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| Rear gear ratio https://slantsix.org/forum/viewtopic.php?t=59432 |
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| Author: | Chunter [ Thu Apr 07, 2016 6:17 pm ] |
| Post subject: | Rear gear ratio |
I have a 66 Dodge Dart with Clifford engine modifactions , mild stall speed converter and 904 trans , looking for advise on rear gear ratio for street / strip , dart has stock rear in it now, not sure what gears are in now . |
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| Author: | sandy in BC [ Thu Apr 07, 2016 6:55 pm ] |
| Post subject: | |
Your tire size is an important consideration. |
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| Author: | ceej [ Thu Apr 07, 2016 8:30 pm ] |
| Post subject: | |
3.23's are a nice compromise gear set. Tall enough to drive, short enough to launch with. Going shorter won't make much difference to your ET unless you build for higher rpm, It will decrease fuel economy out on the road, and the engine will be turning pretty fast at freeway speeds if you go past that to 3.55's. What parts did you use from Clifford? Just want to make sure the combination is right. There have been some unfortunate results from some of those combinations. CJ |
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| Author: | Chunter [ Fri Apr 08, 2016 8:10 am ] |
| Post subject: | rear gear ration |
thanks for quick response , 28 inch tall rear tires as far has Clifford stuff we went all in ,intake ,headers, magnaflow mufflers,complete engine rebuild clifford cam 447 lift on intake and exhaust ,268 duration ,S.I. comp valves intake 1.7 exhaust 1.44 exhaust ,1.6 roller rockers ,Mallory ignition, timing set at 30 degrees total advance right now ,Weber 2 bbl carb , just put holley 4 bbl carb on it to see if it would help with more power (it did not by the way ) ,block and head milled to Cliffords spec to raise compression . the dart runs and drives great, just does not have the power we expected . the dart has 293 stock rear in it now, hoping gear change will give this car some balls also has 904 rebuilt trans with i think 2000 stall speed converter .thanks for the help Chunter |
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| Author: | GTS225 [ Fri Apr 08, 2016 12:40 pm ] |
| Post subject: | |
How much highway driving are you anticipating with this car? Roger |
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| Author: | wjajr [ Fri Apr 08, 2016 2:21 pm ] |
| Post subject: | |
28" tall tires + 2.93 rear gear with 2000 rpm stall will be a dog off the line. Just a few things to ponder: I have 24.5" tires (205/70R 14) + 3.55 rear gear and ~2400 stall, Holley 390, full duels to bumper, 9.5:1 compression, cam with a little more lift and a little less duration and my car ain't no scolded dog off the line. But get it into the mid 4000 rpm range and it pulls hard to 6000 rpm. You need to get a much higher stall converter which would be inefficient in daily driving and generate a lot of heat, and or change one or both tire diameter and rear gear. If you cam has some overlap, it won't produce much power until rpm reaches a point where there is enough air flow through engine for scavenging to take place, in other words, better cylinder fill and combustion takes place. My car turns 3000 rpm at 60 mph 3500 @ 70, 4000 @ 80... Its not a comfortable high speed long distance cruiser because of the buzzing engine out on the interstate. If you have vacuum activated secondaries on that Holley, their opening will be governed by engine demand depending which spring is installed. I use a light spring for early opening (crack open around 3200rpm) where gearing is on the low side. Taller gearing such as your set-up would require a later or higher rpm opening point so as not to lean out engine causing a stumble or flat spot under WOT. To give you a benchmark, my car runs the 1/8th in 10.3 seconds at 68.3 mph 200 feet above sea level, and never gets out of second gear. What vacuum readings are you getting at idle, and what is that rpm? |
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| Author: | slantzilla [ Fri Apr 08, 2016 3:36 pm ] |
| Post subject: | |
Have you had it b on a dyno or drag strip? Seat b of the pants power is very elusive to measure. If you put on a 4bbl and gained no power you are doing something wrong. Make sure those rockers are oiling the valve tips and springs too. |
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| Author: | Chunter [ Fri Apr 08, 2016 5:37 pm ] |
| Post subject: | 3.55 |
Think I am going to put 3.55 gears in it because of tall tires ,looking for more off the line not a lot of highway driving your continued input is appreciated will be putting new rear in it in the next few weeks once I make my mind up on gear ratio. I have not had it on a Dyno or at the track . Will get back to you Monday on vacuum readings . Thanks Chunter |
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| Author: | sandy in BC [ Fri Apr 08, 2016 6:58 pm ] |
| Post subject: | |
With those tires 3.55 is a good choice. I run 3,55 with a 30.5" tire (275/75/15) and an overdrive trans. It ends up the same as a 225/75/15 and 3,23 gears My Valaint had 235/60/14 and 3.55 gears and was awesome off the line.....it also had an OD 5speed for the highway. Chose the speed where you want max torque in 3rd gear.....then do the math. |
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| Author: | Chunter [ Wed May 18, 2016 5:55 pm ] |
| Post subject: | |
Well I went with 355 gear and still not happy with how the car pulls of the line ,really not sure what I might be missing . The car runs and drive good just no low end power . my engine vacuum is about 13 inches at idle which I think is low but not really sure what it should be . I have tried moving the timing around but does not seem to make a difference. Right now timing is at about 15 btdc at idle and and 35 total advance using Mallory distributor with no vacuum advance, seems to run best at these numbers . I have Holley 450 four barrel carb with heavy secondary spring in it right now car does not bog down or hesitate. Had a webber 2 barrel on it originally. I put the four barrel on it because I thought it may help with the power , I don't think it helped much maybe more top end power but that's it . I would really like to get the tires smoking from stop sign to stop sign ,am I expecting to much from this motor combination or am I missing something thanks chunter |
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| Author: | DusterIdiot [ Wed May 18, 2016 7:04 pm ] |
| Post subject: | Yep... |
Quote: I have Holley 450 four barrel
Unless this is a 10:1 SCR engine with a fairly good sized cam...I have never seen the Holley 450 do better than the 390 for street/strip....(and back to back it got average gas mileage to a 600 with the secondaries locked out...)If your actual compression ratio and cam are closer to stock... put the black spring in the secondaries... 13" idle in gear is kind of abissmal... you might take a step back and pull the timing cover and make sure that your cam is properly degreed in... if you did dot to dot method, you probably left your torque band back in the garage... what cam is in it? (If it's a clifford grind...that's another problem entirely...) Most slant cams like the intake lobe centerline in the 100-104 range depending on the grind.... After all that, I'd probably be looking at the ignition curve as the next culprit....stock distributor is going to rob you of some power.... Let us know what you actually got... |
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| Author: | Joshie225 [ Wed May 18, 2016 7:54 pm ] |
| Post subject: | |
Chances are excellent that the compression ratio is not high enough and that the cam is retarded from optimum. About 12-15° initial timing and 28-30° total timing is what you want. There's not much to do here except to pull the head to measure the compression ratio and pull the timing cover to degree the cam. What is the torque conveter stall speed, observed? |
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| Author: | Chunter [ Thu May 19, 2016 5:03 am ] |
| Post subject: | |
we are running clifford cam and did build to clifford specs and used his parts for the total build. The cam is grind # 268 mech . intake and exhaust lift are 447 and duration is 268 on intake and exhaust.I know the carb is a little bigger than recommended but did start with weber 2bbl . intake valves are 1.7 and exhaust valves are 1.44 , 1.6 roller rockers ,9:1 compression , head and block work where done to clifford specs , had our machine shop speak with larry clifford for that .the machine shop said they put timing chain in dot to dot , I was thinking of pulling cover to check , is dot to dot good or does it have to be degreed in . Thanks again for your help and fast response |
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| Author: | DusterIdiot [ Thu May 19, 2016 6:32 am ] |
| Post subject: | x2 |
Quote: Chances are excellent that the compression ratio is not high enough and that the cam is retarded from optimum.
I can't find the full specs on that cam, but it smacks of being a copy of the Isky 268 cam...If the engine isn't stock, then it has to be degreed....and if looking for tire scorching torque then some advance needs to be dialed in.... running the DCR calc on a 9:1 225 slant six and this cam installed straight up the DCR is 7.42.... which would be fine for a stock engine... but you didn't build a stock engine... Advancing the cam to 102 puts the DCR at 7.72 (objective for daily driving is about 8 DCR....non daily driving runs on super performance engine can go to 9...) If we shave a bit more off the deck or head then we get a perfect 8 DCR, and a SCR of 9.4... (we would need your machined specs and measurements to determine what needs to be done to correct this...) Did you or your machinist do any port work on the head...if not, that's a bottle neck in this equation to.... |
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