weimer20

I'm putting together an engine soon for my 73 and will be using a set of aluminum heads. I understand that you can bump the DCR up a little with aluminum and remain pump gas friendly. How much though? I screwed this one up on my last build of an iron head 383 and now have to run super while flirting with "the rattles". Don't want to make the same mistake twice. Any thoughts / opinions?

63mako

The rule of thumb is 8.5 max with aluminum heads, 8.0 with iron. That said to run that much you need everything perfect. Quench, operating temp, air/fuel mix, timing curve all have to be right on. 8.25 is a safe ratio. In Cali you don't have 93 octane available. That will reduce the amount of DCR you can safely run. I would shoot for 8.0 in your situation.

SLVRSHRK

I've got 7.8DCR and 9.5SCR on mine and it runs with all 36' of timing in at 2500 RPM without issue (93 octane Amoco/BP). 180' thermostat and I replaced all the seals and radiator (3-core HD) when I built the motor.

Heads are old '69 041 cast iron GM units. Quench is at .040".

If you are going to run at the upper edge, you really need to make sure get it ALL right. Not too tough actually, just requires a bit of forethought.

Good Luck

couperdecar

my car buddies have always said you can run 1 point higher with aluminum compared to iron

Grinchia

I am NOT any authority on this, but I run a big block with an L88 cam, SCR of just over 11. I used to add lead to 91 octane, but have found that I get no pinging even without the additive, so I don't use it any more. The only time I've had pinging was one time when the distributor accidentally got turned a little too far while adjusting some things and I inadvertently took a short road trip with about 20 degrees of advance at idle

That said, it's my understanding that big cams with large overlap bleed off a lot of compression and are therefore more forgiving than milder cams when it comes to high compression.

BarryK

I don't know enough about the subject to debate it with you but on my '65 with the original motor (stock) the compression is 11.0:1 and I run 93 octane pump gas with zero pinging or knocking from detonation with the timing at 36º all in by 3000rpm and the factory vacuum advance which gives a max timing of up to 52º. No snake oil additives or anything, just the regular 93 octane from the pump available from any gas station around me.
Dropping down to 8 or 8.5 CR seems to be a drastic drop to just run pump gas.

EDIT: ok, my '65 L76 motor has the 30-30 factory cam which is a pretty radical cam from the factory for street use, probably the most radical cam they ever put in short of the L88, so based on what Grinchia said perhaps the large overlap of the cam is what lets me get away with the higher compression level on pump gas. Don't know......

js292

I beleive your 11;1 ratio is the static compression, not the dynamic which is a lower ratio.

63mako

Static compression is the compressio ratio you get if you lower the piston all the way down, close both valves and bring the piston all the way up. Dynamic compression is the actual working compression the engine has after taking into account the intake closing point and overlap. The later the intake valve closes after TDC and the more overlap (time both valves are open) the less DCR you have.

Those old school performance grinds had slow ramp rates and a bunch of overlap because of it, bleeding a lot of cylinder pressure off, especially at low rpm, lowering your cylinder pressure in the lower rpm where you are most likely to see detonation. That is why they pull hard at the upper end and really need a lower gear rear end to be effective, you get the engine into the operating range of the cam faster.

The newer grinds have faster ramp rates that can't handle as much static compression as the older grinds, if you compare duration @ .050 and still run on the modern lower octane fuels. The older high performance engines need careful tuning, even with the slow ramps to run on todays gas. The modern fast ramps and higher lift cams will pull more power out of less compression in a wider operating range, but the same duration @ .050 can't run as much static compression.

Example: A high performance modern cam might have a .050 intake duration of 242 and a total intake duraton of 304. The old school grind might have a .050 intake duration of 242 and a total intake duration, as in the cam Barry has, of 342 (slower ramp rates). Big difference in overlap and actual intake valve closing point. Also a big difference in the amount of static compression you can run on 93 octane. You might be able to run 10 to 1 with the modern cam with iron heads and be able to run 11 to 1 with the old school cam with iron heads due to the difference in intake closing point and overlap even though both cams have a 242 duration @ .050

I don't know if this is confusing as I am not real good at putting my thoughts into words sometimes.

BarryK

good info, thanks!
I don't know a damn thing about cams so always interested in learning new stuff.

63mako

Edited my post trying to clarify a bit. Don't know how well I did though.

MotorHead

Anything over 360 deg duration, you are in trouble

63mako

Barry's cam has an intake and exhaust duration of 342 from base of lobe to base of lobe. .000 to .000. The 1970 LT1 cam has a total intake duration of 317 and exhaust duration of 346. The L88 cam has an intake duration of 354 and exhaust duration of.......................360
( information from Corvette by the Numbers by Alan Colvin) Think them L88's had a little bit of overlap! That is how they handled over 12 to 1 compression

68/70Vette

A couple of anecdotes:

I have a ZZ4 with 10:1 compression ratio. Gasoline here is 91 octane. I don't seem to have any pinging problem. For instance, driving it around on the street, lugging the engine at a low rpm in a high gear, doesn't seem to cause any pinging.

Years ago, I had an L88 clone, and it had a 11.5:1 compression ratio (the original 12:1 compression was lowered by milling the piston domes 0.100 inches). It did have a mild pinging problem that went away when mixing 5 gallons of 100 octane av gas with 15 gallons of pump gas.

63mako

Better, modern combustion chamber design on the ZZ4 vortec's, Peak power at lower timing setting. Both help reduce detonation.

edit: Think the recommended octane rating on the L88 was 100 octane.

weimer20

I'm looking at using the comp cams xr-276hr. Hydraulic roller cam with 110 deg lobe center, 59 overlap, Int dur. 276, Exh dur. 282. Static comp works out to 10.3 - 1 with the combo I'm looking at. I come up with a DCR of 8.27 - 1. Ya think I can get away with that using aluminum heads and 91 octane California crap gas?? I want to be able to get all my timing in without the knock and ping thing going on. Heck, if I can get away with using 89 octane even better! Gas is getting to be like liquid gold these days. Again, I appreciate all the input from all of you.

js292

I'm too lazy to look it up but what are the spec's on the LS7 cam vs L88 cam.

nastee383

This is real world, here in Florida anyway. I have an 11.07:1 396 sbc with an 8.30DCR, 244@.050 hydraulic roller cam and alum heads. I run 93octane fuel with 36 degrees total plus 13 vacuum advance. No problems with ping so far. It has about 500+ miles on it.

I also have an 11:1 383 sbc with an 8.19DCR, 242/248@.050 hydraulic roller cam and alum heads. Same fuel and same total timing but a bit less vacuum advance because I was getting the "lean surge". No problems there either. This motor has been in service for 2 years and 7000+ miles.

yellow-fever

I am using the CC XR276HR retrofit cam in my 454 with GMPP aluminum heads and pistons with 22cc domes. Quench is about .045 which gives a SCR of about 10.5 and a DCR of about 8.4 (block has been decked). I can get away with 93 octane and 33 degrees of timing without detonation but I'm right on the edge.

63mako

I think your border line with 93 octane. I would bump the cam up if your drivetrain has a low enought first gear total ratio. My 6" rod 383 @ 10.4 to 1 needed a XE 288 HR (236 242 duration) 114 LSA to get to 8.25 to 1 DCR. 93 octane will work in it. There are a couple different calculators out there that figure differently and even the head gasket bore or rod length moves your numbers noticably. I would rather give up a little power than have ping issues or possibly detonation at high RPM you can't hear.

63mako

Nice, Careful planning and properly matched components. I have seen streetable 11 to 1 motors that will run on 93 with no issues and also 10 to 1 with cams you would think will work fine that ping. Careful planning, Dyno tuning, Proper A/F, Proper timing curve, good quench, lower operating temperatures and luck are all contributing factors.