250 build questions

Doing a restomod 71 Bronco, installed is a 74 250 carter RBS carb, duraspark 2, stock manifold. Pulls great on the bottom end, but falls on it's face at 2800 rpm. and if i push it past 2800 my gas mileage gets real bad real fast. The exhaust is 1-3/8" P/o did it(waiting till new transmission is in befor going to the muffler shop). I use this truck as a sometimes commuter, mostly trails and rock crawling. What size exhaust should I be running? 2-1/4"? Not going to headers, and I really want to keep as much low end torque as possible. Is there a carb that will perform better especially off road ? Or should I re jet, its always rich and i'm driving mostly between 4000-8000 elevation. Will those help with my top end woes?
My ultimate goal is a fun rock crawler, that I can still daily drive and hopefully 18+ MPG.

I am building a new engine, and would Like input on my plan.
69 250 block crank rods
255 v8 pistons .040 over
82 200 Biggest log head milled .050 and milled for either Howell injection or motorcraft 2100 ???
The I'm trying to decide on cam and timing set, Leaning toward the 82 200 cam with the early 69 250 timing set? I also have a 66 170 I can steel parts off of????

I am getting ready to install a ZF5 speed and that should put my cruse 2000 rpm instead of 3000, so I want an engine with max torque from 600-2400 rpm but that will still turn to 4000 rpm if I need it, and any help is a big help! thanks in advance.

Good to see you here!
welcome
WoW, some pretty tall orders
but
we think U can do it. Just may not B the way ur visualizing it. The Carter is v e r y difficult to re-jet.

Ever think of the 300/4.9?
The lrger exh diameter & compression increase is highly recommended. So is a modern cam, no custom grind needed (but both Iscadarian & Schnider R very reasonable AND accomidating). U R at a pretty hi elevation. May B in CO? Lots of bronk guys there (bent8) and parts. Congrats on the i6 & 1V use. The bronk is a brick, they're not for hi MPGs. A search over @ FTE has a thread (by guy in AZ w/name MPGs) that has some good info, he went 4 it and no longer runs a bronk (that may B one answer 4 U). Things, as U no, done to increase performance can increase milage...

What sz tires R U thinking, what % is on/off rd? U have 3 engines (170/200/250)?
EDIT:
forgot MPGs is over here & he did it on a stang, still a good thread to research.
Make sure U visit (Jeff's Bronco Grave Yrd's) classicbronco dot com. This is THE site for the i6 but they R for the bronks. Just watch the egos over there, they may pin ur ears back, but in the ol days they'd do more'n that when learnin U wanna keep a six. Things R way better but B 4warned.

Howdy NevadaSmith:

What a great movie that was. First, your current rig. What is your initial advance set at? It should be in the 16 degrees of advance. 250 tend to knock when lugged because of their huge deck height so don't lug the engine. How long since the RBS has been rebuilt? The RBS is quite a good choice for a Bronco 250 and your planned use. What choke are you using?

Upgrade the exhaust system to a 2" pipe system with a turbo type muffler. It will be a great upgrade with both your current engine and your new one.

Your upgrade to the better trans with OD will solve several of your current drivability issues. And complement your new engine as well.

On your new engine, what is your compression ratio goal?

FYI- Measure deck height on the '69 core block before disassembly. They are typically down in the bore .150". The 255 V8 pistons will only close up the deck height .085". That still leaves .065" deck height. Add the thickness of the head gasket of about .050" and the deck clearance is about .115"- Way too much for good squench effect- but way better than stock. My point is, depending on the price of 255 V8 pistons as compared to stock small dips pistons you may want to skip the more expensive pistons. And, dished pistons are more knock resistant than flat topped pistons. Something to think about.

You could do both 255 V8 pistons and deck the block .065" in an attempt to achieve a zero deck height, but now your CR will be too high. In that case don't mill the head and consider machining a "D" shaped dish into the top of the pistons to mirror the chamber shape in the head. That will give you a combination that is doable, not too expensive and attains a much better squench effect for a 250 engine.

On the cam, IIWIYS, I'd go for Comp 252H. Isky and Schneider offer similar grinds. While the advertised duration is not that different than a stock cam, the aftermarket have a much fatter profile, opening faster and staying at max lift longer than the stock cams. The '69 250 timing set is about the best you can do for a 250.

That's my two cents, for what it's worth. Keep us posted on you progress and your plan.

Adios, David

I believe my exhaust is 2" and my 250 pulls good and seems to rev about as high as I want operating at 4300 elevation and above. It is in northern NV and I am in TX so can't check it. Don't have a tach yet so not sure of how high it revs. My Bronco was an original 170 and as far as I know mine is the original exhaust. I don't have to rev it high I can just keep shifting to go faster and you will really enjoy that 5 sd od it makes a big difference in the way it drives.
Sounds like a real nice Bronco you have. I have a spare 1979 250 and a late model 200 with the big log and have been contemplating a built 250 for mine.

David, thanks
Currently timing is set at 10 initial and 34 all in. The RBS is freshly rebuilt, just before Xmas, and other than running rich, and a small hesitation on hard acceleration seems to be working well. The choke is electric with thermal tube from the exhaust manifold and it is hooked up correctly. It is adjusted so that it is full off at 80f on my non contact themomiter. Thanks for the tip on 2" I wasn't sure if that is the right size.
On too the new engine.
I had not thought about zeroing the block and opening the piston dish, another good tip. I was looking to being in the 9-9.25 to 1 range. Would I better served with the after market cam or better valve springs and 1.6-1.65 rockers? And staying with the best of my 4 stock cams?
Bmbm40
I'm in northern Nevada, actually Fallon give me a shout if you make it back up here, might even have an old suntune colum tach for you
Chad
I thought about going EFI 4.9L and decided against it for weight and clearance issues, that may have been a mistake. I actually have 4 "falcon" engines 2 250's a 200 and a 170.
I'm running 235/85r16's 32's now but my next tires will be 315/85r16 or 9.00-16 about 35"
18 mpg isn't a deal breaker for me just one of the goals I'm building towards, I get 15 around town 11 hwy now so I don't have a long way to go. Currently driving habits are about 40% in town, 40%off road,20% hyway

Howdy Back All:

I'd suggest that you try a base timing of 15 bdc and assess for comparison to where you are now.

On an after market cam you'll get .400" to .425" lift compared to .368" on a factory 250 cam, you can use stock springs shimmed .030" with stock type adjustable rocker arms and still be ahead of a stock cam- without the considerable expense of high ratio rockerarms. The aftermarket cams lobe profile will be considerably different than the stockers profiles. If you were just upgrading your existing 250 and didn't want to go to the trouble and expense of building another block then the high lift rocker arms would be a good idea.

Adios, David

I am in CC and have bought parts from a Bronco collector in Fallon it might be you. Thanks for the offer of the Sun tach I have been thinking about getting one, please PM with price and we can work out the details.
On classicinlines.com you can find the dyno results #3, #3A and #3B. I'm kind of liking that 140 ft lbs of torque at 2500 rpm in #3A and I think the prime contributor is direct mounting the carb. Stock torque is apparently around 111 at about 4000 rpm as noted in #3. #3C shows some really good numbers but with a lot of work done and parts. I am thinking about the #3A scenario with 9.1 compression and mild cam but maybe not headers until I find the long tube style I want like a Hooker 6602. And for me have decided on the Autolite 2100 it is said to have superior low end torque. Have a couple on hand so far.

I've been to Fallon, Hawthorne too. You can see a long way from there. This response offers nothing of use to you.

Ledslinger29
Ha, no not helpful​ at all, other than it made me laugh!
Actually big thanks to everyone for the suggestions, I really appreciate it!

"...I'm in northern Nevada, actually Fallon give me a shout …"
still got the hi sand dune w/the races on it?

If you have 1 3/8" exhaust, I'd start there. It'd be downsized out of the 1.75-2" head pipe. Outside of any tuning problems it's probably choking itself through that pipe after 2800. 1 3/4" is oem for a 170. I have 2 1/4" on mine.

After building my 250 I would not recommend going above 9.5:1 scr/8:1 dcr with the stock head. I'm at 9.6:1 static and 8.1 dynamic with .065" quench distance, 256/262 cam, .020" deck height, 8cc D dish pistons that mirror the chambers and the engine runs on the edge of pinging under load if it's timed where you really want it. Running a manual transmission will help you. I'm fighting my AOD shifting into 3rd as soon as it can. In my defense the engine was built around an Erson cam that put the compression ratio's @ 9.3-4/7.7-8

Chad, yes Sand Mountain Nevada still there, but I try to avoid it, way too many of those sxs crashing into each other.
Seth yes it does neck down right below the starter, the PO told me it was from a Toyota Tercel 1.5L muffler and all, and I think it was ok on the worn out 170, but I'm going to get the exhaust redone or get a complete system from while horses, after I get the 5spd in so I only do it once.
Thanks

If you are looking for torque from an idle to 2400 rpm the intake valve needs to close early, before 50* ABDC.
To also have a wide power band you are looking for high lift (More than a stock 250 cam with an optional 1.6 ratio rocker).

A Jones cam with a H62279 lobe would fit the bill.
The .050" duration is 192* with a 248 advertised duration.
The valve lift is .418" with a 1.5 ratio rocker, .446" with a 1.6 ratio rocker.
Normally the lobes would be on a 110 LSA but if you are using a header either long or a shorty the LSA could be decreased to 108 for addition low end torque. A rock climbing special.

Even at your elevation I would keep the Static compression ratio at 9:1 with this cam combination.
As mentioned earlier the Ideal piston should have a mirrored "D" dish and be at zero piston to deck clearance with .050" quench distance or less.

The Howell injection is immune to rock climbing extreme vehicle angles and would be better suited than a carb.

Nevadasmith":2exd51v4 said:

Chad, yes Sand Mountain Nevada still there, but I try to avoid it, way too many of those sxs crashing into each other.

Click to expand...

I think it wuz just a straight drag when I cruzed by there in '77/8? I broke down in Tonapah fora wk'n 5 days then & my cousin took off for a visit to his Annapolis, MD buddies who haunted "Sand Mt, a mi hi sand dune" after movin there a lill earlier. I never made it - skedadelin onto Mono Lake and the Serras after parts for the Dart slant6 wagon came in on the Greyhound. Incredible vehicles in the 'junk yards' around there! Some great folks too...

HOwdy Back:

"Even at your elevation I would keep the Static compression ratio at 9:1 with this cam combination."

Hey pmuller- why so conservative on static cr? In your case, at sea level and very moist air, I can understand and agree, but at and above 4,000 ft elevation, 9:1 is on the conservative side. 9.5:1 would be fairly safe, assuming a well tuned carb and properly set up ignition timing. What is your rationale for the 9:1 recommend?

Adios, David

CZLN6":2dztlhb4 said:

HOwdy Back:
Hey pmuller- why so conservative on static cr? In your case, at sea level and very moist air, I can understand and agree, but at and above 4,000 ft elevation, 9:1 is on the conservative side. 9.5:1 would be fairly safe, assuming a well tuned carb and properly set up ignition timing. What is your rationale for the 9:1 recommend?

Adios, David

Click to expand...

With the cam I was suggesting the DCR would be right at 8 if the SCR was 9.

Having extra margin makes the timing and fuel tune less critical. It also depends on what octane fuel the OP wants to use.
At high elevation locations the pump octane available at the pump is sometimes lower than for us closer to sea level.
So that would be anther question to ask?

Otherwise if high gas octane is available and the OP doesn't mind paying the difference between regular or premium then yes the DCR can be as high as 8.5 at that elevation.
That was my thinking.

What are your thoughts?

Howdy Back All:

Pmuller- My 250 with a 9:75:1 cr in a Comp 260 cam, 1.08 Autolite, 4speed OD, and a recurved DS II, on regular gas, Initial set at 16 degrees BTC, etc... and nary hint of pre-ignition. That is the experience that causes me to be a little more adventurous with CR.

NSmith- FYI, I spend a good bit of time smoothing and polishing the combustion chambers. That causes them to reflect heat and eliminates hot spots on edges. I still stand by my recommend that given your plans, a cr goal of 9.5:1 is reasonable.

What's next for your project?

Adios, David

David
I'm certainly am not going to argue with experience. 9.5 scr it is!
Thanks for qualifying the combination you used to get there.

Btw, the discrepancy in the static C/R for my build came about b/c the Felpro gasket was measured between 46-47 thou uncompressed at it's thickest and at the fire rings, not .050" compressed, after the cake was baked so to speak. I figure it's .044-.046" compressed. Bumped me from 9.4 to 9.6 @ .040" over cylinders figuring 61-62cc chambers. I have a corteco/rol gasket that is thicker I will install if/when I swap the head for tbi. I'm at sea level btw.
Good luck!

What are your, all ya'lls, thoughts on the sharp edge of the chamber after surfacing a head? Should that be blunted with a fine file, emery etc?

I don't mean to thread hijack, I think this is relevant