Rear gears for camshaft swap - Ford Mustang Forum
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post #1 of 3 (permalink) Old 12 Hours Ago Thread Starter
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Rear gears for camshaft swap

How undriveable would an essentially stock 1999 mustang gt be with 270/274 .55/.55 Comp Cams and the stock rear gears on a manual transmission.

Not planning on doing this, just wondering.

On top of that though, for that cam profile on a daily driver, would I be better suited to 3.73 or 4.10 gears? Most driving is 35-55 mph with a 3 hour trip at 70-80mph twice a month or so.


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post #2 of 3 (permalink) Old 10 Hours Ago
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Well, to start with, Comps QA is terrible and would not be anywhere on my choice pick list...secondly, the advertised duration is just that...advertised and not the true spec or ramp profile of the cam..... could range from smooth to oh crap in traffic.....only the cam grinder can tell you. As far as gears, assuming you have 3.55' 4th gear, that would equate to approx. 400 rpm difference per .25 gear with the 3.73....a lot of $ for no little......4.10....about 1000 rpm difference....take 20% for're about 800 rpm higher in 5th.

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post #3 of 3 (permalink) Old 9 Hours Ago
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Matt, it's a good question, but the answer isn't simple. Basic math would say that since the 4.10 is spinning about 9% faster than the 3.73 gears, you'd get 9% worse gas mileage and 9% more torque multiplication, but this is the real world and like so many other things, it's not that way.

Your engine won't have to work as hard to turn the rear axle, but it will be spinning faster. That means you use less throttle, but more RPM. In the end, you'd just have to try it to find out. I'm certain you'd lose a few MPG, but hard to say how many. My best guess is between 2 and 5 mpg, from the gear change alone. Cam will make you blow a lot more fuel out the exhaust, so it's hard to guess how much that would slaughter your mileage.

Another consideration is your transmission's first gear. If you are already geared steep enough that you don't use it much, then adding more gear just makes it that much less useful. On the other hand, if you're in 1st at 60 mph, the steeper gears would make a lot of sense. A lot of times, swapping gears just one step like this is pretty much useless and not worthwhile compared to the expense, unless you're fine tuning for track use. Don't subscribe to the hype of "But what if I did _BLANK_ to my car?" in this case.

Or do! If you really feel the need for a holy terror that's more of a weekend car, and less of a daily driver, get with it, and start swapping parts. You've gotta start somewhere, right?

But for street use, your car is all about average power, not peak. Idle to redline. Many cars with hairy cams seem to feel faster as you get on it, but that's often because you're robbing the bottom end to improve your peak horsepower. Since you intend to daily this, I have to think that the expense of cams and rear gears would be counterproductive to what you're using it for. You'd lose driveability, mileage, and throttle response, in favor of a little harder pull on the top end. Don't forget, you'll need to spend some money tuning the thing once you make the changes, too. A hotter cam will significantly mess with your fuel curve.

So bottom line here: What do you hope to accomplish, and how does your car drive NOW? You've got the answers.

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