SneakyPete

Hey all. I have been trying to put a combo together for my 383 with Bob Madera from Marine Kinetics. He discovered that my current motor has way more compression than I thought (11.2:1). So my simple head and cam swap will necessitate a new build with lower compression pistons (and I want to make a more stout short block) for 2008 season. I am going to build the new motor over a period of time while still using my current motor. I have 64cc Vortecs on the current motor now, and will put on the 65cc AFR 180 Eliminators I just bought (rather than let em sit on my shelf while I get the new motor together) , with a Vic Jr intake and 1.5 rollers. The current cam is a Comp flat tappet 12-268-4 w/ .479 in lift and .480 ex lift.
Intake Exhaust
Duration At 0.05 224 230
Valve Timing At 0.006
Open Close
Intake 24 64
Exhaust 78 22
I was thinking of switching to the XM278 flat tappet with the following specs
Valve Timing At 0.006
Open Close
Intake 29 69
Exhaust 80 32
Duration At 0.05 234 244
.498intake lift and .500ex lift

I haven't pulled the vortec heads yet, but my plugs had no evidence of detonation w/ 91 octane and 32 degrees of timing on the old combo.

I will probably use a 50/50 mix of 93 and AvGas, and the carb will be tuned by Nickerson, and compensated for the avgas.
My question is do you think the 278 cam will be okay (and the overall combo w/ 11:1/cr), due to its longer duration, use of aluminum heads, and higher octane.

Michael1

It would be much easier to tell if you had cam specs for .050" lift.

11:1 for a marine engine is probably too high, since they are under such high load all the time. You can compensate with a big cam, but then you get into water reversion problems, since most big cams have a lot of overlap.

Michael

SneakyPete

The 278 specs at .050 are 234 (intake) and 244 (exhaust). As far as the reversion goes, I have stainless marine exhaust with 27 inch risers. They are basically shaped like an upside down U. The exhasut comes up thru the center of the manifold, rises 6 7/8 inches thru the riser, turns 90* and travels 12 inches straight back, then turns 90* straight down for about 9 inches. The water mixes at the bottom of the U, 27 inches back, so reversion should be warded off (hopefully).

jmherbert

I am running 10.2:1 with Dart iron heads and a Crane 284H (228@050). I run 92 octane and have zero signs of detonation.

That being said, if I were to do it again, I would have the pistons machined a bit to get the ratio around 9.5:1. I have the feeling I am on the very ragged edge of the limits. I called Dart, and for what its worth they said that 92 octane should be good for my application, and that their chamber design allows higher comp ratios than GM heads, including vortech and fast burn.

Call AFR and see what they say.

articfriends

iTrader: (1)

If your working with bob madera why don't you just buy the cam from him and have it custom spec'd to your application? Smitty

SneakyPete

I am going to buy a cam from Bob, but he advised that he would have to adjust it so much to compensate for the hi c/r that the performance would be compromised. My plan is to buy the cam from him for the new motor, so there is no handicap anywhere in the motor, everything is built to compliment each other. The motor I want to build won't be ready until probably the 2008 season. I was just thinking of throwing a cheap $125 cam in my current motor for this season until I get the "real" thing built, kind of like a stop-gap measure.

Ryan00TJ

iTrader: (8)

With the aluminum heads you can normally run one full point higher compression that iron heads as the aluminum acts like a giant heatsink. I've ran 10-1 on two prior SBC's with aluminum AFR's with no problems. 36* timing, 150* water temps, 89 octane and sometimes 87 is all that's available.

At 11-1 I don't think you should have a problem as long as the engine is kept cool, highest grade pump gas available, tune is on. You might back off the timing a few degrees to help things out if needed.

What type of drive? What realistic rpm will the motor WOT at?

SneakyPete

I don't know how high it would rev, but 5500 would be as high as I would spin it. The old combo went to 4400 rpm with a 25p mirage. The drive is an alpha, and I will be nice to it.

Ryan00TJ

iTrader: (8)

I believe the XM278H to be too wild for your setup. It has
61* overlap @ .006" and 15* overlap @ .050". This compared to your current cam of 46* @ .006" and -1* @
.050". As you can see from the specs the XM278 is going to be very hard to get to idle behind an Alpha One that needs a low rpm idle speed to shift. Just going off the specs and overlap your going to have to idle that cam 900-950 minimum to stay lit.

I made a bad cam choice and went with a large solid lifter 257/269 @ .050". It was a pure PIA! It would not idle under 950 rpms and had to be tuned rich as could be to stay lit at that rpm. This was also behind an Alpha One. It was also a safety concern as if not throttled right the motor would die. This meant only I could drive the boat. Combined with the AFR heads the boat screamed but the drawbacks of the large cam were not worth it.

If you are going to stay around of under 5500rpms then the XM278 is way overcamming the engine. With those heads and SCR, peak HP will be far past the 5500 mark meaning you will not be getting the full potential of the cam. The XM278 is designed for Jet Boats that don't have to worry about idle speeds. I would also ditch the Vic Jr as it is not suited for a motor that will never see over 5500rpms. Your leaving easy power on the table with it vs a Performer RPM or RPM Air Gap especially in the midrange.

Just adding the AFR heads is going to net you a heckuva noticeable increase in power, at least mine did! Your current cam combined with the AFR's might suprise you. If you do want some added topend, the Comp XM270H is not a bad choice and will still be Alpha friendly. I don't think you should have any reversion problems but it's always safe to check.

HTH

SneakyPete

Ryan,
The desktop dyno showed a small increase in HP with a corresponding small loss in TQ with the 278 so I think you are right. This is probably the stupidest reason for changing a cam, but I would like a little chop in the idle. The 268 is super smooth, and will idle under 500rpms. I think I will follow your advice and stick with the 268 and see how it does with the new heads, the new exhaust, new intake, and new carb.

Stupid question for ya--I just got the AFRs today. I noticed the stem of the intake valves stick up past the retainer, much more so than the exhaust valves. I can't remember it being like that on my vortec heads. Is this going to necessitate different size pushrods for intake and exhaust?