Hello and Merry Christmas,

Hello Everybody, I have a 1985 TPI stock 4 plus 3 Corvette I installed a larger cam the original is
202/216 @ 0.5
403/415 114.5 separation
The new cam is
212/226 @ 0.50
483/520 112 separation
1.5 rockers
I was watching a video about camshafts and the presenter Myvintangeiron 572 said that on the lobe separation angle can cause idle problem. My original cam was 114.5 my new cam is 112. Is this enough of a change to give me poor idle?

The car is running like it has a vacuum leak.
Thank you for the help

 

Merry Christmas! That cam should work well with the stock chip with a bump in initial timing and idle speed - it might need a couple pounds increase in fuel pressure, but should be "OK".

If you're running the stock iron heads, hopefully you changed the springs and made sure they could handle the .520 lift. If you didn't I suspect that is your problem - about all those stock heads could handle (if I member correctly) was .480-ish lift. Much more than that, you'll go into coil bind and start bending pushrods (at the least).

If you had the heads worked, or aftermarket heads that will handle the lift, double check your vacuum lines and go through the minimal idle speed procedure with the minimal idle at 550 rpm (or as low as it will stay running) and the base timing at 8-degrees and see how that works for you. For the procedure - see Vadar's post linked below.
https://www.corvetteforum.com/forums...post1548489870

 

This is such a great response. Thank you so much. I didn't change the springs. And I tried to use the original timing. Thank you. Once I get the 89 squared away I will use this information.

Merry Christmas and thank you

 

 

Is that the ZZ9 roller cam or flat tappet cam? As mentioned you need to change springs and make sure you do not have coil bind or retainer to seal interference. You will need a tune to make this cam idle smooth, maybe it will just require a higher idle RPM of at least 650 RPM in my opinion.

Lobe separation really does not determine how the car will idle, overlap does and to determine the overlap you need to look at your .050 numbers and the lobe separation.

Your stock cam had (202+216)/2 -(114.5 x 2) overlap =-20 degrees or essentially no overlap and a smooth idle

The new cam has (212+226)/2 - (112 x 2) = -5 overlap so it has 15 degrees more overlap that is why it will idle rougher than the stock cam at the same RPM.

FYI these are .050 numbers the actual .006 numbers will show the total overlap which will not be negative.

The point is the newer cam has 15 degrees which will make more power but idle rougher.

 

Thank you. I need to read this a few more times
I am under the assumption that the L98 is not a vortec. Am I correct.

Thank you
Joe

Merry Christmas

 

Do you have a vacuum gauge? If so, how much vacuum (-inches of mercury) does it pull in a warm idle? I would assume it should pull around 16-17 inches of vacuum, maybe a bit more. The actual vacuum level should offer a clue as to what's going on.

A vacuum leak should raise the idle speed and not really affect much else other than the fuel trim (once in closed loop).

Are the valves adjusted properly?

FWIW: I can idle just fine at 700 rpm with a 224/230 112 LSA cam/1.5 rockers and only 12 degrees of advance. Not 12 initial, 12 degrees period (11.95 to be exact). I set my initial to zero and tune accordingly for various reasons. Its an 88 4+3 car with a steel Centerforce flywheel (not sure about the flywheel weight, but similar to stock I think).

In this configuration, I see around -6 psi (manifold pressure) in a warm idle. Atmospheric pressure is around 14.73 psi or 30 inches of mercury, so:

14.73 psi - 6 psi = 8.73 psi or 17.77 inches of mercury (manifold pressure).

30 In Hg - 17.77 = 12.23 inches of vacuum. 12 inches is relatively low considering the cam (but intentional). I can pull more vacuum with more timing up to around 15 inches of vacuum, if desired.

Point being is that you can idle just about anywhere with respect to timing and rpm with stable timing and reasonable if not rich mixture control.

Note: My idle settings are intentionally skewed in order to artificially increase the load in order to help with over-fueling with 63 lb. injectors @ 54 psi and stock displacement. Once the fan turns on, the extra load will be just enough to get the O2 cycling with the Integrator and clean up the exhaust a bit.

Some example logs with maladjusted tuning as described above: https://datazap.me/u/tequilaboy/chec...155-693-95-220

Note: I'm not suggesting that you duplicate my settings, only want to point out that there is a wide range of timing and mixture that will work.

 

I want to thank you for this. I have not run the car in a few months. But, from what I remember the vacuum was over 16 inches. I have a TunerPro RT file on the car. As I recall the IAC was not good. I have to look at it again.

Thank you so muc

Merry Christmas