Rollers will give you about 10hp with your combo in my opinion.
They are quieter but more important to me was reduced temps!

 

I am actually glad you brough that up. I like the original High Energy Versions vs the later Extreme Energy Versions. It has more to do about lobe life and failure rates more than anything. They steepened the ramps, in the extreme energy versions, which shortened the adv duration, which is a good thing, except there are way too many instances of flat cam lobes after this happened. The ramps are so steep they approach Hydraulic Roller levels, but without the roller. If those steep ramps are what you want, just buy a HR, so it will live.
Otherwise stick with the original 268H not the XE version. Clips are from Comps Catalog. Much easier to compare.




50* hydraulic intensity (steepness) between adv duration and .050". Proven reliable.



6* Shorter adv duration numbers vs similar .050", or vice versa. But only 44* hydraulic intensity (steepness) between advertised and .050" . These steeper lobes seem to be behind a lot of the flat cam complaints. It does help TQ production a little, but at what cost.

 

I forgot to mention 1.6 RRs.
They should be safe w/stk pistons!

 

Good explanation, thanks, Leigh, I'll take the reliability advice. I have the older Lunati Voodoo 213/219 cam in my '64 Lemans, same idea as the XE I think. I love that cam for the street and have had no problems with it, but they don't make 'em anymore.

 

I was going to ask about advancing the HE cam 4 degrees to make up the lost dynamic compression, but I found this dyno test video that shows that it doesn't make much difference.

 

Leigh, you sound like you know your stuff.
I have always been curious as to how much compression my L48 has, but didn't know how to figure this out.
My car has 1980 L48 350 of course
following specs:
Eddy 5089 cylinder heads 64cc chamber volume/intake port volume 185cc, 2.02 intake valve diameter
hr276 Roller Comp Cam with 224/232
1.5 roller rockers
2101 Eddy intake with a eddy 750 carb

of course the Holley long tube headers with straight 2.5" exhaust into some Flowmaster 40's.


I am estimating 370HP based on Comp Cams specs for the Camshaft, possibly a little more not sure.
On the CR, I am guessing 9-9.2???
Short block is original, not messed with other than a better oil pump. 46,000 original miles.

 

What is the compressed thickness of the head gasket you’re using? That will affect your compression ratio.

My setup is very similar to yours and has a computer estimated peak of 347 horsepower, however I would be interested to put the car on a dyno to see the real world numbers.

Here is a compression ratio calculator on Summit’s website. A stock L48 has a 4 inch bore and 3.48 stroke. Yours heads have 64cc chambers, pistons have a 12cc dish (allegedly), .025 deck clearance, whatever your head gaskets are, and 8 cylinders (obviously.)

 

According to this my CR is appx 9.4

 

This what I was approximating it with:
Camshaft Part # 12-423-8 Grind # XR276HR-10 Duration @ 0.006": 276° / 282° Duration @ 0.050": 224° / 230° Max Lift w/ 1.5RR: .502" / .510" Lobe Separation: 110° Lobe ID#'s: 3314 / 3315 Intake Centerline: 106° Engine Type: Chevrolet 350 Small Block Bore: 4.040" Stroke: 3.480" Displacement: 356ci Compression: 9.25:1 Heads: Dart S/R Intake: Edelbrock performer Carburetor: Holley #3310 Vac. Sec. Exhaust: 1 5/8" Headers Max Torque: 427 ft*lbs @ 3700 RPM Max Power: 373 HP @ 5200 RPM Manifold Vacuum: 12" Hg @ 800 RPM & 14" Hg @ 1000 RPM w/ no load.
Also they are kind of dead on with the vacuum at 800 rpms 12"Hg
Granted mine is not bored .40 over

 

see post #2 3rd line, likely 8.6 CR
pistons are 18.6cc dish and you likely used a .040" felpro
If so, a slightly big cam for that compression, and DCR could be in the high 6s.
Flat-top pistons would help a lot with that cam!
Or a milder cam would crispen up the throttle and add TQ.

 

Not 100% on this but the places I have read, say that 1980 L48 had dished pistons with the 12cc

 

I see a lot of internet quotes like that too.
But I tend to believe manufacturers specs as more accurate.
But I can not find an old TRW/Speed Pro catalog to check the specs.

 

I measured the cc's of my 77 and came up with 16.9 cc's. Go to post #31.

https://www.corvetteforum.com/forums...er-size-2.html

 

I am curious about this too.
I did find a picture of an original 1978 L48 Corvette cast piston.

4 valve relief eyebrows, round dish in center, heavily beveled edges.

No current piston manufacturer, that I can find, still produces round dish pistons.
Because they do not work, they have no quench (turbulence), and they more easily produce detonation than more modern piston designs.
The four valve eyebrows alone can easily be 5cc, and with the circular dish, and the beveled edges, 18cc seems about right to me, vs 12cc.

From Keith Black catalog:


So what the manufacturers do with new pistons, is they go to a D-cup dish when needed, to keep the compression ratio the same as original.
So the ICON pistons I quoted above are for that application, and yield the original 8:1 CR. With a D-dish. And they have more quench, so much better resistance to detonation.

Icon D-Dish

These state an 18cc dish, and yield the original 8:1 CR, so although they have a completely different (more modern) dish shape, I believe 18cc is closer to reality then the internet myth of 12cc.

Unless anyone has any better numbers or source?

The lack of any quench with these pistons, and their being more prone to detonation, yields the "common" suggestion of 10:1 CR and 8.4 DCR way too aggressive. Since you are limited to about 9.4 CR anyway, with 57cc heads, and thin gaskets, and that is known to work well, that is probably high enough.

Even then, with these dished pistons, the best you can get, is 9.4CR, and a high 7 DCR, maybe 7.9. But it works.

Do what you can to get the compression up, hopefully 9.4, and keep the cam at 260* adv duration, and you'll have a nice responsive engine. But you will never have terrific quench. So it might need premium gas.

 

I found this pic of my engine, before 64cc heads where put on.

 

Absolutely correct. Original stock pistons have no effective quench zone as has been stated many times. A d-dish or flat top are only piston designs that have an effective quench. Of the two, I prefer d-dish. Smaller combustion zone promotes better burn vs the larger area on the top of a flat top piston. Also promotes turbulence during compression which mixes the incoming charge better creating a more consistent mixture of air and fuel for a more complete burn.
my d-dish pistons with 12.5 cc dish will not detonate, or at least I have not ever been able to irregardless of mixture or load or temperature. It's very detonation resistant at my 10.6 CR. I usually run 88 octane to 90 octane.
in my area you can get 85 and 91 ethanol free. So I mix the two often, by alternating which fuel I purchase when about 1/2 full in the tank.

I used to race motocross on a yz250. That 2 stroke engine was extremely detonation prone due to its .055" quench. I couldn't get a high enough octane to prevent detonation.
I took the head off and had .015 shaved off to get a quench of .040. It never detonated again. I ran 91 octane with no problem. Where as before I could run 95 octane and it still detonated.
Quench works.

 

Okay I found the specs on the manual to my car. This is what it states:

 

I'll make a BIG assumption. Let's just say you know for sure that your short block is "GOOD" for more miles. If you're not sure? I'd just pull it out and go a whole other way that would include more cubic inches. 383 is probably the best way to go for a car still running 3.08s and attempting to do this on a small budget.

I do realize you are thinking the original 350 short block is good enough for a simpler/cheaper route. In the video they use iron vortec heads but just about any 64cc aluminum head on the market today would work as well without needing a vortec intake. The cost would be a little more but losing 50pds off the nose of your car seems worth it to me.

This seems like a great route and tough to beat combination for a less expensive extra 130HP. The cheapo crate engine they start with has the same overall specs as an L48. The cam is a little aggressive for an automatic car with a stock converter though. This would work better for a 4 speed car or an automatic with 2,000-2,500 stall converter.

 

That video confirms what I was talking about with dyno tests.
Stock L48 235HP gross, and 365HP modified.
Minor part differences.
I would lean to a little smaller cam, and better TFS aluminum heads.
Their version will need some rear gear, and rpm.
With better heads, it would make the same power with a smaller cam, have better lower end, and like stock gears.
Your choice.

 

As far as torque is concerned the most fresh charge into the combustion chamber via high speed inake charge and good flowing heads and a short duration cam with little overlap is the successful equation,
but that would be a compromise to high RPM HP.
More overlap on the cam will allow higher RPM's allowing HP to build. How you get that overlap is important as well. Split durations are a waste unless the exhaust or heads are suboptimal, then it may be necessary.
High durations gives up a lot of low RPM torque unless compensated with higher CR. Or if the CR is excessively high may be required to bleed off CR to avoid detonation.
Overlap degrees is another way to manipulate the RPM limit. Say a 110 vs a 112 or a 108 vs a 110. Then lower intake and exhaust durations can be used without losing the more ideal overlap duration.
I would say best flowing heads, intake and exhaust you can afford along with appropriate cam to your desired performance goals are the best ways to make the car you want.
You can get away with more overlap via any of the above ways with a higher stall torque converter on an automatic and still have a nice driver.