71 Skylark 350 gear upgrade questions

[Joyman]

Hey you Buick pro's out there...


I have done some research on my 71 Larky, and I have to make some decisions soon for upgrades...


My mechanic/storage place here in cold IL says that the engine is very strong, no major issues detected other than likely the wrong octane gas and a carb in need of cleaning and tuning...
And likely a small bit of timing adjustments. Points need to be converted over...


Anyway, her number is coming up soon in the rotation for working on cars before the spring comes...


But before I get into all of this, I have done research into her guts from the factory and since then...


She is a numbers matching 1971 Skylark Custom Convertible 350 CID Rochester 2-bbl with a THM-350 3-speed automatic...
There is a small chance that she had a 4-bbl from the factory, I will not know until I can see some ID codes on the valve covers when they come off...


Since then, she has been upgraded to a Holley 4-bbl 650 CFM...
SHe was upgraded to Stage 1 in 1987, which for sure is the cold-air induction and the limited restricted exhaust... I am not sure if the manifold was upgraded to Stage 1...


She had original 14" tires, in 1987 upgraded to the 15" tires she is sporting now...


Most likely, the gear ratio was pretty weak cheese from the factory... THIS IS WHERE I THINK THE EASIEST UPGRADE COULD BE...
With the 2-bbl, gear ratio was 2.56... with the upgrade to 15" tires that equals about a 2.74...
With the 4-bbl, the gear ratio was 2.73... with the upgrade to 15" tires that equals 2.93...
Yes, these are why the power seems so limited but yet she seems to have potential...


I have to figure out that original carb setup, that will give me the compression so that I can match the gas octane better for what she needs...
Can anybody shoot me the compression options/tables for 1971...????



The factory GS 350 CID with the 3-sp Auto had a factory 3.08 rear end... many were upgraded to 3.42...


So, here is my question/conundrum...


I know that this is a small block and never will be a track burner, but c'mon this car is sweet and should show more *****.
With the shift points from 1st to 2nd in the way-upper 30's already, why not gear-up the rear end and give her some more jump for those RPMs??
I for sure still want her to be a cruiser, with the 3-speed she runs a bit high-strung on the highway, so I do not want to get too aggressive.


But what do you think of this combo...:


1- I already discussed with this mechanic to replace the points system for better reliability, performance, and efficiency...


2- He is going to run thru the carb, clean and adjust as needed.. also, plugs and associated wires and some timing checks...


3- What about the hotter rear-end upgrade to a ratio in the 3:42 range??? That will give me the "goose" factor around town...
The GS 455 engines had 3:08 and 3:42 from the factory with the 4-speed auto trannys... the 3:42 were also POSI tractions...


Overall, this way she won't be frankensteined on the engine or anything else too crazy.. just the same juice getting the tires easier...
Still the same classy Skylark but a bit better off the line...


What do you think? Please let me know issues/comments/suggestions.... thanks !!


MJ

[carmantx]

Hi. I read through your stuff there and have a few things to add.

1. 3.42 is a nice gear for the Buicks. I have used them for years in mine.
2. An original type quadrajet would provide best fuel economy and performance. They were 750 cfm don't waste money tuning the other carb. Check my website. I can build what you need.
3. There is no Stage 1 350 from Buick. So it could not be upgraded to that spec. TA performance makes a "Stage 1" 350 intake so many people are confused by that. It is an aftermarket dual plane intake.
4. The other best thing you can do for performance is "power time" your Buick 350. recurve the distributor to bring all advance in by 2800 rpm, and set timing to about 32 degees advance when all in. Don't worry about initial advance setting.

Hope this helps.

[Bob]

I agree with the above. One thing I notice is converter stall is not mentioned and can make a huge difference in off the line jump. You might want to consider a little higher than stock stall, maybe about 2000 rpm. Just as a note, a 14" wheel with a 70 series tire is virtually the same diameter as a 15" wheel with a 60 series tire.

Here are some links which may help:
http://www.teambuick.com/reference/l...iles/100-a.php
http://www.teambuick.com/reference/w...tire_howto.php
http://www.teambuick.com/reference/calculators.php

[Joyman]

Team Buick team... okay, great info so far, thank you a ton...

Here are two replies from local guys that I know with some more experience than me.. take a read and please reply with insights/corrections/etc....

Also, anybody have insights as to proper fuel (89-91-93 octane) for my current setup and my likely compression ratio???

#1> my old neighbor Jimmer--

When you increase the tire diameter from 14” to 15”, the final drive ration decreases. Pi R squared. Moving to a larger tire will seem more sluggish off the line, but will increase the top speed. If you are happy with the current tire size, look up the size and circumference for the speedometer gear calculation. Your current speedometer should be close, but changing the rear ratio will give inaccurate readings.


You can determine the exact rear end gear ratio by having the rear tires off the ground (up on a lift is ideal, but a jack will do in a pinch) and the transmission in neutral. Mark a starting point on the driven tire, and then count the turns while rotating the driveshaft. I’m betting it is the 2:56.


You are on the right track looking to change the rear end ratio to “liven up” the car around town. The most important question in the decision is ……………………………….drum roll………………………………………..Do you ever plan to take this car on a highway trip? My experience was to check out other cruise nights further away to meet new people and see different cars. My Torino had a 4:11 gear the previous owner installed. If I went down to Lombard or Elmhurst, I am in the right lane screaming 3500 RPM at 55 MPH and pissing off every car and truck behind me. Not a fun experience. On the other hand, I left the stock 2:79 gear in the Cougar and it loafed along at 2200 Rpm at 60MPH, passing gear kicked out at 80. If I wound have kept the car, I think 3:25 would have been ideal for a cruiser. My gut feeling is 3:42 might be a little too far for a cruiser. When you do decide (final answer) the speedometer gear will need to be changed. This should not be a big deal. It is cable driven from the trans to the speedometer, you just put a different nylon gear on the trans end. There are gear ratio calculators on the web, a 27” diameter tire w/3:25 would turn 2450 RPM at 60 MPH.

It sounds like you found the right guy to work on your car. There is no substitute for experience with classic cars. It will save you time and money compared to someone guessing or learning.

A few questions…

The carb seems to be the right size for the engine, what intake are you using? If it is the factory cast iron unit, ask Matt if he thinks an Edlebrock Performer would help you attain you performance goals. A put one on my Cougar’s 351C and loved it. You can always paint it to look stock.

Upgrading the ignition is a great idea in terms or both reliability and performance. Is he recommending The Pertronix and recurved springs or a modern distributor?

Lastly, will he run a compression check to determine the overall health of the engine? I would highly recommend this (although he said the engine runs strong) before putting any money into the car. You can spot several internal ($$$) problems concerning piston rings or bad valves. Bottom line, all cylinders should within 10% of each other, otherwise you are in my opinion – wasting you money.
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#2>> my engineer cousin Marky --

First thing you want to do is put a Crane Electronic Ignition in place of the points


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Then you want to ask your guy whether he thinks that a Holley 650 is too much carb for a street car where performance off the line is more desirable than top end. If it has vacuum secondaries, it might be enough to put in a stronger spring so that the secondaries open a little later. That will give you good throttle response. It would help him choose if you could figure out from your paperwork whether the cam is stock, or if it was upgraded (probably stock). If there are no vacuum leaks, and these two things are right, you will already like it a lot better. You might like it better still with a 600 CFM Holley (that is all we have on the Hot Rod Lincoln, and it made an honest 360 HP).


As far as axle ratios, go, most people go too far. Without overdrive, you probably don't want to run much over 2650 RPM while cruising. I like it lower as it helps economy & noise, but you probably aren't planning on driving to the Grand Canyon any time soon.


If you are interested, you can take the tire size, and work backwards to see what the engine RPM is now, and what it would be if you kept everything the same, but changed the axle ratio.


If you arbitrarily say 2,300 RPM at 65 MPH cruise:


3rd gear on a TH350 is 1:1, so the engine, driveshaft, and input to the diff turn 2,300 times per minute. With a 2.73:1 axle ratio, the engine turns 2.73 times for the wheels to turn once. If the tire diameter is 24", the circumference is 3.1415927 times that. There are 12" to a foot, and 5280 feet to a mile,


(2300 revs/min)/2.73 *(1hr/60min) *(24"*3.14/rev) * (1 ft/12") * (1 mi/5380 ft) =


something around 60 MPH I expect.


3.42 vs 2.73 represents a 25% increase in ratio, which means a 25% increase in engine speed (all other things being equal). If your tires are 24" in diameter, the engine RPM would go from 2,300 at 60 MPH to 2875 RPM. that is pretty busy, and would certainly hurt your fuel economy. If you know what size the tires are on the car (e.g. 235/55R 15) we can figure out the diameter.
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So what do you folks think so far??? Thanks so much! I want to make the best-informed decisions...

[carmantx]

Best Buick information I EVER received was:
Only listen to real Buick people for advice. You will save money and enjoy a better ride!

It isn't a chevy, It's a Buick.

[bob k. mando]

First thing you want to do is put a Crane Electronic Ignition in place of the points


an interim solution would be to pull an HEI unit off of a ~75 Buick 350. custom curving and timing is always going to be better than the OEM.



When you increase the tire diameter from 14” to 15”, the final drive ration decreases.


basically correct, but i think you confused the tire dia for the wheel dia. i'm going to be REALLY surprised if you have 15" dia tires on your car.

wheel diameter is only peripherally related to tire diameter, the tire dia being a function of both the wheel it's mounted on and the profile ratio in the sidewall.

for instance, a common tire size would be 215 / 70 R 15. this number decodes as a tread width of 215 millimeters, a sidewall height of 70% of the tread width ( 215mm*.7 == 150.5mm ) and the wheel diameter in inches ( 15" ). so you have to know all three pieces of information in order to calculate a thumbnail diameter for the tire, which you can then use to calculate the perimeter of the tire using Pi*d. d will equal the wheel diameter plus twice the sidewall height AND you'll need to convert metric to US.

of course, every manufacturer is slightly different in their spec for a given 'number' tire, so you should always use a tire tape or stagger gauge to accurately measure the tire size. also, as the tread wears, the diameter decreases. a slicked bald tire can be ~an inch shorter than the exact same tire with new, with full tread depth on it.


a change in tire size on the drive wheels can necessitate changing the speedo gear JUST AS changing the ring and pinion would. but the key is overall tire diameter, not just the wheel diameter.

[Joyman]

Great insights, guys... this is really helping me...

Couple more monkey wrench questions and updates...

The current tires are BF Goodrich P255/60R15... the Skylark is in storage, however I have a full-sized spare that tape measures the outside diameter at just over 80 inches...

What is the likely compression ratio on these things? The mechanic/storage guy eluded that the higher octane gas may actually have an adverse effect on performance depending on the compression..
If I remember correctly, he said something like "for 8:1 use 87 octane... but 10:1 would do well with the 93 octane.."
Does that sound correct? How would I ascertain my compression ratio?

Likely I will not go ahead with a gear ratio change before the summer cruising months... I don't think I should rush into anything...

But I am going to have the points converted, the carb's jets/ports cleaned and adjusted, and those should lead to overall better timing and performance...
What about new/better spark plugs?
What grade oil are people running in these birds?

I guess that I should have a compression test to make sure that "her heart is healthy enough for sexual activity" and other financial upgrades...!

And also figure out if her speedometer is accurate with the change from 14's to 15" tires...

Finally, what about adding a tach? in the dash, the analog clock has been replaced twice already and it still doesn't work, just the second hand... it's always 5pm in Larky!
I have seen tachs in the OPGI catalog that seem to fit in that spot, how difficult is that? I really don't want to hang one off the steering column or dangle by the tranny hump...

Many hanging questions, any other thoughts out there my Buick friends??
Thanks in advance.....

[Bob]

Here is a link to the compression ratios:
http://www.teambuick.com/reference/y...ine_number.php

Which can also be found in the Chassis Manual here:
http://www.teambuick.com/reference/l...ssis/index.php

[bob k. mando]

What grade oil are people running in these birds?


Rotella 15w-40 is excellent, cheap, and more closely follows the 1970 era formulations with lots more zinc than modern blends.

use a good filter, Wix, AC-Delco, Napa Gold etc. NO FRAM.



Does that sound correct? How would I ascertain my compression ratio?


in 1971, all Buick engine options were ( nominally ) 8.5:1 ... this is a middling low comp ratio.

you wouldn't be worried about "high compression" for a GM engine unless it was a model year 1970 or earlier ( with a few minor exceptions like the Corvette ). iirc, Ford kept their higher CRs until 71. highest compression available on a Bu 350 was 10.25:1 on the 1968 to 70 four barrel engines. Buick controls CR with piston dish, so you can get that by swapping your pistons out.

so far as his general point, yeah he's kind of correct. i really don't think you're going to observe any noticeable difference in power output between high octane and low though. for me, the bigger thing has always been that there is usually far more attention paid to the refining process for the quality blends and they have less **** in them.

for higher comp ratios, definitely you will need the higher octane. that's to stop detonation, though.


something to keep in mind if you do a high comp rebuild, 100LL Aviation Gas is usually a cheap(er) alternative to the +100 octane race blends. it usually splits the difference between Premium and Race fuel prices.





The current tires are BF Goodrich P255/60R15... the Skylark is in storage, however I have a full-sized spare that tape measures the outside diameter at just over 80 inches...


the spare is also a BF Goodrich P255/60R15? with zero tread wear?

because my spreadsheet says that should give just under an 85" circumference.

and if you went to a 255 / 70 R 15, you'd have 89 1/4".

whereas, if you went down to a 225 / 70 R 15 it would be ~86".



Likely I will not go ahead with a gear ratio change before the summer cruising months... I don't think I should rush into anything...


if you can mount your own tires, you can play with some pretty big gear changes just by swapping in junkyard takeoffs to experiment with. especially if you've still got the 14" rims.

a 195 / 60 R 14 would be about a 73" circumference.

what you're looking for is the "final drive ratio" that you want compared to the tire size that you actually intend to mount in the back.


And also figure out if her speedometer is accurate with the change from 14's to 15" tires...

somewhat unlikely. the 14" rim probably mounted a higher ratio sidewall BUT 255 is a massive tread width for these older cars.




But I am going to have the points converted, the carb's jets/ports cleaned and adjusted, and those should lead to overall better timing and performance...


this is the best place to start. take care of the basics before you go nuts on the performance stuff.

replace the pickup sock in the fuel tank, put a new fuel filter on, make sure all the rubber for the fuel and spark wires is in excellent shape, etc.