Author Topic: Chicken vs. Egg: Choosing a cam AND a gear ratio  (Read 12108 times)

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120mm

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Chicken vs. Egg: Choosing a cam AND a gear ratio
« on: March 17, 2017, 07:38:41 AM »
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.
« Last Edit: March 17, 2017, 08:17:46 AM by 120mm »

machoneman

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #1 on: March 17, 2017, 08:21:12 AM »
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.
Bob Maag

unclewill

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #2 on: March 17, 2017, 08:39:37 AM »
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!
1969 Ford Cobra, 482 side oiler, BBM aluminum heads, FiTech EFI, Edelbrock 7105, Comp 292H, CR 4 speed, 9", 3.50

Drew Pojedinec

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #3 on: March 17, 2017, 09:05:20 AM »
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

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #4 on: March 17, 2017, 09:15:50 AM »
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.

Quote
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.

Quote
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.

Quote
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

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #5 on: March 17, 2017, 09:22:00 AM »
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

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #6 on: March 17, 2017, 11:26:35 AM »
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

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #7 on: March 17, 2017, 12:30:02 PM »
Sounds like a good street combo to me...
1969 Ford Cobra, 482 side oiler, BBM aluminum heads, FiTech EFI, Edelbrock 7105, Comp 292H, CR 4 speed, 9", 3.50

machoneman

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #8 on: March 17, 2017, 12:52:06 PM »
Jay's 2008 H/R article may be of help:

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

unclewill

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #9 on: March 17, 2017, 01:09:11 PM »
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. 
1969 Ford Cobra, 482 side oiler, BBM aluminum heads, FiTech EFI, Edelbrock 7105, Comp 292H, CR 4 speed, 9", 3.50

120mm

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #10 on: March 17, 2017, 01:20:51 PM »
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.

« Last Edit: March 17, 2017, 01:25:06 PM by 120mm »

unclewill

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #11 on: March 17, 2017, 01:29:21 PM »
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.
1969 Ford Cobra, 482 side oiler, BBM aluminum heads, FiTech EFI, Edelbrock 7105, Comp 292H, CR 4 speed, 9", 3.50

120mm

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #12 on: March 17, 2017, 01:42:08 PM »
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

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #13 on: March 17, 2017, 04:15:55 PM »
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

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #14 on: March 17, 2017, 04:18:13 PM »
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...
Jay Brown
- 1969 Mach 1, Drag Week 2005 Winner NA/BB, 511" FE (10.60s @ 129); Drag Week 2007 Runner-Up PA/BB, 490" Supercharged FE (9.35 @ 151)
- 1964 Ford Galaxie, Drag Week 2009 Winner Modified NA (9.50s @ 143), 585" SOHC
- 1969 Shelby Clone, Drag Week 2015 Winner Modified NA (Average 8.98 @ 149), 585" SOHC

   

ericwevans

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #15 on: March 17, 2017, 04:33:05 PM »
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.
Eric Evans

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1960 Falcon, 306 SBF

unclewill

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #16 on: March 17, 2017, 04:42:36 PM »
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.
1969 Ford Cobra, 482 side oiler, BBM aluminum heads, FiTech EFI, Edelbrock 7105, Comp 292H, CR 4 speed, 9", 3.50

blykins

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #17 on: March 17, 2017, 07:25:56 PM »
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. 
Brent Lykins
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HTM101

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #18 on: March 17, 2017, 07:44:29 PM »
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.   
« Last Edit: March 17, 2017, 07:49:46 PM by HTM101 »

Autoholic

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #19 on: March 17, 2017, 08:06:29 PM »
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
« Last Edit: March 17, 2017, 08:31:05 PM by Autoholic »
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120mm

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #20 on: March 17, 2017, 09:53:13 PM »
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

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #21 on: March 17, 2017, 10:48:03 PM »
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

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #22 on: March 18, 2017, 07:55:48 AM »
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.

---------------------------------
Ross
Bullock's Power Service, LLC
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, Erson SFT cam, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 461 cid FE, headers, Victor Pro-flo EFI, Comp Custom HFT cam, 3.50 9 inch

120mm

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #23 on: March 18, 2017, 08:15:08 AM »
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

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #24 on: March 18, 2017, 08:20:58 AM »
I love the amount of detail you put into that. Learning has occurred.

Drew

Except for all the spelling/typos LOL
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blykins

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #25 on: March 18, 2017, 08:39:20 AM »
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.
« Last Edit: March 18, 2017, 09:06:54 AM by blykins »
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bn69stang

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #26 on: March 18, 2017, 09:16:51 AM »
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
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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #27 on: March 18, 2017, 09:30:59 AM »
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.

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #28 on: March 18, 2017, 03:23:10 PM »
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



















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Autoholic

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #29 on: March 19, 2017, 12:18:20 AM »
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.
« Last Edit: March 19, 2017, 12:22:46 AM by Autoholic »
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unclewill

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #30 on: March 19, 2017, 09:34:29 AM »
You may find some Cobra replica owners near you who will take you for a ride in their cars so you may experience their setups firsthand, very valuable information indeed.
1969 Ford Cobra, 482 side oiler, BBM aluminum heads, FiTech EFI, Edelbrock 7105, Comp 292H, CR 4 speed, 9", 3.50

120mm

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #31 on: March 19, 2017, 11:02:23 PM »
You may find some Cobra replica owners near you who will take you for a ride in their cars so you may experience their setups firsthand, very valuable information indeed.

I'll keep an eye out for some Cobra replicas today, when I'm at work.


Heo

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #32 on: March 20, 2017, 01:02:48 AM »
 ;D ;D ;D



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Autoholic

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #33 on: March 20, 2017, 04:17:51 PM »
Where do you work? That does not look like anywhere in America. The street is far too unorganized.
« Last Edit: March 20, 2017, 04:19:27 PM by Autoholic »
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120mm

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #34 on: March 21, 2017, 01:24:33 AM »
Where do you work? That does not look like anywhere in America. The street is far too unorganized.

Lovely Herat, Afghanistan.

Yesterday was a dust storm and today was a rain storm. In a couple days it will be hotter than blazes.

Drew Pojedinec

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #35 on: March 21, 2017, 07:04:51 AM »
Ahh, I have many friends that "vacationed" there for extended periods.  They didn't spend that much time in town tho, they mostly hiked in the mountains.  :)

Autoholic

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #36 on: March 21, 2017, 07:02:20 PM »
Are you from Afghanistan or just working overseas?
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jayb

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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #37 on: March 21, 2017, 09:44:41 PM »
Joe, see his post in the new members thread.
Jay Brown
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Re: Chicken vs. Egg: Choosing a cam AND a gear ratio
« Reply #38 on: March 21, 2017, 10:46:15 PM »
Are you from Afghanistan or just working overseas?

I'm working there, temporarily.

I also belong to a Land Cruiser forum, and this is Land Cruiser heaven, so there's tons to talk about in reference to the vehicles.

FEs and Cobra replicas... meh, not so much. ;)