Under the Hatches
Fine-tuning for Maximum Performance:
A Checklist of What you Need to go FAST
By Matt Fairbrass, Licensed Mechanic, D/Tech Ed, Whiteside Mechanical, Port Carling, Ontario

Greetings fellow boaters,
In this article I will try to cover some aspects of fine tuning and performance enhancements you can do yourself or have done by a competent mechanic or shop. I will share some of my secrets and tips on how to max out your power-plant (yes, even a Dippy and a Midget can go faster with enough tweaking!). My friend Mike Windsor has a saying that goes: “When was the first boat race held? When the second boat was built!”
Not too long ago, I had the opportunity to work on Ed and Linda Skinner’s lovely Ditchburn launch. Well, you say, they can’t be that fast, and they have a round bottom and all that stuff. Well, I will tell you that I clocked Ed at a smoking 25 MPH with my GPS after we spent a couple of days doing some relatively minor repairs and some other adjustments - and this was on a 1950 Buchanan Flathead 6 engine that had never been apart.
This topic could take up volumes, so I will try and stick to the basics. While much has been learned over the years about power-to-weight ratios, gearing, propellers, aerodynamics, hull resistance, and (our favourite) hydrodynamics, it is interesting to note that while speed records are still being broken on land, sea, and air, they are many times now only by fractions of seconds. Most racing nowadays has classes of machine or limitations as to what you can do and cannot do to your machine. They even have fuel limitations in some race circles, so you need to not only win but win with the amount of fuel that all racers are allocated. As we are primarily interested in our vintage boats here, it stands to reason that we are mostly running the flathead style of engine as well as some earlier V-8s from the late ‘50s to the late ‘60s, but let’s not forget the Ford flathead V-8s - now there was a game changer!
In many cases engines are not always performing at peak performance or have dropped off over the years due to many factors. I often hear people say things like, “when I first bought the boat she did 35 mph at 3200 rpm now all I can get is 25 mph and she won’t go over 2800 rpm.” Well, let’s look at this for a moment. What could be wrong here, I ask? Well, tuning for a start - overlooking the fact that the propeller might be damaged or the hull is covered in gunk or the boat is water logged. Let’s look at the engine itself. I will break down the major components that you want to watch for while tuning your machine. The factory specs are what you really want to stick to, but adjustments can still be made. Here is the simple list of first things to check before we start tweaking the mill - that’s racing talk. These things need to be in order before any serious tuning can take place.
Clogged and dirty flame arresters will run the engine too rich, waste fuel and ruin performance, so be sure to keep them clean.
Incorrect timing of ignition, not set to specs, seized or stuck mechanical advance system not allowing proper timing through operating rpm range.
Cold running engine. Stuck thermostat(s) (open) oil dragging robbing power, engine will not achieve N.O.T. (Normal Operating Temperature). Hot running engine. Stuck thermostat(s) can cause vapour locking of carburetor, hard restarting after a run. Bad water pump, rusted block and passageways.
Sticking choke mechanisms, carburetor is dribbling fuel on down drafts and puddling on updrafts, running rough will not respond or hold adjustments.
Spark plugs. Incorrect heat range, worn out, wrong gap, resistors burned out internally if equipped. Wires, distributor cap, rotor points worn out or shorted.
Compression. Poor compression, weak rings, burned valves will never allow an old engine to perform as new. You should have at least 110 to 125+ P.S.I. compression
Valve train. Worn camshafts, weak valve springs, worn timing chains, bent push rods and incorrect valve adjustments - all can rob power.
Tip 1 Indexing the spark plugs. This is the process where you try to have the open end of the spark plug electrode facing into the air/fuel mixture stream as it enters the combustion chamber. First have about three times more plugs than you need. Why so many, you say? Well, you are going to take your new spark plug and observe where the grounding electrode has been welded on. Mark this point on the side of the spark plug and note where your intake valve port is located. The idea is
that once it is threaded in, your mark lines up with the port. The only problem is that the threads all vary from plug to plug as to where they bottom out and stop. You must, of course, have them tight and don’t try to make it too loose or too tight. Cool eh? Some gear heads swear by this old school trick.
Tip 2 I am going to get heck for this one but here goes anyways. Being VERY careful and having a fire extinguisher on board, warm up the engine and go for a full throttle run and note the rpm of the engine. Now remove the flame arrester and repeat the same run and compare your rpm notes. Odds are you will find an increase in rpms. (This can be especially noticeable on the V-8 engines with the chrome flat pancake type arrester). Why is this, you say? Well, first of all you are allowing in more air to the carburetor as there is now no obstruction that is limiting the flow of air, BUT BE CAREFUL - you can also seriously lean out the engine as the flame arrestor is often designed in conjunction with the carburetor manufacturer as they try to achieve a happy balance between safety and performance, the arrester IS part of the carburetor design. You will now need to pull the spark plugs and observe the colour of the tips to see that you’re not running too hot. If you are a serious gear head then you actually do a high speed shut down where you shut off the engine at full throttle and coast in so you don’t get any false burn patterns on the plugs. I really don’t recommend this in a boat, but this IS race tuning. You can sometimes stack the arresters to allow more flow of air and still be safe. Some serious tuners will even remove the choke plate and shaft as it also can pose a restriction. NEVER RUN YOUR BOAT WITHOUT SOME SORT OF ARRESTER !!!
Tip 3 Distributor re-curving. This is where some real tweaking comes in. Your distributor has an advance mechanism that works automatically and is preset at the factory to do its thing dependent on rpm only. (In cars a vacuum unit is also used as it senses load as well.) As our engines are old and have seen many years, the adjustment of this mechanism can go out of spec. Rusted and weak springs and seized mechanisms are all too common and can noticeably affect performance. You can, however, change those specs to suit your performance needs. This is where you first need to know the factory specs, and second have access to a distributor machine. They are quite rare but indispensable here at the shop. They run the distributor at various rpms and will also be able to detect weak point tension, bad condensers, and worn shafts and allow you to set dwell angles. Also they can determine if each lobe on the distributor cam is firing exactly as all the others. In other words all cylinders are firing at exactly 10 degrees and not one at 10 another at 12 and another at 8 degrees. Yes, I see this more often than not – impossible, you say - not when your shaft is bent causing the cam to wobble!
Tip 4 Exhaust restrictions. Too many sharp bends and incorrectly sized pipe will rob your engine of power. Remember that the exhaust pipe is also carrying cooling water and that will displace any combustion gasses that are also trying to get out further restricting flow. Also be sure that the transom through hull fitting is not restricted. Many old fittings are cast and can be rather thick and can severely restrict flow. I have seen this first hand on several original boats.
Tip 5 Ignition systems. High energy ignition is probably one of the most important things you can do to get the most bang for your buck. The hotter the spark, the more efficient the combustion. As compression pressure increases in the combustion chamber, the spark has a harder time to ignite this highly compressed air fuel mixture. Just because a spark plug has a good arc in the atmosphere laying on your block, it does not mean that it is performing well under pressure in the combustion chamber. Ignition breakdown is very common and not always that easy to detect why it is happening. Some older engines even had dual plugs fitted and had two sets of coils and points that fired simultaneously to give that extra hot spark so that no fuel is left unburned. An oscilloscope can help greatly in detecting these issues and I use mine often to diagnose anomalies in the ignition system especially at higher rpms.
Do not use carbon core (shielded) plug wires on any points ignition system, and stay away from resistor spark plugs as well! Any plug with an R in front of the type is usually a resistor plug. Our old engines never ran them as these were designed to help get rid of unwanted radio and T.V. interference as electronics came into wider use.
Tip 6 Fuel additives. While this area is worthy of mention, I will not dwell on this topic too much - remember that we are running older design engines and that they often run much colder than their modern counterparts. As a result, many of these additives will simply not work well just because our engines’ combustion temperatures are just not hot enough to create the chemical reaction with the fuel. Also, most available today are for fuel injection and not for carbureted engines. I still do recommend the Nitro Clean by Kleen Flo as it does contain a lead substitute that helps the valves lubrication.
So? Money is no object...
Any further modifications start to delve into dismantling of the engine and reworking the internal components. I will list some of what can still be done, but remember that the stock engine is designed to give maximum power and reliable service over many years. A drag car can develop 1000+ Hp, but it can only do it for several seconds or so as the components are stretched to their breaking point with major parts being replaced between races. I will stick to the basics here and I am sure nobody wants to know how to supercharge a Dippy or Midget - do they?
Porting and polishing. This is the process of removing any factory burrs or rough edges as well as smoothing out any tight turns and restrictions in both the intake and exhaust systems. Poor fitting manifolds can benefit from this process where ports simply do not line up properly creating restrictions and turbulence. These ports can at times be greatly enlarged by die grinding.
Camshaft reworking. If the factory “stock” camshaft is too “mild” and does not produce the desired lift and or duration, etc., a major power increase can be had - but be careful if you go this route. There is very little available for our average flatheads, and this is a performance enhancement that is really more towards the V-8 engine such as the Fords Chevy’s and Chrysler inboards. They can range from a very mild upgrade to wild, but remember always that when you change the factory camshaft you usually sacrifice something else. If you go too radical you can have starting issues and certainly idling problems and stalling. This can be especially problematic with the hydraulic transmissions such as the Velvet Drives, as they cannot be eased in like an old non hydraulic unit. An engine idling too high due to too much cam can actually snap a drive shaft as you need to keep the idle higher than factory settings just to keep it running. Once you engage the transmission into forward or reverse, the result is a violent engagement - then bang. If you’re going this route then be sure to run an old school reverse gear where you can manually ease the engagement. Sure you will wear the clutch plates faster, but, hey, we’re racing.
Shaving the head and block. This is what will raise the compression and thus create a bigger bang, to put it simply. The process involves removing material from the head and or cylinder block to create a smaller area to squeeze the fuel into. This can have its drawbacks as well as now you must be very careful that you do not hit the valves on the head of some flatheads and the pistons of some overhead cam engines. One major drawback is you sometimes need to run a more exotic fuel with a higher octane just to make it run.
Boring/oversizing. This involves enlarging the cylinder bore and fitting a bigger piston to allow more air/fuel mixture into the combustion chamber this increases the C.I. or Cubic Inch displacement of the engine, but you must be very careful especially with old marine engines, as the block walls are getting very thin after 50-70 years. I do not recommend boring much over .030 from standard to be safe, but that’s just me.
Another area that I have noticed not being given too much consideration is hull loading. While this is best left to the experts, I have often seen restored boats that are very stern heavy at speed. I very often see too much weight aft and the hull really hogging down, creating a big wake. All the throttle in the world will not let her plane out or run flat, whatever the style of hull may be. I like to see batteries near the engine and fuel tanks as forward as possible. This includes all the safety gear, and, yes, big Uncle Bill and Auntie Edna too!
Happy and Safe boating.