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Discussion Starter · #1 ·
i have been trying to tune my 78 and i have a question for someone that might know. i've rebuilt the motor using vortec heads a mild cam (called it an "rv" cam) and a holley 600 carb.
oh and i swapped the transmission with a 700r4.
all this and she's sluggish in cruise. she's all right from idle to cruise, but once the overdrive kicks in and you put your foot in it... she's a dud.
 

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Ignition curve specs and are you running vacuum advance?

Can you also define dud and your rear gear ratio? That's a mild (but good) motor in a heavy car.

good luck!
 

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Whats your rear end gear?
 

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Discussion Starter · #4 ·
the rear end is 3.08
i installed a mr gasket centrifugal weight kit to work with advance.. maybe i'm doing something wrong.
i read lars thesis on timing advance and i understand i need 36 deg. of advance at around 2500rpm, but the weights or the springs seem to have little or no effect.
 

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Discussion Starter · #5 ·
yes i am running a vacuum.
also from a stand still it takes off pretty good. at around 50 when it's in overdrive and you punch its almost non-responsive. i have to accelerate slowly to get to move.
 

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You still need to play around with the springs more to get the advance you're needing. Remember that the weights are connected so your springs do not have to be symmetrical. One weight can have a weaker spring than the other to achieve a middle ground that two like springs make you miss.

How are you controlling your converter lockup?

Have you properly set the detent cable and how did you set it?

Have you measured your vacuum at idle and at 2500+ RPMs?
 

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Discussion Starter · #7 ·
with vac disconnected i get no advance when i accelerate it, no matter what combination of springs i use. i even used the "weakest" set and i get no advance.
the only advance i get is when i connect the vac advance and accelerate.
 

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3.08 is a real high gear for use with overdrive, it probably shouldn't be in 4th at 50. What is that, 1500rpm? GM had to do some kooky tuning stuff to get early 80s cars to run acceptably with 3.08 & overdrive. If you're not gonna change the gear it's best to run around in 3rd and only use OD at 65

The way you're setup the last thing you want is a performance advance curve. The engine is being lugged too hard to accept a bunch of timing. Get a rear gear above 3.55 and it'll start accepting timing. The 36 total mechanical plus initial is correct.

The important thing with your setup is mostly to verify the vacuum & mechanical advances are not stuck and are working properly.

If your mechanical advance is freezing from corrosion it can be freed but you'll be fighting it every few years. Just replace the distributor base and don't look back.

Good luck!
 

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with vac disconnected i get no advance when i accelerate it, no matter what combination of springs i use. i even used the "weakest" set and i get no advance.
the only advance i get is when i connect the vac advance and accelerate.

Can you get the weights to move with finger pressure? There might be something catching or binding that's keeping them from functioning properly.
 

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Discussion Starter · #10 ·
well i found part of the problem, the centrifugal advance was stuck (or maybe thats the way they work in those year models).
in any event it wasn't advancing. i just replaced it and everything seems to work. time for a road test.
 

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3.08 is a real high gear for use with overdrive, it probably shouldn't be in 4th at 50. What is that, 1500rpm? GM had to do some kooky tuning stuff to get early 80s cars to run acceptably with 3.08 & overdrive. If you're not gonna change the gear it's best to run around in 3rd and only use OD at 65

The way you're setup the last thing you want is a performance advance curve. The engine is being lugged too hard to accept a bunch of timing. Get a rear gear above 3.55 and it'll start accepting timing. The 36 total mechanical plus initial is correct.

The important thing with your setup is mostly to verify the vacuum & mechanical advances are not stuck and are working properly.

If your mechanical advance is freezing from corrosion it can be freed but you'll be fighting it every few years. Just replace the distributor base and don't look back.

Good luck!
Good info, FYI I use a 373 gear for just about every OD trans vette diff I build. Works good on the st or highway but I would drive your car for awhile before you take on a diff build and the expense.:thumbsup:
 

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Good info, FYI I use a 373 gear for just about every OD trans vette diff I build. :
Does this apply to say a T-56?

Or, since I'm talking about manual vs automatic, is it a whole different game of selecting gears for both the tranny and the diff. based on what you want to do with the car?:huh:
 

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Discussion Starter · #14 ·
wow. what a difference a properly working distributor makes.
it did better in the road test. anyway i drove it to the shop where i bought the 700r4. they are tweaking the tranny. lets see what the results are tomorrow. they are pretty helpful and said they will get it right.
i'll see what it feels like after they are done with it. if i need to change the rear 3.08 gear i will probably do it during the winter.
 

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ok this may be a very stupid comment so please just brush it aside ..had a long day and i could be missing something ..but when you say overdrive do you mean 4th gear ? and if so at 50 mph ...is it down shifting properly ..cause with that gear ratio 50mph should be dropping to 2nd?..maybe to be in the proper rpm range to feel peppy..?

do you have the same lack of power at 50 mph if you run through and manuallt shift up through your gears?
 

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Does this apply to say a T-56?

Or, since I'm talking about manual vs automatic, is it a whole different game of selecting gears for both the tranny and the diff. based on what you want to do with the car?:huh:

Street small blocks tend to want to cruise in the 2000-2500 RPM range. Stockers like the lower end, 400hp builds like the upper end.

Go lower or higher and you not only suffer mileage penalty, but a handful of driveability & tuning issues pop up. Generally cruising below 2k is accomplished thru fuel injection, carbs can be finicky bout continuous low RPM.

So whatever your transmission is, figure the cruise RPM based on tire height, OD ratio, and rear gear. Then adjust rear gear accordingly.

To the OP, I wanna change my ignition advice. 36 total is correct for standard small block heads. You have vortec heads which for some reason run the best at more like 32 total. Vortecs make less HP at 36, honest.

Good luck
 

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Discussion Starter · #17 ·
i'm glad there are smart people out there.
thanks for the input about the vortec heads.
the tranny shift is off, thats because when i originally took it in so that they could do their adjustments they were dealing with a sub-performing motor and they tried to adjust it accordingly.
i'll have to wait until later on today to see how she shifts and performs now that the
 

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I started out with a 3.08 rear in my 77 with a 383 and 700R-4. 1400 rpm @ 60 mph and unless I kicked it into passing gear, it had no pep at all. Switched to a 3.70 rear and it made all the difference in the world. Now it is @ 1800 rpm @ 60 mph, right at the start of my cam's power band. Much more fun to drive. Art
 

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I have my total timing set at 33 right now. I have Vortec heads on my 405. Todd is right, at 36 I was having problems.
 

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Discussion Starter · #20 ·
well, the transmission shop replaced the governor and adjusted the throttle valve yesterday. i haven't had time yet, but i'll tune in the timing sometime tomorrow.
driving it home yesterday it certainly drove and felt a lot different. it is at around 1700rpm in cruise (overdrive), around 60mph. when you stomp it, it down shifts and goes :D. i'ii drive it around tomorrow tune it and play closer attention to rpm and mph. i'll see how the 3.08 rear does.
 
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