Question: A Curve Kit??

[401KVet]

I just replaced the stock cat. converter with a aftermarket Hi Flow. I think I can feel it being a little more responsive.
I know this guy who knows alot about Vette's and seems to be a pretty good mech.
He said if I let him put a curve kit in it, I would get better gas mileage and it would run better.
I haven't talk to him much about it yet, but what's the deal with a curve kit? Is what he says is true?
If so, why didn't GM do it?

Thanks, and Merry Christmas to everyone!

 

[baxsom]

the distributer recurve kit makes the timing come on faster.
do a little searching around on timing here. you will find that these cars run their best when the full mechanical timing of 36 degrees is in by 2500-3000 rpms. the stock weights and springs wont allow the timing to come full in by then.

from what i understand it was set like that for emissions reasons.

 

[TimAT]

Bax is correct. Emissions. The curve kit will have 3 sets of springs, some weights and a couple of bushings. The springs are different tension so you can tailor the curve to get exactly what you want. Vacuum disconnected and plugged, set it up so you get 36* TOTAL at 3000 RPM, and try to stay in the 10-15* range at idle.

 

[401KVet]

Does a curve kit actually increase the horsepower any?

 

[minifridge1138]

It can improve fuel efficiency and throttle response.

 

[petes74ttop]

Paper by Lars.

Here's a 101 on changing the curve in the distributor from a trusted source. You also have to make sure the advance bushing is set correctly so that it stops at the limit. http://home.comcast.net/~chadwick.robert/howto.pdf
Pete.

 

[howard blackburn]

if you use the comcast.net method you must not reconect the vacuum advance. just plug the pipe. i read somewhere the increase is around 10 hp. vacuum advance is only an economy device.whith the mechanicl advance set at 36. reconected vacuum will take it way past this.

 

[Vettehead Mikey]

if you use the comcast.net method you must not reconect the vacuum advance. just plug the pipe. i read somewhere the increase is around 10 hp. vacuum advance is only an economy device.whith the mechanicl advance set at 36. reconected vacuum will take it way past this.

Huh? Where did you read that? Lars' paper says quite the opposite. :confused

 

[Rodney Gold]

If there is no vac advance at idle (dont use ported vacuum) the car will run a lot hotter in stop and go traffic. Vac advance wont be effective unless there is vacuum , and full bore acceleration = almost no vacuum.
If you "really" dont want to use vac advance , you can recurve the dizy so initial timing is high and the amount of centrifugal advance is lower.
I have found MSD dizzys the best , some other makes are made in china and it shows with bushings and plastic parts I don't think have good long term survival rates. Had a 454 with a hesitation off idle and after a lot tinkering with shooters , plug wires etc , took out the cheaper aftermarket dizzy abnd snuck in a MSD ...all gone.

 

[Vettehead Mikey]

He said if I let him put a curve kit in it, I would get better gas mileage and it would run better.

Throwing bits at a distributor and hoping you've got it right is chancy at best. Unless he's got a proper distributor machine and will include re-mapping the vacuum advance I'd leave well enough alone.

 

[howard blackburn]

Huh? Where did you read that? Lars' paper says quite the opposite. :confused

ok lar. theirs no s. info from CCCUK -. oh, my wife is from toronto.

 

[Vettehead Mikey]

oh, my wife is from toronto.

My deepest sympathies. Not really sure what the rest of your post was supposed to say :confused

 

[minifridge1138]

For full details, see:
vettetech

and click on the Timing and Vacuum advance section.
There is a lot of good information from JohnZ and Lars.

If you read all of those articles, you'll come away with a lot of knowledge.
Some highlights from the articles that I found useful:

If you use timed or ported vacuum for your vacuum advance, then there will be no advance at idle. Less advance = hotter exhaust temperatures. Ported vacuum was a way to increase the temperature of exhaust gases at idle and therefore burn up more of the carbon to reduce emissions.

You're supposed to disconnect the vacuum when you map and adjust the advance curve of the mechanical advance and when you set your base timing. You're supposed to reconnect it before you drive.

Total timing = base timing + mechanical advance all in between 2500 and 3000 rpm.
Total timing should be between 34 and 38 degrees.

Idle Timing = base timing + vacuum advance. Idle timing should be around 34 degrees.

For the engine to run most effectively, all 3 timing systems should be configured correctly: Base, mechanical advance, and vacuum advance.

During light cruising, you'll be pulling some vacuum advance and some mechanical advance. A GM V8 at light throttle cruise performs best when timing is at 52 degrees btdc.

I strong suggest reading the actual articles, rather than relying on my little summary.

Good Luck!!!

 

[JohnZ]

if you use the comcast.net method you must not reconect the vacuum advance. just plug the pipe. i read somewhere the increase is around 10 hp. vacuum advance is only an economy device.whith the mechanicl advance set at 36. reconected vacuum will take it way past this.

Nope, bad information - whoever told you that doesn't understand ignition timing and advance systems; read this:

http://www.lbfun.com/warehouse/tech_info/timing & vacuum advance/Timing101Article.pdf

 

[Bills money pit]

My question is on an 81, with the computer (ECM) determining advance, can any improvements be made to the distributor such as curve etc?

 

[petes74ttop]

For full details, see:
vettetech

and click on the Timing and Vacuum advance section.
There is a lot of good information from JohnZ and Lars.

If you read all of those articles, you'll come away with a lot of knowledge.
Some highlights from the articles that I found useful:

If you use timed or ported vacuum for your vacuum advance, then there will be no advance at idle. Less advance = hotter exhaust temperatures. Ported vacuum was a way to increase the temperature of exhaust gases at idle and therefore burn up more of the carbon to reduce emissions.

You're supposed to disconnect the vacuum when you map and adjust the advance curve of the mechanical advance and when you set your base timing. You're supposed to reconnect it before you drive.

Total timing = base timing + mechanical advance all in between 2500 and 3000 rpm.
Total timing should be between 34 and 38 degrees.

Idle Timing = base timing + vacuum advance. Idle timing should be around 34 degrees.

For the engine to run most effectively, all 3 timing systems should be configured correctly: Base, mechanical advance, and vacuum advance.

During light cruising, you'll be pulling some vacuum advance and some mechanical advance. A GM V8 at light throttle cruise performs best when timing is at 52 degrees btdc.

I strong suggest reading the actual articles, rather than relying on my little summary.

Good Luck!!!

Well done, you summed it up in a few words. PG.