Truck: 1989 F-250 with dead 7.5L gas engine, 5-sp manual transmission. Engine has lost almost all compression on two cylinders -- rings are broken.

Would like to avoid replacing above with another gas guzzling 7.5L and just go with a 351W or 302 while retaining the heavy duty 5-sp. Has anyone done this?? And does this list of things to do / adapt look feasible:

New motor mounts
Swap engine, engine harness and computer from "donor" vehicle


Obviously the bell housing is an issue--- but will the 351, 302 bell housings bolt up to the heavy duty 1989 5-speed? And can the (approximate) 12" clutch, pressure plate from the 5-sp be retained somehow?

Alignment issues (lengthwise along the frame) with the smaller engines in place of the 7.5L? (or are adaptor motor mounts avail to account for any differences)

 

you need a small block trans to put a small block engine in the truck.


everything else is plug and play from one to the next, no issues there.


302 would be an outright stupid swap, 351 isn't exactly smart either. you have one of the best gas engines ever made....id keep it. a large part of the poor mileage in a stock 460 is; it was designed in the dark ages of the 80's when designers/manufacturers didn't know how to make power AND comply with emissions. lucky for you, there are remedies for all that today.


rebuild the 460 (I'd stroke it) with the following mods
-port heads and intake (or go aftermarket if your pockets allow)
-mill heads for compression
-cam
-roller rockers
-long tubes
-adjustable fuel press reg


and enjoy a mileage bump with a very serious power increase. the 460, in this form, gets about the same mileage of a 351 with the same mods, as long as you don't use the extra power everytime you get a chance, and keep the rpm down. cubic inches, in and of themselves, don't cause poor mileage. using the extra cubic inches to rip around is when your mileage decreases. just like a turbo doesn't decrease mileage, until you decide you want to rip around utilizing the power potential. drive modestly, rip once in a while, enjoy best of both worlds.

 

To expand on what '89F2urd said about the trans, your trans does not have a removable bellhousing, so there's no way to put that trans on an engine unless the bellhousing matches up. If you want to put a small block (or 300 six) in it you'll need a new trans with a small block bellhousing.

You will also need the correct flywheel for the engine you get. And I'm not sure, but I'd guess you'll need a new clutch as well.

But I'm going to diverge from what '89F2urd said about engines. First of all the statement that cubic inches don't necessarily use more fuel. That's not correct. If an engine makes more power than you need, you get less power out of it by throttling it. The thing is, closing the throttle not only limits the amount of air that gets in, thereby limiting the power. It also sucks up power used to pull air past the partially closed throttle. All engines make their best efficiency (measured as power per amount of fuel burned) at wide open throttle. So an engine that is run at closed throttle isn't as efficient as one that's at WOT. That's one reason why the 300 six gets such good mileage. It's also one reason diesels get better mileage than gassers (diesels don't have a throttle).

That said, an engine swap isn't cheap, so if you compare the cost of rebuilding your 460 or swapping in a small block (that will also likely need work unless you pay for one that was already rebuilt), you might find that you can buy a lot of gas for that 460 with the money you save.

If you're set on swapping in a smaller engine, getting a donor vehicle with the engine and trans you want is probably the easiest way to go. As long as you are pretty close in model years you should be able to swap most stuff over (wiring and electronics are what get the messiest if you are off too far in model year).

As to what engine, it depends on what you want. The 302 is more of a high-revving sporty engine. Fun and decent mileage in a light F-150 with a light shifting trans, but probably not so fun in front of a heavy duty trans in a heavier truck.

People say the 351 doesn't get any better mileage than a 460, but that hasn't been my experience. I got 12 - 15 mpg with the two 351s I've owned, compared to 9 - 10 from my 460. Granted I could probably improve my 460s mileage some if I built it carefully, but you can improve on the 351 the same way. Dollar for dollar the 351 will never have the torque of a 460, but unless you want the torque of a 460 (I don't, at least for the fuel cost) a 351 is a better option for fuel cost.

The one that intrigues me is the 300 six. I know it would have way less power (and torque) than just about any other option (including the 302). But from what people say, the torque curve is much more usable than that of a 302 (at least in a heavier truck) and it promises the best mileage. I know a lot of people will say it's a dog, but I drive like an old man, so I don't think that would bother me. But I'm only speculating, I've never driven one.

So you pays your money and takes your choice, but I'll repeat what I said above. It's awfully hard to save money by swapping engines, so if that's your goal, run the numbers first.

 

Many thanks for the informative comments.

 

if your even contemplating putting a 302 into a hd truck,then it strongly suggests your driving the wrong vehicle.

part with your truck.sell as is/part out/scrap and get yourself an f150-300-3.08 combo.........that's if you need/"must have" a truck at all.
after struggling trying to keep a 7-9 mpg vehicle alive (probably less with poor compression) ask yourself how much more enjoyable life would be if you had a little 4 cyl car pulling in 30 mpg instead (you've only got to answer it to yourself.)

 

if you don't agree with me, we can go on and on and on about the topic...I don't intend on doing that, so ill try to keep it concise.


first, ill use a 351 vs a 302 as an example. ill start by saying its wildly accepted that a 351 gets better mpg than a 302 in our trucks. but how? it has more displacement!?!? the short stroke of the 302 means it has to rev to make power, something you've already indicated later in your post. it has to rev x rpm to make y hp, and so does a 351. so the 302's x rpm to make any given y hp vs a 351 is higher in the rpm band, as the 351 takes greater advantage of each combustion event, due to its increased stroke (the only difference between 302 and 351 is stroke). fuel consumption is increased exponentially as a result of rpm, because engines lose efficiency as engine speed increases for a myriad of reasons; drag on internal components, harder (not enough time) to evacuate exhaust and fill the cylinders with fresh air, parasitic loss as rpm increases, among other reasons. so, the lower you can keep engine speed, the more efficient an engine is regardless of the throttle blade position.


next: a throttle blade at WOT has the least parasitic loss vs any other throttle position, but that doesn't mean that part throttle is burning more fuel or WOT the most efficient in terms of fuel consumption. the higher the rpm, the greater the parasitic loss AND loss from drag inside the engine regardless of WOT or not. WOT must maintain its richest AFR to prevent catastrophic failure. no matter where you are in the rpm range, WOT AFR in our trucks goes ~13.5, even though there is no more available power by doing so at low rpm vs a slightly cracked throttle blade. with part throttle, comes part throttle AFR....the optimal AFR at any given rpm for any given load is wildly variable and impossible to discuss, however, low rpm (cruising and/or cruise acceleration) AFR's can be (and are) all the way in the high teens, while still producing power that is very close to WOT at that same rpm, if you were to go WOT all the sudden. WOT at low rpm is a waste of fuel....if there were things inside the engine that were variable, instead of dynamic, then WOT would be a possibility all the time. but that engine doesn't exist, and would be wildly expensive and unreliable if it did. the throttle blade is sufficient in creating the variable conditions needed to keep a gasoline engine happy.

while you could argue that a small part of the increased efficiency of a diesel comes from the lack of a throttle blade, the lack of a throttle blade has very little to do with the added mpg capability of a diesel. thermal efficiency accounts for 90+% of the mpg advantage of a diesel over gas. this thermal efficiency comes from: high compression ratios, turbo's (turbos are thermodynamic devices, harnessing waste thermal energy), higher BTU of diesel fuel, and the inherent torque production as a result. torque production which allows the engine to generate x power at a very low rpm. if you were to take a gasoline engine and bump it to 17-21:1 compression ratios, it would perform very similar to a diesel. but, that is just about impossible to do given the volatility of gasoline.





so, does higher displacement automatically equate to guzzling more fuel? no, it doesn't. a 460 will make x power at y rpm. a vehicle requires x amount of hp to do y amount of work...a 460's engine speeds can be much lower to provide the x hp to do the y amount of work you ask from it. a 460 will burn more fuel, if it is asked to do more at any given rpm than a smaller engine could, but in real world daily driving/cruise scenarios it can be used in a fashion that gets comparable mileage to a substantially smaller engine. the abysmal thermal efficiency, associated with a 460 in stock trim, and lead feet of the drivers, is the leading cause of its poor mpg.

 

lol i don't think so sorry.you can baby a 460 all you want,it's still going to be a pig.about everyone who reads this who owns a 460 knows it pretty much gets the same poor mpg,driven hard or easy and their attempts to get anything sensible have failed so they just run 'em and have fun.the economy cant be be controlled much,but the fun factor sure can.

 

considering I, and countless others, got 9-10 mpg for years with a 302 (and even worse), and 9-10 is within a 460's common capability....I think I don't need to validate anything.

I also said; in stock trim....getting rid of what causes all of these engines to be so miserable in the mileage department is first and foremost. however, a 302 cant be fixed without a larger crank. funny how that works....increasing displacement increases mpg....

 

a 302 f150 truck should pull in mid teens average just behind the 300-6 mid to upper teens.
real world hd 460 trucks pull 8. yes i know they can peak more but i don't claim my diesel gets 17 mpg because i did it once while all interstate solo running.(high reeving 302 engines don't belong in hd trucks.you could never ask reasonable economy from such a turd setup lol.)

im sorry your 302 didn't do so well,but on average they do much better than yours did.i had a 300-6 truck that didn't do so well either but one before that,that did much better.i knew because that truck didn't pull in the economy it should have,that it wasn't reflective of what the majority of 300-6 trucks normally get.

regardless though,this guys clearly driving the wrong truck for his needs.the only way he'll get ahead and save $ will be to cut his losses and move on.rebuilding a 460 and then driving the economy killer truck more will only lead down the same road he's having with more hardship at the pump.

 

I agree, no point in dragging this out. So I will agree that an engine that needs to spin faster to make enough power to move the truck will use more fuel than one that can move the truck while spinning slower. But I contend that at least a 351, and possibly even a 300, can make enough power for most people at low enough rpm that they will do the job with less fuel than a 460.

If you need more power, you're going to have to spin those smaller engines faster, which is why a loaded 460 doesn't burn much (if any) more gas than a loaded 351. I'm not arguing that a 351 (let alone a 300) will tow a 10,000 lb trailer using less fuel than a 460. But they will still tow the 10,000 lb trailer, and they'll use less gas when they're not.

As to it being widely accepted that a 302 uses more fuel than a 351, I disagree with that too. Personally I don't know how they compare, but I've read more people on here saying the 302 gets better mileage than a 351 than those who say the 351 is more efficient. To me that makes it clear that, whether or not it's true, it at least isn't widely accepted. (I'm guessing that the difference of opinion on that comes from usage, people who are comparing empty half tons say the 302 is more efficient, people who compare loaded 3/4 or 1 tons say the 351 is more efficient.)

But bottom line is that the smallest engine that can readily do the job will get the best mileage. If you under-engine a truck for the load you aren't doing yourself any favors regarding mileage either. But if a 351 (or 300 or 302 for that matter) is big enough for you, it'll use less fuel than a 460.

 

delete, wrong thread

 

delete, wrong thread

 

deletaroni

 

This is a five year old thread that was resurrected accidentally...

 

He found it “informative” but didn’t like the answers? One and only thread. Never had another post