SO last year I rebuilt the engine in my 1973 L48. Bottom end stock with the factory dish pistons. Installed new rings and bearings. Had everything checked out at machine shop. Someone suggested I install a set of 305 HO heads for increased compression and more power. So that is where I am now. I installed the 305 HO heads along with a lunati vodoo cam, specs:
•Advertised Duration (Int/Exh): 262/268
•Duration @ .050 (Int/Exh): 219/227
•Gross Valve Lift (Int/Exh): .468/.489
•LSA/ICL: 112/108
•Valve Lash (Int/Exh): Hyd/Hyd
•RPM Range: 1400-5800

Also has an edelbrock performer intake with the stock quadrajet carb.
Would like some more low end power. Was thinking maybe trying a set of Vortec heads. Like to keep cost below $1000. Would the vortec heads be a noticable improvement? Any other ideas?

 

Absolutely get rid of those 305 heads doesnt get much worse. Plain horrible you will lose power

You can get a used intake off ebay no reason to buy new

Vortecs will be night and day difference;you can get em through Summit or GM ready to take larger springs ready to go.


Youre moving forward!

 

305 heads are not good for performance. Go Vortec. You will regret the 305 heads.

 

Not sure what you SCR is w/dished pistons?
Combined w/stk heads and you have a slug.
Even w/Vortec heads, SCR might be too low for your cam?

I'm not an expert on this, but the Forum has people that are!
Hopefully they will chime in and get you straightened out.

Use the search and do your homework before you do anything!

R

 

Vortec heads with a .015 shim head gasket will get your compression ratio a little over 9:1. With that cam it will seriously wake the motor up. I would also get some longtube headers and a full true dual exhaust setup as that would be the next big limiting factor on power and is a relatively cheap $/hp ratio.


I did something similar (stock 350 bottom end, Brodix Aluminum heads with 64cc chambers, .015 shim head gasket, lunati 268 cam, performer rpm intake, headers) and I am very happy with how it turned out. Feels like double the horsepower of the stock motor ( and it probably is considering stock it was under 200 horsepower).

 

Even with vortech 64 cc heads and a .015 felpro shim gasket you are only going to get 9.0:1 if the deck has not been shaved down and the pistons still reside .025" down the bore at TDC.

The problem, as I see it, with the stock dish pistons is that they have no quench pad. So good quench is pretty much lost at TDC during the power stroke. This can and does lead to detonation problems with cast iron heads and high temps or high torque conditions.

If your 73 has good gearing it will be better, but if you have a 3.08 rear ratio and a TH400 it's not ideal.

Given the piston top design and cast iron heads like vortecs then 9.0:1 might be close to max CR and pump fuel without running into problems. So it may be your best option in the engines current configuration.

If the piston tops were flat tops or D-dish type then Running 10.0:1 or even close to 11.0:1 would be possible with aluminum heads.

Aluminum heads cut down to maybe 60cc's or so is probably possible without detonation issues with a 3.08 and a TH400.

In any case look to the flow capability of the head for more low end torque vs just the compression ratio.

More fuel and air in the given time alloted by the cam is going to lead to more torque. So flow and velocity are going to be king.
Runners of 180cc's or so and a good flow of maybe 220 or better in the .500 range.

Too big of a runner and the air slows down affecting the amount of fuel air charge that can get into the cylinder during the intake phase. Particuarly at higher RPM's. Too low of flow, same problem.

I'm running AFR's with 180 cc runners shaved down to 56 cc's giving me a CR of 10.6:1 with my 11.2 cc D-dish pistons and getting about 440 HP by 5800 RPM.
This is on a stock bore 350 with a 219/219 @ .050 cam. So a little less duration than the cam you are using. I could at this point run more duration, but like the bottom end torque.
With a TH-350, a 3000 stall torque converter and a 3.55 rear ratio I run 87 octane fuel with no detonation issues.
Probably my biggest problem right now is trying to get the tires to hook up. One I can live with.

 

The vortec heads are a nice cheap upgrade. I got mine from Summit. $340 each! I also have the dished pistons and plan on changing those to flat tops some time.
Jim

 

So I guess a set of Vortec heads sounds like the ticket. I am running a 3:36 rear and a stock Muncie 4 speed if it make any difference.

 

Trans and diffy make no difference in what you are doing with the engine. If dollar$ are a big deal, you might want to just go back to the heads you took off (better than those 305 turkeys, anyway). The problem with the 305 heads is the poor flow through them...not the CR.

The best you can do is buy some modern aluminum heads for your engine. Less expensive alternative is to install used Vortec heads. But, make sure you prepare heads/engine as needs for them to work together.