6T5RUSH

Well,

I've finally got my baseline established for my fresh drivetrain. To bring those of you up to date, I took my 327/300 horse motor and after rebuilding the motor from the ground up, I swapped in the LT1 solid cam and lifters for the '70 Vette. I had about 60-70 miles on her strictly from setting the distributor by sight and ear from the initial run in.

A good neighbor and a fellow Vettenut mapped out my ignition (I did help by taking notes and being his "gopher" for tools). Here's where my timing has been set up. This will be my base line starting point. I'm going to set this up in an EXCEL spreadsheet and graph her:

Idle: 9*
1,000 R's: 9.5*
1,500 R's: 20*
2,000 R's: 24*
2,500 R's: 29*
3,000 R's: 33*
3,500 R's: 35*

The idle R's were adjusted to 950.

We then adjusted the idle mixture scews, turning them in till they started to starve the motor then backed them out 1/4 to almost a 1/2 turn.

A trip onto 16 Mile and I can tell she just feels "crisper". We put a vacuum guage on her but she's fluttering a little. This is being interpreted as my lash is just a hair tight. I originally set the lash cold .021 Intake, .026 Exhaust...but I set 'em kinda tight (first time I've done this). I'm going to relash her cold adding .001 to both intake and exhaust. This is my first experience doing this. So I know I'll get better at this by just doing it.

We'll keep you posted as I put some more miles on her.

Jim

SWCDuke

One thou of lash isn't going to do anything, and it will probaby loosen a thou or two during runin.

Set the initial timing at 12 so you have 38 total WOT advance (initial plus full centrifugal). Also, you can probably bring it in faster with lighter springs. The limit as to how fast you can bring it in is detonation.

Also, if you have not already done so, you need to swap the vacuum can for a NAPA/Echlin VC 1810. The 300 HP can doesn't provide full advance until 16", which is less than your idle vacuum, which should be about 12"@900 with about 30 degrees total idle timing, but there will be some vacuum variation due to the relatively high overlap. The VC 1810 provides 16 degrees @ 8", so full vacuum advance is locked in at idle, which is what you want. If not the idle could be unstable due to vacuum can "dithering".

The initial 1.5 turns out from the seat adjustment on the AFB should be about right if you have proper total idle advance of about 30 degrees (initial plus full vacuum plus a couple of degrees of centrifugal with a quick curve). The final 'best" position should be within a quarter turn of 1.5 out.

Duke

6T5RUSH

SWDuke,

Thanks Duke. I did replace that can with the Echlin NAPA 1810. You think I should just drive her as is and not touch the lash till I have a few more miles on her? I'm running the 600 cfm Holley (it's a list 2818 that was rebuilt/restored). The jets are the factory stock set up, 65 fronts, 76 rears.

I did have a lighter set of springs installed at start up. We checked and found that it was "all in" at 2,300 r's. Thought that might be too agressive for the street, so we switched to a stiffer set of springs. In fact, the package of springs received from LICS specifically stated to use the heavier springs for this application...I didn't, but during last night's tuning session we switched to the heavier set and the result was it moved the "all in" up the r band to 3,500.

Like JohnZ said, take notes of every change you make. That's what I'm doing.

Thanks again Duke.

Jim

Jim

SWCDuke

Forget what I said about the carb mixture screws. That's for an AFB. Holley's are different. Use the Holley idle mixture adjusting procedure in the '64-'65 Corvette Shop Manual supplements or later CSMs.

Check the valve lash after you have a few hundred miles. Concentrate on the carb and ignition map tuning.

Reset the initial timing to 12 or 38 total WOT timing (vacuum can disconnected). If it doesn't detonate above 3500 then your total WOT timing is okay for the fuel octane you are using. If you get detonation back it off to 36. Now try the lighter springs and check for low rev WOT detonation.

The optimum centrifugal curve is the fastest one that doesn't cause any detonation. To check for low speed detonation floor the throttle in second or preferably third gear at 1000. If it doesn't detonate before the centrifugal curve is all in then you can go with a faster curve. Try the lightest springs first. If it detonates replace one with a slightly heavier spring. Keep experimenting until you find the fastest centrifugal curve that doesn't detonate.


Duke

JohnZ

Sounds great, Jim - a better way to set the idle mixture screws is to connect your vacuum gauge, engine idling with vac advance connected, and adjust the idle mixture screws (working back and forth between the two screws) to get the highest steady vacuum, tweaking the idle speed screw as necessary to keep idle rpm where you want it. "Steady" is relative, as you'll have more gauge needle fluctuation with the LT-1 cam than you had with the 300hp cam. You're gonna love it!

6T5RUSH

SWDuke,

Do I understand you correctly that you can mix as well as match the springs. In other words, install 1 light one and 1 heavy one...as long as your goal is reached, right? If so, that adds another dimension in performance tuning.

JohnZ,

Thanks much for that insight on tweaking the Holley. She sure has more grunt!

My "sewing machine" sound is not quite as evident as I'm using aluminum rockers...but she's there!! SWEET!!

Jim

dropTopTx

Can somebody explain in more detail how to determine if the configuration is causing early detonation? Exactly what does it sound like?

SWCDuke

Yes.

6T5RUSH

perryth,

It's a sound like pinging, metal to metal sound. Detonation is a death sound to a motor. This is something you will here when you lug out in your gears (like SWDuke says lug out in 2nd or 3rd at a 1000 r's. If your timing is too advanced, you'll hear it.

Jim

SWCDuke

Advance your timing about 10 degrees and experience it yourself!

Duke

dropTopTx

I actually started to add to my first post "Without advancing mine so I can hear it first hand"

But I might just try it real quick so I know for sure.

JohnZ

It's kind of a metallic "tinkling" sound - like dropping ball bearings on a china dinner plate; you'll know it if you hear it (and you don't want to hear it - it's your engine screaming for mercy)

SWCDuke

Another way to experience it is to let your fuel tank get down to near empty, then pump about two or three gallons of regular.

Once you experience it drive moderately to the nearest gas station and fill it up with premium.

Detonation causes shock waves that bounce around in the combustion chamber. The "knocking" we hear is the engine structure "ringing" in response to these shock waves.

Damage is not caused by the mechanical pounding, but by a rapid increase in temperature of piston crowns and valves, which can cause them to break. Detonation shock waves can increase the rate of heat transfer to the combustion chamber boundaries by up to an order of magnitude. Light transient detonation is not a problem, but sustained moderate detonation can be damaging and heavy detonation can damage valves or pistons in a matter of seconds.

Duke