AIR TIME

thanks too .

cstraub

Here are the before and after numbers on Rotax's heads:

Numbers are at 28" of of a SF600 Bench. This is flowed with a 4.500" fixture. Intake had radius fixture and exhaust is with 1 7/8" pipe.

GM #12363390 Alum Oval Port Head-stock
Intake Valve 2.250 " Exhaust Valve 1.800 "
Lift: inches Flow: cfm Lift:inches Flow: cfm
0.100 74.0 0.100 71.0
0.200 143.0 0.200 126.0
0.300 207.0 0.300 158.0
0.400 250.0 0.400 178.0
0.500 284.0 0.500 200.0
0.600 309.0 0.600 218.0
0.700 322.0 0.700 232.0
0.800 0.800

GM #12363390 Alum Oval Port Head-ported
Intake Valve 2.250 " Exhaust Valve 1.800 "
Lift: inches Flow: cfm Lift:inches Flow: cfm
0.100 85.0 0.100 73.0
0.200 161.0 0.200 141.0
0.300 230.0 0.300 175.0
0.400 283.0 0.400 208.0
0.500 332.0 0.500 234.0
0.600 354.0 0.600 257.0
0.700 361.0 0.700 274.0
0.800 363.0 0.800 287.0

Just referencing the numbers at .500" you are looking at a cam that needs about 6 to 8 degrees of split favoring the exhaust @ .050". The heads at 70% I/E ratio are actually really good for a BBC.

Power out put, based on the Super Flow formula of CFM x .2552 x No. of Cylinders = Possible HP at 100% VE. I will reference .600" and 309 cfm. HP could be 630HP

Now look at the ported heads. We reference .600" lift and 354 cfm and we have the potential of 722HP.

Now is it easier on a machine to run it at 100% capacity or 80% capacity. Making HP relies on how much air you can get in the engine. It is easier to make power with more air because other parts don't have to be maximized. If these engines were both going to run at 5200 rpm, 1 had the ported heads and one the "out of the box" heads the engine with the ported heads would have the smaller camshaft because the head is doing more work. Therefore the camshaft doesn't have to do as much. Long term this engine will be less maintenance.

I hope this helped some more.

Chris

Now

LAKE EFFECT

I replaced my ZZ cams during my rebuilds last year with the Crane 168741's. I really didnt notice any big improvement over the GM cams though(maybe 100 to 150 rpm). I dont think it was worth the investment. Two very important points, if your going to swap cams though: do the lifter swap, the stock lifters drop pretty far down the guide on the new cams(I checked). I know there pricey, but its a gamble not worth taking(I used the 13532's, either will work). But more importantly make sure you buy the billet camshafts(gold cores), and not the induction hardened(grey core). Why you ask? Because the induction shafts can and will break(@ the dist gear end, where the two pieces are joined). I learned the hard way. After talking to Crane, they admitted that the cam could break in marine applications due to constant rpm, and harmonics(especially @ 4000rpm). The exact rpm I was running all weekend on a long cruise. Fortunately it just kills the distributor and oil pump, and the valvetrain stays in time. I was able to swap the cams out in the boat and with a strong magnet get the broken piece fed thru the journals. Surprisingly a very clean break, and Crane replaced the cams with billet ones. FYI, this happened to me one week before the July 4th weekend, and the only cams Crane could overnight me were Mercury HP525 cams(#OEM 0888), which have all the exact specs as the 168741's, except they are ground on a 114 lobe separation, as opposed to the 112 of the 741's. I guess that means I have HP525's in my old 311!

fandango

Ditto on what Lake Effect has said. I have read alot about the difference on construction on the 731 cams. Am doing the exact same thing on (2) brand new zz502 short blocks. Purchased (2) 168731 cams from Jegs this week and they showed up with the black color (different hardening process) and have a somewhat history of breaking. Called Summitt and they pulled every 731 cam. As of right now they had 5 in stock. 4 were copper colored, the good ones ( I purchased (2) and (1) was black in color (buyer beware). The boxes in the warehouse were then marked in color so you should be able to dictact what color you want. If you want more info, call Crane and ask for Dave Maxwell in tech. He will confirm problems.

Ed

TROUBLE MAKER

Sounds like the HP525 OEM cam might be the best bet with the 114 degree lobe separation... Is that the actual part number (#OEM 0888)???

ratman

i have a used 741 hyd roller cam if you decide to go with the 741cam. ratman

boatn70

and im still sitting on as full set of lifters and cam from a zz502, if interested, all brand new never run.

Donman

FYI :

The 731 cam that I would like to sell is a billet ( gold ) cam.

Is $150 shipped, a fair price? That`s what I was thinking of asking for it. It looks brand new. Still have the cam card too.

LAKE EFFECT

When Crane set me the new cams, they came with new cam cards, and thats the part# on the cards. OEM0888

TROUBLE MAKER

Thanks...