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Discussion Starter · #1 ·
So im building my 68 SS clone up to be a wicked pro touring car which i plan to take all over america. since there isnt a whole lot of information out there about overdrive transmissions and the effect on fuel economy i figured id create a post and see what solutions folks have come up with. i know theres some stubborn folk who will post on here about how little they care about fuel economy but this car is purpose built for mostly highway driving with some street performance and some strip every now and then (i dont care about winning really just like going fast) and believe me the back seat gets used lots. so in the past couple years ive done a few things to this car. and in the next few months i plan to do a few more things. currently it has an old 454 out of a dually pickup truck its low compression stock cam, a tremec t56 .050 od 6th and a 2.97 first with 3.73 rear gears, holley 750 holley street dominator single plane intake, 1 7/8 headers with 2.5" exhaust (which is in rough shape) the fan is direct drive. this combination got me 18mpg on the highway keeping my foot out of it. i believe that after i build and install my 496 around 600hp with 2" headers 3" exhaust, quadrajet 800 with edelbrock 2.0 manifold, dual electric fans, vintage air, rebuild the diff, freshen up the suspension with some tubular control arms and get the alignment nice and dialed in, throw some fat rubbers on the ass end of er and take it for a burn i can break 20mpg. my hypothesis is that i cruise at about 80mph at 2500rpm (if my tach is right) if i go much slower then i begin to lug a little bit up hills. i chassis dynoed this car before the trans swap, it had a th350 and it got around 270hp with about that in torque. i believe that my 496 will get at least double that allowing me to drop my revs to 2000 or even lower and safely cruise the speed limit.i have no idea what cam to put in it yet. what do you all think? i hope to get the swap done and the rest of the work completed before august. i hope to take my grandpa to bonnyville for speed week, there is so much for me to learn about how engines behave and ideal operation conditions. i will upload some pictures eventually most of them are on my phone.
 

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I can't see how more cubes and a bigger carb will net you better mpg all else being the same, but I've been wrong before...
 

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Discussion Starter · #3 ·
i figure if i go with a cam that isnt crazy on overlap, have some lift and mild on the duration, sortof like an rv grind but a little more suited for a 35-3600lb car that isnt going to tow ill have plenty to launch the car when i need it, plenty of passing power but at cruise with my vacuum advance and curve set up right and my rochester tuned properly should sip fuel with all that torque. i think that 600hp is plenty for my application even if it was something like 550. i doubt that even with the big wide tires on the back ill be able to put all of that into forward motion if i pin it. i believe the 3.73 is right for my application. i have to rebuild the diff but i am hoping it has enough life in it for me to do some tests with my new engine before i rebuild it in case i decide the ratio needs to be changed. there is a limit to how wide the rear tires can be anyways and really once you start to spin it doesnt matter if the tires go 30mph or 150mph you are still spinning and the chicks still think its cool. i want this thing to be effective off the line. i want to feel like marty mcfly except not in a crappy unrealistically fast delorean. there are a ton of cans of worms for me here. i have no idea what to tune my rochester to in order to be even close to what i need to be on the 496. has anybody attempted to do what im doing with a setup similar to mine?
 

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Not sure what tire size you are running, but 80 MPH with a 28" tall tire and the 0.50 OD should be somewhere around 1,800 rpm at 80 MPH...


I can't see how more cubes and a bigger carb will net you better mpg all else being the same, but I've been wrong before...
I see what he is saying, with the current engine, he doesn't have the torque to "cruise" at such a low RPM. Bigger carb, but he is talking about a Q-jet, with smaller primaries... If the carb is tuned so that he can cruise at highway speeds without getting in to the secondaries, then this could help mileage.

I'd consider either an electric fan or a clutch fan setup, but may no be a noticeable mileage improvement.
 

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Discussion Starter · #5 ·
sorry for the delayed response. the tires i currently have on her are 235/60/16. i will be going wider on the rear. by how much im not sure yet
 

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Discussion Starter · #6 ·
so the biggest thing im trying to decide lately is what cam and heads and intake manifold i should use to fit my purpose. should i bite the bullet and pull the trigger on some afr 290s or should i freshen up my cast iron oval ports. as far as a camshaft goes im not quite sure how to cam something so that it operates at peak performance on the highway. a camshaft grind has a given rpm range. i understand that i need to be "up on the cam" but by how much? say i run a grind that is good for 2000-5500, am i going to be "up on the cam" enough to be efficient? another question i have is how much torque and down low performance do i really need before it stops benefitting me, like at what point do i say "ok this is enough low end performance, lets start working on the mid and top range here" say my engine makes 650lb of torque at like 2500 rpm but falls flat on its face by 5000 or so but when i hit a hill the size of everest i keep the same throttle position. the car is a chevelle, not a freight train, the weight of the car is all it will ever "haul" so i guess what im trying to figure out is what grind is going to allow me to have just enough torque in the cruise rpm range so that when i hit big hills my rpms will drop as little as possible and be able to have the rest of the torque in the higher rpm range for screwing around. damn my head is spinning, it is really hard to find real world FACT information about this stuff
 
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