Hot Rods Cam/gear ratio/converter/etc relationship

What I'm reading here is you're looking for "How to figure this out for myself", rather than just input on what to use. I think the guys really are trying to do that. They've told you, first figure out what you want to do with the car. Then match up components to try to meet that. Bigger cams with more lift and duration will shift the power range higher up the rpm scale. For that you need lower gears (higher numerically) and a looser converter. This will give you greater acceleration at the cost of higher rpm's on the highway. How big of a cam, how low of gears and how loose of a converter all depend on what you want out of it. You can go completely stock, slightly more aggressive than stock, or far more aggressive than stock, that's what you have to figure out. As far as what combination of components will work best for you personally, that's quite subjective, the only real way to determine that is to do it and see how you like it. It doesn't matter if some combination has worked perfect for someone else, they aren't you. You're going to have to make the best choice to start with, try it, and if it didn't meet what you're wanting, try something else. I don't think there is another way to learn this stuff. Read the magazines, read books on the subject, hang out on internet forums and read what people have to say, go to cruise nights and talk to owners, that's all good stuff and eventually things will start to come together, but at the end of the day you're going to have to pull the trigger on some components and put it together and see for yourself. Experience is the best teacher, and sometimes the only way to learn.

 

well said....

 

You nailed it perfectly! I just ordered some books and now I have some home work to do. I need to reach a certain level before I can even utilize the great advice I've been given. Thanks a lot Guys, keep the advice coming please.

 

2.73 gear, that goes against all the bench racer BS I hear. But that may be something I'm interested in. A mild high torque big cube mill with a properly spec-ed converter to make it happen. I'm very interested in any info on that you would share.

 

I put a lot of miles on my 55 chevy with 2.75 gears, and a mild blown 454. It didn't drag race very well that way, but moving up to just 3.25 gears put it in the low 11s. which is pretty quick for a car that was that mild and easy to drive so many thousands of miles.

Like we've said, you have to start somewhere, and see how you like it.

 

29" tire? Isn't that rather tall? 27" seems better. I have 215/70R15s, which are 26.9."

If the OP has a 56/w overdrive for the highway, why doesn't he put some real gears in the Chevelle? At the very least, I would consider using the factory ratio used behind L78s.

 

Anybody who figures out how to do what you ask on the first time, every time will have found the Holy Grail of hot-rodding.... LOLOL

 

Unless the differential is 100% locking, like a spool, they are designed to slip a bit when cornering. There are a lot of cars and trucks out there with limited-slip differentials that go around corners just fine. My 1969 L78 SS 396 Chevelle, 1988 Mustang 5.0, and my 2017 Ram 1500 are just three examples.

 

I had a couple of Pontiac wagon 400 tow cars that had a 2.56 rear. They would pull away from a stop just fine.
The key here was stock cam, stock, tight converter. No headers..No hot rod parts..It all worked together in harmony, as it was intended to do.

Edit: I forgot to say this was pulling an open trailer and 3600# car. 10mpg.

 

I put Eaton TurTrac differentials in everything or the equivalent, that I own that has two rear wheels, ant in the front of 4x4 and FWD stuff, too. My Model A has a DPI GoldTrack and the Wife's German road missile has a Quaife. These are all versions of the same function model.

They have seamless, no-surprises operation, require no additives or maintenance. They just provide sure-footed traction under all conditions except ice. No differential helps with ice.

 

When you mention peanut port heads, smog era, detuned engines come to mind. I wonder if the newer FI BBC heads might be a better alternative? Heck, a complete engine can be pulled at Pick and pull for 250/300 bucks.
I’d think you could build a fun driver with 400HP and 500ft/lbs with a mild cam under .500 lift, and run a stock converter and 3.08’s.
I only base my unprofessional opinion on the ‘70 LS5 I have with a TH400.
Pretty much idles like any other factory vehicle and can still wear out the rear tires with a factory stall and 3.08s.

I guess a looser converter to say a 2000 stall would be as much as I’d go, again for a cruiser and a fun ride.

Only personal advice I can give you, is stay away from a big cam for an “idling sound “.

Now, the only time I took my truck to a real strip, I ran 14.11 on a hot muggy night in Sacramento. Felt like I was running through a field of mud the whole way.

 

@scotty t many OT Chevelles came with BBC 400 and 12 bolt from the factory with decent power especially if you tweaked it a little. High 14 second 1/4mile was impressive then.
I don't know the actual figures but most came with a 3.08 or 3.3something rear gear.
Good all around car.

 

3.55 was standard for 1969 L78 396 Chevelle, but options ran from 3.07 to 4.88. 12-bolt rear end, of course.

 

One of my cars has a roller cam 350, muncie 4spd and a 3.36 rear gear. It runs/drives great and gets about 20mpg. BUT it only weighs about 2800#. I’ve never had a big block other than work trucks or built a hot rod with an automatic so this new territory for me. I’m in no hurry and I have a better budget than i’ve ever had so i want to study and learn so i don’t have to do things three or four times to get it right. I’m taking all the advice and learning all that i can before i start building, i’ve ordered two books that were recommended, they should get me up to speed

 

The formula is [RPM x Tyre Diameter] ÷ MPH, then ÷ 336 = overall ratio
So you need to be honest about what RPM you will cruise at [without your ears getting beaten to death]

Choose a converter that has a stall lower than the cruising RPM to avoid cooking the transmission.
Then calculate the ratio from the formula.

Then tune your engine to that [which requires being totally honest to the cam grinder]

 

I like the way you stress be totally honest! I’ve heard so many people talking so much shit about what they’re going to do with their cars.

 

Honesty is not only critical when communicating with those making the parts, you also need to be honest with yourself.

It doesn't matter how much horsepower and torque an engine makes at 5250RPM. If you spend a whole lot of time there, you are going to break the engine, if you don't mash the car into a guardrail first.

My freeway cruising RPM is 2250. I built my engine to have its torque peak at 2500, with a manual transmission.

Were it an automatic, I would have chosen a converter with a stall speed below 2250, with lockup.

 

The "standard method" for choosing a street cam, is to ask the cam grinder "what is the biggest cam you have available?"

 

Trouble maker

 

Using the online calculator, I calculated that my cruise rpm at 65 mph will be approximately 2200. I chose a torque converter stall speed of 2,000 because the rpm range of my cam is 1800-5000. Note that the Hughes torque converter stall speed is big-block based. It will be lower behind my .060 over Ford 292 Y-block. Stall speed tends to be higher behind big blocks and heavier vehicles. As stated, you don't want stall speed to be higher than your cruising rpm.

 

"My car runs hot, and gets 8mpg."

I cannot tell you how many times I have heard something to that effect, only to discover a computed 2500 cruise RPM mismatched to a 3500 stall RPM converter.

 

Scotty is this going in the Malibu I saw sitting under Patricks lift where he said he hated yellow cars?

 

I'm no expert and all I have for reference is an OT Mopar but . . . 440 big block, maybe 8.5:1 cr, stock iron heads, mild cam, 4 bbl, 727 with about a 2000 stall.

I originally had a 2.71 rear and the setup was more fun than the 318 it replaced but it was kind of a slug off the line in a 2600 or 2800 lb car. I swapped in a 3:55 rear end and that woke things up a bit . . . this is more fun . . . but now it kind of felt like she was wound a bit tight at 65+ mph. I finally bit the bullet and put a gear vendors behind it which drops the final ratio back down to a 2.71 range and gets out of the hole rather nicely. Just food for thought.

 

A 3.55 rear is a good compromise on the street. I run a 355 SBC worth about 425 HP, 10:1 with a Lunati 268/276 Voodoo cam. 4-speed has a 2.52 1st gear. My rear is a 3.89 and I'm turning 3200 RPM at 65. Any less gear and it lugs badly around town.