351M ideas?
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351M ideas?
I would like to go back to the 351 from a 400 to get back the 5 mpg that I lost. I love the power, but I went from 15-17 to 8-11 mpg. I am pretty sure that flat=top pistons are unavailable for the 351M. So here's my idea.
Mill the block -.040. That should give me a nominal .020 positive deck height. Mill the tops of the pistons for that .020 to get close to a flat top. ( Will .020 do it, or is there no way to get that much to make it a flat top? ) Then put on the aussie quench heads. I want quench but with decent compression, no more than 9.5-1.
Or does anybody know where I can get flat-top pistons for this thing. I've looked high and low, but I can't find pistons that have enough compression height. My other option is to figure out how much longer the rod needs to be to run the 351 C flat-top and find a rod that is close enough, or get some custom ones made. Either way I'd rather do it the less expensive way. And going to a 351W is not an option. I'm tired of trying to swap engine families in these trucks. It always turns out to be more headache than it's worth.
Mill the block -.040. That should give me a nominal .020 positive deck height. Mill the tops of the pistons for that .020 to get close to a flat top. ( Will .020 do it, or is there no way to get that much to make it a flat top? ) Then put on the aussie quench heads. I want quench but with decent compression, no more than 9.5-1.
Or does anybody know where I can get flat-top pistons for this thing. I've looked high and low, but I can't find pistons that have enough compression height. My other option is to figure out how much longer the rod needs to be to run the 351 C flat-top and find a rod that is close enough, or get some custom ones made. Either way I'd rather do it the less expensive way. And going to a 351W is not an option. I'm tired of trying to swap engine families in these trucks. It always turns out to be more headache than it's worth.
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If you're building a street 351M, I'd just run the stock rods, crank, and pistons for the M, and build it 9:1
Believe it or not, even 9.5:1 is pushing the CR with today's gas.
I hear you on the mileage my friend, my 400 gets the same, best I can hope for on highway in 2WD is 11-12 mpg. I'm thinking of going to a more efficient aftermarket aluminum intake, and 4-barrel carb with smaller primaries than the Autolite 2-barrel has. Perhaps an Edelbrock or Qjet carb. The GM QJets were on a few Ford engines at one time, they are really good for mileage as they have tiny primaries.
Believe it or not, even 9.5:1 is pushing the CR with today's gas.
I hear you on the mileage my friend, my 400 gets the same, best I can hope for on highway in 2WD is 11-12 mpg. I'm thinking of going to a more efficient aftermarket aluminum intake, and 4-barrel carb with smaller primaries than the Autolite 2-barrel has. Perhaps an Edelbrock or Qjet carb. The GM QJets were on a few Ford engines at one time, they are really good for mileage as they have tiny primaries.
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All things equal, a 400 should get better mileage than a 351M, since it is not working as hard to get the same job done. I bet if you built a 351 and put all the same parts in and on it that you have on the 400, it would do the same or worse.
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I'll have to disagree there. The 400 is 15% bigger in cubic inches than the 351, that's why it gets less mileage. Remember, even at idle the 400 pistons are going down the bore 1/2" further per intake stroke. That's pulling in that much more air and fuel than the 351, hence the decreased mileage. Both engines will run around the same rpm under load at part throttle with the same gearing, with the 400 pulling more fuel the whole time.
You won't find any engines that much larger, getting better mileage than smaller engines in similar tune, with similar gearing. It just doesn't work out that way, that's wishful thinking, really. Every time I've built an engine of increased size, the mileage went down, not up. I've taken a few vehicles from 350 to 400 to 455-470 cubic inches, same vehicle, but 3 or 4 different engines, the mileage always goes down.
Otherwise, there would be huge V-8's in everything, getting better mileage than 4-cylinders, right ?? ut-uh...
You won't find any engines that much larger, getting better mileage than smaller engines in similar tune, with similar gearing. It just doesn't work out that way, that's wishful thinking, really. Every time I've built an engine of increased size, the mileage went down, not up. I've taken a few vehicles from 350 to 400 to 455-470 cubic inches, same vehicle, but 3 or 4 different engines, the mileage always goes down.
Otherwise, there would be huge V-8's in everything, getting better mileage than 4-cylinders, right ?? ut-uh...
Last edited by cantedvalveFord; Dec 20, 2003 at 02:42 PM.
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From: Georgia
cantedvalveFord really knows his stuff when it comes to this stuff. Take his advice. I have a edlbrock aluminum intake and holley 4160 600 cfm carb on my 351M. It has been rebuilt .30 over honed and decked, with roller rockers and cam. If I could find out how to tune the carb, i'd bet i'd get around 12-14 mpg. Good luck with your 351M.
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From: (near) Denver USA
Dropping from 15-17 mpg to 8-11 mpg is a loss of 35-50% (depending on which numbers you compare). The displacement difference between the engines can only account for 15%. So where did the other 20-35% go?
Of course, if you made modifications to the 400 to improve its volumetric efficiency beyond that of your previous 351M (e.g., camshaft, intake manifold, carb, headers, and exhaust system), then the engine will consume even more fuel than the difference in displacement alone might suggest.
On the other hand, if driven conservatively, you could use the increased torque at low rpm to shift up earlier, and you could get the same or perhaps better mpg than you got with the smaller displacement engine, especially in city driving where you have to shift more often.
One of the biggest factors in mpg is driver behavior. If you're loving the 400's power too much, your right foot is probably consuming a lot more mpg than the engine's increased displacement.
So do you drive the truck the same way with the 400 that you did with the 351M? Do you shift up at the same rpm? Do you accelerate from stops at the same rate? Do you take hills at the same speed? Do you carry (or tow) the same loads at the same speeds?
Keep in mind, modifying the 351M for performance, and using that performance, might well give you less mpg than what you had before.
Of course, if you made modifications to the 400 to improve its volumetric efficiency beyond that of your previous 351M (e.g., camshaft, intake manifold, carb, headers, and exhaust system), then the engine will consume even more fuel than the difference in displacement alone might suggest.
On the other hand, if driven conservatively, you could use the increased torque at low rpm to shift up earlier, and you could get the same or perhaps better mpg than you got with the smaller displacement engine, especially in city driving where you have to shift more often.
One of the biggest factors in mpg is driver behavior. If you're loving the 400's power too much, your right foot is probably consuming a lot more mpg than the engine's increased displacement.
So do you drive the truck the same way with the 400 that you did with the 351M? Do you shift up at the same rpm? Do you accelerate from stops at the same rate? Do you take hills at the same speed? Do you carry (or tow) the same loads at the same speeds?
Keep in mind, modifying the 351M for performance, and using that performance, might well give you less mpg than what you had before.
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Joined: Dec 2003
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From: Fayetteville, AR
I swear by synthetic lubes! THEY WORK! If you want splurge a little and buy the $4.50 Qt oil for one oil change. That's all it took for me. My daily driver (2.3 '89 Stang 170K miles) benifited greatly from the synthetic blend and Lucus oil stabilizer.
And the right foot might have more to do with it than you might think. Pulling out in traffic and sticking it to the floor... just for a second or two you know... I love my 400M's!
And the right foot might have more to do with it than you might think. Pulling out in traffic and sticking it to the floor... just for a second or two you know... I love my 400M's!
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Thanks for the replies.
As for the missing mileage, I'm sure that went to the 4bbl, edelbrock intake, big cam, ported heads, headers etc.
My only thing is that when I had the 351 it got 10-12 when you were running it wide open pulling a trailer. Then when you were cruising empty, it would go to 15-17 as long as you kept the speed at 65 or less. My 400 will no doubt run circles around the stock 351, but it does so at mileage that hurts the pocket book and makes those long distances between gas stations a proposition that requires carrying 5 gal cans in the bed to ensure that you will make it to your destination. The worst I ever got with the 351 was 9 mpg with a 12,000 lb trailer on it towing down dirt roads back and forth to our ranch. The worst the 400 ever got was pulling the same trailer with a similar load, it averages 4-5 mpg same trip. Now there is no arguing that the 400 doesn't have to be run as hard to get the same work done. But the jist of the problem is 1/2 the fuel economy. Therefore I am open to as many options as possible.
I am willing to give up some of the power to get back at least double digit fuel economy. And I will try the synthetic lube to see if that helps.
As for the missing mileage, I'm sure that went to the 4bbl, edelbrock intake, big cam, ported heads, headers etc.
My only thing is that when I had the 351 it got 10-12 when you were running it wide open pulling a trailer. Then when you were cruising empty, it would go to 15-17 as long as you kept the speed at 65 or less. My 400 will no doubt run circles around the stock 351, but it does so at mileage that hurts the pocket book and makes those long distances between gas stations a proposition that requires carrying 5 gal cans in the bed to ensure that you will make it to your destination. The worst I ever got with the 351 was 9 mpg with a 12,000 lb trailer on it towing down dirt roads back and forth to our ranch. The worst the 400 ever got was pulling the same trailer with a similar load, it averages 4-5 mpg same trip. Now there is no arguing that the 400 doesn't have to be run as hard to get the same work done. But the jist of the problem is 1/2 the fuel economy. Therefore I am open to as many options as possible.
I am willing to give up some of the power to get back at least double digit fuel economy. And I will try the synthetic lube to see if that helps.
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From: tukwila
efi
Would you consider EFI? Anyone EFI'd a 400? Keep the power and gain a little in mpg? it's something I've been looking into for a while and haven't come across yet. There was a post earlier this year about the required manifold to adapt already exisiting EFI systems but I didn't see it resolved conclusively.
All the best whatever you decide and happy holidays.
All the best whatever you decide and happy holidays.
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