Hey guys,

I'm running a 383 with a SuperRam, old AFR 190cc heads, and a Lunati Voodoo cam (211°/219° @ .050", 112° LSA). I've realized the Voodoo is too mild for my engine, so I'd like to move up to something bigger. I happen to have a CC XR282HR (230°/236° @ .050", 110° LSA) laying around, and I also happen to be about to take my timing cover and intake base off for something else (a whistling crankcase vacuum leak that seems to be coming from one of the two places). It would be easy to swap the cam out then, but I'm concerned my compression may be too low for a larger cam - I'm at 9.37:1 right now, and I know the bigger cam will further reduce my dynamic compression ratio. I've considered swapping the head gaskets too, but by going down to a .026" gasket (which leaves me at .039" clearance on my tightest cylinder), I only gain about half a point of compression. I am also not sure if I can get the heads up off the studs with the engine in the car.

Would it be worth it to swap in the 282HR with my current setup, or would I end up with poor driveability and power with such a low DCR?

 

If it were mine I'd swap the head gaskets to pick up the compression and run it.

 

I agree, run the cam and swap the gasket.

 

Thanks. Do any of you guys have experience getting studded heads off in-car? Not sure if they'll clear.

 

 

I would probably look at something between the two cams. I bought my car with the stock heads and a 236/244 @ .050 112 lsa cam, and while it made 408 rwhp, it had no power below 2500 rpm, even at 10.3:1 compression. I recently had the heads ported and milled to bump the compression to 11.5:1, and I gained over 100 ft/lbs of torque at just 2500 rpm. Before I had to keep the revs above 2500 rpm while cruising to keep from bogging down, which means I was driving 80 mph in 5th gear. Now I can drive 70 mph in 6th, doing just 1800 rpm (I have a 3.90 rear end). It's definitely worth the effort to bump the compression.

 

You should be able to pull the studs while the heads are on. They should just be finger tight and take minor effort to loosen them. Good luck.

 

This is a graph from comp cams on that cam in a 350 with 9.25 compression, seems to be doing OK. I think you will be fine. Your DCR will be around 7.2 with a SCR of 9.9. Not great but still OK.

 

I do agree with your feedback and thought process. I think the 9.9 CR is boarderline but should work in a 383 with his cam. If I was building new I would have more compression and a different cam.

 

Dang, amazing how a couple of degrees advance makes that much difference. My torque curve didn't have that shape until AFTER the compression bump.

 

Good info guys, thanks.

Thanks for the info. That's a pretty dramatic change from another point of compression, although that cam is 6/8 bigger than mine. Is that a 350 or a 383?

Yeah, I've pulled a few studs before to re-seal, but it's such a pain making sure oil doesn't get down into the coolant passages and vice versa. Also, a few of my studs are kinda stiff going in (really should have chased the threads in the block when I had it out of the car, but oh well), so I'd be nervous about the possibility of not being able to get them back in. I'd rather not mess around with taking the studs out, even if it meant sticking with my current cam.

That graph looks great. 340 lb-ft at 2500 and 9" Hg (about 30.5 kPa) at idle, and that's on a 356. Granted, carbureted doesn't quite parallel my SuperRam setup, but it couldn't possibly be less driveable going from a 356 to a 383.

Couple degrees advance? I don't see any advance numbers, am I missing something?

EDIT: Wait, 9" of vacuum means 30.5 kPa below ambient, not 30.5 kPa absolute pressure. So that's about 70 kPa on the MAP sensor at idle, compared to 30-40kPa currently. Am I gonna notice a big difference there? And will I be fine for now with stock manifolds? I've been delaying on the headers because all I can get in CA is shorties.

 

347. Stock bottom end, stock LS1 heads (I have a 2004 C5 LS1). The heads were ported and milled to bump the compression.

Actually, the Victor Jr intake is a single plane intake. You should see better low rpm torque but less top end hp.


Look at the intake lobe centerline. 110 deg lsa - 106 intake lobe centerline = 4 deg of advance ground into the cam. And don't change the cam without getting headers - a cam that big REQUIRES headers. Although the super ram might partially compensate for the headers, they will still easily be worth 20-30 hp with your setup, mainly by extending your effective rpm range.

 

Compression is a two sided sword. Too much and you have to add race gas, too little and your looking at taillights all the time.
I rebuilt a 427 that came from my 1968 Corvette convertible, I was trying to find power any place I could find it. Turbo's and super Chargers were not ready for prime time in the early mid 1990's. I went and read anything I could find on the subject and settled on a old fashioned high compression 427 engine. My compression ratio is 12.25-1 now and has been for over 20 plus years, when I crank it to check the compression I get numbers in the 240-245 range. When my pistons are at TDC they rise out of the block by almost 3/4" and fit very snugly into the appropriate pockets in the Cylinder heads I chose. The heads I have are "Closed Chamber" used on Corvettes in the 1960's and early 1970's. Open Chamber heads are the norm today but I love those cast iron heads I have, I spent over $1000 rebuilding these heads.
There are a ton of problems with running HIGH Compression engines.

First: Cranking the engine will require a special gear drive starter to bring it to life
Second: Getting the correct fuel for your setup will be an issue, you can no longer stop and dump in cheap gasoline. You will need octane and lots of it! My engine requires 106 octane to run properly and this is important when it gets warm outside.
Third: Your Ignition system will need to be upgraded as it is hard to light off a plug under high compression.

Do not get me wrong, there are answers to all the issues today. The one biggest mistake was to use 100 octane "Low lead" Aviation Gasoline because it was readily available and inexpensive. After about a year of this gasoline the spark plugs had huge lead deposits on them. I pulled both heads and re-did them because of this. Pulling Big Block IRON heads off the car while the engine is in ain't fun, those darn things are heavy. Aluminum closed chamber heads are still not available so I still used the old heads. By the way the reason I am hung up on the closed chamber heads is because they don't have the knock issues that open chamber heads have. My ignition system is a MSD 6AL with a "Multiple Retard Box" which allows me to pull the timing back up to 20* which makes the car crank like a Chrysler.

Over the years I finally got smart and I bought a Methanol/water injection system from SNOW Performance. It injects the mixture when my engine is under a load and really bumps up the power while cooling down the combustion process. The gasoline Octane is bumped up by the methanol up to the maximum octane of 115. This engine of mine really likes the high octane and the extra cooling, I wish I had installed this device back about 20 years ago. On my car with it's carburetor made it a bit challenging, SNOW Performance uses a 1" aluminum spacer in which they mount their nozzle to spray the fine mist into the intake system. I have an original L-88 hood under which there was not enough room for that 1" and not wanting to damage the hood I decided to drill, tap and mount two spray heads under the carburetor going through the sides of the EDLEBROCK RPM Plus Air Gap intake manifold. It makes people think that I am using Nitrous when the hood is open as the lines are there in the open.

The extra heat generated by a High Compression engine will generate substantially more heat than before, this is a real pain as I went to a Aluminum Radiator and dual Spal fans on a shroud. Next I went to a higher flow water pump, it was marginal and that pushed me to another great find. EVANS Cooling NPG is a coolant that won't boil until it gets over 375 degrees and it requires no pressure in the system to work. This coolant works very well in hot running engines and has lasted over 20 years in my car without hurting the radiator or damaging anything else.

High Compression is fun and easy to do, just be very careful how far you go doing it. I would like to drop mine a point or even two for longevity. When I went to the racetrack one day I could not stop spinning the rear wheels until I was past the 1/8th mile mark on a 1/4 mile track. I have enough power to generate enough speed to scare the hell out of myself, I just don't let anybody drive this car as it can get out of control in a big hurry if you are not careful. The one thing I did that mention that I would strongly suggest is investing in a MSD Knock Sensor system. Simple (cheap) insurance to protect what you can't hear over my long tub headers and chambered exhaust system with the top down. By the way if my water/methanol storage tank gets low you just add another bottle of the Blue windshield washer fluid into the tank and you are ready for another five-hundred miles.

Good Luck but carefully weigh each option before jumping in with Higher than normal compression! If I were you I would change the head gasket and see what happens!

 

How are you getting 245 psi on a compression check at only 12.25:1 static compression? 245 psi indicates a dynamic compression ratio of over 16:1! And that's calculating it at sea level. No wonder you have to run race gas and use a high torque starter.

 

It does not work that way since the compression of air heats the air and causes expansion which raises the cranking compression.

 

Not that much. The most I've ever seen a 12:1 motor make on a compression check was 160 psi and that was with a cam in the 230-ish range. Either he doesn't know his true compression ratio or he's running a cam with about 180 deg duration.

 

OP are you worried about "really" passing smog? It wont wiht the larger cam
Long tubes will make a difference.

 

No chance of passing? I'd heard a good tune could get cams that big to pass?

Sadly, long tubes are an auto fail on smog here. And I'm having trouble tracking down even a set of smog-legal short tubes.

 

Do you have to pass a tailpipe test or a computer test?

 

I think this cam is too big to pass the sniffer unless you have a bare bones tune to lean it out enough just to stay running for the sniffer and then switch to another custom tune for regular driving.