Well I'd say I dont have enough gear to have fun. I can go fast sure, my rearend is 2.56. I like teh mpgs tho and was thinking the mod I should do is a stall converter. What rpm would be good? What brand? Any other changes required? The car is a mostly stock 72 skylark,with 350, it has a stage 1 intake and headers and is power timed but thats about it. Its healthy but taking off is its weakness with that backend.
You won't gain anything except destroy the drive ability of your car. I would go to a 3.08 gear which is the best compromise of economy and performance. If you want to keep the 2.56 gears, you could go to a switch pitch 400 transmission and activate the high stall with a vacuum switch like the control box from Bruce Roe offers. That would give you the best of both worlds. Putting an off the shelf high stall converter in a basically stock engine is a waste of money. Driving a car with a loose converter around town will get old really fast.
I'm new to stall and switch pitch converters. While reading online I was under the impression a stall converter acts normal unless you hold the brake and wind it up. http://www.badasscars.com/index.cfm/page/ptype=product/product_id=78/prd78.htm Based on your response sounds like maybe I was wrong? Switch pitch sounds like a good way to do it. Have any sources / links of known good ones I could read up on? As an aside, anyone have a 3.08 rear in the MN area that they arent attached to?
The th350 has a lower first gear 2.52 vs the 2 48 of the th400 so have the th350it built with a 700r4 low gear 2.75 it requires a simple machining procedure most all trans shops can handle it. I myself would do a 323 gear its not to revy on the hiway will still give you a good down shift and obviously provide a deeper first gear coupled with the 2.75 trans gear to make the most of the low po 350, also 350's love timing put a strong advance curve in it this will make a lot of difference down low, also.make sure the Qjet is opening as soon as possible with out a bog , then after all that MAYBE look into.a 2200 or so 11in converter not some BanM or TCI Saturday nite special, but a quality unit,
Thanks I dont have plans to rebuild the tranny, if it ever needs it I may do that. But had condsidered a different rear diff. 70mph with 3.23s isnt bad huh? I do road trip for 3 hours one way sometimes. I currently get 20mpg highway on no ethanol 91 octane with is the only no eth we can get here. My total timing is 36 but its in a 3000 rpm, maybe it should be in earlier?
A high-stall converter will always have more slippage than a low-stall converter, even if that isn't immediately noticable in the tach. And slippage means more heat in the transmission, and a less efficient coupling between the engine and the rest of the drivetrain (read: worse MPG). For regular driving, you want the converter to be at/near stall speed when you are maintaining speed. If you had a 4000RPM converter when driving down the highway at 2200RPM, you would be slipping like crazy and burning up the transmission fluid and losing lots of MPG. I'm a huge advocate for the 2.56 gears... They allow you drive on the highway like a boss getting good MPG and spinning the engine nice and slowly. But for stoplight fun, they require a lot of low-end torque up front. In my car, I added Rhoades lifters and I gained a ton of low-end torque - granted, however, it was a fairly healthy 455. My first step is to ensure that you are making some good low-end torque. If you have that department pretty much maxed out, then I'd look at things like a 3.08 or 2.93 rearend, maybe a switch-pitch TH400. If the budget is there, a 3.42 rearend + 200-4R (rebuilt to handle "Buick" amounts of torque) is my favorite combination behind a 350- a nice short ratio in first gear, plus the overdrive and lockup converter. I bet you could get 22+MPG with that combination, and have a lot harder launch as well. (But you'll need a much heftier budget for that...)
Do you mean that you don't get full mechanical advance until 3000RPM? I would time the engine so you have more like 45 degrees around 2000 RPM (or something like that). That is initial advance + mechanical advance + vacuum advance = 45 to 50 degrees during cruising...
Wow if this is true then yes I can use alot more timing. I have the crane adjustable vacuum advance "kit" on it.
My thoughts as well, take off torque is what I need to maximize, and budget friendly of course. The option I would love is a stick shift and clutch! Yeah can you say budget and skills?? Without dynoing is knowing your torque kind 'seat of the pants' only? Thanks for all the input guys, keep it coming!
Welllll..... Since you brought it up..... [DISCLAIMER: some VERY BROAD generalizations below, just to help the discussion, and there is always more to meets the eye- but I'm trying to keep it simple here!] A torque converter actually multiplies torque. So between your engine and axles, you essentially have the following torque multipliers: Torque converter (maybe 2*) First gear (maybe 2.52*) Rearend (2.56*) Your wheel diameter (RECIPROCAL OF tire diameter in inches / 2 / 12, which gives you the radius in feet) So if your engine is putting out 250 lb-ft of torque, that could be 250*2*2.52*2.56 = 3225 ft*lbs being split between your rear axles. If you switch to a manual... you lose the 2x from the torque converter. So, just a bit more: - A higher stall converter will have more torque multiplication. - HOWEVER the higher the engine RPM, the lower that torque multiplication becomes (it's on a curve, and at high RPM is pretty much 1:1 minus efficiency losses). So that's why a higher-stall converter will push you harder off the line. You might get a 2.5x or 3x from a really high stall converter (however at the cost of efficiency). Based on that, I would leave the automatic in there. But you can also see that a shorter tire will essentially give you more acceleration as well. If you change from a 27" diameter tire to a 25.5" tire, it would be kinda like going from a 2.56 to a 2.73. Also, on your other point, if I remember correctly the Crane adjustable vacuum advance can only be used to decrease advance, NOT to increase it. You cannot get more advance than with the factory vacuum advance unit. But this has me wondering if you can put some lighter springs on your mechanical advance - that might help your low-end torque considerably and make the whole gear-change discussion kinda moot. -BC
Thanks. My tires are Cooper Cobra 255/60/15 which I believe are about 26 inch diameter. I think smaller looks poor so even though thats a cheap fix I probably wont go there. Increased torque from an auto seems odd, I know the theory is an auto sucks up HP, also uses more fuel, increasing torque at the same time seems strange. The crane kit does limit, not decrease just set where max happens, it is how you adjust how much at a particular rpm as I understand it. I do have light springs from the kit which play a part. I think right now I have one of each a light spring and a mid spring. (Id have to look its been a while) here is a pic of one: Summit says: "These Crane adjustable vacuum advance kits let you tailor your ignition to the advance you want. The adjustability provided by these kits permits you to run the maximum amount of ignition advance throughout the rpm range, without incurring detonation."
Torque multiplication is at maximum at the stall speed of the converter when the car has not moved yet. As the speed between the 2 main parts of the converter (impeller and turbine) equalizes, torque multiplication drops off. As far as the Crane adjustable vacuum advance unit goes, unless you limit the advance in the canister, you will get about 20* out of it. The stock distributor frequently had springs that would not allow full mechanical advance until upwards of 4600 RPM. At typical cruising RPM, only part of the mechanical advance was "IN". The stock vacuum canister would provide another 14-18* of advance resulting in a cruise ignition timing of approximately 42-45*. Under heavy throttle, all the vacuum advance would drop out and at the same time, increasing RPM would bring in the rest of the mechanical advance. When you speed up the rate of mechanical advance with lighter springs, the result is ALL your mechanical advance is in at cruise RPM. That requires less vacuum advance to get to that mid 40's target, typically 8- 12* maximum. Failing to limit the advance available from the canister may result in surge from over advance at cruise, and part throttle ping when the vacuum advance can't drop out fast enough.
Just to be clear. This is what the crane kit is correct? The black piece in the kit limits the vacuum advance? Thanks
Yes, the black cam, but you should not follow the Crane instructions, you should mount it the way it is in the pictures in the timing thread. It's right there in the 1st post. That involves drilling and tapping a hole in the arm.
Thanks yep thats what I have done. Sounds like I need to be more like 45degrees ar 2000 rpm though....
So is 45 degrees correct or this: "Most Buick V8's run best at WOT, with a total timing of 30-36*, all in at 2500 RPM, or less."
Both are correct. At WOT, engine vacuum will be at or near 0. The vacuum advance canister works on vacuum. The canister has a spring that opposes the vacuum pull. The vacuum overcomes the spring and advances the timing. When you nail it, vacuum drops to near 0, and the spring overcomes the vacuum and pulls that timing out. At WOT, the only timing you will have is your initial + mechanical advance. At part throttle light load cruising, the engine will have less cylinder fill and more timing will increase fuel economy. That is the purpose of vacuum advance, to give you better economy at light load cruising. In general, the engine will like between 40 and 45* under those conditions. The stock distributor weights and springs do not allow full mechanical advance until 4600 RPM. At a cruise RPM of 2000-2500, only part of the mechanical advance is in. The vacuum advance supplies another 14-18* of advance under those conditions. When you nail it, the vacuum advance drops out, and with rising RPM, the rest of the mechanical advance comes in and provides the WOT timing. When you change springs to speed the rate of mechanical advance, the result is ALL of your mechanical advance is in at cruise RPM. Under these conditions, the stock vacuum advance provides too much additional advance, and the engine can surge or ping at part throttle. The solution is to limit the amount of advance in the canister to 8-10* so that your cruise timing is in the 40-45* range for best gas mileage.
Both? At WOT there is no vacuum advance (because there is no vacuum in the manifold) - only mechanical advance. So your total advance = initial + mechanical. So if I=12 and M=20, you have I+M=32 degrees total advance. At cruising RPM (say around 2500 RPM), you have full (or mostly full) vacuum advance and you still have mechanical advance. So at cruise you have Initial + Mechanical + Vacuum. Maybe you have I=12, M=20, V=14 so I+M+V=12+20+14=46. Make sense? -Bob C.
