I have a 250 "small six" out of a 1971 Mustang. It was running when pulled from the car. I am going to build it with fuel economy in mind, running an automatic, probably an AOD, and an 8" rear end. I am going to put the 2 bl carb flange on the head, when it is off. I see that the pistons from a V-8 with a taller compression height are recommended for a 250. I think the 71 model had 9to1 compression and might not need any more. Maybe just a suitable cam, a good rebuild, and a 2 bl carb. I have an Autolite carb, and a little Carter 2bl from a Dodge 318. Looking for some suggestions.
Rebuilding a 1971 250
- Thread starter 250six_71
- Start date
8) for a cam for this engine here are three i recommend in no particular order;
1: CSC-264-HSP-12
2: CSC-264-HDP-10
3: CSC-264-HDP-12
the first one is a single pattern cam with 112 degree lobe centers, the second and third ones are dual pattern cams with 110 and 112 degree lobe centers respectively. any of the three will do nicely, though i prefer dual pattern cams with engines, like ours, that have small exhaust ports. you can find the specs on these cams here;
http://classicinlines.com/products.asp?cat=115
as for the carb, you probably want the autolite in my opinion. it flows better, and is a bit more reliable, and easier to rebuild.
i would get the compression ratio up to about 9.5:1, and finish off the engine with a good header, but you will probably have to build your own header, unless you buy one from classicinlines and modify theirs. i understand that mike(azcoupe) is working on a header for the 250. you might pm or email him for more information.
dont forget to add a good electronic ignition to the mix. mike has some good systems for sale at classicinlines, or you can cobble together your own system like many of us have. i have used a duraspark dist with a chrysler ignition box and an msd blaster lll coil. check out www.gofastforless.com for more ignition ideas.
1: CSC-264-HSP-12
2: CSC-264-HDP-10
3: CSC-264-HDP-12
the first one is a single pattern cam with 112 degree lobe centers, the second and third ones are dual pattern cams with 110 and 112 degree lobe centers respectively. any of the three will do nicely, though i prefer dual pattern cams with engines, like ours, that have small exhaust ports. you can find the specs on these cams here;
http://classicinlines.com/products.asp?cat=115
as for the carb, you probably want the autolite in my opinion. it flows better, and is a bit more reliable, and easier to rebuild.
i would get the compression ratio up to about 9.5:1, and finish off the engine with a good header, but you will probably have to build your own header, unless you buy one from classicinlines and modify theirs. i understand that mike(azcoupe) is working on a header for the 250. you might pm or email him for more information.
dont forget to add a good electronic ignition to the mix. mike has some good systems for sale at classicinlines, or you can cobble together your own system like many of us have. i have used a duraspark dist with a chrysler ignition box and an msd blaster lll coil. check out www.gofastforless.com for more ignition ideas.
Howdy 250six:
And welcome to the Forum. It's clear you've done some reading here already. The albatros of building a 250 for increased efficiency is the huge deck height that most have. Mine measured out to .150". That along with a modern head gaskets extra thickness make for miserable combustion efficiency. Here's an article I put together some time ago about dealing with the issue. This may be overkill for you in your situation. If so, let me know, and I can scale back to some more basic suggestions in building for fuel economy. Before you start the article do you know about "Quench" and combustion efficiency?
****************************
"Achieving Zero Deck Height Using 255 V8 pistons.
A zero deck height can be achieved on a FoMoCo 250 engine by using modified 255 V8 pistons ( Silvolite 1176), offset grinding the crankshaft journals.020” for a stroke increase of .040” to 3.95”, and then decking the block to zero.
The 255 V8 pistons have a pin height of 1.585” as compared to 1.5” for the stock 250 pistons. The difference eliminates .085” of the typical .150” of deck height in a stock engine. The offset grinding of the journals reduces deck height an additional .020”, but increases the stroke by .040” for a final compression stroke of 3.95”, and a combined reduction of .105” (.085" + .020") of the deck height.
I would plan for an overbore of .020” or .030” depending on the condition of the block bores.
The next step will be to deck the top of the block to zero while using the new parts and with the machining complete. Decking of from between .030” and .045” will need to be removed to achieve the goal of zero deck height. All of these steps will raise your engines compression ratio to a too high level.
To reduce CR and to add to combustion efficience the next step is to mill a D-shaped dish into the top of the new pistons mirroring the shape of the combustion chambers in the head. The purpose is to increase combustion efficiency and to maximize bore-to-quench ratio. This dish should have a volume of approximately 7 to 8 ccs. When assembled with a NAPA Victor head gasket with a compressed thickness of .045” the CR will be less than 10:1 depending on how you finish the combustion chambers.
Do not mill a late model head until all other machining and parts modifications are done and all measurements have been taken and calculated. Mill the head only enough to achieve a level mating surface. The 10:1 CR was figured assuming stock 62 ccs volume chamber. If more CR is needed, mill the head last to achieve the goal CR. If less CR is needed, open the chambers by unshrouding the intake valves. With optimized chambers and piston dish and a performance cam a CR of 9.3 to 9.7:1 is reasonable. The longer duration of a performance cam will bleed of cylinder pressure and lessen the likelyhood of pre-ignition."
In general, when building with MPG in mind, more compression ratio is best. Use as much as you can manage. What is your elevation. This is especially true with an Auto trans. All of the cam profiles that Mike lists are more performance oriented than economy- Too much lift and duration to get economy cylinder pressure. He may have a better suggestion for you.
For best economy and driveability with an auto trans, I'd be looking for an Autolite 2100 1.23. The annular discharge venturi booster and adjustible accelerator pump arms will be pluses when tuning for economy.
Anyway, give this some thought and give us some feedback on your thoughts.
Adios, David
And welcome to the Forum. It's clear you've done some reading here already. The albatros of building a 250 for increased efficiency is the huge deck height that most have. Mine measured out to .150". That along with a modern head gaskets extra thickness make for miserable combustion efficiency. Here's an article I put together some time ago about dealing with the issue. This may be overkill for you in your situation. If so, let me know, and I can scale back to some more basic suggestions in building for fuel economy. Before you start the article do you know about "Quench" and combustion efficiency?
****************************
"Achieving Zero Deck Height Using 255 V8 pistons.
A zero deck height can be achieved on a FoMoCo 250 engine by using modified 255 V8 pistons ( Silvolite 1176), offset grinding the crankshaft journals.020” for a stroke increase of .040” to 3.95”, and then decking the block to zero.
The 255 V8 pistons have a pin height of 1.585” as compared to 1.5” for the stock 250 pistons. The difference eliminates .085” of the typical .150” of deck height in a stock engine. The offset grinding of the journals reduces deck height an additional .020”, but increases the stroke by .040” for a final compression stroke of 3.95”, and a combined reduction of .105” (.085" + .020") of the deck height.
I would plan for an overbore of .020” or .030” depending on the condition of the block bores.
The next step will be to deck the top of the block to zero while using the new parts and with the machining complete. Decking of from between .030” and .045” will need to be removed to achieve the goal of zero deck height. All of these steps will raise your engines compression ratio to a too high level.
To reduce CR and to add to combustion efficience the next step is to mill a D-shaped dish into the top of the new pistons mirroring the shape of the combustion chambers in the head. The purpose is to increase combustion efficiency and to maximize bore-to-quench ratio. This dish should have a volume of approximately 7 to 8 ccs. When assembled with a NAPA Victor head gasket with a compressed thickness of .045” the CR will be less than 10:1 depending on how you finish the combustion chambers.
Do not mill a late model head until all other machining and parts modifications are done and all measurements have been taken and calculated. Mill the head only enough to achieve a level mating surface. The 10:1 CR was figured assuming stock 62 ccs volume chamber. If more CR is needed, mill the head last to achieve the goal CR. If less CR is needed, open the chambers by unshrouding the intake valves. With optimized chambers and piston dish and a performance cam a CR of 9.3 to 9.7:1 is reasonable. The longer duration of a performance cam will bleed of cylinder pressure and lessen the likelyhood of pre-ignition."
In general, when building with MPG in mind, more compression ratio is best. Use as much as you can manage. What is your elevation. This is especially true with an Auto trans. All of the cam profiles that Mike lists are more performance oriented than economy- Too much lift and duration to get economy cylinder pressure. He may have a better suggestion for you.
For best economy and driveability with an auto trans, I'd be looking for an Autolite 2100 1.23. The annular discharge venturi booster and adjustible accelerator pump arms will be pluses when tuning for economy.
Anyway, give this some thought and give us some feedback on your thoughts.
Adios, David
I am at 2500 foot elevation, and I think that lets you use a bit more compression ratio. I don't know about milling a dish in the piston. I think enlarging the head chamber could be done with a carbide bit. I have a Pertronics in the distributor, it was working OK. So, if I changed pistons, I still need to deck the block . What about a good timing chaIn? I have a couple different Autolite carbs, a 500 and a350, electric choke. Would it be better to find a different head, than trying to change this one. When was the bigger log made?
Howdy Back:
Q- I am at 2500 foot elevation, and I think that lets you use a bit more compression ratio.
A- Yes, 9.2 ot 9.5:1 should be easily manageable given a performance/RV/Mileage cam and cleaning up the chambers.
Q- I don't know about milling a dish in the piston.
A- this would have to be done in a machine shop with a fairly sophosticated, computor controlled milling machine.
A- I think enlarging the head chamber could be done with a carbide bit.
Q- Yes, this is hand work- and tedious. It will require careful cc measuring along the way and in balancing out each chamber so that they all end up the same.
Q- I have a Pertronics in the distributor, it was working OK.
A- The '71 distributor will be compatible with either the Autolite 2100s or Holley 2300s. The Petronix conversion is a good upgrade.
Q- So, if I changed pistons, I still need to deck the block.
A- If your goal is to attain zero deck height to achieve a better quench effect to improve combustion efficiency, then the answer is "Yes". If zero deck height is not your goal, then there is no need to use the 255 V8 pistons either. Just stock 200/250 small dish replacement piston and mill the head to achieve your goal CR. This is a much cheaper, easier alternative, just not as efficient.
Q- What about a good timing chaIn?
A- About the only thing you want to be sure of is that you get a timing set for a '71 250, or earlier. Later sets have the cam advanced for better emission numbers, but less performance. I bought mine from NAPA, but CI may carry them by now. There are, currently, no double roller timing sets available for 250 engines that I am aware of. It is a need.
Q- I have a couple different Autolite carbs, a 500 and a350, electric choke.
A- Either an Autolite 2100, 1.23 or a Holley 350 will work. Both are in the 350 cfm range. I prefer the 2100 for mileage and daily driving as it is a simpler carb with annular discharge venturi boosters. Its only downside is that jets and part are getting harder to find. You will be surprized how close it will be to right on your 250. In any case, an automatic choke is a good idea for economy. Just be sure to set it as lean as possible.
Q- Would it be better to find a different head, than trying to change this one.
A- A '78 and later head will have bigger intake valves, 1.75" to your 1.65", induction hardened seats and a slightly larger volume intake tract- 1,240 to 1,340 cc. IF mileage is you goal none of these issue will matter. Larger valves and greater volume may make a difference for performance, for economy you are more interested in flow velocity. Hardened valve seats can be added to your '71 head.
Q- When was the bigger log made?
A- '69 - '74 = 1,240 cc. '75 - '79 = 1,34 cc. '80 = 1,420 cc. The '80 EOxx casting had some taps and bungs added that added to volume, but may not have helped flow. AKT- while the '71 250
advertized a cr of 9:1, it has the same dimensions as the '72 and later 250 which are advertized at 8:1. What FoMoCo advertizes and what they do are sometimes similar.
Have you given an thought to cam selection for an engine rebuild with "Fuel economy in mind"?
Keep it coming.
Adios, David
Q- I am at 2500 foot elevation, and I think that lets you use a bit more compression ratio.
A- Yes, 9.2 ot 9.5:1 should be easily manageable given a performance/RV/Mileage cam and cleaning up the chambers.
Q- I don't know about milling a dish in the piston.
A- this would have to be done in a machine shop with a fairly sophosticated, computor controlled milling machine.
A- I think enlarging the head chamber could be done with a carbide bit.
Q- Yes, this is hand work- and tedious. It will require careful cc measuring along the way and in balancing out each chamber so that they all end up the same.
Q- I have a Pertronics in the distributor, it was working OK.
A- The '71 distributor will be compatible with either the Autolite 2100s or Holley 2300s. The Petronix conversion is a good upgrade.
Q- So, if I changed pistons, I still need to deck the block.
A- If your goal is to attain zero deck height to achieve a better quench effect to improve combustion efficiency, then the answer is "Yes". If zero deck height is not your goal, then there is no need to use the 255 V8 pistons either. Just stock 200/250 small dish replacement piston and mill the head to achieve your goal CR. This is a much cheaper, easier alternative, just not as efficient.
Q- What about a good timing chaIn?
A- About the only thing you want to be sure of is that you get a timing set for a '71 250, or earlier. Later sets have the cam advanced for better emission numbers, but less performance. I bought mine from NAPA, but CI may carry them by now. There are, currently, no double roller timing sets available for 250 engines that I am aware of. It is a need.
Q- I have a couple different Autolite carbs, a 500 and a350, electric choke.
A- Either an Autolite 2100, 1.23 or a Holley 350 will work. Both are in the 350 cfm range. I prefer the 2100 for mileage and daily driving as it is a simpler carb with annular discharge venturi boosters. Its only downside is that jets and part are getting harder to find. You will be surprized how close it will be to right on your 250. In any case, an automatic choke is a good idea for economy. Just be sure to set it as lean as possible.
Q- Would it be better to find a different head, than trying to change this one.
A- A '78 and later head will have bigger intake valves, 1.75" to your 1.65", induction hardened seats and a slightly larger volume intake tract- 1,240 to 1,340 cc. IF mileage is you goal none of these issue will matter. Larger valves and greater volume may make a difference for performance, for economy you are more interested in flow velocity. Hardened valve seats can be added to your '71 head.
Q- When was the bigger log made?
A- '69 - '74 = 1,240 cc. '75 - '79 = 1,34 cc. '80 = 1,420 cc. The '80 EOxx casting had some taps and bungs added that added to volume, but may not have helped flow. AKT- while the '71 250
advertized a cr of 9:1, it has the same dimensions as the '72 and later 250 which are advertized at 8:1. What FoMoCo advertizes and what they do are sometimes similar.
Have you given an thought to cam selection for an engine rebuild with "Fuel economy in mind"?
Keep it coming.
Adios, David
CZLN6":3vgrifu1 said:
David,
You got the timing set wrong. The "later sets" are Retarded, not advanced.
And there is a way to install a SBF double roller timing set on a 250. But it does require some modifications to do it. We're running one now on Kelly's Falcon. So far it's working very well!
See http://fordsix.com/forum/viewtopic.php?f=5&t=58314&p=451409&hilit=SBF#p451409
Later,
Will
Howdy Back:
Thanks Will. My forgetter is getting better and my rememberer is broke. Good ketch! My point was to get the early, straight up timing set and not the altered, later one.
I did read your solution to a double roller cam timing set. Very creative and effective. But, until one is commercially available, I hesitate to suggest such an involved solution for a simple engine rebuild. I'm looking forward to future updates on the success of your inovation.
Adios, David
Thanks Will. My forgetter is getting better and my rememberer is broke. Good ketch! My point was to get the early, straight up timing set and not the altered, later one.
I did read your solution to a double roller cam timing set. Very creative and effective. But, until one is commercially available, I hesitate to suggest such an involved solution for a simple engine rebuild. I'm looking forward to future updates on the success of your inovation.
Adios, David
andyman":3ajjtd7u said:
8) yes, remember that most things designed to improve fuel economy also improve performance as well. the big difference is going to be things like cam timing, and fuel systems where you will have to choose performance over fuel economy. for instance in cam selection. where you would pick a cam that works best in the 1000-4500 rpm range for fuel economy, you would pick a cam that works best in the 1800-6000 rpm range for performance. same with fuel and induction systems. a single carb, or two very small carbs work nicely for fuel economy, where as two or three larger carbs are good for performance. for instance an engine built for economy would use a smaller one barrel carb such as the autolite 1100, or even two of these carbs to balance the fuel mixture, or you might use a small staged two barrel carb such as the smaller holley/weber carb. for performance you might step up to two autolite 2100 two barrel carbs, three one barrel carbs, etc.
Howdy Back Andy:
And apologies to 250six for hyjacking his thread. But Andy asks- "im building a 1972 250 for performance......should i also deck the the block if im using 255 pistons with the valves eyes? im looking for hp not economy?" Yes. Optimizing the Quench effect should be a goal in any engine build where optimun efficiency is the goal. Is it nessecary? No, but it is best.
Using the 255 V8 pistons and milling a "D" shaped dish that matchs the shape of the combustion chambers in the head gives an opportunity to not only tighten deck height, but also increases the quench to bore ratio, farther increasing the benefit of quench effect. FYI- Quench effect squeezes the intake a/f mixture out of the recesses of the bore on the compression stroke at high velocity, creating turbulence. The stock dished piston does these to some degree, assuming zero deck height, but the mismatch of chamber to dish shape is less than ideal. Matching the dish shape to the combustion chamber shape creates a more complete and efficient combustion and burn of the intake charge. Milling the "D" shaped dish into the V8 pistons does two things; it helps to reduce CR to a more manageable level (10:1 or less) and increase the ratio of quench are to bore. Getting to zero deck height is a critical part of achieving an efficient quench.
Adios, David
And apologies to 250six for hyjacking his thread. But Andy asks- "im building a 1972 250 for performance......should i also deck the the block if im using 255 pistons with the valves eyes? im looking for hp not economy?" Yes. Optimizing the Quench effect should be a goal in any engine build where optimun efficiency is the goal. Is it nessecary? No, but it is best.
Using the 255 V8 pistons and milling a "D" shaped dish that matchs the shape of the combustion chambers in the head gives an opportunity to not only tighten deck height, but also increases the quench to bore ratio, farther increasing the benefit of quench effect. FYI- Quench effect squeezes the intake a/f mixture out of the recesses of the bore on the compression stroke at high velocity, creating turbulence. The stock dished piston does these to some degree, assuming zero deck height, but the mismatch of chamber to dish shape is less than ideal. Matching the dish shape to the combustion chamber shape creates a more complete and efficient combustion and burn of the intake charge. Milling the "D" shaped dish into the V8 pistons does two things; it helps to reduce CR to a more manageable level (10:1 or less) and increase the ratio of quench are to bore. Getting to zero deck height is a critical part of achieving an efficient quench.
Adios, David
Quench, also known as "mechanical octane". 
A stock 250 at 9.0:1 may detonate badly while a 9.8:1 motor with the same timing and fuel could be good.
On a 250, rather than open up the pistons, I would use 255 pistons, zero deck the block, then open up the chambers by relieving the chamber all the way out to the cylinder wall. That will manage compression and get decent quench while improving the breathing.
And while you're at it, you need to work the valve pockets and ports. The 250 will need all the breathing room it can get.
A stock 250 at 9.0:1 may detonate badly while a 9.8:1 motor with the same timing and fuel could be good.
On a 250, rather than open up the pistons, I would use 255 pistons, zero deck the block, then open up the chambers by relieving the chamber all the way out to the cylinder wall. That will manage compression and get decent quench while improving the breathing.
And while you're at it, you need to work the valve pockets and ports. The 250 will need all the breathing room it can get.
Lots of good info. It sounds like the 255 pistons and zero deck are needed in a 250 build-up. I could rework the chamber in the head to reduce compression. I need some recommendations on a economy cam. I have the Comp cam book, they have a 252H with206/206 at 50 and lift 425/425; and a 260 H with212/212 at 50 and lift 440/440, both at 110LC. I have heard of an Erson cam in between these two. Would a dual pattern cam be better? Any suggestions?
250six_71":x8puh1a2 said:
8) these are the cams i recommend, they are ground by clay smith and available from www.classicinlines.com i think they are better choices than the cams you have looked at as these are designed specifically for the small six rather than lobe designs that have been pulled off the shelf and used in a variety of engines.
1: CSC-264-HSP-12
2: CSC-264-HDP-10
3: CSC-264-HDP-12
follow this kink to these and other cam choices;
http://classicinlines.com/products.asp?cat=115
i prefer dual pattern cams for our little engines over a single pattern design, unless you are going to use classicinlines aluminum head, as they favor exhaust flow which these engines need more than intake flow, even though the intake side is also rather choked off.
I got the 250 on an engine stand yesterday, and took off the head. I will be putting the direct 2 bl mount on the log head. I checked the Classic Inlines site, and they were out of stock. Maybe some elves will make some more soon.
Today, I took one piston out to measure the bearings, they were stock size, but worn down to the copper.
When the piston came out, the top ring was in 2 halves, been running like that for quite a while , by the looks
of the wear on the rings and the piston lands. No wonder the pertronic didn't help it!
The motor will need an over bore and some new pistons, I'll use those 255 pistons. The bearings look original, and have no oil groove.
Just a small hole in the bearing. Do the new bearings have a groove?
Today, I took one piston out to measure the bearings, they were stock size, but worn down to the copper.
When the piston came out, the top ring was in 2 halves, been running like that for quite a while , by the looks
of the wear on the rings and the piston lands. No wonder the pertronic didn't help it!
The motor will need an over bore and some new pistons, I'll use those 255 pistons. The bearings look original, and have no oil groove.
Just a small hole in the bearing. Do the new bearings have a groove?
I just went and measured it. I started with a 1/8 welding rod, measured at .125, and a steel straight edge, wouldnt fit.
Then I used a .070 material and feeler gauges to make up to .100. Very close, another feeler gauge of .02 filled the gap.
So, .102 deck hight , piston pin is at 1.500, this engine had the original steel shim gasket, and has original bearings and pistons
(badly worn). Those are the rod bearings I looked at and they don't have grooves.
I am still looking for a cam, for economy. Isky has a 256 Supercam , pretty small, but might be OK.
Then I used a .070 material and feeler gauges to make up to .100. Very close, another feeler gauge of .02 filled the gap.
So, .102 deck hight , piston pin is at 1.500, this engine had the original steel shim gasket, and has original bearings and pistons
(badly worn). Those are the rod bearings I looked at and they don't have grooves.
I am still looking for a cam, for economy. Isky has a 256 Supercam , pretty small, but might be OK.
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