Hey guys, I was wondering what you guys thought I should do to get a little more giddy up in this old truck. It's got a 300 with 4 on the floor. My acceleration is abysmal, she'll barely go faster than 40 up an incline of any kind, and will not go faster than 65 period. This would not bother me nearly as much if my fuel economy was more than 11 mpg. If I'm going to get bad mileage anyways, I figure I should try and have her go a bit faster. Everything is bone stock. Should I even bother messing with the intake or exhaust before I do work to the head?
Thanks,
Mike

 

Mike sounds like tune up time. I've never owned a 6 but a lot of guys love them. I doubt you will get much better than about 12mpgs on the ol truck, but unless you have mighty tall gears it should run 80 or so.



John

 

I had a 300 in my 65 when I first got it. That was a good engine and I could get 95 out of her all day. I didn't get rid of it until I bought a slide in camper and she just couldn't handle the hills. The old girl was tired, but she still ran good w/o a serious load. I suspect you may need a tune up to start. If that solves the problem, good to go. If not...

 

Do yourself a favor and start with a compression check. A tune up will do very little to a motor with burnt valves.

 

The good thing about these motors is that they have a set of gears for the timing set and not a chain. That keeps the timing accurate and eliminates a potential problem. So following up on the compression is the next step. The design flaw is with the intake manifolds; the two end cylinders tend to run lean because the other cylinders get the fuel first. If the compression seems low the most simple check is the valve seals - they get brittle over time and can cause a substantial leak potential. They are pretty easy to replace. Likewise with a good tune up; new plugs, wires, cap & rotor will make a big improvement if the existing stuff has been on it for a while. The fuel filter should be replaced too, if the motor starves for fuel it can't make power. Finally make sure the timing is right including the static setting and the vacuum/centrifugal advance. The motor needs advance as the rpms increase to get to it's potential.

I've had enough 300s, they should get into the mid teens at least if you're not beating on it or hauling a lot of weight when it's tuned properly.

 

Valve seals have NOTHING to do with compression numbers. NOTHING.
now if you mean valve seats, yea those play a major role....


Yes, 15mpg should be the least it gets. That's what I average with a 350hp 351. It's not unusual for a 300 to get 18+. Although max seems to be around 22 depending...

I'm with everyone else saying it probably needs a tune up. One thing I didn't see mentioned yet was to check the ignition timing. Usually you can run 12 degrees advance without any problem. The more the merrier, but don't go too far or it will detonate under heavy throttle. If you really get into the engine, an advance timing set may help give it some extra grunt. Especially since a regular 300 never goes past 4,000rpm anyway.

If you want to do upgrades, intake and exhaust. Source a late model EFI exhaust manifold that has dual outlet. They flow as good as a header does from what guys say. And I'm not familiar with the intakes much. I know there are 4 barrel options, but I would recommend trying to find a 2 barrel intake (if they exist) and running an autolite/motorcraft or Holley 2 barrel. Would give you a lot of gains over the 1 barrel.

Speaking of exhaust, check the current setup for any restrictions. Like a bent or crushed pipe or kinked tailpipe.

300s have been called a lot of things, but gutless isn't one of them... They used to be powering cement mixers and dump trucks way back when. Many many industrial applications as well.

 

2x.... should get better mileage than 11 MPG unless ya got short rear gears....3.50 and below is a good compromise between acceleration and MPG. 3.90 or numerically higher gears will significant reduce efficiency. For a 300, a 3.25 or numerically lower would be typical. Go back to basics with a tune up, chassis lube, and then inspect the rolling resistance at each corner. Is anything dragging the driveline? Meaning, can the front wheels actually freewheel or maybe the rear brakes are dragging?

Second, maybe you need to adjust your driving style. Do you floor it to every stoplight or stop sign? Rabbit starts? Do you anticipate stops by shifting into neutral or letting off the gas to let'er coast? Try some "hypermiling" techniques and determine how much MPG you can actually eke out.

Third, uncork what power is already there with EFI manifolds (as described above) or long tube headers plus a free-flowing exhaust. Given the power range of a 300, up to 2.25-inch, dual exhaust is recommended.

.

 

Mike, Get a hundred dollar bill for tip and get 351Cleveland C4 to tune your truck, he can get mpg that no one else can get, so it would be worth the money. You would have it back shortly with gas saved.


John

 

Thanks for the compliment John. I know you are skeptical of my claims but I assure you, it's true.

 

My 69 in it's stock form was a 300, C4 auto and 4.10 gears. It got high teens in mixed use, low 20s when I could do state routes long distance, and mid to high teens on the freeway. It could also tow decently, but the manual drum brakes were the real limiting factor.
Get a set of Autolite 45s, new wires, rebuild the carb, and put in an electronic ignition with new cap and rotor, and you'll see impressive gains. In moderate performance circles, this is generally referred to as stage 0. It is crucial to have the basic stuff functioning properly so you can know the gains from modification.

 

Valve SEALS have everything to do with part throttle acceleration. Basic ability of the port to maintain vacuum, flow of the air/fuel mixture, air to fuel ratio, all things associated to the ability of the motor to make efficient power. Valve SEALS also have quite a bit to do with the condition of the valves; run it lean for long enough and the valves will warp. How can the seals cause it to run lean? By allowing unmetered air to enter the combustion process and cause the air fuel ratio to have excessive quantities of air. Warped valves from excessive air entering the combustion process = low power levels from low compression. Since we're talking about a 40+ year old vehicle it is a possibility that the seals have been leaking.

 

I think you are confusing valve SEALS with valve SEATS.

The SEALS are the rubber pieces that are outside the chamber, inside the valve spring. They are to control oil entering the valve guide and chamber. they do not do anything for valve control or pressure holding.

 

Lemme add an accelerant to this bout of a discussion by inserting "valve guides".

Ding-ding. Box!

.

 

The SEALS WILL cause a vacuum leak if they become brittle or crack. I'm not confusing the seals with the seats. To put this in context...the seals are a relatively simple replacement compared to anything else discussed with valve seats (or guides). Pull the valve cover, then the rocker arms, then the springs and replace the seals. I've replaced more than one set of seals to calm down a mysterious rough idle and to make a motor run better because it pulls enough vacuum and oil past the seals.

No, they're not typically replaced in the context of a tune up but valve seals do wear out, they do get cooked and brittle over time, and they will cause a power loss.

 

As long as the valves are seating, worn seals play no part in compression, because they are isolated during compression. If the seals allow oil to deposit onto the valve, then this can cause compression loss because the valves wont seat.

If you have low compression, and only change the seals but not service the valves, the compression problem will not go away.