Cam vs CR effect on fuel economy

[aribert]

Greetings all:

Last year my fuel economy with a stock '78 200 smog motor (w/ a Weber 32/36 DGAV) averaged 23.5 mpg for the driving season (I kept a record of the mileage at every tank full). Over the past winter I swapped out the stock smog motor cam for a Clifford 270M that I had many years ago in my '61 170 engine.

At the same time I also had the head milled .060 and I unshrouded the valves & balanced the chamber volumes, blended the valve pockets, opened up the hole on the intake log. http://i290.photobucket.com/albums/ll27 ... urface.jpg I also re-profiled the accelerator pump cam. http://i290.photobucket.com/albums/ll27 ... stroke.jpg Non engine related, I swapped out the 3.20:1 rear axle for a 3.0:1 (and changed the speedo gear to match and checked the odometer with both freeway mile markers and GPS).

This year my best mileage was 18.5 mpg with an average mpg in the high 17s. My typical driving pattern has not changed. I drive about 50% uncongested freeway, 10% congested freeway and about 40% stop & go on surface streets.

Is this primarily attributed to the cam? Does CR increase have an effect in decreasing fuel economy? I expected the lower revs of the 3.0 rear axle to minimize the increase in fuel consumption due to the increase in cam. I realize that re-profiling the accelerator pump cam would reduce fuel economy but only during acceleration (before on hard acceleration uphill I was hearing detonation and my narrow band O2 sensor (I realize that a narrow band O2 is not ideal but that is what I have to work with) was showing very lean). Now the detonation appears to be gone and at the start of hard acceleration the gage dips to mildly lean and then transitions to mildly rich.

Your thoughts? BTW, this is a bit too much cam - a noticeable loping idle in the next lane is not what I am after and the roughness when cruising at about 35 to 40 mph in 3rd gear is also not what I am after. So I am considering swapping out the cam - suggestions for a milder cam? I have my stock '61 mech cam, several '65, 66 stock hyd cams in addition to the '78 smog motor cam or what else is out there to buy?

 

[Asa]

Increasing the compression ratio will (moderately) increase the horsepower. Provided you stick with high octane fuel to gain the boosts associated with that, it should increase the milage (moderately)

I would definitely look at the cam.

A mild thought might be where you're getting your gas. This year I believe I noticed the first sign that said "15% ethanol" on a normal gas pump.

 

[80broncoman]

Where are you on cranking compression??

If you are not maxed out for pump gas you might want to advance the cam.
it will bump cranking compression. But then so would a smaller cam.

 

[CZLN6]

Howdy Bert:

As usual, I need more info. Do you recall what the volume of the balanced chambers was? What head gasket did you use? Given the work to the chambers and a composite head gasket you likely didn't raise the CR too much. I forget what transmission you're using? What ignition? what is the initial advance setting?

You made too many changes all at once to be able to put the blame on any one thing. My guesses are 1) the cam? Do you recall the lobe center angle? Too much overlap will reduce cylinder pressure, increase lopeyness, and hurt mileage, but increase top end. And, 2) going from a 3.20 to a 3.00 rear gear may be lugging the engine too much at highway speeds. What is your highway cruise engine speed? What was it with the 3.20 rear gear? At what rpm does the cam really come alive? So likely there is a mismatch between your gearing, CR and the cam given your daily driving habits.

My assessment is that you probably need more CR to obtain better mileage. A mill cut of .060" will make up for the extra thickness (.025") of the composite head gasket you likely used, not to mention the extra volume you created by unshrouding and balancing the chamber volumes, both good moves. Given that, my guess is that your static CR is just slightly over 8:1. Given the relatively small diameter bore, along with dished pistons these engines can easily handle 9:1 on typical 85 octane modern gas. The cam further lowered cylinder pressure. With the 3.00 rear gear more CR would be needed to offset the higher gear ratio.

Very interesting. Keep the info coming.

Adios, David

 

[aribert]

Thanks for the replys. I've put the car away for the season, but since we have had only one road salting w/ following rain and dry yesterday, I took the car out to be better able to answer some of the questions.

Asa: I am not aware of any E15 blend here. Fuel economy on my F150 has stayed consistent at about 15.5 mpg over the past several years so I don't think the fuel is a factor.

80broncoman: Cam was installed straight up. Checking the degreeing on assembly, the cam appears to be 2 deg adv. I run mid-grade 89 octane.

David: chambers at 53cc. Detroit brand head gasket, from memory it measured about 0.005 to 0.007 thicker than published values for uncompressed Fel-Pro head gasket. 3.03 trans; point ign, about a '72 dist, stock, with mech adv only, initial advance set at approx 18 BTDC (was at 12 or 13 BTDC prior to engine mods, engine appears to want even more adv at idle - I'm concerned about too much adv at higher rpm with out having the total adv in the dist reduced - afraid I might not hear detonation at freeway speeds); All I know about the cam is what is shown on the cam card that is in my posting. I don't have a tach in this car so I did a temp hook up yesterday. My typical shift points 1st gear at 3700 to 4000, 2nd at about 3500. Engine rpm in 3rd: 35 mph: 1800 rpm, 60 mph: 3000 rpm, 75 mph 3750 rpm. Cam appear to come alive (modestly) at about 2800 to 3000. I have installed performance cams in both my '92 Isuzu 2.3L 4 cyl and my '71 Triumph 2.5L 6cyl and they both are more noticeable when the cam is in the sweet zone. Hot idle is at 900 rpm. I don't mind the loping idle too much but the rough engine felt when cruising at 35 mph in 3rd is annoying and something I want to minimize. I made a short video of the engine at idle yesterday but have not figured out where to upload the file.

I have some concern about raising the compression. As background I raised the compression on my Triumph to a 9.3:1 by milling off 0.135 in and I traveled 60K uneventful miles before the head gasket burned thru. In my Falcon with the original 170. bored 0.040 over Clifford cam, original 1908 carb and stock 3.50:1 rear axle; I drove 180 uneventful break-in miles before taking it for the first run on a freeway where in it began to labor and by the time I got to the next off ramp it would not idle and sounded like there were a hand full of ball bearings in the engine. First time since my parents got this car in '66 that it did not make it home under its own power. Upon tear down, the piston skirts were blued and skirts and bores were deeply gouged. I got another block, had it bored 0.020 over, repeatedly checked the piston / bore dimensions assembled it with with the milled 170 head and with a longer break in on surface streets. First longer (a bit over 10 miles) freeway run it labored and seized at just under 300 miles. I then re-ringed a worn out 200, used my milled 170 head and drove "slowly" on the freeway. After seizing the second engine, I could hear a distinct change in engine sound and sure enough the 3rd engine began to change tone as the speed got over about 60 mph. I immediately backed off and only ran the car on surface streets the balance of the season till I got my low mileage, bone stock '78 smog motor. So I am a bit spooked after spending just under $3K on the two seized motors (first one I had all dynamic parts balanced) even though my Triumph engine runs good at 9.3:1 CR with an idle of 1000 rpm and no issues when loafing in any gear at 1700 rpm (not that there is any reasonable power at that rpm).

 

[xctasy]

There's an old saying that a good judge doesn't need more info, just enough to make a judgement.


David Vizard said that the idealized compression ratio verses Octane tating is problematic when

a) high low duration stock cams are used on engines with very high compression.

b) Its very easy to get too much effective and cold cranking compression with a stock cam than it is using an aftermarket cam. He also stated that with certain carbs, timing is touchy...just a few degrees too much 50 thou timing, and an engine will suffer carb calibration issues which are based on too much intake duration for fuel to aviod a fuel standoff situation.

The problem is not then compression ratio, but how the valve clearnaces, camshaft indexing on the cam shaft chain is tuned to maintain a cold cranking compression value low enough to run on pump gas. You can influence the camminess of the engine by valve lash and retard and advance on the stock 144-170-200 US timing gear. Using the JP twin roller gear set, you cansit the tiing almost anywhere as long as the cam doesn't hit the conrods when advanced and retarded (not an issues with the 170 as it is with the 200)

The carb and ignition then get tuned to suit.

The problem you are dealing with is most likely a Weber 32/36 carb jetting power valve /squirter jet issue, which is also present on the Holley Weber 5200 series. Powerband and I have had the same problem with certain versions of two carbs which should be the same, but are not. I rememer his 2.8 version of the US 5200 HW carb being totally different on his 170 compared to any other 5200 carb from other 2.3 Pinto/Lima engines. The reason is that the emulsion tube and circuits were subtly different between similar looking versions, and that some parts are not even listed in the Ford, Holley Weber, or Weber listings, and it then gets impossible to bench mark the right jetting. Tom at Stovebolt is on record as saying that there are so many differences in the similar looking Weber 32/36 DGAS verses the handed revrese image Holley Weber 5200 and Weber DFEV/DFAV that "they bear no comparison at all", even though in many cases they use the same jets, same venturi and throttle sizes. I have to affirm that, although its important to index and discuss those diffferences so a person having a problem can isolate the problem.

Is your carb a
1.DGV, the manual choke (early Capris, Pintos, Cortinas with 1600 ohc or 2000 OHC)
2.DGEV, the automatic (electric) choke (some versions, aftermarket)
3. DGAV, the automatic (water heated) choke (four versions, a} Weber, b} Bressel, or c} Holley G180) d} Borg Warner),
4. DFEV/DFAV, the mirror-image to DGV series (European Escort, US Fiesta and Euro XR2 OHV, and some 2V 1600 OHV Kent)?

Some of the reverse image carbs, the seven other versions,

5.the Holley Weber 5200,
6.5210,
7.5220,
8.6200
9.6500
10.6510
11.6520

have myriads of changes, venturi diameter, cfm variances, the whole gammit of changes. And sometimes parts from those get substituted. Jets interchange, air correctors, but some of the circutry doesn't

See http://datsun1200.com/modules/mediawiki ... Carburetor


Compression upgrades in the 270 dgree level with stock 2-bbl carburation require a very carefull mean best torque timing, as your effective compression with the 270 cam has gone way up, and your carb needs to be tuned to enrichen just the right amount, and and ignition need to be tuned down to avoid too much total advance, and the right ramp of intial to part throttle advance. Your mpg loss is most likely a combination of ignition and ported verses manifold vac issues, and the way the primary to secondary circuit is enriched up on your particular Weber 32/36.


My advice is to try another used Weber 32/36, or perhaps a Holley Weber 5200, and do plug readings or Colortune readings. Your cam timing makes the idle and progression circuitry of a Weber 32/36 critical. Though I've been around Weber 32/36's since I was 10 in 1980, and done about four Weber carb swaps (including an alloy head 1725 Sunbeam Alpine engine with Cortina 2-bbl 32/36 carb), I still don't know enough on how to identify the early non drilled accelerator jet Weber used in some 1971 to 1974 2000 cc Pinto engined European (not American Pinto) engines. I've been bitten by the early versions, as they have some major traps for tuners.

Small engines with small intake volumes like our 170 heads are more influenced by cam timing than larger engines, and the Weber is a great but often very finicky carb to tune, as Weber's were made by many different licence holders Bressel in Spain, Borg Warner, Holley, and there are four types.

The US versions of the 5200 had similar variations, and its possible to ruin a whole engine just by putting in the right jetting in the wrong carb.

I had two carbs on my Pinto 2000 carbed 1980 Cortina. Factory emissions rating was then same as the US Pinto 2000, somthing like just 88 hp in the Aussie version, which also had 1973 Pinto carb emissions gear. The factory NZ non emissions engine would make 98 hp with the fan clutch on, 102 hp with the viscous fan clutch unhooked. If I used the stock emissions carb, it would run great, but the non emission carb, which should have given it an extra 10 hp, was just a dog. Each was a factory European 2000 Pinto series OHC carb, but one had no secondary enrichment circuit and it caused over heating every time it was used. One was an emissions carb, the other a stock non emissions carb. Both were made by Bressel, but each had subtle changes. The emissions one had a vapour return line, the other none.

Hence often the the Weber misses an entire section of secondary jetting, and will never work because the wrong version of the four generic types is used for an application that should have had a secondary emissions enrichment circuit.

Any Weber may need just a good vac signal, good distributor curve to run, but some are causes of major problems, and the 270 degree cam timing is enough to push a marginal Weber carb over the edge. Since stock timing was only 240 degrees to begin with, the cam needs specific carb and distributor work to suit.

In your instance, your at around the 270 duration mark with just 170 cubes, so the jetting neeeds to be perfect to avoid lean missfire. The vac porting with the Weber and the earlier 1-bbl 170 head requires a special combined brake booster and vac fitting. You won't have power brakes, but you may have vac window wipers and a canister like early Fords ran. If you do, you need to be carefull on how you set your Weber up

 

[aribert]

xctasy:
Thanks for the reply. My carb is a DGV5A - correction to my first posting. I do not have a spare carb to swap with, nor do I know of any vehicle using this carb as an OEM fitment. I have been pleased with the carb on this vehicle in the past and am focused on improving the current drive-ability and fuel economy a bit compared to where I am now. I am not trying to get every last pony out of this engine - just trying to pick the low hanging fruit.

Because I am adding A/C late this winter/spring, I want to resolve my potential cam issue before mounting a condenser up front. If I need to acquire a milder cam, I am willing to do so, or reuse one of my existing stock cams.

I tend to agree that having a custom distributor advance curve might be in order - especially since the engine seems to want more stating advance. I see that you are referring to a 170, I am running a 200 with a late (large log) head and stock dished pistons - my 170 (with a lot more compression than this one) seized - see my previous posting in this thread.

As for plug cuts, I find using the narrow band O2 sensor to give me better results - I live in a major metro area and it is not convenient to shut down the engine at a given rpm, coast to the side of the road to do a plug reading. I have triple DCOEs on my Triumph and I started out carb tuning with plug cuts and a Colortune system - I find the using the O2 display to be a more reliable tuning aid. I am hopeful that I will have the discretionary cash to buy a data logging wide band O2 sensor this spring - I have been saying this for a couple of years now. I piggy back the narrow band O2 display on my '92 Isuzu (vehicle with full engine management) in the winter months to help train my eye a bit to improve my expectations of the gage at various engine loading and rpms.

 

[xctasy]

Ah yes, clearly the D8 headed 3.3, sorry about me not reading your details.

Your running a fuel pressure regulator with your stock Fairmont/Granada fuel pump?

(Carter did make a return line fuel pump that the Weber 2-bbl 1982-1992 Aussie Falcon XE and XF used, and the line fits the blocked line on your Weber carb. It allows reduced fuel pressure even when a 5 psi fuel pump is used. The Pinto engined Cortina used that return line, and a Bosch version of the duraspark electronic ignition and a 9% taller diff to gain 6 miles per gallon at 55 mph for 1980 onwards models. When optimised by Ford in this manner, it gave a huge boost in fuel economy just because the spark was reliable and the carb float was under little pressure. That year, the 1980 Euopean emissions regulations kicked in, and the 1980 cars for Scandinavian markets ran US style TVS trees and other standard emissions stuff to meet there US 1973 spec emissions standards, yet were more economical. The 4180 4-bbl HO carb used in the manual 5.0 1983-1985 Mustangs and the 460 F250 truck used return line fuel pumps much the same)


The carb sounds fine, so I guess its time to look at getting your ignition advance curve reset to suit your cam and carb. The key is to have a shaper igntion curve, with quite a lot of static advance, and 34 degrees all in by 2800. Our 121 to 147 hp Aussie and non emmissions New Zealand spec 2-bbl Falcons ran 9 degrees static, then 34 all in by 3000 rpm.

Of you extrapolate the existing mark of tdc, 5, 10 btdc on your existing cam cover to 15, 20, 25, 30, 35, 40 btdc, and wait till dark with a timing light, you can determine your unloaded advance curve at present. I'd be supprised if your not running about 45 degrees advance right now at 3000 rpm, which is most likely your roughness problem.



See pages 3 to 5 of the 8 page PDF from Dave Andrews pdf at First Fives http://www.firstfives.org/faq/timing/ti ... ngines.PDF

( I hang out with BMW 5 series owners, because they modify little I6's,I get TRX tires for the 81 Mustang from them, and the Diesel 524 engine is shared in the Fox body Lincoln LSCs and Hash Sevens)

 

[CZLN6]

Howdy Back Bert:

Sorry for the delay in responding. I did run some numbers on your new head and came out with a CR of 8.3:1 approximation. That's certainly safe given todays typical pump gas.

On the DuraSpark II recurve there is a good 'How-to" article on Mike's Classic Inlines home page.

X has given you enough information for several good judges. Still no final judgement. I have nothing more to offer, but I wish you well and will be anxiously following your progress to a better outcome.

Merry Christmas.

Adios, David