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

[120mm]

I'm in the process of planning an FE-powered Cobra Replica build. When I talk to the kit company about what rear axle ratio to get, they ask me about my engine; primarily the type and cam. When I research what kind of cam to put in it, everyone asks what my rear axle ratio is.

Assuming the transmission is either a Top Loader or TKO 600 RR, which have remarkably similar gear ratios, exc. the extra gear in the TKO, how do decide both, when neither is set in stone?

I live in a rural area, will probably seldom/never go on the interstate with this car; I may do some track day events, but will mainly terrorize country roads.

I am going to build a mild FE. My budget will allow either a 390 with a hotter cam, Edelbrock or Streetmaster heads, or a stroker with stock heads.

So what I want to do is to figure out my cam and rear gear for both engine builds, and think about which I really want before pulling the trigger. Would love to get help/advice from this informed body of people.

[machoneman]

I'll comment on all but the cam choice specs.

If you don't drive on the highways, the Toploader will do just fine and be a lot cheaper with say a Lakewood bell or a OEM bell, than a TKO and QuickTime bell. But don't think for a minute the two trannys are near identical as one gets to choose not only different 1st gear ratios with a TKO but also 5th gear ratio choices. With a Toploader, it's two 1st gear choices and none for 4th gear as all are 1:1 in fourth.   

On rear gear ratio, you could easily live with gears at 4:00+ simply since highway cruise mileage isn't an issue. But in a light, high powered Cobra, one must be careful as overpowering all but true slicks on such a car is very easy to do. That and ensuring that your overall 1st gear ratio X the rear gear ratio isn't so high that 1st gear becomes unusable. Think of a farm tractor, heavy truck etc. where the vehicle moves only 5-15 feet and the engine, in 1st, is already red-lined! Most agree a 10:1 ratio or a bit less is ideal.

On the cam, my only comment is this: to me it makes no sense to install a mild-to- barely wild cam. No highway, likely weekend short mileage driving = maximum cam with maximum rpms for your engine.

You may want to hit a few true Cobra sites (Club Cobra) to get more info on overall gearing x tire traction issues from actual owners with similar set-ups. JMO!

 



I'm in the process of planning an FE-powered Cobra Replica build. When I talk to the kit company about what rear axle ratio to get, they ask me about my engine; primarily the type and cam. When I research what kind of cam to put in it, everyone asks what my rear axle ratio is.

Assuming the transmission is either a Top Loader or TKO 600 RR, which have remarkably similar gear ratios, exc. the extra gear in the TKO, how do decide both, when neither is set in stone?

I live in a rural area, will probably seldom/never go on the interstate with this car; I may do some track day events, but will mainly terrorize country roads.

[unclewill]

I have a running 427 SO in the classifieds if you want it - make me an offer I may just take it!  It runs a Comp Magnum 280H which I find to be a great street cam with lots of midrange and a lumpy idle.  For a light street car around town I would focus on mid range torque 3-5k rpms - power where you can use it.  On the track or wide open roads maybe go for higher rpms.
Video:  https://www.youtube.com/watch?v=N5w1c6CMk80
I also have a Weber 8V intake for sale which would look awesome on a Cobra!

[Drew Pojedinec]

Will, I just watched your videos..... impressive scenery around there.
If I lived in your neck of the woods, everything would be getting 2.73 gears and I'd be getting arrested.

[120mm]

I'll comment on all but the cam choice specs.

If you don't drive on the highways, the Toploader will do just fine and be a lot cheaper with say a Lakewood bell or a OEM bell, than a TKO and QuickTime bell. But don't think for a minute the two trannys are near identical as one gets to choose not only different 1st gear ratios with a TKO but also 5th gear ratio choices. With a Toploader, it's two 1st gear choices and none for 4th gear as all are 1:1 in fourth.

I should've stated "Wide Ratio" Toploader. A WR Toploader and a TKO RR have nearly the exact same gearing. Of course the TKO has one more gear, but for argument's sake, they can be treated equally, since I'll probably not use 5th in a TKO very much.

 Lakewood no longer offers an FE to Toploader bell. Stock bellhousings seem a bit too sketchy for safety's sake, imo. So it seems a Quicktime is the only currently available choice. Unless someone knows more than I do and would care to chime in.

On rear gear ratio, you could easily live with gears at 4:00+ simply since highway cruise mileage isn't an issue. But in a light, high powered Cobra, one must be careful as overpowering all but true slicks on such a car is very easy to do. That and ensuring that your overall 1st gear ratio X the rear gear ratio isn't so high that 1st gear becomes unusable. Think of a farm tractor, heavy truck etc. where the vehicle moves only 5-15 feet and the engine, in 1st, is already red-lined! Most agree a 10:1 ratio or a bit less is ideal.

Since both 1st gears are around 2.8, that would make 3.25, 3.31 and 3.40 the winners in the common 8.8" Ford rear end ratios. My early Hipo Mustang ran 3.25s with a T-10, so that makes sense.

On the cam, my only comment is this: to me it makes no sense to install a mild-to- barely wild cam. No highway, likely weekend short mileage driving = maximum cam with maximum rpms for your engine.

I "get" how that might work with the 390 build, but I'd think a completely different cam would be appropriate for the stroker with stock heads.

You may want to hit a few true Cobra sites (Club Cobra) to get more info on overall gearing x tire traction issues from actual owners with similar set-ups. JMO!

I'm currently on all of them. And they are all over the map in the types of builds, to the point of being too diverse. Plus Cobra guys tend to really not to talk about "budget" anything. I figured there might be some old hands around here who know how to make "less than hand grenade" moderate build engines.

[120mm]

I have a running 427 SO in the classifieds if you want it - make me an offer I may just take it!  It runs a Comp Magnum 280H which I find to be a great street cam with lots of midrange and a lumpy idle.  For a light street car around town I would focus on mid range torque 3-5k rpms - power where you can use it.  On the track or wide open roads maybe go for higher rpms.
Video:  https://www.youtube.com/watch?v=N5w1c6CMk80
I also have a Weber 8V intake for sale which would look awesome on a Cobra!

I'm actually looking at a 390 shortblock with 10:1 pistons and a Crane H10 272/272 .533/533 cam already in it.

Looking at the specs, it is supposed to make good power from 2500 - 5000 rpm.

[chilly460]

Why would you not use 5th?  I run 4.10 in my Marauder with the TKO500, anything over 55-60mph is comfortable in overdrive, and I run a fairly tall 275/60/15 tire.  I imagine in a Cobra you run a shorter tire, and if you go with the road race 5spd with shorter .82ish overdrive you'd be right in the sweet spot for easy 60mph cruise but enough rpm to pull that light car around. 

[unclewill]

Sounds like a good street combo to me...

[machoneman]

Jay's 2008 H/R article may be of help:

http://www.hotrod.com/articles/ccrp-0808-ford-390-fe/

[unclewill]

With that combo I'd go 4.10 in town and 3.50 for the open road, with the top loader 4 speed.  You can use the stock style bell because you will be shifting before 5500 rpm anyway. 

[120mm]

Why would you not use 5th?  I run 4.10 in my Marauder with the TKO500, anything over 55-60mph is comfortable in overdrive, and I run a fairly tall 275/60/15 tire.  I imagine in a Cobra you run a shorter tire, and if you go with the road race 5spd with shorter .82ish overdrive you'd be right in the sweet spot for easy 60mph cruise but enough rpm to pull that light car around.

With that combo I'd go 4.10 in town and 3.50 for the open road, with the top loader 4 speed.  You can use the stock style bell because you will be shifting before 5500 rpm anyway. 

4.10 just sounds too low to me. It seems to me that 1st gear would become rudimentary.

Stock bells for cars are the same price as Quicktimes from what I can find. Stock bells for trucks are out there at decent prices, but am told they require adaptation to run in a car, but haven't seen an explanation of what adaptation is needed to get them to work.

I'd buy 3.55 gears as a possibility. I'll be running 26.5" tires, so a 3.55 ratio gets me 2700 rpm at 60 mph. That's not bad, actually.

Jay's 2008 H/R article may be of help:

http://www.hotrod.com/articles/ccrp-0808-ford-390-fe/

Their Street build is similar to what I'm thinking, though I don't need 500 hp. I'll be using stock rockers, and probably stock stroke. I'd be delirious with 400hp/450tq, frankly, as long as I can do it under budget.

Sounds like a good street combo to me...

At worst, I could try it out, and if I don't like it, swap in a new cam.

[unclewill]

Around town you want 0-100mph why gear it so tall?  If you were slogging down the highway for hours on end 2700@60 makes sense but around town 4.10s would be much more fun, especially with a mild 390 under the hood.  Just one man's opinion.

[120mm]

Around town you want 0-100mph why gear it so tall?  If you were slogging down the highway for hours on end 2700@60 makes sense but around town 4.10s would be much more fun, especially with a mild 390 under the hood.  Just one man's opinion.

Because I don't "do" "around town."

I live in a rural area, and like I said in an earlier post, this car's main purpose will be to terrorize rural roads. And maybe do some light "for fun" track days.

[plovett]

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

[jayb]

I think the different questions from the kit car company and the cam company relate mostly to your cruise RPM.  You want the engine to be happy at cruise, not "lumpy" like it may be at idle.  So, you need to figure out what speeds you will be most likely spending the most time at, and what size tires you will have, and then pick the gear ratio and cam from there.

For example, let's say you are going with the toploader, which is a 1:1 final drive.  You have selected tires that are 26" in diameter, and you figure your cruise speed will be 65 MPH.  Your engine RPM is calculated as (MPH X Gear Ratio X 336)/(Tire Diameter).  If you plug in 65 MPH, 26" tire diameter, and a 3.50 gear ratio, you will be at 2940 RPM at cruise.  Thus, you can pick a pretty healthy cam, because most cams will be running well at that speed.

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.

These are just examples, of course, but these are the kind of things you need to think about, and why the kit car company and the cam company are asking these questions...