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Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1420957 Tue Aug 17 2021 07:39 PM
Joined: Mar 2014
Posts: 2,560
J
'Bolter
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.


Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: What Ford 9" rear-end ratio should I go with?
Hotrod Lincoln #1420967 Tue Aug 17 2021 08:38 PM
Joined: Dec 2020
Posts: 37
C
ColbyG Offline OP
'Bolter
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?

Last edited by ColbyG; Tue Aug 17 2021 10:32 PM.

colby

1951 Chevrolet 3100: Photos [photos.app.goo.gl]
–Engine: 219 to 250 to 350 (waiting to be dropped in)
–Currently: Readying a 1987 S10 frame for mounting the body.

1956 Chevrolet 6400
–Engine: moved the 250 from 3100 - newly installed
–Currently: Adjusting Brakes & Installing New Headlamp Switch

1963 Chevrolet C10 Fleetside
–Engine: EMPTY
–Currently: Sitting in Barn
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1420982 Tue Aug 17 2021 11:54 PM
Joined: Feb 2004
Posts: 23,246
H
'Bolter
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1421026 Wed Aug 18 2021 01:49 PM
Joined: Mar 2007
Posts: 1,397
T
Crusty Old Sarge
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.


Craig

Come, Bleed or Blister something has got to give!!!
59' Apache 31, 327 V8 (0.030 over), Muncie M20 4 Speed, GM 10 Bolt Rear... long term project (30 years and counting)
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1421043 Wed Aug 18 2021 04:51 PM
Joined: Mar 2014
Posts: 2,560
J
'Bolter
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.


Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1421062 Wed Aug 18 2021 06:42 PM
Joined: Feb 2004
Posts: 23,246
H
'Bolter
"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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1421253 Fri Aug 20 2021 03:12 PM
Joined: Apr 2005
Posts: 2,886
C
'Bolter
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.


Evan
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1421330 Sat Aug 21 2021 01:17 AM
Joined: Feb 2004
Posts: 23,246
H
'Bolter
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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