So with $3,500 to put in the engine what combo do you guys think I should go with?
And now back to our regularly scheduled topic:
What is reusable from your last engine? You said the cam went flat; did that trash the crank, block, pistons, etc.? Do your rods have the 3/8 bolts or 7/16 bolts? If you have the 7/16, you can put some decent worry free power to them. I think people way underestimate what stock parts can handle. If you are on a budget, no point in spending money just for overkill on something you are not stressing anyway. It depends on how hard you are going to kick it in the guts. It would suck to have rod break and take out a really nice expensive head.
I wouldn’t recommend a longer rod in your application. Longer rods will reduce friction by reducing piston skirt to cylinder wall loads, but might increase the tendency to detonate because of increased dwell time at TDC. A shorter (stock length) rod will favor your lower flowing heads by dwelling at BDC longer, allowing more cylinder filling. Shorter rods also dwell at TDC less (not much) compared to a longer rod, possibly reducing tendency for detonation.
According to Chevy High Performance: Cylinder-Head Flow-Bench Database, the GM 049 iron oval port heads flow:
Lift Intake Exhaust E/I
.050 37 24 64.9%
.100 59 52 88.1%
.200 124 98 79.0%
.300 188 128 68.1%
.400 232 152 65.5%
.500 250 177 70.8%
.600 264 187 70.8%
.700 264 192 72.2%
.800 265 192 72.5%
Avg. 206 147 71.4%
Note: 2.06/1.72-inch valves, 122cc chamber, 253cc intake port. Exhaust flow numbers are with pipe installed. CHP recommends installing the larger valves if horsepower is your goal.
I am now thinking of recanting my former statement and recommending upgrading to the larger 2.19/1.88 valves along with a good 5-angle valve job. I would leave port work for an off-season upgrade, with the exception of slightly blending the transition between the as-cast port and the machined surface of the valve job. Get it together and see how it runs, then contemplate the costs/benefits of port work vs. aftermarket cylinder heads. Realistically, I think you would only gain about 40CFM max (if that) with extensive porting. That would get the flow in the realm of 990 or 188 castings but could make better power because of more quality airflow through a smaller port (increased velocity). My opinion here is that if you have to do extensive porting to meet your performance goals, you have the wrong head to start with.
Looking at the flow numbers, the intake port flow stalls at .600 lift and the exhaust port flow stalls at .700 lift. Opening the valves beyond the stall point of the port does not yield any additional flow, however the valve is held open at peak flow for a longer duration. The Exhaust to Intake Flow Ratio is a little weak (<75%). A low E/I ratio is indicative of a weak exhaust port and not of a strong intake port. Choosing a cam with a split pattern will help the head flow characteristics which is not being helped by the large diameter of your header primary tube diameter.
Cam, Carb, and Compression:
Camshafts
The cam that I would select is most likely going to be controversial to what others would suggest for your application. It is my belief that if you are not taking full advantage of your cylinder head’s flow potential, you are leaving horsepower on the table. However, as was stated earlier, you need to get your engine to produce good power in an RPM range where it will be usable in your application. If you can at all manage it in your budget, I would strongly recommend a hydraulic roller cam/lifters. My choice for cam would include as aggressive a tappet acceleration rate as possible, to provide maximum torque. You will also need good valve springs.
The intake lobe @ .050 lobe lift duration should be in the 218 to 224 degrees range. That should produce a peak power in the high 4000RPM to low 5000RPM range. Here are some intake lobe examples from the CompCams catalog:
Extreme Energy Lobe #3192, 276* Advertised Duration @ .006 tappet lift, 224* Duration @ .050, .378 Lobe Lift, .643 Theoretical Lift w/1.7 Ratio Rocker @ “0” Lash.
XFI Intake Lobe #3014, 268* Advertised Duration @ .006 tappet lift, 218* Duration @ .050, .356 Lobe Lift, .605 Theoretical Lift w/1.7 Ratio Rocker @ “0” Lash.
XFI Intake Lobe #3015, 274* Advertised Duration @ .006 tappet lift, 224* Duration @ .050, .358 Lobe Lift, .609 Theoretical Lift w/1.7 Ratio Rocker @ “0” Lash.
Again, some exhaust lobe examples from the CompCams catalog:
Extreme Energy Lobe #3194, 282* Advertised Duration @ .006 tappet lift, 230* Duration @ .050, .389 Lobe Lift, .661 Theoretical Lift w/1.7 Ratio Rocker @ “0” Lash.
XFI Exhaust Lobe #3036, 282* Advertised Duration @ .006 tappet lift, 230* Duration @ .050, .355 Lobe Lift, .604 Theoretical Lift w/1.7 Ratio Rocker @ “0” Lash.
You can gain a little lift by going to a 1.8 ratio rocker. Be advised that the geometry of the rocker will dictate where the added lift is applied to the valve. Some high-ratio rockers will apply the increased ratio at peak lift while reducing low-lift. Some will increase low tappet lift numbers at the valve, increasing your @ .050 duration (not advertised duration) and will make your engine perform like it has a bigger cam installed. Please see
http://www.socaljetboats.com/engine-mechanical-electrical/1-7-vs-1-8/msg223042/#msg223042 for additional info on rocker ratio.
Ready for more controversy? I like tight centerlines, and have run cams in the 106* to 108* in just about everything I have done. More overlap, by decreasing the lobe centerlines, is necessary for top end horsepower. It has been stated many times that large overlap numbers are not “wet” exhaust friendly. This is another compromise that you will need to make. Can you live with running “dry” exhaust (not recommended in your family boat)? OR! Can you handle, as a captain, operating a valve to shut off the water to the headers at low RPM? You can also shim the spring in the Bassett T-valve to increase its cracking pressure, as a secondary failsafe, but you may hurt your engine getting that right. My new thinking on wet exhaust is to run .028 MAX ID jets in the headers with as short a run of hose as possible between the Bassett T-valve and the header water lines so that the pressure in the line drops faster, but that is a whole different topic.
Tight centerlines increase power output, but vacuum at idle and cruise is decreased along with fuel economy. Wide centerlines, 112* to 114* will improve idle quality, increase vacuum at idle and cruise, and decreases fuel consumption. Wide centerlines, and therefore less overlap, will also prevent reversion as the intake and exhaust valves are open at the same time for a shorter period of time as well as at lower lift levels. Basically, wide centerlines will be more jet boat and user friendly. Again, if it was me, I would have the cam ground on a 108* centerline and advance the cam 2* to an intake lope centerline of 106* to start. Also invest in a good true roller timing chain that will easily allow you to adjust cam timing. A gear drive or belt drive will eat away at your budget and can be added later if you like. I like gear drives personally because, if set up properly, will provide rock steady timing that will never changed like a chain or a belt. With that being said, you will be shocked at how much your chain will stretch after just a few hours. You might even consider advancing your cam one more degree to 105* to compensate for that stretch as the cam timing will retard as the chain stretches.
Carbs
What carb and intake do you currently have? Since you are limiting your RPM to the low 5000 range, I would consider an Edelbrock RPM AirGap to maximize torque and economy throughout your RPM range. However, if you wanted to maximize your top end horsepower to reach your speed goals, a good MODERN single plane would be your best choice. If you want to go to the expense and complexity of a tunnel ram, that can be made to work well and give you the associated cool factor. Just keep in mind that you might only be touching the affective operating RPM range of a modern single plane, which could hurt your cruising speed economy and not really help you on the top end. My thinking on carbs has always been to have “just enough” carb for the application to increase midrange drivability. The new school of thought is “BIGGER IS BETTER” when it comes to drag racing and jet boats. It seems that I have been reading recommendations of minimum 850CFM and even square bore carbs pushing 1000CFM. Yet others claim going to a Dominator will give your more horsepower at the same CFM rating. Not too long ago, the carb companies were claiming better drivability and more torque from the big square bore carbs.
As for me, I am a mechanical injection kind of guy. I think going back to carbs would be a step backwards for me.
Young Padawan, ready you are not that step to take. Learn you must, together your engine to keep, before that step you take.
Compression
I would for your compression ratio to be 10.5:1 with your iron heads. Advertised compression ratios never come out where you expect so make sure you measure everything. I would make every effort to get your quench distance down in the .040 to .045 range by zero-decking your block. A tight quench distance will increase burn efficiency, power output, and resistance to detonation.
Another word of caution. When you upgrade to aftermarket heads, your compression ratio with your iron heads will crash big time when you install aluminum aftermarket heads. 1.) The combustion chamber sizes of aftermarket heads all seem to be larger than 122cc. 2.) Aluminum will conduct more heat away from the combustion chamber, reducing burn efficiency. The general rule of thumb when switching to aluminum heads is that you run one more point of compression or one more pound of boost to compensate. Aftermarket heads are not exactly a bolt-on upgrade that you can just slap on in an afternoon and get more horsepower. That too should also be planned. Not all head manufactures offer this, but Brodix offers angle machining (not angle milling) of their heads at time of order to produce rather small combustion chambers. This will allow you to get the compression ratio back up to where it needs to be without decking a huge amount off of your new (expensive) heads.
After spending all this time writing and examining these numbers, I think that it is possible to get to the 500HP mark. 550HP is definitely unrealistic until you start getting into head work and even then, without spinning the RPM up, you are not going to make a lot more than 500 (best case). I would now start considering adding a nitrous kit. Even with stock internals, your engine should handle a small fogger system. Just remember nitrous is addictive and you WILL find the limit the weakest link in your engine.
That centrifugal blower is looking better and better all the time. Building an NA motor is like driving around with half of your plug wires off. Maybe it is just me, but I would put my whole budget into having CSU convert my carburetor into a blow through, a boost retard ignition, and a Procharger (or Vortech); then bolt it onto a sound stocker, and have some fun learning your entire boat. Then build the engine that you want and crank up the boost as budget permits. That is until you reach the limits of your hull.
I hope that this helps.
Cheers,
Joe
