Help! ? on heads

[MTC_75vette]

I have an 1975 L-48 and plan on buying aluminum heads I was looking at the edelbrock rpm's and jegs brand. The edelbrock have: 170cc Intake Ports
64cc Combustion Chambers
2.020''/1.600'' Valves
1.460'' Valve Springs, .575'' Max Lift
3/8'' Rocker Stud

The jegs have:
197cc intake ports
64cc combustion chamber
2.055'' Int/1.60'' Exh Valves
1.440'' Springs, .680'' Max Lift


What will give me better performace i am afraid the 197cc intake ports will be too large for my set up.

I currently have comp. 1.5 roller rockers, Edelbrock Perma Star polished billet Manifold, Edelbrock 650 Thunderbird 4 barrel carb., Edelbrock tri-angular Elite air cleaner, super comp. headers, Flowmaster Delta 40 dual exhaust, comp high energy 268 cam, 2400 stall converter, Dewitt aluminum radiator, rebuilt rear with 370 gears.

This winter I am adding the aluminum heads, flat top pistons looking for about 10.5-1 compression, msd ignition, aluminum pulleys.

Should I get a new crank or will the stock crank be alright?

Thanks for your help!

Mike

 

[Vette79]

Well, I'm not going to provide to much information and wait for someon else to chime in. This past spring I did a top end rebuild of my 79 L48. Spent a lot of money with marginal results. I think with everything you have already done, the addition of some quality heads should really open her up. You want to be sure that you get the right size head and gasket and along with everything else that you have done you then need to figure out what your compression ratio will be. You may have to shave the heads to get best CR. I had my AFR 180CC Eliminator heads milled to 60cc. There is a compression ratio guide to use where all you have to do is plug in the numbers and it will tell you what your CR will be. My brother and I hope to have the vette on a DYNO within a few weeks. We still need to tune it and find out why it's lacking. With everything we did and spent you think it would take flight...lol.

 

[MTC_75vette]

Well, I'm not going to provide to much information and wait for someon else to chime in. This past spring I did a top end rebuild of my 79 L48. Spent a lot of money with marginal results. I think with everything you have already done, the addition of some quality heads should really open her up. You want to be sure that you get the right size head and gasket and along with everything else that you have done you then need to figure out what your compression ratio will be. You may have to shave the heads to get best CR. I had my AFR 180CC Eliminator heads milled to 60cc. There is a compression ratio guide to use where all you have to do is plug in the numbers and it will tell you what your CR will be. My brother and I hope to have the vette on a DYNO within a few weeks. We still need to tune it and find out why it's lacking. With everything we did and spent you think it would take flight...lol.

What is your timing set at now? I saw all the work you put into that motor it should fly!

 

[Vette79]

27.7 without vacuum advance and 13.7 with vacuum advance.

 

[Rodney Gold]

I have a similar setup , Car explodes outa the blocks......I take on superbikes with it from robot to robot and beat em....mostly..
I have a 270H Cam , edelbrock 64cc ally RPM heads , 2600 TCI stall , 3.55 rear gears , roller rockers , edelbrock performer manif , ceramic coated headers , holley 770 street avenger , SS exhausts , shift kit in the TH350 and a hurst promatic 2 shifter. Electronic Dizzy and a MSD 6al
Car does 12.9 on stock BF goodrich 255/60/15's in drive. (short shifting 1-2 5000, 2-3 4500 on a less than ideal track - I live in South Africa , poor startline , no trackbite ) Prior to TQ swap , I was running a 1600-1800 stall , did 13.4
Just installed the Promatic so keen to see what it does now I can hold it in gear , gonna use slicks next time too. Im using stock block , pistons and crank
Dynoed at 270kw and 560 nm torque which I think is over optimistic , I worked out that if the car weighs in at 1600 kilograms with me in it , I am getting at least 300 HP to the ground
I run 38 degrees total on 95 octane pump gas . I dont worry at all about initial timing , just make sure that I'm all in at 3200 RPM, I use full manif vacuum for vac advanced after I set the total.

 

[topless82vette]

I have a similar setup , Car explodes outa the blocks......I take on superbikes with it from robot to robot and beat em....mostly..
I have a 270H Cam , edelbrock 64cc ally RPM heads , 2600 TCI stall , 3.55 rear gears , roller rockers , edelbrock performer manif , ceramic coated headers , holley 770 street avenger , SS exhausts , shift kit in the TH350 and a hurst promatic 2 shifter. Electronic Dizzy and a MSD 6al
Car does 12.9 on stock BF goodrich 255/60/15's in drive. (short shifting 1-2 5000, 2-3 4500 on a less than ideal track - I live in South Africa , poor startline , no trackbite ) Prior to TQ swap , I was running a 1600-1800 stall , did 13.4
Just installed the Promatic so keen to see what it does now I can hold it in gear , gonna use slicks next time too. Im using stock block , pistons and crank
Dynoed at 270kw and 560 nm torque which I think is over optimistic , I worked out that if the car weighs in at 1600 kilograms with me in it , I am getting at least 300 HP to the ground
I run 38 degrees total on 95 octane pump gas . I dont worry at all about initial timing , just make sure that I'm all in at 3200 RPM, I use full manif vacuum for vac advanced after I set the total.

you run a 12.9 and take on superbikes and beat them? I would think at that your car would be losing pretty bad. Well unless your version of a superbike isn't what ours is .... Not slamming you but slow ones I've seen run 11's-12's and generally do 0-60 in what a blink of an eye. I can see say a new ZR-1 giving one a hell of a run but even it would lose too ...... They don't call them crotch rockets and organ donors for nothing

 

[Rodney Gold]

Robot to robot , IE short distance is where I get em
Off the line , after a while they catch up and pass , on a 1/4 mile I wouldnt have a chance , the stock big boys here are 10's

On the roads , most of em cant get away fast enough or are up on their back wheel and so on.
I got a lot of traction

 

[JohnZ]

27.7 without vacuum advance and 13.7 with vacuum advance.

Can you expand a bit on what that measurement/setting is?

 

[Hib Halverson]

I have an 1975 L-48 and plan on buying aluminum heads I was looking at the edelbrock rpm's and jegs brand.
(snip)
I am afraid the 197cc intake ports will be too large for my set up.

Your fear is your best answer on the head issue.

I currently have comp. 1.5 roller rockers, Edelbrock Perma Star polished billet Manifold, Edelbrock 650 Thunderbird 4 barrel carb., Edelbrock tri-angular Elite air cleaner, super comp. headers, Flowmaster Delta 40 dual exhaust, comp high energy 268 cam, 2400 stall converter, Dewitt aluminum radiator, rebuilt rear with 370 gears.

This winter I am adding the aluminum heads, flat top pistons looking for about 10.5-1 compression, msd ignition,

Before you buy, you need to math-out your new CR using chamber displacement, head gasket thickness, deck height, bore and stroke and the displacement of any valve reliefs in the pistons.

Should I get a new crank or will the stock crank be alright?

It depends on the rpm range you plan to run. Below 6000 rpm, you'll be fine

 

[Vette79]

Can you expand a bit on what that measurement/setting is?

Using a digital timing light (Actron CP7529), these are my readings. I believe they are my degrees of advance. I'm all in at 3K rpm. It's very accurate and also displays your actual rpm's. We may need to continue to adjust our timing with everything that we have done with this top-end rebuild. Also, the fuel air mixture ratio as well. We have found that we have oil coming through the PCV. I had to install an oil catch can.

 

[TimAT]

27.7 without vacuum advance and 13.7 with vacuum advance.

Here may be apart of your problem.

Chevrolet engineering and lots of racers have worked this out. Disconnect and plug your vacuum advance line. Check your initial timing at idle. Should be 8-10* (guessing). I saw in a later post you have a timing light that will read advance- run the engine up to 3000 RPM and see what your total timing is at that speed. WITHOUT the vacuum. get a curve kit and set it up so there's 36* TOTAL with no vacuum. Don't worry about the initial setting, just set it so you get 36* @ 3000.

And read this:
Technical Information Bulletin Rev. G 9-11-05
How to Set Your Timing for Peak Performance

by Lars
Lafayette, CO


“90% of all “carb problems” are ignition timing problems.”
(Lars Grimsrud)

This tech paper will discuss setting the timing on a Chevy V8. This procedure also applies to other GM V8s.

The procedure outlined here differs from the Service Manual, and is based on my years of experience doing this work in the quickest, least painful, most economical way while keeping the level of quality high. It is recognized that other people will have different methods of doing things, and may disagree with specific methods and procedures that I use.

How to Set the Timing
When you think about it, setting the timing at idle speed makes no sense at all: You don’t operate your car at idle, and timing changes as the rpm changes. Fact is, the timing spec at idle speed is provided as a simple way for most people to set the timing, and is not a good procedure for optimum performance.

Small block Chevys (and most other GM performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance hose disconnected) is all in by 2,500 – 2,800 rpm and is set to about 36 degrees. If you have an adjustable timing light, this is very easy to check. If you don’t, you need to scribe a 36-degree mark on your harmonic balancer. Here’s how:

Measure the circumference of your harmonic balancer using a sewing tape measure (or other flexible tape measure). Get it as accurate as you can. Take this measurement and divide by 10. The number you get is the distance to 36 degrees. Measure this distance CLOCKWISE from your existing harmonic balancer timing mark as viewed from the front of the engine and place a clear mark on the balancer.

Remove your distributor cap and rotor. If you have a points-style distributor with the stock, factory, heavy springs in place, remove one of the springs. Disconnect the vacuum advance. Install the rotor and cap. Loosen the distributor hold-down clamp bolt just enough so that the distributor can by turned, yet leave it snug enough that the distributor will hold its position.

Start the engine. If you’re using an adjustable timing light, set the light to 36 degrees advanced. Now rev the engine while observing the timing marks with the light. You will notice that the stock line on the balancer will move up towards the timing plate as rpm increases. Continue to increase rpm until the line does not move any further (centrifugal advance is “pegged out”). Once the timing is “pegged out,” the line on the balancer should line up with the “0” mark on the timing tab. Rotate the distributor to achieve this.

If you’re using a non-adjustable light, perform the same process, but align your new 36-degree mark with “0” mark on the timing tab.

Shut it down.

Pop the cap and rotor and re-install the spring, if you removed it. Put everything back together, but leave the vacuum disconnected. Start it up. For future reference, make a note of the timing setting at idle. This is your new curb idle timing spec. Now give the engine a few quick rev’s past 3,000 rpm and verify that the full timing (36 degrees) is coming in. If it’s not, you need to change to a softer set of springs until you get full 36-degree advance before 3000 rpm. (NOTE: A stock set of springs will often not allow full centrifugal advance to come in before redline rpm. If you have heavy stock springs installed, don’t rev the engine beyond its limits to try to force full advance in.) I suggest obtaining Mr. Gasket kit part number 927 or 928: Use the gold springs on HEI systems. For points-style systems, use one black spring and one silver spring – these springs will get your total timing all in by 2500-2800 rpm, providing very good throttle response and power. The black & silver spring combo can also be used on MSD distributors if you widen out the spring hook ends.

Hook up the vacuum. Re-set your idle speed and idle mixtures if necessary to lower the idle speed. Now do a road test.

The 36-degree 2500 rpm advance curve is optimum for performance, but may require premium fuel. Lug the car around, and punch the throttle at low rpm while listening for detonation (“engine knock”). If you’re getting any audible knock, you MUST retard the timing. Retard the timing in 2-degree increments until engine knock stops. Engine knock will seriously damage engine components if not corrected. If you get no knock, you may see slightly improved performance at 38 degrees total timing. This is particularly true if you’re running at high altitude.

If you have no engine knock under acceleration, but the car “chugs” or “jerks” at cruising speed (light throttle application), you are getting too much vacuum advance on top of the mechanical advance. You may need to change out the vacuum advance diaphragm with a unit producing no more than 16 degrees of vacuum advance. See my paper on “Vacuum Advance Control Units Facts and Specs” for more info on this.

Your timing is now set for best possible performance. Make note of the new setting, and use this for your future tune-up work.

Lars’ Suggested Timing Specs for GM V8 Performance Applications:

• 36 degrees total timing (vacuum advance hose disconnected), all “in” by 2500 rpm
• 18 degrees initial timing at idle (vacuum advance hose disconnected). Note that it may not be possible to achieve the 18-degree initial spec with the 36-degree total without modifying the distributor advance stop system. It is more important to achieve the 36 total than to hit an exact 18 initial. However, if your initial timing is very low (below 12 degrees) with the 36 total, it is important that you repair or modify your distributor in order to achieve correct engine performance
• 16 degree vacuum advance control unit with a pull-in spec that allows the full range of vacuum advance to be pulled in at the engine’s idle manifold vacuum level. Connect to manifold vacuum for most applications (this will allow the engine to idle with actual timing at idle of 34 degrees).

Questions, Comments & Technical Assistance
If you have questions or comments regarding this article, or if you notice any errors that need to be corrected (which is quite possible since I’m writing this from memory…), please feel free to drop me an e-mail. Also, if you need any technical assistance or advice regarding this process, or other maintenance issues, feel free to contact me:

V8FastCars@msn.com
(This has been re-printed with the authors permission.)

 

[Vette79]

Thanks Tim. We're running 36d total @ 3000 with vacuum disconnected.

 

[Jack]

I have an 1975 L-48 and plan on buying aluminum heads ...I currently have comp. 1.5 roller rockers, Edelbrock Perma Star polished billet Manifold, high energy 268 cam,

This winter I am adding the aluminum heads, flat top pistons looking for about 10.5-1 compression, msd ignition, aluminum pulleys. Mike

L48 has dished pistons ... L98 aka ZZ4 heads work real well with them with their 58cc chambers ... yes they have rather small runners about 163cc but that's fine for your 268 cam. Also, L98 heads fit your intake. L98/ZZ4 aluminum heads are very good quality ... they are GM p/n 12556463.

With L48 dished piston and GM head gasket p/n 10105117 (~ 5.8cc & 0.028") and L98/ZZ4 head, compression ratio will be about 9.7:1.

L98/ZZ4 heads match well w/ L48 dished pistons but probably not the best choice if you truly do install flattops.

If you do install flattops, Brodix IK180 is a real good aluminum cylinder head choice that'll match well w/ your 268 cam & your intake & flattops.

Your L48 cast crank is plenty strong for the level build you describe.