Hello,

I have a 1974 Corvette with the L-48 350 SB engine. I've rebuilt the engine with non-stock parts and it's running pretty well but due to my lack of experience I can't figure out where I should time it. Can anyone help? I've tried to time it with a vacuum gauge to get the most vacuum but that sets the timing way too fast or advanced. Also, about how much Horse Power would you guess this engine might have now?

Here's what's in the engine:

-Holly 600 cfm carb
-Edelbrock Performer EPS Intake
-Stock Exhaust Manifolds
-Cherry Bomb Glass Packs on dual exhaust
-New HEI Distributor, plugs, and 8.5 wires

**Pistons:
Brand Speed Pro
Manufacturer's Part Number ZH618CP
Part Type Pistons
Product Line Speed-Pro Hypereutectic Pistons
Summit Racing Part Number STL-H618CP

Bore (in) 4.000 in.
Bore (mm) 101.600mm
Piston Style Dome, with two valve reliefs
Piston Material Hypereutectic aluminum
Compression Distance (in) 1.560 in.
Dome Height (in) 0.125 in.
Piston Head Volume (cc) -3.50cc
Wrist Pin Style Press-fit or floating
Pin Diameter (in) 0.928 in.
Piston Ring Thickness 5/64 in. x 5/64 in. x 3/16 in.

** CAM Kit:
Brand COMP Cams
Manufacturer's Part Number K12-238-2
Part Type Camshaft and Lifter Kits
Product Line COMP Cams Xtreme Energy Cam and Lifter Kits
Summit Racing Part Number CCA-K12-238-2
UPC 36584024408

Cam Style Hydraulic flat tappet
Basic Operating RPM Range 1,300-5,600
Intake Duration at 050 inch Lift 218
Exhaust Duration at 050 inch Lift 224
Duration at 050 inch Lift 218 int./224 exh.
Advertised Intake Duration 262
Advertised Exhaust Duration 270
Advertised Duration 262 int./270 exh.
Intake Valve Lift with Factory Rocker Arm Ratio 0.462 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.469 in.
Valve Lift with Factory Rocker Arm Ratio 0.462 int./0.469 exh.
Lobe Separation (degrees) 110
Grind Number CS XE262H-10
Computer-Controlled Compatible No
Lifters Included Yes
Lifter Style Hydraulic flat tappet
Valve Springs Included Yes
Outside Diameter of Outer Spring (in) 1.254 in.

**Rings:
Brand Sealed Power
Manufacturer's Part Number ZE251K
Part Type Piston Rings
Product Line Sealed Power Performance Piston Ring Sets
Summit Racing Part Number SLP-E-251K
UPC 053059132604

Bore (in) 4.000 in.
Bore (mm) 101.600mm
Top Ring Included Yes
Top Ring Thickness 5/64 in.
Second Ring Included Yes
Second Ring Thickness 5/64 in.
Oil Ring Included Yes
Oil Ring Thickness 3/16 in.
Oil Ring Tension Standard
File Fit No
Gapless No
Top Ring Material Iron
Top Ring Facing Material Moly
Second Ring Material Iron
Second Ring Facing Material Cast iron
Oil Ring Material Stainless steel

**Oil Pump:
Brand Sealed Power
Manufacturer's Part Number Z2244143
Part Type Oil Pumps
Product Line Sealed Power High Volume Oil Pumps
Summit Racing Part Number SLP-224-4143
UPC 053059000293

Oil Pump Style Wet sump
Oil Pump Volume High-volume
Oil Pump Pressure Standard-pressure

**Gasket Kit:
Brand Sealed Power
Manufacturer's Part Number Z2601000
Part Type Engine Gasket Sets
Product Line Sealed Power Gasket Sets
Summit Racing Part Number SLP-260-1000
UPC 053059465689

Gasket Set Type Full gasket set
Head Gaskets Included Yes
Intake Gaskets Included Yes
Valley Pan Included No
Oil Pan Gasket Included Yes
Rear Main Seal Included Yes
Exhaust Manifold Gaskets Included Yes
Exhaust Pipe Gasket Included No
Valve Cover Gaskets Included Yes
Valve Stem Seals Included Yes
Water Pump Gaskets Included Yes
Timing Cover Gaskets Included Yes
Timing Cover Seal Yes
Water Neck Gasket Included Yes
Fuel Pump Gasket Included Yes
Distributor Gasket Included Yes
Fuel Injector O-Rings Included No
Upper Intake Manifold Plenum Gasket Included No

**Crank:
Original Factory Stock Crank

**New 800hp aluminum radiator
**Freshly rebuilt Rear-end 3.70:1 (i think)

The engine had no wear and didn't need to be bored over or anything. The reason I had it rebuilt was because it had a slight knock that turned out to be a wrist pin. No other engine wear, it only had 62,000 miles on it.

Engine is running great, except on idle it seems to smell a little rich, but when I check the plugs they are not black like an engine that's been running too rich. Also when I first start it and put it in gear it seems to want to quit unless it has warmed up for about 15 minutes. It does this regardless of wither the choke is on or not.

The valves have been set and rechecked several times and they are good. The engine has about 2500 miles on it since rebuilt, so I would think everything should be seated in by now.

I'm thinking it might be timing because it doesn't seem to have the power that I would think it should have. Also the quitting when you first put it in gear before it has warmed up for 15 minutes.

Oh, and I have the lightest secondary spring in the Holly Carb, but I've watched and the secondaries won't engage if you rev it in park, so I think this spring is ok (not too light).. The secondaries seem to be kicking in the way they are supposed to.

This engine is pulling between 14 and 15 vacuum. Is that low for this configuration?

Thanks in advance for any advice!

Jason

 

I'll let other with more knowledge of the cam and pistons comment on what that did for performance but you might want to post what heads you've got. If stock, I don't think you have much of a total HP gain over stock.

For timing, follow the instructions that are posted in one of the stickies up above. Basically, power time the engine to have the mechanical advance all in by 3K-3.5K and don't worry about the initial advance. Forget trying to time it by vacuum.

 

Hi Mike,

Yes, they are the original stock heads. The console says the stock compression was 8.5:1. My intentions was to raise the compression some, that's why I used the dome pistons.

I also forgot to mention that when I did a compression test I got 165 on each cylinder, give or take a lb or two.

Thanks,
Jason

 

You want to have a total amount of mechanical timing (without any vacuum advance) of 32 to 34 degrees BTDC, all in by maybe 2800 or 3000 RPM. (Some guys would push it to 36- that's a bit much, in my opinion. You will never have problems at 32). With an unmodified '74 distributor, you will have anywhere from 18 to 22 degrees of mechanical distributor advance. So your initial timing (with the vacuum advance disconnected and plugged), can be from 10 to 14 degrees, at idle. The vacuum advance will probably add about 15 degrees, during low load conditions.
You should be able to buy a timing tape at a speed shop. It wraps around your harmonic balancer and will be marked so you can determine those numbers with a timing light. You will probably have the small (6 3/4") balancer on that engine, unless it has been changed.
I have had better luck modifying the weights in the distibutor than changing the springs. With springs light enough to give you an "all in" speed of 2800, the distributor can start to advance right at idle. You need to carefully grind off the backside of the counterweight part of the advance weights. This can be a time consuming trial and error process. Or you could just pay the speed shop to set up your distributor for you.
All carburetors have a method for enriching the mix when the engine is cold. I am most familiar with the Quadrajet, but of course the Holley has an adjustment procedure as well. There will be some sort of vacuum operated choke pull off, that must be carefully set. Look the instructions over carefully, and you will be able to straighten it out.

 

I second getting tape and remarking timing. not unusual for stock mark to be off. if you are running 73cc heads and .125 domes I think you might be around 9.7-1. this should give you a noticeable different from stock.
I have a very similar settup only it is bored and estimate 300-350 hp. very respectable difference from stock 180 l48. you should see similar. when tuned correctly

 

You need to 'dump' the concept of setting timing at idle...that method is what GM recommended, and it was chosen for: 1) simplicity of initial adjustment, 2) minimizing hydrocarbon emissions at idle, and 3) to [effectively] de-tune the engine so as to minimize warranty claims due to engine abuse.

Instead, since you want to maximize engine "performance", you should accept the concept described above which sets the maximum level of mechanical advance {32-34 deg.} and also sets the rate of mechanical advance [via the selection of springs/weights for that mechanical advance mechanism]. Setting those characteristics properly will maximize engine performance while [safely] eliminating the possibility of pre-ignition damage. Once mechanical advance is set correctly, then the measured engine vacuum at idle is used to select the proper advance can for the distributor. If all that is done correctly, the 'initial' advance (should you even decide to check it) will be somewhere in the range of 12-18 degrees advanced when at idle and with the vacuum can feed line being blocked. Once the advance can is added back into the system, spark advance at idle should read around 30 degrees or so. But, as long as the engine starts well and runs well at idle, "initial" timing really isn't used to set the engine up for 'performance' operation.

 

That might 300-350 gross, but is not comparable to 180 net HP. Apple and oranges.

 

Hi,

Thanks for the info!

I finally figured out why it wanted to quit while cold when I put it in gear... I really didn't want to mess with he carb screws because I've heard so many horror storries about adjusting carbs. But I decided to have my Dad hold the break and put it in gear. Then from the really rough idle I started backing the screws out an eight turn at a time until the idle smoothed out. That did the trick! I readjusted the idle to 750 rpm and pulled it back in gear and it idles perfect...

So I'm guessing it wasn't letting enough gas through the idle circuits to keep the engine going when I put it in gear?

Jason

 

After all that it was the idle screw!
I'm speechless!

 

The comments on timing are still valid.

 

Yes, I still need to get it timed right rather than just guessing at it. All the replies have been really helpful! I need to get a newer timing light and I'll probably tackle the timing tomorrow after work!

Thanks,
Jason

 

You might want to get a 'dial back' timing light then you don't need the balancer tape.