Honestly right off the bat ive got 2000 bucks to put in it. I want to to do full roller. Motor is completely stock at the moment. Pump is stock JS right now. Going in next week for a b impeller unless you have a different opinion. I dont even know what rpm range i expect. Like i said im starting this with little to zero knowledge. I cam from building jet skis lol
I have been stewing over this for some time now. I’m making a judgment call here; I would recommend a hydraulic roller over a solid roller if you are adamant about going roller. HR cams are much more user friendly than solids. Solids will require much more attention to adjusting lash etc. and do not like a lot of idle time. I have had good luck with the Crane solids, but I have heard that the hot ticket to making solids last is Isky needle-free EZ-Roll lifter. The bad news is that a cam, lifters, pushrods, rockers, valve springs, and probably locks & retainers will eat up most, if not all, of your $2000 budget, not to mention any machine work, gaskets, etc. that will be required to make a bigger cam fit. The really bad news is that your heads will require some machine work to reduce the valve guide height to get much over .500 lift as well.
If that doesn’t scare you away, contract with a reputable engine builder, who understands race jet boat engines, to handle your heads. It would also be a good time to do a performance multi-angle valve job while you have the heads off. A good multi-angle valve job alone will most likely net you a few more horsepower.
If your short block is in good condition, you will be compression challenged, which will ultimately limit how big (duration) of a cam you will be able to run. The engine will be a dog as the delayed intake closing reduces combustion chamber pressures. The only way to make substantial gains in compression is to change the pistons. Flat milling the heads .030 could yield a half point of compression Angle milling is an option and could yield slightly more compression, but it overly complicates things and then the intake manifold becomes married that set of heads. In my opinion, substantial milling of your heads is simply not worth the reduction of the head’s rigidity which could lead to warping and blown head gaskets. If that happens, there is no more material that can be safely removed from the heads necessitating the purchase of new heads.
Lower compression isn’t necessarily a bad thing as it will force you to keep your duration down to a more jet-boat-friendly level of less-than 225 degrees at .050 on the intake side. Durations that short will also dictate how much lift the lobe profile will be able to have. Although I have never even considered running logs in any kind of performance application, I believe that a cam of 224 degrees @ .050 duration would be pushing the limit for reversion. Advancing the cam timing would help retain cylinder pressure and would possibly reduce the chance of reversion. Be careful, advancing a cam will decrease piston to intake valve clearance.
Personally, I like an aggressive lobe profile that opens the valve quickly with as much lift as I can get. Aggressive lobe profiles will typically have high lift to duration and the difference between advertised duration and duration at .050 will be less. Mid .650 valve lift is about the most valve lift I have seen for a lobe with 224 degrees @ .050 duration utilizing a stock ratio rocker. Bottom line, I am a big fan of big lift.
There are going to be many variables that influence exhaust lift and duration. If your heads have a weak exhaust port or a poor flowing exhaust system, increased lift and/or duration can help overcome these deficiencies. Some engine builders prefer to do this with rocker ratio rather than at the cam lobe. While on the topic of rocker ratio, all brands of rockers advertise similar ratios. The reality is that even though advertised ratios may be the same across brands, valve lift at any given amount of tappet lift may be different. Some high-ratio rockers increase low lift numbers while others increase high lift numbers, but not both. If a high-ratio rocker increases valve lift @ .050 (tappet lift) duration, the engine will “see” the equivalent of a bigger cam @ .050 with no change to total advertised duration. But that is another story for another time.
Lastly, is lobe centerline. I have always liked a tight lobe centerline around 106-108 degrees, typically installed 4 degrees advanced. A tighter lobe centerline really seems to make an engine pop harder but you have to have the compression to back it up as it increases overlap. Increased overlap will bleed off cylinder pressure at lower RPM and will further aggravate an already bad reversion situation.
As hooked on tight lobe centerlines as I am, the current trend in jet boat cam lobe centerlines is in the 112-120 degree range. Even though a wider centerline will reduce tendency for water reversion with wet exhaust, every application that I have discussed cam timing for has been in a performance (9 second and faster NJBA) application with dry exhaust.
If you are still not scared away, I would start by contacting some cam grinders, particularly those who have been pushing modern lobe technology, and ask a recommendation for your application. A good cam grinder will be able to tell you exactly what each grind will require. Then think about what other changes to your budget and engine you are willing to make, i.e. carb, intake, exhaust, pistons, etc. all while thinking about the butterfly affect and the potential can of worms you are about to open. Lastly, make sure that you check every clearance yourself as it is going together. Just because “it should fit” doesn’t mean that it will fit.
Good luck,
Joe
