gear ratio coice

[scott h]

according to the tag on the diff my gear ratio is 3.31 however i rotated the driveshaft approx 3.75 times to one rotation of the wheel. does that make sense.
i would like some advise on changing this gear set. my motor likes to run best around 3500 rpm anything less than 3000 is a problem. here are my approx. speeds at 3000 rpm- 1st=22mph 2nd=34 mph 3=50 mph 4th =64mph and 5th =98mph :shift: I wind up spending alot of time in 2nd and 3rd gear and think that a 500 -700 or so rpm bump would get me into a higher gear quicker. if i were to regear what ratio shoud i choose.
i don't know if i could even use 5thwith a regear without making all the other gears too tall

[pgermond]

Scott if the tag says 3:31 and it came from Unique, more than likely it is correct. I'm running 3:31 with a 4-spd wide ratio top loader. At 70 mph I'm spinning 3,000 rpm. For me it's perfect. If you do a lot (and I mine a lot) of wide open road cruising, then a 5-speed might be okay. If not, I would look at changing the tranny rather than the rear gears.

my $0.02

[TurnpikeBoy]

It is a chore to change the gears in these rears; it would be far easier to swap a trans or to modify your engine setup. That said, the easiest thing of 'em all is just to use those lower gears in the trans you have.

I've got 4.09's in mine and truly love 'em. Tremec's first gear is admittedly way too low (3.27), so makes for an awful short first burst - but 2600 rpm 5th gear cruise at 75+ mph is just right. I do still cruise local streets in the lower gears and wind up using 5th only out on the open road.

I don't know what you run for power, Scott, but I have a 331 with AFR185's, the Ford Z-303 cam, and an Edelbrock Performer RPM; Torque below 2000 rpm isn't huge but can still spin 'em at will. And above 3000?.............


-Roger

[scott h]

i have a 351 W built by Dutton performance It has edbrock performer heads and victor jr. intake, and comp cams extreme energy cam, intake duration is 284 and exhaust is 296, their info page suggests 2800+ rpm stall. it really does need to run at about 3500rpm i was thinking that maybe a different cam would be a better way to go, i would probably need a new intake too. i sure to like the way this motor sounds and runs but would also like to run at less than 3000 rpm. Any suggestions on cam choices? I had a feeling that geting to the gears would be difficult. Seeing how i don't use 5th right now i was thinking why not just put a 4 speed in, is there significant diffrence compared to a toploader and a tremec in gears 1-4?
My dilema should point out to those of you planning your builds right now the importance of engine , tran and rear end combos. I was not experienced or knowledgable enough to make the best choice for my needs.

[TurnpikeBoy]

If those duration figures are base numbers, that timing, while biased toward top end power, is not extreme; numbers at the .050" lift territory would likely wind up around 226* - 230*. Again, biased toward top end power but certainly livable. There are many cams, even in the Extreme Energy series, that will go a looong way toward generating low end power; look for something in the 220* - 222* duration (@ .050" lift) range with lots of valve lift - like, say, .500" and up. This will lend itself toward a lower torque peak.

The Victor Jr. is a hi-rpm manifold by design - usually the 351's don't mind them because they put up more torque than the 302's to begin with, but bear in mind that manifold was designed for the upper end of the rpm scale. For your intents the RPM Performer Air Gap is a much, much better choice; they only give up power above around 5500 - 5800 rpm and even then it's only about 10 hp. They offset that by giving lots more power everywhere else; I think I recall a MM&FF article about a 302 picking up 30 ftlbs or so thruout the range with the Air Gap. You can find it, If I remember right, in the AFR website. I'll try to post a link to it.

Ask Dutton what kind of compression ratio they built - you can safely run 10.0 to 10.4 to 1 on 93 octane and aluminum heads. If they built it low (say under 9.5:1) you can pick up decent power here - have the heads cut to bring this up some, maybe a half-point or so. If it's already up there, then check your timing; something on the order of around 34* TOTAL (and check this - don't rely on guesswork!) should help things out down low. 15* or 16* initial (idle speed) timing usually works well; you will have to try it and see what works best for your car.

Last - what carb is on the car? If you've got a 750 cfm or bigger this could be changed now (to a 650) to pick up some bottom end power. If it's an 850, definately change it to something smaller. Usually the troubles here aren't that the carb is too big - it's that the tuning done on it focuses only on idle and wide open, leaving everything else way too rich. Smaller carbs are typically set up somewhat leaner to begin with, and the switch here can bring mixture strength back to more-useable ranges (with an improvement in economy, too...but that's relative).

Finally, while there is some difference in the gear ratios between a 4 speed and 5 speed, be aware that almost all (Richmond excepted) are 1:1 ratio in 4th gear. Tremec uses a pretty wide spread across all ratios, while some 4 speeds are much closer together but start with a higher (lower numerically) first gear.

Lots to think about. Cams, more than anything else, affect the operating range of the motor. Manifold next. All the others contribute to how much it puts out. Good luck and let us know what you decide on; sounds from here like you already have some thought into it and are headed the right way.

-Roger

[scott h]

thanks for all the good advise. i am running a holley HP 650. the original carb was a basic holley 650 double pumper,all sorts of problems, it ran like a jalopy super embarassing sputtering ,backfiring and missing like crazy, my mechanic put a carb from his race car on it and WOW lots more power. comp ratio is 9.75, suggested timing is base=13 advanced=36

[Justin Upchurch]

Scott,

What do you mean by a problem when running the engine under 3000 rpms?

It sounds like from the specs you gave that your motor should start to clear out around 1800 rpms and could sustain a cruise above 2000 rpms. Are you running a mechanical cam or a hydraulic version? If your engine is running rough or lazy under 3000 or 3500 rpms I think you may have some tunning issues.
My engine builder accidently sent me the .64 OD TKO instead of the .82 version. He later swapped it for me because I really wanted a useable 5th gear. My car has a 3.31 gear with the .82 TKO and I find it to be the ideal gearing for way I drive the car. If you are running a .64 OD in 5th you may want to consider a .82 overdrive instead or just runing in 4th until 75.
If you are spinning the motor under 6500 I would strongly consider sticking with the 3.31 gear, or maybe a 3.55 (the change to the 3.55 would not be much of a difference). If you are running the motor over 6500 you would want to go with 3.55 gear or higher.

Justin

[scott h]

it is a hydrasulic roller cam. i think the cam and intake combo is causing the problems of rough running under 3000 rpm, it feels like it is loading up. the victor jr is designed for rpm range of around 3800 and up. if i am not mistaken my cam need some higher rpms to get in its operational range too. i definitely run under 6500. my wife is going to buy me a RPM air gap manifold for my birthday, i will start with that and then consider need for cam.

[Russ Dickey]

it is a hydrasulic roller cam. i think the cam and intake combo is causing the problems of rough running under 3000 rpm, it feels like it is loading up. the victor jr is designed for rpm range of around 3800 and up. if i am not mistaken my cam need some higher rpms to get in its operational range too. i definitely run under 6500. my wife is going to buy me a RPM air gap manifold for my birthday, i will start with that and then consider need for cam.

Be sure to check your hood clearance. I've heard that the air gap makes it pretty tight on some cars.

[Justin Upchurch]

Scott,

I do not think the intake is your problem. I have a cam with a bit more duration than you, and I run a Super Victor jr intake. My car runs just fine as soon as it is above 2200 and I sometimes cruise in 5th at just 2000 rpms without any semptoms of the motor loading up.
The victor jr. Is a very good intake, the most significant difference between it and the air-gap is the single plain vs. dual plane. I don't think the intake change will make much of a difference if you think the motor is loading up.
If the motor is loading up on you, take a look at your ignition and carburation. What is the timmming curve on your distributor, is it an MSD, and do you have the curve kit that came with the distributor? Where are your float bowls set and how do the plugs look? Is this a mechanical or a vac secondary carb?

Justin

[scott h]

the carb is mechanical secondary, dist is ford (don't know how the advance curve is set up) with msd ign. box. timing is at 35 deg at full advance, i will need to check plugs and carb this weekend. i had the carb adjusted at time of purchase, carb adjusting may need to be my first step.

[Justin Upchurch]

Scott,

Start by getting a new MSD distributor, maybe your wife can get you that instead of the intake. The MSD has springs for the curve adjustment that are very easy to use. Does the Ford distributor have a vaccume advance module?

Are you still running the HP 650 carb?

[JeepSnake]

The Performer RPM Air Gap will result in hood clearance issues on the 427 SC body.

We put one on the 383 Windsor stroker we built for Dad's car, and have had to get a drop base air cleaner to clear the hood. I think it's a 1 1/4" drop, with a 2" element & lid from a 289 Hi-Po type cleaner.

[scott h]

the ford dist does not have vacuum port, i am still running the holley HP, the motor has had this issue since i got it. new carb, plugs and tunings by exp. mechanics have no effect on the need to be running over 3000rpm. i can't shake the feeling that the cam and intake combo are the main culprits. How does the RPM air gap clearance compare with a victor jr?

[TurnpikeBoy]

http://www.edelbrock.com/automotive/man_ford_351w.html

According to this link, the Victor Jr. for the standard 9.5" deck height block has a 5.75" manifold height to the carb pad; the AirGap has an (averaged) 5.75" height.

The potential alternative would be to use the Performer RPM (not AirGap) manifold which averages out to 4.8" at the pad.

One more thing....has anybody checked you out for vacuum leaks? These will cause all manner of rough-running at low engine speeds. Make sure all the vacuum ports in the carb base are plugged and that the intake manifold gaskets are intact.

-Roger

[ralphscott]

Perhaps far fetched, but it happened to me. The cam was not installed properly (crank advance) the cam advanced by one cog past the crank index mark. I had to take the front of the engine down and re-idex the cam with the crank so that the index marks were aligned. Made a huge difference in how the engine ran. I have an Edelbrock RPM preformer which comes on at 1500 rpm with little torque off of idle. Valve lift .572, Duration .296, lobe separation 108 degrees. Ask the builder if they applied crank advance, a lot of drag racers used to do that to get a little more out of the engine.

Ralph

[Justin Upchurch]

The air gap will fit, but you will need to run the drop base like Zach suggested. My intake is an inch taller than the airgap, I just run the drop base that Alan suggested with a three inch filter.
Roger may be onto something with the vacuum leak. With your combination you should be around 300 lb feet of tq by 2500 rpm if not more.
Can you describe the problem in more detail? Is the car just lazy under 3000, or does it miss and strugle wheny you give it gas under 3000.
Your intake and cam should be fine.

Justin

[scott h]

it is more like a miss, it can be compared to how a motor runs when very cold and choked up a bit, or if it were loading up.

[TurnpikeBoy]

Does it want to stall if you try to let it idle? Do you have to keep stabbing the throttle to keep it running? Does the throttle response, when first moving off in first gear, seem sluggish and not in sync with the amount of pedal you're giving it, but then run pretty much OK at higher rpm?

If yes, look HARD for a small vacuum leak. Once had a piece of the intake gasket - about 1/8" wide across the top of one intake port - get sucked out during an auto-cross event; car ran very poorly at lo rpm, but ran OK once up over about 2500 or so. Had no guts at all under that, so had to cruise home in 4th. gear, to keep revs up. All from a gap of about 1/16" thick by 1/8" wide, between the intake manifold and the cylinder head.

[scott h]

idle is ok once warmed up (carb is chokeless). it is responsive and does not require excessive throttle