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Posted By: ColbyG What Ford 9" rear-end ratio should I go with? - Wed Jul 28 2021 02:36 PM
Howdy!

I am putting a SBC 350 (it was bored out 30 over by the previous owner), a Borg Warner T56, and a Ford 9" rear-end in my 1951 Chevrolet 3100. It's all going on a 1987 Chevrolet S10 frame.

The T56 gears are 2.66, 1.78, 1.30, 1.00, 0.74, and 0.50.

I plan to have 20" rims on it with tires of a 28" or 29" diameter.

I need to rebuild the Ford 9" and I am wondering, what ratio you guys would recommend I put in the rear-end?

I don't plan on racing it, but making it a cruiser that will have a little bit of get-up to it if necessary. I would also like to have decent gas mileage. As we all know, it's not cheap right now, so that is somewhat important. Being those things said, I was thinking a 3.73, but wanted to get y'all's opinions before moving forward.

Any advice?
HMMMMM- - - - -big rim diameter, Band-Aid tires, and two overdrive ratios- - - - - -I'd suggest a rear end gear no higher than a 3.73, and a 3.90 or 4.11 would probably be better. Getting the final drive gear too high will result in only being able to use the OD on the Bonneville salt flats, or somewhere else where you've got miles to accelerate, no hills, and no speed limits.
Jerry
Posted By: TUTS 59 Re: What Ford 9" rear-end ratio should I go with? - Wed Jul 28 2021 04:20 PM
I would imagine it all comes down to what RPM power band your cam develops. If the gear is to tall it will lug the engine, to low and it works to hard. Use an RPM calculator to help find that sweet spot everyone looks for .

https://spicerparts.com/calculators/engine-rpm-calculator

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator
Posted By: ColbyG Re: What Ford 9" rear-end ratio should I go with? - Thu Jul 29 2021 03:14 AM
TUTS_59, how do I figure out more about the power band / cam? I actually know nothing about that.

And I don’t own any gasoline vehicles, so what RPM should I be shooting for on a SBC 350 around 70MPH?
Posted By: ColbyG Re: What Ford 9" rear-end ratio should I go with? - Thu Jul 29 2021 03:17 AM
Hotrod_Lincoln, you said you’d suggest “a rear end gear no higher than a 373” then you said “a 3.90 or 4.11 would probably be better.” Was there a typo in your original reply or am I reading that wrong? Thank you.
Posted By: Fox Re: What Ford 9" rear-end ratio should I go with? - Thu Jul 29 2021 04:53 AM
No, it’s not a typo. Gear ratios can confusing for some. Think of a bicycle back gear. The little gear spins VERY FAST when you’re in that gear = high speed.
The big back sprocket= power.


Put it in low! You want power and a big gear, hence a low ratio.


Remember this:

Low and slow= power...a big back gear ratio(further away from 1:1 ratio) —-4.11 : 1, 4.56: 1a low (slow and powerful) gear ratio, but a higher number. The low can be thought of as power.

High and fast= speed...a small back gear ratio (closer to 1:1 ratio)—-2.73 :1 , 3.23 : 1, 3.41: 1 ratio... a high(fast) gear ratio but a smaller number. The “high” can be thought of speed.

That’s why he mentioned racing at the Bonneville. Some of those cars can’t get going by themselves because they have ratios that are high. No power, but LOTS of speed.

Heed Jerry’s words, you’d be wise to!

Confused much?
Posted By: Fox Re: What Ford 9" rear-end ratio should I go with? - Thu Jul 29 2021 05:01 AM
[quote=TUTS 59]I would imagine it all comes down to what RPM power band your cam develops. If the gear is to tall it will lug the engine, to low and it works to hard. Use an RPM calculator to help find that sweet spot.


Think of the bike analogy again:

If you try to climb a hill with your chain on the little back sprocket (the speed gear) your legs are going to DIE because it’s so hard (engine lugging)!

If you try to go as fast as you can with the chain on the big back sprocket, you are going to be pedalling like a son of o’ gun and won’t be moving quickly at all (ENGINE SCREAMING)!
Posted By: TUTS 59 Re: What Ford 9" rear-end ratio should I go with? - Thu Jul 29 2021 12:41 PM
ColbyG,
it would certainly help if you knew what camshaft you had in the engine. Camshafts are developed for different load ranges across a given RPM band, you wouldn't want a race developed cam in a truck used for towing. Typically a SBC starts to make power at 1500-1800 RPM, with a midrange power band of 2400-3000 RPM. With an overdrive as high as you are considering it could drop the engine down to 1600-1800 RMP at speed. This setup would be fine on a flat open Interstate, now through in a few hills and your doing a lot of down gearing.

As an example, my 327 was built with a higher RPM cam (2200-6400 RPM), with a 3.08 rear and 27 inch and final drive of 1.00 my RPM at 65 MPH would be around 2500 RPM, change the rear ratio to a 3.42 and now were at 2750. Keep in mind that I built my engine for around town (traffic light to traffic light) and an occasional trip to our local drag strip.

The key to this is knowing what RPM range your engine was built for. It's time for HRL (Jerry) to come in and provide some knowledge.
Posted By: Joe H Re: What Ford 9" rear-end ratio should I go with? - Thu Jul 29 2021 12:56 PM
It takes a lot of torque to push a brick at 70 mph, your power band needs to peak at the rpm you are planning to run in while pushing said brick at 70 mph. Don't look at the horse power, look at where to torque is. I would want the motor be in 1800 to 2000 rpm range at highway speeds, so 3.73 - 3.90 - 4.11 would be about right. Carburetors also don't like low rpm in heavy trucks at 70 mph, the air velocity is pretty low so pulling the required fuel is pretty tuff. If you had a 454 or 500 inch Cadillac, then you could run around all day at low rpm due to the torque.
Posted By: TUTS 59 Re: What Ford 9" rear-end ratio should I go with? - Thu Jul 29 2021 04:23 PM
Here's a list of Chevy 350 "Stock" torque ratings. Just food for thought.

Year HP@rpm Torque@rpm(ft. lb.)
1975 145@3800 250@2200
'76-'77 145@3800 250@2200

'78-'79 170@3800 270@2400 350 V8 w 4 bbl

Corvette w 4 bbl (from Chilton's 1963-1982 Repair Manual)
1969 300@4800 380@3200
1970 300@4800 380@3200
1971 270@4800 360@3200
1972 200@4400 300@2800
1973 190@4400 270@2800
1974 195@4400 275@2800
'75-'76* 165@3800 255@2400
1977 180@4000 270@2400
1978 185@4000 280@2400
1979 195@4000 285@3200
1980 180@4200 255@2000
1981 190@4200 280@1600
1982 200@4200 285@2800 (TBI)


There were of course factory performance options so these are not the only data specs out there. Many of the later year 350's had more power as fuel injection became the norm.
Craig has done an excellent job of explaining torque, RPM, and why lots of people end up building a rig with totally incorrect gear ratios for the kind of driving they're expecting to do. "Lugging" happens when an engine is unable to accelerate when the throttle is opened wider because the torque the engine is developing can't overcome the drag caused by the weight of the vehicle, or the steepness of the hill it's trying to climb, etc. At that point, the driver needs to select a "lower" gear to get the engine back into the proper torque range to handle the load. Today's modern automatic transmissions with 8, 10, or more speeds allow the computer to figure out the proper level of RPM and torque to make the engine run most efficiently for the operating conditions and select the proper gear to keep the engine in its desired torque band. The person driving a manual transmission vehicle has to do that by experience, or "seat of the pants" driving, and it's easy to engage the wrong gear for the conditions at hand.

My father had a favorite saying- - - -"automatic transmissions are smarter than all drivers, and most mechanics!" That was true back in the 1950's and 60's when he was providing us kids with a place to live and food to eat by being the first guy in Nashville Tennessee to work on automatics, and it's definitely true today. Manual transmissions are entertaining toys for the guy who just has to do something with his right hand while driving other than scratch his personal equipment, but they're not the best at keeping the engine in the proper RPM and torque range.

A transmission with two overdrive ratios available where the driveshaft is turning faster than the engine will lend itself to overloading and lugging the engine down very easily. Don't go too "high" with the rear end ratio (numerically low, like 3.08- - - -2.73, etc. ) or you'll end up with an engine that's almost always running too slow to develop enough torque to pull the load. Performance (and gas mileage) will suffer terribly!

Dang it! I just fat-fingered a key on my laptop and erased two paragraphs! I was explaining how something as simple as changing the height of the rear tires on my daily driver (a Ram 1500 that gets 1k+ miles a week in several states) made a huge difference in being able to keep the transmission from constantly shifting in and out of OD and keeping the torque converter locked in. It also keeps me from getting speeding tickets, as the former 2K RPM "sweet spot" for cruising with the taller tires was 75-80 MPH! Now it's 65-70. I also get better gas mileage with the engine turning a little faster and staying in the proper torque band more of the time without being lugged down.
Jerry
Posted By: Doc.Hall Re: What Ford 9" rear-end ratio should I go with? - Thu Jul 29 2021 07:49 PM
Ahhh, Fox, You and I have had a lot of experience with bicycle racing. A very good analogy, in my younger years I pushed a 52 front with a 11 tooth rear. This was track bike racing in Chicago (a track bikers dream), my knees are still in fair shape which is a miracle. I broke my back in Dec. but, I'm still training to compete in the Spring Senior's race in 2022. You have to keep going. PM me I would love to hear your history. Ed
Back to the original question- - - -your 350's best torque range is going to be determined by the camshaft profile and the compression ratio of the engine. A "torquer" engine is going to have fairly short time when the valves are open on each cylinder (called "duration") and a moderately high "lift", the amount of distance the valve moves from its closed position on the cylinder head, and its fully open position that allows the gases to move in and out of the cylinder. Cams with a duration of 270 degrees of crankshaft rotation or less on the intake and exhaust valves will have a smooth idle, plenty of low sped and midrange torque and good all-around performance at variety of engine speeds. As the duration and lift increases, the power band shifts toward higher speeds and low and medium speed power diminishes. The two biggest mistakes rookie engine builders make is "Too much cam" and "Too much carburetor". Doing race engine stuff on a street engine is an exercise in futility, no matter how the hotrodders protest that "It runs great"- - - - -compared to what? For a long time, Chevy dealers offered a new buyer a free STD- - - -so they could have something that "runs"!
smile
Jerry
Posted By: coilover Re: What Ford 9" rear-end ratio should I go with? - Thu Jul 29 2021 10:38 PM
Years ago I stopped using high and low to describe gears. With ten people five would call 4.11's high geared and five would cal them low so I switched to calling them shallow or deep geared. With a 2nd overdrive of .50 and nothing radical engine wise I would go with a minimum of 4.11 and probably 4.56. Look up the rpm your engine is at maximum torque and then choose a gear that will hold it there at the speed you desire.
Posted By: TUTS 59 Re: What Ford 9" rear-end ratio should I go with? - Fri Jul 30 2021 12:06 PM
As Evan and Jerry have said torque is what moves you down the road, horse power just gets you there quicker. My 327 was rated at 320 ft lbs of torque @ 2400 RPM, so a 3.08 puts it in it's sweet spot. A 3.42 would give my better off the line power but my cruising RPM would be higher, it's a give and take sometimes.
Posted By: SimS Re: What Ford 9" rear-end ratio should I go with? - Sat Jul 31 2021 11:20 AM
Reminds me of another circle track saying: Horsepower determines how fast you can hit the wall; Torque determines how far you can get through the wall.

SimS
Posted By: Jon G Re: What Ford 9" rear-end ratio should I go with? - Sat Jul 31 2021 05:44 PM
I agree with Evan but I'd go with 4.56...with a little hesitation

With a 2.66 first and a .50 6th gear that's the only rear end ratio I can see working anywhere close to right. Unless you could find 4.71 gears. Those would be closer and once upon a time there were 4.86 gears for the 9 inch. 4.86 would be almost perfect in my opinion.

You can use the Tremec calculator (or there are about 100 other ones out there). Here's Tremec's:

https://www.tremec.com/calculadora.php

Plug in your desired rpm (I used 1900 and I believe that is an okay target in your case--2000 would be better for hills and wind), your tire diameter (I used 28), your axle ratio (I used 4.56) and the ratio for every gear in your transmission (you already posted those). With all the factors you mentioned and using a 4.56 rear at 70 mph you'll be turning 1900 rpm in 6th. That in my opinion is the lowest rpm you need to consider with a carbureted 350 and an AD pickup.

If you used 4.86 gears with 28" diameter tires, your rpm at 70 would be apx 2050 and at that point you're right where I think you need to be. However your starting line ratio would be 12.92 and that might be more than you want. However the original AD starting line ratio was 12.08, though so you are not too far off (that is in case you ever wanted to use it like a truck). If you don't care about using it as a pickup your ratio could run down to 10.5 or so. If you plan on putting 1000 pounds of "stuff" in the bed and driving without burning up your clutch, you ought to at least have an slr of 11.

If you want more reasons I think this is what you should use, please let me know.
Posted By: Joe H Re: What Ford 9" rear-end ratio should I go with? - Sun Aug 01 2021 01:00 PM
Those first four gears are going to go by really fast with a 4.56 rear ratio, you may not even need the first two gears. So you can gear it with low starting gears ratios, or low high gear ratios, you have to decide what type of driving you are going to be doing the most, or how much shifting you really want to be doing.
Posted By: 49choptop Re: What Ford 9" rear-end ratio should I go with? - Mon Aug 02 2021 08:51 PM
I have my truck set up with similar components except the motor is a stock gen II LT1 with fuel injection and rear tires are 26". Rear gear is a 3.50 and that is just right. First is not too low and 6th is a highway only gear. With your 28 or 29 rear tires 3.9 or 4.11 seems about right.
Posted By: MiraclePieCo Re: What Ford 9" rear-end ratio should I go with? - Thu Aug 12 2021 05:20 AM
3.73 will make you happy - good acceleration around town, but easy on the RPMs when you're in OD.
Posted By: panic Re: What Ford 9" rear-end ratio should I go with? - Thu Aug 12 2021 03:04 PM
3.XX numbers work well @ high cruising speed, but you'll be reaching down frequently between 50 and 70.
Posted By: ColbyG Re: What Ford 9" rear-end ratio should I go with? - Mon Aug 16 2021 07:52 PM
Man, you guys are so helpful with all of this info! Sorry I have been MIA the past few weeks.

I can give you guys the cam info. I have almost no idea what this stuff means, but this is what the card says (photo of it also attached):

Part #: 12-600-4
SN#: 308997-19

Engine: Camshaft, CS 279T H-107 T Thumper

Valve Adjustment: INTAKE HYD EXHAUST HYD
Gross Valve Lift: .479 .465
Duration @ 279 297
.006 Tappet Lift

Valve Timing OPEN CLOSED
@.006 INT: 36.0 BTDC 63.0 ABDC
EXH: 76.0 BBDC 41.0 ATDC

These specs are for CAM Installed at 102.0 INTAKE CENTER LINE

INTAKE EXHAUST
Duration @ .050 227 241
Lobe Lift: .3190 .3100
Lobe Separation: 107.0


Now that you have the cam info, is there any more info that y'all can offer? Also, I have decided that I'm going with an EFI system instead of the 4 barrel carburetor.

Thank y'all!

Attached picture Comp CAM SHAFT Info.jpg
Posted By: TUTS 59 Re: What Ford 9" rear-end ratio should I go with? - Mon Aug 16 2021 08:23 PM
That's a NASTY cam, lots of thump. A cam with those numbers will need a higher RPM power band to work correctly. It will work great with a lower gear, not so much with a tall ratio.
Posted By: ColbyG Re: What Ford 9" rear-end ratio should I go with? - Mon Aug 16 2021 09:30 PM
TUTS_59, can you explain a bit more of what you mean?

The guy that built it just told me that idle is about 800 RPMs and I'd need to go to about 3000 to 3500 RPMs for shifting. That seems high, but I don't know anything about CAMs, so that's why I'm asking for more info/education.

Thank you!
Posted By: TUTS 59 Re: What Ford 9" rear-end ratio should I go with? - Mon Aug 16 2021 11:58 PM
Your cam is developed to run at a higher RPM to create more top end horse power. A lower geared rear end will get the vehicle to that RPM range quicker, with a higher geared rear end it takes longer to reach that RPM range.

Your cam is similar to my cam in my 327, it's meant to have more top end horse power with the sacrifice of low
end torque. It takes a higher engine RPM to move the truck.

A lower gear would give you the performance the cam will designed for and the higher geared overdrive would allow you to cruise at highway speeds.
Start out with a 4.56 ratio rear end, and maybe you'll need to go a bit lower- - -possibly a 4.88. Get used to running the engine at or above 3K RPM all the time. If I were doing a dyno test on an engine with that cam, I wouldn't bother making a pull below 2500-2800 RPM, and I'd expect the peak HP to come in somewhere north of 7K. Below 3K or so, it won't make enough torque to get out of its own way. The name "Thumper" on the cam card should give you a clue what to expect. If you're planning to pair the engine up with an automatic transmission, use a torque converter with a stall speed around 2800-3000 RPM.
Jerry
Posted By: ColbyG Re: What Ford 9" rear-end ratio should I go with? - Tue Aug 17 2021 03:58 PM
TUTS_59, what rear-end gears do you run in yours? And would you recommend I do the same for simple highway cruising? I won't be racing it.

Hotrod_Lincoln, I will be running at Borg Warner manual 6 speed in it (the T56 gears are 2.66, 1.78, 1.30, 1.00, 0.74, and 0.50). So with a Thumper and since I'm not racing it, would y'all recommend I get another cam or will it do? I'm wanting to cruise it and hopefully get decent gas mileage (say 15-18 MPG) with an EFI setup (probably a Holley Sniper).
Posted By: TUTS 59 Re: What Ford 9" rear-end ratio should I go with? - Tue Aug 17 2021 06:24 PM
Colby, I'm not running an overdrive, I have a Muncie M20 4 speed. I have an 8.2 10 bolt rear end with 3.08 gears. I may go deeper later on 3.42 or so. With a 255/60 15 (27 inch tall) I will be about 2400 RPM at 65 MPH.

I built my engine to run street or strip. I would like to run it on the local drag strip once in awhile.
Posted By: ColbyG Re: What Ford 9" rear-end ratio should I go with? - Tue Aug 17 2021 06:55 PM
TUTS_59, ahhhh, I understand. With my cam, would you also definitely recommend a 4.56 or so in mine? I do believe that I will learn more once I get it all together and running, but also, I would like to save as much time and money as possible and if I don't have to change the gears from my first choice, that would be ideal.
I'm a little crippled up from a motorcycle wreck a few years ago, but I think I could outrun anyone who tries to GIVE me that cam and make me run it in a street machine. It's totally unsuitable for anything short of a bracket racer, or maybe a round track car. The accessories that it needs such as double valve springs, lightweight spring retainers, hardened pushrods, guide plates, oversize valves, and ported and polished cylinder heads, are going to cost somewhere over $1K, after the cost of the cam is factored in. Then you'll have an engine that belongs on a race track, not in a street machine.

The biggest mistake most rookie engine builders make is "too much cam"- - - - followed by "too much carburetor". You're halfway there.
Jerry
Posted By: Jon G Re: What Ford 9" rear-end ratio should I go with? - Tue Aug 17 2021 07:39 PM
Hi Colby, as far as gas mileage goes, please remember your engine is just an air pump. It spins around and doing so it sucks air in and forces it out. Air mixed with fuel that explodes at the top of each compression stroke. The more times you spin your engine around to travel a specific distance, theoretically the less your gas mileage will be because you're passing more air/fuel through it. There are some qualifiers to that statement, but if gas mileage is your goal low rpms with high torque is a good starting point. With that cam, you're going in the opposite direction (as Jerry...HotRod Lincoln and others) already said. That camshaft is going to push torque up into the high rpm band.

Take a look at your 5th gear---.74. Now take a look at your 6th gear---.50. Craig told you he runs around 2400 rpm at 65, and he is right about that...but Craig has no overdrive gear so he's running 1 to 1 in fourth. And so will you. It is when you go to 5th and 6th (if you can even use 6th gear) that things change. If you had Craig's 3.08 rear end at 2400 rpm in 5th gear, you'd be going about 88 mph and at 2400 in 6th, you'd be going about 130 mph. So obviously that rear end is worthless to you.

Now with a 4.56 rear end, at 2400 rpm in 5th gear you'll be going 60 mph and at 2400 rpm in 6th gear, you'll be going 88 mph. (assuming you have 28 inch tires, which is the lower of the two tire sizes you mentioned)

So if you assume 2400 rpm is going to be the happy spot for that camshaft, a 4.56 rear end will sort of work for you, but I honestly doubt you'll be doing much with 6th gear. You'll have to drop down to about 1900 rpm in 6th to be going 70 mph and that cam and your engine are not going to like that. Not even a little bit. What you're going to learn is that in 5th, your engine is running too fast for you to drive at 70 and in 6th it is running too slow. I made this same mistake once and it was a big mess. Ate gas like crazy and was genuinely irritating to drive on the highway.

Just think it all through. That cam is for a race engine, and if that's what you want go for it. If you want a 350 that will work with those transmission ratios you have, then get more of a stock cam (one that will pull your torque down to a lower rpm range) and at least a 4.56 rear end...4.11 if you must, but with a 4.11 in 6th gear at 2000 rpm (a nice target for good gas mileage), you'll be zipping along at 81 mph.

Now I've owned an S10 and while it was nice, it was not engineered to drive at speeds over 90 mph for any length of time unless you had a road as flat as a pool table with no wind. And that wasn't with an AD body sitting on it.
Posted By: ColbyG Re: What Ford 9" rear-end ratio should I go with? - Tue Aug 17 2021 08:38 PM
Hotrod_Lincoln, the engine is already built and ran well for the last owner (I saw it run), it's just not installed right now. Do you think I should stick with it or bite the bullet and try to give it a rebuild with a new cam?
Swapping to a milder cam doesn't involve a rebuild- - - -I can do it in 4-5 hours in a running vehicle, quicker if it's out of the car and on an engine stand. For a little better performance without killing all the low end grunt, I'd suggest the Comp Cams 268-H and a set of their matching lifters. That cam was still pulling strong on several dyno runs we did on a 350 with rams' horn exhaust manifolds and a Quadrajet in the 6500-7K RPM range and it had a good idle and impressive midrange power. That engine was in a fairly heavy-bodied Chevy Monte Carlo. The kid who owned it ate 5.0 Mustangs for lunch at stoplights on a pretty regular basis.
Jerry
Posted By: TUTS 59 Re: What Ford 9" rear-end ratio should I go with? - Wed Aug 18 2021 01:49 PM
I believe the key to making this all work for you is doing your research. You no doubt have a set goal in mind, something that has power and able to cruise at highway speeds with good gas mileage. It is possible to come up with a combination that meet most of these goals but your may need to compromise on some things as well. If you are set on using the 6 speed trans you will definitely need the lower geared rear end (4.56 would be a good starting point) the easiest compromise would be the cam shaft.

As an example: I am building my truck not as a daily driver but a Hot Rod, something that will see a lot of traffic light to traffic light and drag strip use. The compromise for me is the rear end gearing, a higher gear so that I can drive it on the highway occasionally. A good work around would also have been a 5 speed, that would have allowed me to use a lower gear and still drive it at highway speeds. I chose the 4 speed for other reasons, 1) it's COOL, 2) I had it already, 3) it was given to me by a family member that has since passed on. (we all have our reasons)

As Jerry has said it wouldn't take long to swap out the cam and replace it with the correct cam for your needs. Cam shaft changes are fairly routine with racers as they look for the right balance of performance. I think at this point the cam swap is your best route. Contact a reputable company (comp Cams, Crane...) and talk with one of their Tech Reps. Explain your needs and ask what cam the would recommend. It is very possible to have a setup that gets close to your original plan.
Posted By: Jon G Re: What Ford 9" rear-end ratio should I go with? - Wed Aug 18 2021 04:51 PM
While you're studying, you also can (a) figure out what year of 350 you have from the casting numbers and (b) go to the GM Heritage pages for a bunch of information (example: https://www.gmheritagecenter.com/do...hevrolet-Trucks/1969-Chevrolet-Truck.pdf) Change the year number in that https string to match the date of your engine...let's guess for example that you have a 350 produced in 1975. Then go to that year's info and you'll discover the 350 in 1975 produced 250 lb/ft of net torque at 2400 rpm...which is a pretty nice torque curve for what it sounds like you're trying to accomplish. It starts to flatten around 2000 and by 2400 has hit peak and begins the downward trip. If low rpm/higher torque is your goal, the way the 1975 350 was set up could be good for you. But if you look at the high torque option for 1969, you'll see the high torque engine without pollution equipment produced 355 gross or 310 net lb/ft at 2400 rpm with a curve that looks about the same as the one in 1975. More torque at the same rpms. And while you're on the 1969 pages, look up at the optional engines for the El Camino and you'll see the Turbo-Fire 350 had higher compression and was set up to produce a gross torque (I didn't see a net figure) of 380 at 3200 rpm...a move toward hot-rod status (7% more torque but at 33.33% more rpms--and clearly that won't help your gas mileage). And you'll learn if you study this stuff a bit in 1969 the overdrive 3 speed was available but only on C10 trucks and only with a 4.11 rear end ratio. My point is this: we have no idea what the person who set up your engine did, what he used, what he was trying to accomplish. However, if you want a little power and you still want gas mileage, you might find a nice compromise with a stock GM camshaft.
"There's no free lunch"- - - -an engine with the current "thumper" cam had better have domed pistons that give a static compression ratio of 10.5:1 or better. That doesn't mean the actual compression pressure will be that high- - - -the intake valve doesn't close until the piston is WAY up the compression stroke, so what's left of the cylinder volume has to be squeezed more tightly just to get back to stock compression pressure. Basing an engine building plan on camshaft profile alone is absolutely the wrong way to go about things, and correcting someone else's blunders by installing a milder cam might make things worse if the whole engine was built initially by someone who knew what he was doing. High compression pistons and a conservative cam profile will cause detonation that will melt the tops of the pistons in very short order, and the only way to avoid the problem is to retard the ignition timing to the point where the engine has very little low end torque. The factory muscle car engines of the 1960's were built back when 100+ octane gasoline with lots of tetraethyl lead in it was available at every corner gas station.
Jerry
Posted By: coilover Re: What Ford 9" rear-end ratio should I go with? - Fri Aug 20 2021 03:12 PM
Jon and Jerry have put the head of the nail directly on the center of the hammer face. Back in the muscle car era we "cured" many an ill running car by going to a shorter cam and a smaller carburetor. Our roundy track super modified was known as the one to beat and it was running a 288 Crane roller while most others were over 300 with the 305 Lunati being the top choice. The long cam cars were faster at the end of the straightaway BUT the flagman is exactly half way down the front straightaway so guess who got there first. Buzzing a long cam deep geared ride was acceptable with 30 cent gas in the sixties but kind of silly now.
Back in the early 1950's when every salvage yard was full of WW II surplus trucks with 270 GMC engines in them, the dirt track racers scooped them up and there were lots of go fast accessories like multi-carb intakes, Vertex magnetos, and 12 port heads available for them. Dad's flathead Ford powered 34 three window coupe never had much of a problem beating them, because they were so torquey they didn't quit spinning their wheels until they were halfway down the straight. By then the Fords were getting set up for the next turn. The Jimmy 270's and 302's did do very well at Bonneville, where they had room to stretch their legs- - - -especially the ones with the Wayne 12-port cylinder heads.
Jerry
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