Effects of a hotter cam?

[CobraSix]

Just curious about possible side effects of a hotter cam.

As you may or may not know, I'm running a Comp Cam 260H cam. fairly mild, but it doesn't shift the power curve a little bit.

With this shift, should my idle vacuum be lower then normal? I can't really get it above 18" Hg, but that is at about 18 *BTDC.

Also, should my gas mileage suffer ( used to be at 25 MPG on the highway, not I'm like 18 MPG on the highway, with a T5)?

Right now I'm running at 14* BTDC. According to Comp Cam the cam runs advanced of 4* BTDC, so is my engine running at 18*BTDC even though I'm set a 14* BTDC?

I guess I can retard my timing some more again and get back down to 8* BTDC, but then my vacuum is around 14" HG. So basically, is that normal for a hotter cam?

Slade

 

[Anlushac11]

8)

Yes, depending on the cam you may experience a loss of vacuum. This is typical of performance cams but I would not expect it to be enough to affect your systems in the car.

And yes, it depends on how the car is setup and which cam but yes you could loose some gas mileage. But you are also running a higher compression which should also make the engine more effecient so it should balance out sorta. Are you also running higher gears?

Your complaints are common for a performance engine with cam and headers but your setup also should be pretty driveable. I think your mileage has gone down more than it should have. Are you sure you arent accidently slipping into the glossy eyed grin while rowing through the gearsa nd giving it gas?

 

[CobraSix]

I thought about the lead foot phenom...and yes, it is hard to lay off the gas with the new cam and T5 tranny (rear is the same 3.20 as before). But this mostly highway driving, 40 miles one way to work.

I don't have any systems other then the Dizzy running off of vacuum (manual brakes) Car still runs very strong, just seem to be using a lot of gas. I expected a drop of efficiency, but figured I might gain it back with the T5. I didn't expect to drop 8 MPG. And I'm even running at the same highway speed as before.

I am going to retard the timing back to about 6 *BTDC (since my cam runs 4* advanced) and see what that does to my mileage. I won't go by ear and the vacuum gauge, I'll do it just by the timing light as see what happens. I have been timing it to run smoother, but maybe that is where i'm messing up, since I do have a hotter cam, I may just have to get used to the lope and time it down again.

Slade

 

[addo]

Was the higher mileage with your old block? What kinds of hot compression readings can you pull from this motor?

Cam timing and ignition timing are related, but don't affect each other's actual values. If your cam is 4° advanced, it doesn't add 4 degrees to your ignition setting. You are on the Duraspark now, correct?

Excessive fuel consumption can be due to cam timing and (interrelated) over-exhausting. The more the overlap, the greater this effect can potentially be felt.

What kind of colour are your plugs? Would a lower vacuum reading cause your power valve to be more open, more often? So many questions.

Regards, Adam.

 

[MustangSix]

Greetings from Europe. Will not be home until later than expected.

More cam = more power = more fuel.

18" of idle vacuum with the 260 is good and may increase as the engine breaks in.

Cam timing and ignition timing are not the same things. The 4 degrees advance on the cam is for the cam relative to the crank. Timing is for the distributor relative to the crank. Leave your initial timing between 10-14 degrees as you had it.

 

[rbohm]

8) something else to consider here is, if the engine is running lean it can contribute to poor fuel economy as you run into a lean miss problem. usually you have to open the throttle a bit more to compensate and you use more fuel to run the same speed. also make sure the speedo is calibrated correctly. the wrong gear means the odo is reading wrong and that could give you a false impression of good or bad fuel economy.

 

[Anonymous]

At the same compression as before, adding longer duration cams will reduce manifold vacuum and gas mileage. That's inevitable.
But, what can be done to recover some of it is:
1. Lean out the carb slightly. Even just the idle screw will help. Your power valve will be opening sooner than it used to, so this will help recover some of the lost fuel.
2. If you get into the carb, put a lighter spring on the power valve (or try cutting off 1/4 or 1/2 turn from the one you have). Ths will bring it back toward the original economy. Most carbs use manifold vacuum to close the power valve, so lower vacuum will let it leak more.
3. Recurve the distributor. Use a lighter spring in the primary side, leaving the heavier one in the secondary (or add a heavier one to the secondary - worst case). Don't get too crazy with the static advance, or you'll end up with overheating problems.

A word of higher tech: advancing the cam another 2-4 degrees from where it is now will make the power peak earlier in the RPM range, although there will be slightly less power at that peak. If you have tall rear gears, this will actually help the MPG, too - a trick from Ford's 1979-1980 lineup...

 

[Anonymous]

Hey Slade,
When you say "According to Comp Cam the cam runs advanced of 4* BTDC" Could they be speaking about advanceing the cam 4 degrees at the cam gear? I was going to ask you guys about this anyway. I know in my V8 days we would advance the cam at the cam gear which really helped more than advancing the cam with the distributer. How do these straight sixes run with the cam advanced at the gear. I know with a V8 you can buy Cloyes gear sets for this as well as other manufacturers. Do they make similar gear sets for our sixes?
Dave

 

[Anlushac11]

Yes

http://www.fordsixparts.com/Timing%20Sets.htm

 

[CobraSix]

according to Comp Cams, they say they build into the Cam a 4* advance. They leave it up to you advance or retard the cam from there.

Slade

 

[LEROY POLL]

HELLO SLADE

.....FIRST THING......I HAVE ENJOYED THE PAST YEAR WATCHING YOUR PONY GROW WINGS... AND GREAT WEB/SITE.

.....I HAVE THE SAME CAM IN MY 200 CU IN. MOTOR. I FOUND THAT AT HYWAY SPEEDS I GOT BETTER MILEAGE...BUT AROUND TOWN JUST A LITTLE DOWN. MY MOTOR LIKE 2500 RPM'S ON HYWAY. I WENT TO A TIRE/ WHEEL/GEAR CHANGE THAT I HAD 2150 AT 65. THE GEAR WAS A 2.83 W/ 14 IN WHEEL AND 26 1/2-- P/195. THIS MADE MY MILEAGE DROP TO 18'S. THE MOTOR SOUNDED BETTER...BUT IT WAS NOT RUNNING THE BEST RPM. MY MOTOR'S TORQUE SEEMS TO COME ON STRONG AT ABOUT 2400 RPM... AND VERY STRONG UP TO AROUND 4000 RPM.

.....I WENT DOWN TO A 24 INCH TIRE AND THE RPM WENT TO 2,450 AND MILEAGE WENT UP TO 22.2 MPG. I BELIEVE THE CAM WAS THE ONLY THING THAT WOULD MAKE THE DIFFERANCE. MY VACUUM GAGE WENT FROM 13.5* TO 15.5* AT THE SAME SPEED 65....!!!

..... YOU MIGHT TRY JUST GOING BACK TO HIGH GEAR (4th) FOR A FEW DAYS AND SEE IF THE MILEAGE CAN GO UP. IF IT DOES...THEN PUT ON A SMALLER TIRE......OR DROP A GEAR TO GET THE CRUSING RPM UP TO ABOUT 2500 RPM. I BELIEVE THIS CAM LIKE THE 2500 RPM AREA.....!

.....ALSO RBOHM SAID DID YOU CHECK YOUR SPEEDOMETER? THAT WILL GIVE YOU A BAD READING BUT YOU COULD BE GETTING GOOD MILEAGE.

.....WITH YOUR OVERDRIVE YOU SHOULD HAVE A 2.53 GEAR AND MUCH LOWER RPM NOW AT 65 MPH. I DON'T KNOW YOUR TIRE SIZE BUT IF YOU'RE BELOW 2,100 RPM AT 65 YOU'RE TOO LOW FOR THE CAM. ( OR..... YOU CAN GO 75 MPH AND GET BETTER MILEAGE..... AND A TICKET....) THAT'S NOT THE WAY TO GO.

LOTS OF LUCK....LOL

LIVE IN GRACE

LEROY POLL

 

[CobraSix]

Leroy,

Thanks. I rechecked my timing and my vacuum/mechanical advance last night, still dead on 10* BTDC with about 15-16" HG.

I'll try running the 4th gear. I don't have a tach so I can't really tell what RPM i'm running. I'll see about hooking up my tuning tach for a test run or at least a cheapo aftermarket one for test running.

I know my speedo is off, I checked it using highway markers and with my wife following me in another car. It is about 10% off, so if my speedo is saying 60, I'm goin 66 MPH. Same with the odometer. I've known this since I put the T5 in and I've been compensating for that on my calucations. I am running a 4cylinder T5 which has shorter gears then the V8 version. According to the RPM calculators, I should be running aroudn 2300 RPM at 65 MPH. If I dropped to 4th at 65, I'd be running 2900 RPM.

I did some minor adjustments last night to the carb and timing and then I refilled on gas. I'll see what happens today when I fill back up.

Slade

 

[rbohm]

8) 10% is not too bad, but i learned a lesson on relying on using performance factors to calculate speed. i nice va state trooper informed me about that one day a few years ago in my grand marquis. didnt get a ticket though, but he clocked me at 64 in a 55.

 

[LEROY POLL]

HELLO SLADE

.....HOW DID IT GO THIS WEEK? HAVE YOU FOUND THE REASON FOR THE DROP IN THE MPG.?

.....JUST WANT TO ADD..... I'VE ALWAY LOOKED FOR LEAKS FIRST BEFORE CHANGING ANYTHING. BE SURE AND CHECK ALL YOUR GAS LINES FOR LEAKING....!

LIVE IN GRACE

LEROY POLL

 

[CobraSix]

Leroy,

Still looking at possibilities, but I think I might have found the culprit:

Started out at around 17-18MPG
Went to MSD Box went to 19-20 MPG (With Pertronix/ 45kV coil)
Did a re-tune: pushing 20-21 MPG on a good day.

Last week I started developing a misfire. Did some investigating and found the spark on #2 cylinder was weak at best. move pertronix around, still weak on #2. shifted wires, still weak on #2. Figured it was time for a new cap and rotor, so I decided just to finish my upgrade to DS2 and 8mm wires. Car runs much better now. It looks like mileage increased to around 26 MPG, but I won't really know until I fill up this afternoon on my way home.

So I think it was a cap and rotor that was starting to go bad.

Slade