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Discussion Starter · #1 · (Edited)
As I posted in my introduction Greetings from North Georgia. "Hot Rodding" a...

I'm dealing now with a Chevrolet Straight 6 250CID engine. I'm not sure what it's from, probably a Nova or a Camaro from the late 1970s. Sometime in the 1980s, this engine was swapped into an early 1960's British Limousine. The big issue with this setup is that it's underpowered. I'm not sure if it's just too much car for too little engine, or if the engine is well below it's potential. I suspect the latter, or more likely a combination of the two.

We're now considering upgrades to increase power. I'm looking for advice here, especially from those familiar with this line of engines.


Things we're considering:

Upgraded Intake manifold and Carb (or upgrade to TBI).

I'm pretty sure the carb isn't up to much more than stock power. I'm prepared if someone wants to show me I'm wrong, but I'm guessing a larger carb is in order.

The only alternate intake manifolds I see available right now are for square bore 4 barrel carbs. That with a small carburetor (maybe a Holley 390cfm unit) might not be too much carburetor if other upgrades are made. Or can I run a 350CFM 2 barrel on that manifold with a spacer/adapter?

I think a TBI unit (either aftermarket or something pulled from an early 1990's truck with the 4.3L V6 or 5.0L V8) could also work with that manifold, and might be worthwhile. That should have fewer issues with ethanol blended fuels, but it will require an electric fuel pump, etc. etc. More costly and more labor intensive than just a new carburetor, but probably more fuel efficient and reliable as well.

Upgrading the cam:

I guess the first question is, do I have a damaged/worn cam now? I need to pull the valve covers and put the dial indicator on the pushrod end of the rocker arms and see what the lift is now, and if it's consistent across all cylinders.

If cam replacement is needed, I see there are numerous choices. Would something in the 260-270 degree (advertised, probably 210-220 degrees +/- at 0.050" lift) be appropriate for this engine? I want something that will still idle pretty smoothly, but give some grunt and power. What we used to call the "RV Cam" is what I'm thinking. If I'm going to the trouble of a new camshaft, I'll definitely want to do an upgrade there, not a "stock grind" replacement.

Upgrading the exhaust:

It currently has the original cast iron exhaust manifold into a single exhaust pipe. The pipe looks decent (maybe 2.5" OD? I'll have to measure), but the manifold looks very restrictive.

I see there are header pairs available for this engine that split the front cylinders from the back and could be used with a dual exhaust system for better flow. I'm thinking headers + X pipe + resonators + dual in, one larger out chambered muffler (and maybe even a third resonator aft of the muffler) as an alternative to the full single system that's there now. I'll need something that's both free flowing and quiet. The good news is, with nearly 12 feet of wheelbase and some overhang at the rear for the trunk, I've got room for lots of exhaust goodies to quiet the thing down.

Other than an engine swap, or forced induction, am I missing anything?

Assuming a well executed plan here, what kind of HP and Torque numbers is this thing capable of? Anything close to (or over) 200HP at the crank with 200+ ft. lb. of torque (numbers in the ballpark of the 4.3L Vortec V6)?

Oh, and this thing has a TH350 transmission and some pretty short gears in it (3.90 or 4.10/4.11 if I had to guess). There's a Ford rear axle in the back now. That part of the power train seems pretty well sorted out. The only thing I think it might need, depending on the engine setup and torque curve, might be a mildly higher stall torque converter.
 

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I recently helped a friend install an Offy 4bbl manifold and 500 CFM E-brock carb on the 250 I6 in his 1969 F85 (yes, it's a factory motor). Made a huge difference. Now he needs a better exhaust to match, and he's got a 200-4R on the way.
 

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Do you live in Europe’s Georgia or the United States? If Europe parts will be harder to get, more expensive and take more shipping time. I’m going to assume since you say this is a 1970’s engine that it has an intake manifold that is a separate bolt on piece not the updated head with the intake an integral part of the head casting. If you live in the states I’d recommend that if you want to stay with a Chevy inline 6 that you scrounge up a Chevy/GMC 292 or preferably a V8. The inlines have a lot of configurational and fundamental design issues that stymie getting really big power numbers the 250 being a small engine door a two ton vehicle to start with.

If you’re really hard over on the inline 6 the first thing to do is pickup a copy of Leo Santucci’s book Chevrolet Inline Six-Cylinder Power Manual, it goes into the nitty gritty details of what you need to know which at a 154 pages we can’t duplicate that information here.

Let me say that among the inline 6’s problems is mixture distribution in the long runners of the intake system. It is nye on impossible to solve this problem with a centralized place of fuel introduction be that carburetor or TBI injection. The problem is equatable mixture distribution in a system where the center two cylinders feed directly to the plenum while the out board cylinders have lengthy runners. To solve this either takes multiple carburetors or TBI units or the best would be port injection which simply reduces the intake to delivering air.

The big obstacles your up against beyond the intake manifolding is the paired head ports, low compression, poor chamber shapes, long push rods, mild cam timing. Add to that the 194 and 250 blocks have end corner head bolt integrity problems when you start pulling more power. The stresses on these bolts tend to crack the block in the corners. The 292 is a taller engine with more meat in this area and since intended as a truck/industrial engine carries more beef in the high stress zones of the casting. Down side is the 292 bolts in differently and gets you into mount conversions, otherwise it shares the typical Chevy/GMC bellhousing bolt pattern and overall dimensions common to the big inclines, V8’s derived V6’s and the Chevy V8’s big and small blocks up to but not including the Gen III an on motors.

Bogie
 

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Your overweight barge is gonna need all the help it can get.
‘Considering your right hand tiller you’ll likely need to stick with the motor you have.
‘There’s lots of upgrades available.
Look here.



 

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Discussion Starter · #6 ·
Sorry, I somehowe missed all the reply notifications here. My bad.

It's Georgia USA, about an hour north of Atlanta.

I do have the Santucci book.

We did some digging in this one. When I pulled the valve cover, I found the nut loose (almost off) on the #6 exhaust rocker arm, and the arm was loose and twisted. The valve end seemed to be interfering with the intake valve rocker. And the #6 intake rocker arm nut was cross threaded and not fully down on the stud.

I pulled the head, and long story short, we discovered 3 separate "repaired" cracks in the head. We were planning bigger intake valves (1.84" because 1.90" aren't available quick enough), and there was no way my machinist wanted to expand the valve throats on this head with the pinned and stitched "repaired" cracks.

I'm now waiting on another head core that has been represented as "never cracked" and appears to be a good core. Machine shop is closed for vacations next week, but promised he'd start out on my head first when he returned. The guy is awesome.

We're doing 1.84" intake valves, new 1.50" (1.60" are back ordered and unavailable) exhaust, and the HIFLOW lump kit in the intake ports, along with removing the head bolt boss. New seats and all the standard stuff, walong with milling the head to 72cc chambers. I might even go down to 70cc.

And for intake, carburetion and headers, we've got a Clifford Performance "kit" with a single Weber 38/38DGES carb on the way right now.

We'll run the stock cam until the Comp Cams "High Energy" 260° cam arrives. Then we'll install that. The full specs are 212°/212° at 0.050" lift, 0.489" total lift and 110° LSA. It's a "towing/off road" application.

Anyway, we're making progress. I'll keep posting updates.
 

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Discussion Starter · #7 ·
We're still waiting on the Weber carb to arrive. I've been distracted a bit by another other project. Also have Easter travel plans through Sunday.

I did get the intake and headers mocked up on the old head. I also ran a water supply tube from the gauge sender port on the head to the heat under the carb.

Here's pix of that.


From the side.


From above


From below.

I think this will be better than rubber hoses teed into the heater circuit.

For the return, I plan to go over the valve cover to the spark plug side, and have a "bulkhead fitting" over there on a bracket, with a hose barb to connect a heater hose to that goes into a tee in the heater return line to the water pump. That'll minimize the rubber hoses and keep the rubber hoses away from exhaust heat.
 

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Discussion Starter · #9 ·
I would be concerned about copper tube as it work hardens from engine vibrations and will eventually crack.

Bogie
I had that available. It feels really solid the way it's run and shouldn't move or vibrate. It isn't a long piece, and the 3/8 inch copper seems strong and stiff. I did anneal the ends before I made the single flares.

I'll use a bulkhead fitting on a bracket to support the end of the tubing and the hose barb on the return side, to keep things solid and vibration free. And I might even add a tie down clamp on the return run.

What type of tubing would be best for this? I didn't have galvanized steel or stainless in that size. I thought fatigue issues and cracks were also a problem for aluminum. I have some soft aluminum tubing that size, but I don't think I have enough to do both lines with aluminum.
 

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It’s not an immediate problem but is a risk over time. Illegal to use on aircraft for that reason. If you ever watched the movie “Never Cry Wolf” there is a scene early on where while flying to the wilds of Alaska the engine quits and the pilot grabs some tools and steps out on the float to work on the engine that has no cowl being a DeHavilland Beaver it didn’t come that way from the factory. There is a quick camera sweep showing a copper fuel line on this beater airplane. Living as I do in a airplane centric city at least more so in the 1980’s when this movie came out one could hear the gasp that went up in the audience as that scene flicked by.

Aluminum is not worthy of vibrating environments either. The longer term concern with plumbing made of vibration and age hardening metals in your case is they are mostly under the intake and out of sight, therefore likely not to be inspected for wear, tear and corrosion till something ugly happens.

Steel or stainless steel is a better lasting choice but much harder to work into the shapes you need. Barring their use the next best thing is regular inspections and replacement if looking corroded as that often is an indicator that the material is becoming brittle and hairline cracking.

Vibration isn’t all about the tube doing stiff and tight, vibration is always with us. It is a constant cause of metal fatigue.

Bogie
 

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It’s not an immediate problem but is a risk over time. Illegal to use on aircraft for that reason. If you ever watched the movie “Never Cry Wolf” there is a scene early on where while flying to the wilds of Alaska the engine quits and the pilot grabs some tools and steps out on the float to work on the engine that has no cowl being a DeHavilland Beaver it didn’t come that way from the factory. There is a quick camera sweep showing a copper fuel line on this beater airplane. Living as I do in a airplane centric city at least more so in the 1980’s when this movie came out one could hear the gasp that went up in the audience as that scene flicked by.

Aluminum is not worthy of vibrating environments either. The longer term concern with plumbing made of vibration and age hardening metals in your case is they are mostly under the intake and out of sight, therefore likely not to be inspected for wear, tear and corrosion till something ugly happens.

Steel or stainless steel is a better lasting choice but much harder to work into the shapes you need. Barring their use the next best thing is regular inspections and replacement if looking corroded as that often is an indicator that the material is becoming brittle and hairline cracking.

Vibration isn’t all about the tube doing stiff and tight, vibration is always with us. It is a constant cause of metal fatigue.

Bogie
What are you thoughts on NiCopp, Bogie?
 

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Discussion Starter · #12 ·
What are you thoughts on NiCopp, Bogie?
I was wondering about that myself.

Since I'll be the maintenance guy on this for a while, I'll just add "inspect copper coolant tubing to intake manifold" to the checklist for the semi-anual maintenance (oil change, etc.) on this one.

And I'll get some 3/8" stainless steel tubing and try to have that available and at the ready when this copper starts to corrode and show signs of fatigue issues.
 

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Nickel copper seems to get around the more pure copper fatigue issues, it has become a popular brake line material as it is easier to shape than steel, resists high pressures and corrosion, it appears to have a good fatigue life as well.

A number of guys here have also used it for fuel line. I haven’t but offer no objection or caution about it.

Bogie
 

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Discussion Starter · #14 ·
Much progress has been made on this. If anyone wants a full report with some pix, I'll try to post something up tomorrow.

Unfortunately, I've hit another snag. Spark plugs. When I tried to install the plugs (R45TS, maybe R46TS, whatever I just put in it last month), the threads caught nothing but air. There are threads in the hole, and from what I can see they look to be the correct pitch for my plugs, but it looks like someone helicoiled all 6 spark plug holes in the distant past, and the inserts were removed when the head was cleaned, or by the machine shop that did the valve work and other head work.

I didn't discover this until Saturday morning, and can't do anything about it until Monday. Grrrrr. Somewhere there's a bride who is cursing me because her wedding limo was cancelled on her on the day before her wedding.
 

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Not likely the shop would just take Heli Coils out unless they were unsatisfactory but should have replaced them. Check to see if somebody didn’t retap these for the old Ford 18mm spark plug?

Bogie
 
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