Chicken vs. Egg: Choosing a cam AND a gear ratio

[ericwevans]

On the other hand, let's say you will run with the TKO with the 0.64 overdrive and the 3.50 gears, a 28" diameter tire, and your cruise speed is 55 MPH.  Then, your final drive ratio will be 3.5 X 0.64, or 2.24:1.  This will make your cruise RPM 1478.  A high duration cam will NOT be happy at that RPM, and will make the car run poorly at cruise.

Jay is preachin' the gospel there.  Since I build my stuff mostly out of what is laying around my garage, my truck got a shiny new Tremec 3550 that had been sitting around for 15 years.  With it's .68 OD and the trucks 3.50 rear gear and 32" tall tires it doesn't like OD until it is doing 80 so I never use it.  It would love a set of 4.56 gears which one day she'll get.

[unclewill]

I'm running a close ratio top loader with 3.50 gears and 26.5" tires.  I live in a very rural/remote area where the Interstate speed limit is 80mph and sightlines are measured in miles.  I am very happy with my setup, but I spend a lot of time high speed cruising.

[blykins]

A cam that works for a 390 with good heads will not work for a stroked 390 with factory heads.  Displacement, head flow, and a number of other things will change cam specs drastically.

You need to decide on the engine first before ordering a camshaft.   I would also recommend contacting an FE engine builder for a cam selection as most camshaft companies have not seen an FE in person, much less touched one, much much much less dyno'd 100 of them. 

As for the rear gears, a 3.73 is a real nice all-around gear for a Cobra.  It will work very well for a 4-speed on country roads (read 55-60 mph with blasts) and it will work very well with a TKO for 55-60 (and blasts) plus highway driving. 

If the Cobra guys need a rear gear, that would be my recommendation.  Then when you get that nailed down, decide on what you really want to do with the engine, then have a camshaft chosen for you. 

[HTM101]

120mm,
This will be my 23rd year with a car like the one you'll build.  Mine has a wide-ratio Toploader, the rear ratio is 3.70 and the tire diameter is 26.5".

99% of my driving is on roads identical to those you describe.  In that the roads I travel on are posted at 60 mph or less, I have never needed or wanted a 5th gear.  However, when I occasionally drive on an interstate, an overdrive would be good.  60 mph is 3,000 rpm, 70 mph is 3,500 rpm and at 80 mph the engine is happily roaring at 4,000 rpm.  But, I don't enjoy driving a Cobra on the interstate. Kind of monotonous.   

[Autoholic]

If you ever want to drive near 60 MPH, you'll want an overdrive. Your left ear will thank you for it, or want to kill you for not having it. A Cobra with sidepipes, at even mid RPM's is loud. Like jet taking off loud.

Since you're talking about a Ford 8.8" rear, I'm assuming you're looking at a FFR?

As for the right gear ratios, I'd recommend playing around with a gear calculator to help you decide. I'll happily insert a plug for the one I created and can be downloaded here... and you must download it in order to use it. Typical rear tire size for a Cobra with Goodyear Billboards is either 265/55R15 or 275/55R15. The actual size is listed as 26.5 x 10.5 x 15, but my calculator uses the tire size in modern terms.

https://drive.google.com/open?id=0B3iyR3ENvnVsTTdkQ2ZZenFkTlk

[120mm]

A cam that works for a 390 with good heads will not work for a stroked 390 with factory heads.  Displacement, head flow, and a number of other things will change cam specs drastically.

You need to decide on the engine first before ordering a camshaft.   I would also recommend contacting an FE engine builder for a cam selection as most camshaft companies have not seen an FE in person, much less touched one, much much much less dyno'd 100 of them. 

As for the rear gears, a 3.73 is a real nice all-around gear for a Cobra.  It will work very well for a 4-speed on country roads (read 55-60 mph with blasts) and it will work very well with a TKO for 55-60 (and blasts) plus highway driving. 

If the Cobra guys need a rear gear, that would be my recommendation.  Then when you get that nailed down, decide on what you really want to do with the engine, then have a camshaft chosen for you.

The weight of your sum of experience is hard to refute. Plus, everyone elses' advice is triangulating on this was well.

So, FE-powered Cobra replica with a 3.73 rear diff ratio, it is!

I still have some time, so will continue to ruminate on engine choice.

120mm,
This will be my 23rd year with a car like the one you'll build.  Mine has a wide-ratio Toploader, the rear ratio is 3.70 and the tire diameter is 26.5".

99% of my driving is on roads identical to those you describe.  In that the roads I travel on are posted at 60 mph or less, I have never needed or wanted a 5th gear.  However, when I occasionally drive on an interstate, an overdrive would be good.  60 mph is 3,000 rpm, 70 mph is 3,500 rpm and at 80 mph the engine is happily roaring at 4,000 rpm.  But, I don't enjoy driving a Cobra on the interstate. Kind of monotonous.   

A point for the TKO, then.

If you ever want to drive near 60 MPH, you'll want an overdrive. Your left ear will thank you for it, or want to kill you for not having it. A Cobra with sidepipes, at even mid RPM's is loud. Like jet taking off loud.

Since you're talking about a Ford 8.8" rear, I'm assuming you're looking at a FFR?

As for the right gear ratios, I'd recommend playing around with a gear calculator to help you decide. I'll happily insert a plug for the one I created and can be downloaded here... and you must download it in order to use it. Typical rear tire size for a Cobra with Goodyear Billboards is either 265/55R15 or 275/55R15. The actual size is listed as 26.5 x 10.5 x 15, but my calculator uses the tire size in modern terms.

https://drive.google.com/open?id=0B3iyR3ENvnVsTTdkQ2ZZenFkTlk

Thanks for the calculator.

Well, you are going to have to decide what you want.  Yes, gear ratio and cam duration are interdependent.  Decide what rpms you are going to be at most of the time,  or are most concerned with.  You know the formulas so you can figure this out yourself.  Then, a cam manufacturer can help you out.  They can't tell you what you want.

You also have two different cubic inch possibilities, too. Maybe between 395 and 445 cubic inches. 

There are so many factors.  You have to narrow it down so they can help you.   

JMO,

paulie

This is good info, because I really hadn't realized this key aspect. Thanks!

Drew

[120mm]

I'm going to start another thread addressing the engine choice. Brent is right; cam choice will be different for either build.

Thank you for your help!

[My427stang]

Jay has very good points, and et me play on here too.  BTW I answered similarly on your engine post

1 - You need to look at 1st gear compound ratio AND 5th gear
2 - A decent rule of thumb is 10-12:1 in 1st gear (depending on the weight of the vehicle and cam, also called starting line ratio) and 2.6-3.0 in 5th, for a street vehicle, more dependent on torque cruve of the engine
3 - I like to look at it this way "You have to think about getting 1st gear right, then 5th you need to get close"
4 - With that in mind, cam to the 1st through 4th gear use, and look at 5th, more than likely it's fine, but if not, lean back the other way or play with intake centerline

So here is an example, my 489 FE was a 3.70/TKO-600 combo, 28 inch tall tire, 10.62:1 SLR, 2.36:1 5th gear.  That setup was very fast, no concern with torque even with a car MUCH heavier than yours (70 Mustang with A/C, PS, etc)  However, the 2.36:1 was too tall on the highway.  It didn't cause problems, but I found myself driving real fast when I was with a group.  I changed to 4.10s, and my mileage went up, acceleration went up, and 5th gear was in a more friendly speed and still can go faster than you'd imagine and old Mustang should go. FYI, my SLR is now 11.76 and 5th is 2.63

Remember, an RR means road racing, not a great match for a heavy car IMHO, I like to go deep 1st and a non-RR tranny.  It allows you to be a maniac, then drop into 5th and drive away.

However, a Cobra is different.  It is light, and too much 1st gear will likely take away from performance.  So an RR allows you to controllably launch a very light car with a lot less rear gear, and then you can still have a usable 5th.

I would expect that the lowest gear you'd want is a 3.73 and you'd likely be better with a 3.50 gear.  (10.04:1 SLR, 2.87 5th with the 3.50)  Now, the 3.73 is not TOO deep, but, with the light Cobra, you don't need more 1st, and that same weight affects what it will like in 5th, so IMHO, I'd take advantage of the light car and get below 3:1 in 5th.  However, it's sort of splitting hairs now, because 3.50-3.73, really is all good, either one will run like a scalded ape.

[120mm]

Jay has very good points, and et me play on here too.  BTW I answered similarly on your engine post

1 - You need to look at 1st gear compound ratio AND 5th gear
2 - A decent rule of thumb is 10-12:1 in 1st gear (depending on the weight of the vehicle and cam, also called starting line ratio) and 2.6-3.0 in 5th, for a street vehicle, more dependent on torque cruve of the engine
3 - I like to look at it this way "You have to think about getting 1st gear right, then 5th you need to get close"
4 - With that in mind, cam to the 1st through 4th gear use, and look at 5th, more than likely it's fine, but if not, lean back the other way or play with intake centerline

So here is an example, my 489 FE was a 3.70/TKO-600 combo, 28 inch tall tire, 10.62:1 SLR, 2.36:1 5th gear.  That setup was very fast, no concern with torque even with a car MUCH heavier than yours (70 Mustang with A/C, PS, etc)  However, the 2.36:1 was too tall on the highway.  It didn't cause problems, but I found myself driving real fast when I was with a group.  I changed to 4.10s, and my mileage went up, acceleration went up, and 5th gear was in a more friendly speed and still can go faster than you'd imagine and old Mustang should go. FYI, my SLR is now 11.76 and 5th is 2.63

Remember, an RR means road racing, not a great match for a heavy car IMHO, I like to go deep 1st and a non-RR tranny.  It allows you to be a maniac, then drop into 5th and drive away.

However, a Cobra is different.  It is light, and too much 1st gear will likely take away from performance.  So an RR allows you to controllably launch a very light car with a lot less rear gear, and then you can still have a usable 5th.

I would expect that the lowest gear you'd want is a 3.73 and you'd likely be better with a 3.50 gear.  (10.04:1 SLR, 2.87 5th with the 3.50)  Now, the 3.73 is not TOO deep, but, with the light Cobra, you don't need more 1st, and that same weight affects what it will like in 5th, so IMHO, I'd take advantage of the light car and get below 3:1 in 5th.  However, it's sort of splitting hairs now, because 3.50-3.73, really is all good, either one will run like a scalded ape.

I love the amount of detail you put into that. Learning has occurred.

Drew

[My427stang]

I love the amount of detail you put into that. Learning has occurred.

Drew

Except for all the spelling/typos LOL

[blykins]

Drew, I went 3.70 on my own Cobra with a TKO trans.   (Don't have the car anymore, sold it when I got married.)

First gear is a craps shoot in a Cobra, doesn't matter whether it's a 2.73 or a 3.73 gear.  None of them can use a full throttle (or even much less) application of 1st gear (even with a 225hp 302 donor).  I like to gear the Cobras so that they absolutely will pull your guts out around the top of 2nd gear, through 3rd, etc. 

My Fox Mustang with a 302, 3.27:1 1st gear, with a 4.10 rear gear was really short in 1st gear.  However, the engine was done by 4500.   This echos what Ross was stating about the vehicle weight, 1st gear ratio, and engine rpms.

[bn69stang]

3.50 s to 3.70 s with a TKO 600 with .82 5th .. the 3.50 s become 2.87 s and the 3.70 s become 3.03 s .. that will keep you around your w speed rating  on your tires on a full assault of the throttle , just a thought .. Bud

[120mm]

Drew, I went 3.70 on my own Cobra with a TKO trans.   (Don't have the car anymore, sold it when I got married.)

First gear is a craps shoot in a Cobra, doesn't matter whether it's a 2.73 or a 3.73 gear.  None of them can use a full throttle (or even much less) application of 1st gear (even with a 225hp 302 donor).  I like to gear the Cobras so that they absolutely will pull your guts out around the top of 2nd gear, through 3rd, etc. 

My Fox Mustang with a 302, 3.27:1 1st gear, with a 4.10 rear gear was really short in 1st gear.  However, the engine was done by 4500.   This echos what Ross was stating about the vehicle weight, 1st gear ratio, and engine rpms.

That is great nuanced info.

I don't drag race, or go light to light. Ever. 1st gear is just a way to get moving.

I hadn't thought about maximizing the potential of the 2-3-4 gears. Which are the gears I'm mostly likely to use.

I'm going to start putting some theoretical parts together for a stroker build. But that's the subject for another thread.

[Heo]

I can tell you this I have a Hotrod with a 160 something HP 2v Y-Block
With a Wide ratio toploader, 3,50 rearend. Weight Is probably about same as the Cobra
235 75 15 reartires  First gear is to low. you jump around like a frog if you not
give full throttle and sidestep the clutch and hang on and Cary the front wheels
I always starts in 2nd rev it out and shift to 4th. Cruises comfortably around 60-70 mph
With open lakestyle headers sound is not to bad with no load it almost "quiet" the
wind around the windshield is a bigger problem

[Autoholic]

Something else to think about, especially if you're using my gear calc...

1. The higher the top speed in a gear, the slower it will accelerate. Assuming other variables like power output and tire size are the same in a comparison.

2. The lower the top speed in a gear, the faster it will accelerate. Assuming other variables like power output and tire size are the same in a comparison.

3. Given points 1 & 2, choosing a set of gear ratios becomes a trade off of top speed, acceleration and street manners.

For a Cobra, it's going to be a very fast car even with a "slow" gear ratio configuration. Something I like to pay attention to with the gear calc is the % drop in RPM and RPM after the shift. You want the gear ratios to give you a predictable, roughly consistent drop in RPM from shift to shift. This will make it a little easier to predict how a Cobra will behave, because you'll learn the power curve of the engine and with a relatively consistent drop in RPM's, you have the same RPM spread between shifts. The more predictable you can setup a gear ratio, the better. It's nice to know your shift will always drop say 3,000 - 3,200 RPM. I also try to get my highest gear to have the lowest RPM at say 75 MPH, without getting too crazy.