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BRINGING IT ALL TOGETHER.... A's, G's & E's


4 posters

Engine needs a rebuild, but into what?

Scott
Scott


Number of posts : 1647
Location : Anoka, MN
Age : 52
Registration date : 2008-05-20

Engine needs a rebuild, but into what? Empty Engine needs a rebuild, but into what?

Post by Scott Wed May 18, 2022 2:37 pm

If you look back far enough you might find posts where I bought this engine, and pics of me installing it. But, I made the stupid mistake of trusting the seller. The top end of the engine was, I think, rebuilt, but the bottom end was not. Now it gets about zero oil pressure, and when I started it this year bottom end is knocking like the oil pan is full of rocks.

My plan is to get a short block and take my existing head to a shop to have it checked / rebuilt if needed.

What I have now is an Inline 250 in my 1968 GMC Van that is bored thirty over, an Offy 4 BBL intake, a Holley 390, and a cast exhaust manifold with a 2.5″ dump on it. The head has been ported and polished, and the transmission is a 200-4r from an 87 Monti Carlo. The engine is worn out. The bottom end is knocking like it has a pan of rocks. Right now it gets about 12 miles per gallon.

After the engine rebuild the van will be used for traveling.

My question is if there will be much difference in engine temps or gas mileage if I put in a stock short block instead of one that bored thirty over? What changes should I be making to keep freeway power, but maybe get better gas mileage?

Right now I am leaning toward just putting in a stock short block, maybe getting it bored thirty over if it wont make much difference in engine heat or gas usage.

What do yall think, and if you are happy with your highway power what setup do you have?
wishbone
wishbone


Number of posts : 14
Location : Fredericksburg, Texas
Registration date : 2022-04-18

Engine needs a rebuild, but into what? Empty Re: Engine needs a rebuild, but into what?

Post by wishbone Thu May 19, 2022 12:40 pm

Scott,

I may not know much, but I know what it's like to get ripped off. That really sucks and I'm sorry to hear that. We all have to trust someone at some point, so don't feel bad.

I bought a '64 Chevy g10 with a 250 from a '73-'79 Olds/Chevy/Pontiac. I knew it might need a rebuild and was OK with that. However, when I got it out and to a trusted machine shop, I was told it was previously bored .060 and that's as far as it can go.

Machinist says if he put new pistons, rings and honed the cylinders... it will run. It may smoke a little and not last as long as other rebuilds. Who wants that? Now I need another 250 block.

If I were you, I'd consider having it bored out another .030 and rebuilt. You get to keep all the cool stuff you already have invested. It's much easier than going to a V8. On the other hand, you have the large dogbox so it wouldn't be all that bad.

Good luck... and if you go with a V8, I may want to buy your 250 off you. Provided the machinist tells me he can work with it. Food for thought!

wishbone
Scott
Scott


Number of posts : 1647
Location : Anoka, MN
Age : 52
Registration date : 2008-05-20

Engine needs a rebuild, but into what? Empty Re: Engine needs a rebuild, but into what?

Post by Scott Mon May 23, 2022 5:56 pm

Thanks for the offer Wishbone.. Very Happy I'm not up for the changes needed to go to a small block. I will probably just buy a stock block and put it in with what I have. I forgot to put that the current engine is also equipped with a mild cam. So, a stock block thats not bored over, and stock cam, should help a little with mileage, and maybe shave a little off of what it cost per mile. Everything on it is tuned to work well together, I just hope that the difference in the block doesn't throw off the fine tuning.
wishbone
wishbone


Number of posts : 14
Location : Fredericksburg, Texas
Registration date : 2022-04-18

Engine needs a rebuild, but into what? Empty Re: Engine needs a rebuild, but into what?

Post by wishbone Fri May 27, 2022 5:49 am

I see.

Back in the '70s when gas was hard to come by, I had a friend who converted his 72 Chevy pick-up to run on propane. It was mostly just a unit sitting on top of the carb with a rubber diaphragm in it. On one side ran hot water (coolant), the other side was the propane. The heat from the coolant liquified the propane, fed the carb, and it ran like normal. I'm pretty sure that's how it worked.

He placed a 100 gallon tank in the bed and bragged that, on a full tank he could get out of the U.S. in any direction without stopping. He got great mileage, never waited in line for fuel and propane was cheaper than gasoline... back then. I remember he mentioned propane ran at about 95 octane and there were no carbon deposits on the valves or piston heads.

One extra benefit was he could release a little propane on a warm beer can and cool it down for drinking a cold one!

Those were the good old days and yes... I'm very old. Healthy but old!
AzDon
AzDon


Number of posts : 579
Location : Lake Havasu Az
Age : 66
Registration date : 2014-01-20

Engine needs a rebuild, but into what? Empty Re: Engine needs a rebuild, but into what?

Post by AzDon Sat May 28, 2022 10:39 am

I drove a lot of miles in my 230 powered first gen in the seventies.... Stock with a one barrel carb it would do 17s in the quarter mile while chirping the N50s into second, all while getting about 17 mpg.....
The early vans were designed to run a bit higher RPMs with the sixes and that was reflected in the cooling layout and gearing.....
The 2004r suggests that you want to run lower RPMs, but fuel mileage of a six suffers unless it's got the power to easily pull the gears (low load)....
All three of my Astro vans were "overgeared" as is my 06 Trailblazer and as a result, none of these got (get) particularly good mileage at less than 75 MPH.... All four of these vehicles got/get about 12-13 around town and all were/are factory fuel injected with overdive transmissions....

Matching horsepower, gearing, cooling strategy, and RPM strategy to the weight of the vehicle will get you the best economy.....It's not wrong to believe you can achieve this with a six because the Nova-generation straight sixes are very potent for their size.....The straight six doesn't block airflow like a v8 does and runs much smoother....The exhaust note won't be mistaken for a v-8, but the six will keep up.....
I'm a v8 guy, but I won't bore you with the v8s advantages as I'm sure you've heard them all...
I would say that if you want a good six built, you should choose and buy your parts and work with a local reputable machinist/engine builder on doing a short-block and head separately and bolt the head on yourself after checking the decks for straightness....New oil pump, degreeing your cam and cam break-in etc......
Scott
Scott


Number of posts : 1647
Location : Anoka, MN
Age : 52
Registration date : 2008-05-20

Engine needs a rebuild, but into what? Empty Re: Engine needs a rebuild, but into what?

Post by Scott Thu Jul 14, 2022 9:06 am

On second look the block is .060 over right now, and the cam is an RV cam. I don't know how to read cams, but my guess is an RV cam is more power at lower RPM..? Which goes well with the 200-4r transmission?

I think to really be happy with the outcome I may just do as you said AzDon, just put in a stock inline 6 block and head. From what you said above about the lower RPMs for the transmission, should I put in an RV cam, or ????

Then, the intake with a Holley 4bbl 390cfm / Offy Intake.. Just bolt that on to stock, or do you think that may be too rich for a stock block? If so I can sell off the intake and carb with the modified head. The head is ported to match the intake anyway. Then the exhaust manifold empties into a 2.5" pipe. Would that become a problem? I've heard people say that no back pressure can cause the exhaust valve to scorch?? Something like that?

Wishbone, I had to think about the propane conversion. I've seen that before, and with gas prices on the rise it makes it tempting. But, I don't want to go down the road of customizing again. Cost to much to get it figured out. And, I have put so much cash into the van already without driving it very much its hard to justify. But, once I get past this problem I may be consider changing it over later. I will just have to see where the cost of gas lands.

Thanks for all y'all's help and info..

jrinaman
jrinaman


Number of posts : 889
Location : zelienople,pa.
Age : 58
Registration date : 2011-03-10

Engine needs a rebuild, but into what? Empty Re: Engine needs a rebuild, but into what?

Post by jrinaman Thu Jul 14, 2022 5:57 pm

I am running a straight 6 but went the opposite direction, not caring about mileage in the least. I still get 10-12 mpg. running around town and much better on the interstate. A .030" over bore verses stock is negligible for power or mileage. Your rebuilt short block would be bored over anyways. The 390 cfm. is close to the upper limit but should be fine. A friend just installed dual webers on his bone stock 230 and is fine. They are around 380 cfm combined. He tried the 38/38's (around 470 cfm.) and was a bit much. A few performance mods may not hurt as much as you think. An underpowered motor at full throttle drinks lots of gas! An RV cam probably helps mileage! You need to match your cam (duration) with your cruising rpm's. The bigger the cam, the higher you want the cruise rpm's. A slight bump in compression would work well with an R.V. cam, 0.060 of the head will give you a little more h.p. and even though it uses slightly more fuel, you would definitely get that back by not needing to floor it as often. With a modest cam and compression increase, the 390 carb will be better matched and spend more time at part throttle (primaries only). You didn't mention what rear gears. As 3.36 is most common, it should cruise down the highway at 2200-2300 with the 200 4r. My 4.11's turn about 3000 r.pm at 75-80.

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