks John, that's some awesome info there and thanks for taking the time to calculate it all out for me. I think it's now extremely obvious to me and everyone reading this thread that I've been lugging the engine too much, I adjusted the kickdown cable yesterday to let it rev out a bit more between changes and I'll only use 3rd gear for now till I install a 4.44 (or 3.88) ratio diff.

I had a bit of a look around over at http://holdenpaedia.oldholden.com/Differential_Ratio_selection it shows the lowest rpm with the tallest diff available to be 2390 at 60mph so me at 2100 is way off the mark, no wonder I'm using so much fuel especially considering the cam setup etc.

by the way, I haven't really done any highway driving since I bought it home, it's mainly been around town stuff due to some electrical problems I haven't been game enough to take it on a decent run out on the highway, but i've got that sorted now too.

Once I get the new diff installed etc. I'll get it back on the dyno

[fb delivery]

I have a 3.89 center for sale
$60 Toowoomba or the whole diff for $120
Rob
0438874803

[john253a]

If you haven't been in the fwy then the rear gear isn't the problem unless it's been going into 4th at lower speed

The 700r is ment to only lock the converter above 90kph and converter is normally 1700
Do you have the v8 or v6 box?

[Harv]

I could be way, way wrong here as I don't play with auto boxes much... but doesn't the 700R have an electronically-driven lock-up function on the torque convertor? Is your lock-up connected Skuta, and if so to what? If the box is still slipping at lock-up speed, it could be some of your power loss.

Cheers,
Harv

[john253a]

That would depend on witch 700 box he has
The electronic boxes requires a Compushift electronic controller to control the gear shifts and torque converter
lockup.
As well as a kickdown
And thoes are like 15-2k to get made up


Idd still get it dyno tunned before you change anything and then from the info off the dyno select you final gear ratio from that

[FATBOY]

V6 t700 lockup kit installed in box and wired through brake switch .Make sure you have the tv cable set up properly otherwise you will stuff the box real early cable needs to be really tight.You should be able to drive along and when the box changes upto 2nd gearstomp the pedal and it should kick back down to first gear.Skuta give me a call if you like and ill give you some more info
regards fatboy

[FCV08]

Skuta

Saw you going out Taylor St today.

How was the wagon going?
Cheers

Craig D

[Skuta]

Hi john, you're right, I've been letting it cruise around everywhere in drive so it's been getting into 4th gear around town all the time. At 60 it's been not much above idle.

Thanks fatboy, it has been kicking back ok even with the cable where I've had it but I've adjusted it much tighter now. I'll give you a call this weekend if you don't mind.

Hi Craig, that would have been dad. He took the car to work on the brakes, the seal at the front of the booster was persished. Yay I've got brakes now. I scored well at the swap, got some new spears and other trim, a glovebox insert, and a towbar, I spent more on coke, dagwood dogs and chicken chippies than I did on parts.

Update from yesterday,
So I adjusted the cable so it revs out more between gear changes and I'm leaving it in third. I done about 20 k's around town to see how it would be. It is a different vehicle completely. I'm using way less pedal to get anywhere and it's getting there quicker. So yes again I am pretty confident that by babying the engine has been the cause of my poor economy. I'll get this 3.89 centre from Rob installed and see how I go from there.

Thanks again everyone for the replies. Once I get through a tank of fuel I'll report back with the result.

[newman]

Skuta,

Before you start thinking of diff centre changes, running different profile/size wheels have a play with this



http://www.kabamus.com/garage/gears.html


You can try different combinations of diff ratios or tyre sizes and see what they translate to on the road

Mick

[Dr_Terry]

I believe that the main culprit here is the camshaft.

Economy is not all about just keeping the revs to a minimum.

The best economy is gained if the RPM at a given driving speed is close the the peak of the torque curve. A good stock 179 has its torque peak at 2,000 RPM so best economy is made while cruising in top gear at 2,000.

If you fit a cam that "sounds pretty lumpy" you've increased the available torque (& therefore power) but you've pushed it up the rev range. There's no point cruising at 1,800 rpm if your peak torque is over 3,000 rpm. The motor will be labouring too much. Also the more overlap the cam has, then the more fuel you will need to 'fill' it.

The most economical Holden red 6-cyl engines that I've ever seen, run a stock cam & a single BX Bendix Stromberg carby.

Dr Terry

[john253a]

I like to fit the efi 6 cam
Just seems to be a good overall cam


Back to OP's questions

Idd still dyno the car before you buy anything or change anything
Yes I believe your final gear is too low and need to be higher but just hold off until you know how high

You may find is better to chop down a vn and go with 4.11 or 4.3
Or stuck with what you have and run 3.9 or 4.44(this is what I think you may need if you stick with stock)

But the dyno will tell you where you make peek power and torque and what rpm each is at
Then you look at street signs in area
So you know 40, 50, 60, 80, 100 are about the only limits around anymore
2nd should do 40-50
3rd 60-90
4th 90+
This will change under power but should cruse in that gear at that speed

Look at the ratio of box and work out rmp and try to pick a rear gear that will allow the broadest range of speeds In your torque band

[Skuta]

Bit of an update,

So it's now revving a lot more between gears with adjustment to the kickdown cable and I've been driving around in 3rd, I've only done 150 town K's since changing it around and yes it's a huge improvement, I'm now at around 19 Litres per 100 K's, I'm happy with that for now. So it's extremely obvious that lugging it around in top gear was the problem, thanks for all the replies.