[toobroketoretire]

After installing my TH700R4 I found my '71 would only get 12 mpg at 55 mph but 16 mpg at 70-75 mph which didn't make a bit of sense. I knew my engine loped clear up to 1300 rpm because of it's marine cam but I never thought much about it until I realized I was running around 1300 rpm at 55 mph. Hmm.

So I installed a set of 3.70's and that brought my 55 mph cruising rpm up about 300 rpm and that did the trick as I now get 14-1/2 mpg at 55 mph and the same 16 mpg at 70-75 mph. So it was just a matter of my components being mismatched. And with the 17% lower gears my car is much more responsive in all gears and at all road speeds.

 

[bluedawg357]

Steve do you think that this is due to the operating range of the camshaft?

 

[toobroketoretire]

Steve do you think that this is due to the operating range of the camshaft?

I am thinking it was because at 55 mph my engine was still loping because of the longer duration marine camshaft. I live in an area in which the traffic moves along at 55 mph or less and the roads are all two lane roads with lots of oncoming traffic. So it's very hard to pass anyone for sometimes 20-25 miles. But when I could get the revs higher the fuel mileage instantly improved which made me suspect the low rpm and camshaft were causing the low fuel mileage.

The moment I put the 3.70's in my fuel mileage improved a whopping 22% at my usual 55 mph cruising speed so it appears I was right for suspecting the low rpm and camshaft combination. And with the lower gears my engine is much more responsive in all gears and road speeds. I have to pull a 10 mile long 6% grade coming home and with the lower gears it pulls the grade much easier in 4th gear because it's running 300 rpm faster.

 

[bluedawg357]

What are the cam specs?

 

[toobroketoretire]

What are the cam specs?

The marine cam's specs are 220 degrees intake, 220 degrees exhaust, and .500" lift. Nothing real wild but still a lot more than stock. I'm running it 4 degrees advanced to improve it's low end performance.

 

[toobroketoretire]

Real Good Combination

I have found a set of 3.70's behind a TH700R4 is a terrific combination as my '71 will easily run 120 mph in 4th but get much better performance in the lower gears. And, I also got a 22% increase in my fuel mileage because my engine is operating at a better rpm at cruising speeds.

 

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thats what we are always talking about...matching components for optimum results

 

[bluedawg357]

Probably enough cam to be lugging at 1300 rpm, 220 duration @ .050" lift probably as an rpm range sttarting at 1500 rpm, not that youd notice with the off idle torque of your 454, but enough difference at 1300 rpm to change economy.

 

[[email protected]]

its not just the cam, its also the large runners on the RPM Air Gap, not efficient at low rpm

 

[Hotrod]

I believe it is due to the dist. adv. curve. At 1300 RPMs you are barely into the mechanical advance, although you should have vacuum advance. :479:

 

[toobroketoretire]

I believe it is due to the dist. adv. curve. At 1300 RPMs you are barely into the mechanical advance, although you should have vacuum advance. :479:

I think it was mostly a matter of real slow runner and port velocity. When the TH700R4's were first introduced the CrossFire intake manifolds used real small diameter runners to keep the incoming air velocity high. Then when the Tuned Port Injection came out they too used real small diameter runners (but much longer). So with my large oval port heads and Edelbrock Air Gap manifold my runner velocity was extremely slow at 1300-1400 rpm. So slow fuel was almost certainly coming out of suspension and smearing against the runner and port walls.

By increasing my engine speed by about 300 rpm at my usual cruising speeds my engine became much more fuel efficient as I instantly gained 2-1/2 mpg and the lower gears made every gear much more responsive. And it now pulls the 10 mile long 6% grade a lot easier with that additional 300 rpm.

And I'm sure you are 100% right about the mechanical advance playing a big part of it too. My full advance is by 2500 rpm but at only 1300-1400 rpm I'm sure it was only 1/2 advanced; maybe 10 degrees at best. In the end I think it was just a mismatch of parts as all of the TH700R4's were used with computer controlled fuel and ignition timing AND small-runner intake manifolds to keep air velocity high.