mods to step up horsepower/torque

[nwcove]

have read many posts about guys wanting just a bit more (sometimes more than realistic) performance from their motors.....from rbm's to the big displacement motors.
has anyone tried porting and polishing the intake and exhaust? i know you cant do much with the top of the exhaust port as the timing there is critical, but would smoothing out the intake port(s) and cleaning up the exhaust ports make any noticable difference? (i dont recommend trying it, but if i had a motor.....and a spare powerhead that i could experiment with...maybe!!)

 

[Frank Acampora]

Re: mods to step up horsepower/torque



If you are talking strictly outboards, here is a ported two cylinder and a ported three cylinder engine.

I have ported in line-four cylinder Force engines BUT--- If you port the exhaust, you will actually lose power over stock. Since the exhaust timing is about 120 degrees and the four cylinder has 90 degree crank throws, there is a 30 degree overlap. Since the #3 and #2 cylinders fire sequentially, this overlap bleeds exhaust back into the cylinder.

If you have read my posts, you would know that I am a BIG fan of porting---especially the three cylinder engines. I have done a number of them and can gain up to 10 HP per cylinder. On the Chrysler and early Force engines, I square out the ports and streamling the inter-port webs. My next project will be to invesigate increasing the bypass channel area and porting the piston skirt to take even more advantage of the increased port area. Simply increasing the port area by squaring increases power at roughly the same as stated RPM range. However, ports can be raised (so I have been told, never tried yet) about .060 to increase horsepower but at a higher RPM. So, if you do this, then the engine must be geared and propped to run at a higher RPM.

Note that these are strictly old school crossflow engines. Modifying loop charged engines would take a LOT more research but can be done too.

I have investigted stock reeds and found almost no difference in them, and indeed, fairing the Vee blocks adds no performance at all. Some people are fond of aftermarket reeds but I have not yet tried them. I will eventually though.

 

[JB]

Re: mods to step up horsepower/torque

Back in the 50s I did a lot of mods on Power Products chainsaw engines for my racing karts. Some worked, some didn't.

I messed with port timing and area as Frank did. Got good results just by squaring the ports and feathering passages on crossflow engines. Raising the ports was done only on crossflows. I don't recall the exact numbers, but in general I matched port timing to an O&R RC aircraft engine that peaked at 8,000rpm vs the stock 3600rpm. That required several other modifications to work, mainly with ignition advance and gearing.

On loopers I merely squared the ports but kept the porting angle pretty much stock.

I raised compression by planing heads or the bases of cylinders (This required port timing adjustments).

A second area that gave significant improvement was the volume of the crankcase/bypass. By reducing the volume (we called it "stuffing") scavenging pressure was increased and intake vacuum was increased, increasing the actual volume of the scavenging charge at higher rpm.

Intake was improved with pyramid reed plates, usually doubling intake area, adding piston ported intake or, in some Homelite models, altering rotary valve timing.

Carburetor venturi area was usually doubled or tripled with bigger, or multiple (progressive), carbs mounted on tuned stacks.

Running on a mix of 50% methanol, 50% Fox "Hi Nitro" glow plug fuel those engines screamed. . . and frequently broke.

 

[ondarvr]

Re: mods to step up horsepower/torque

Like the others above, I was into modifying 2 strokes, not much on O/Bs but on the motorcycles I raced.

It is very engine specific as to which modifications are needed and what can be done, so just doing a little porting can be good or bad. Research the options on your exact engine before plugging in the die grinder and removing metal, otherwise you may end up with block that is only good for the recycle bin.

 

[nwcove]

Re: mods to step up horsepower/torque

so it is possible to get power gains by cleaning up the ports, as long as you are carefull not to change the exhaust timing. ( i did port and polish a john deere snowmobile engine, more just opened up the ports to match the gaskets on both intake and exhaust, and smoothed out any casting roughness inside the ports......it made a noticible difference. kec kioritz....what an example of a well built 2 stroke)

might tear down my 4.5 rude this winter......and try to make it a 5.5!!!!.....or maybe a parts engine!

 

[Faztbullet]

Re: mods to step up horsepower/torque

Anytime you raise the exhaust port timing you move the rpm and torque band up but you sacrifice torque on the low end. Engines that are usually ported aggressive are used on small light hulls.

 

[Bob_VT]

Re: mods to step up horsepower/torque

Save your pennies and just look for a larger motor

 

[F_R]

Re: mods to step up horsepower/torque

I would say a whole lot depends on exactly what engine we are talking about. Many OMC motors are already tweaked to get best performance. I have no idea about the 4.5 though. Nor have I tried much along the outboards line. But like JB, I modified my go-kart engines. Some mods helped, and some effects were negative.

From a practical standpoint, I seriously doubt you are going to feel any seat of the pants difference on a 4-5hp outboard on a boat.

 

[nwcove]

Re: mods to step up horsepower/torque

"quote" From a practical standpoint, I seriously doubt you are going to feel any seat of the pants difference on a 4-5hp outboard on a boat.

lol....you mean if i "gasket match" the intake and exhaust, that my 1985 4.5 rude isnt gunna be a demon on the lake????
( i do think, if you have the powerhead apart, and open things up to closely match the gaskets,
then just smooth the ports, you can get a better running, more efficient motor. but if you take it to far......yer gunna be very unhappy).
but i still would like to turn my 4.5 into an 8000rpm , 15mph , rooster tailing beast!

 

[Bob_VT]

Re: mods to step up horsepower/torque

but i still would like to turn my 4.5 into an 8000rpm , 15mph , rooster tailing beast!

How about adapting a small 125 cc 2 stroke motor cycle (dirt bike) engine to your lower unit and you will get your 8000 rpms....

If your current motor hits 8000 rpms I think it would be just like a hand grenade when it explodes

 

[nwcove]

Re: mods to step up horsepower/torque

.....maybe i'll just stick with 0.5 mph trolling!

 

[Sprky]

Re: mods to step up horsepower/torque

Pushing the compression up is the easiest and quickest gain. That motor will perform well on 150 cranking psi and 93 fuel.

If your carb has adjustable needles making other mods will be easy.

 

[nwcove]

Re: mods to step up horsepower/torque

150psi compresion? on a 4 1/2 hp rude? i dont think id be able to pull the rope! lol

 

[Sprky]

Re: mods to step up horsepower/torque

150psi compresion? on a 4 1/2 hp rude? i dont think id be able to pull the rope! lol

You should probably rethink that statement.

 

[nwcove]

Re: mods to step up horsepower/torque

lol.....dont have to rethink it......one bad shoulder.....two bad disks! not sure what the comp numbers are now, but it starts easy! ( i only started the post to help others that thought/wanted a bit more from their motors, and were adventurous enough to try some mods) ...used my " go to" motor as an example.

 

[JB]

Re: mods to step up horsepower/torque

It is VERY important that you recognize most of the above modifications have a negative effect on user friendliness and reliability and were done to get small gains for racing purposes.

The state of tune selected by the maker engineers seeks optimum reliability and user satisfaction.

Be sure you understand what you will be losing when you hotrod an outboard.

 

[Willor]

Re: mods to step up horsepower/torque

Back in the 50s I did a lot of mods on Power Products chainsaw engines for my racing karts. Some worked, some didn't.

I messed with port timing and area as Frank did. Got good results just by squaring the ports and feathering passages on crossflow engines. Raising the ports was done only on crossflows. I don't recall the exact numbers, but in general I matched port timing to an O&R RC aircraft engine that peaked at 8,000rpm vs the stock 3600rpm. That required several other modifications to work, mainly with ignition advance and gearing.

On loopers I merely squared the ports but kept the porting angle pretty much stock.

I raised compression by planing heads or the bases of cylinders (This required port timing adjustments).

A second area that gave significant improvement was the volume of the crankcase/bypass. By reducing the volume (we called it "stuffing") scavenging pressure was increased and intake vacuum was increased, increasing the actual volume of the scavenging charge at higher rpm.

Intake was improved with pyramid reed plates, usually doubling intake area, adding piston ported intake or, in some Homelite models, altering rotary valve timing.

Carburetor venturi area was usually doubled or tripled with bigger, or multiple (progressive), carbs mounted on tuned stacks.

Running on a mix of 50% methanol, 50% Fox "Hi Nitro" glow plug fuel those engines screamed. . . and frequently broke.

Great read. I had cpl question. Is there a way to port exhaust ports an gain power. Like adding 2-3 inches to exhaust manifold with 2-3 inch individual diverters? Also need two new carbs was thinking of going with newer flat slides like off a newer 450-500cc wheeler. Any ideas? Recommendations on squish band clearance? Also can the recirculation lines be deleted or do I really need that recirculation setup?

 

[Willor]

Re: mods to step up horsepower/torque

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If you are talking strictly outboards, here is a ported two cylinder and a ported three cylinder engine.

I have ported in line-four cylinder Force engines BUT--- If you port the exhaust, you will actually lose power over stock. Since the exhaust timing is about 120 degrees and the four cylinder has 90 degree crank throws, there is a 30 degree overlap. Since the #3 and #2 cylinders fire sequentially, this overlap bleeds exhaust back into the cylinder.

If you have read my posts, you would know that I am a BIG fan of porting---especially the three cylinder engines. I have done a number of them and can gain up to 10 HP per cylinder. On the Chrysler and early Force engines, I square out the ports and streamling the inter-port webs. My next project will be to invesigate increasing the bypass channel area and porting the piston skirt to take even more advantage of the increased port area. Simply increasing the port area by squaring increases power at roughly the same as stated RPM range. However, ports can be raised (so I have been told, never tried yet) about .060 to increase horsepower but at a higher RPM. So, if you do this, then the engine must be geared and propped to run at a higher RPM.

Note that these are strictly old school crossflow engines. Modifying loop charged engines would take a LOT more research but can be done too.

I have investigted stock reeds and found almost no difference in them, and indeed, fairing the Vee blocks adds no performance at all. Some people are fond of aftermarket reeds but I have not yet tried them. I will eventually though.

Great read. I have 1999 merc sportjet 120hp in-line 4 cyl 2stroke. Is there a way to gain from porting exhaust ports? Like adding 3 inch to exhaust am adding 3 inch worth of individual diverters or something. I run jet boat so I’m not restrained by an outboard hood. Also what about 2 oval ports rather the 4 squared ports? Any recommendations on squish band. Also I need new carbs was thinking of going with cpl newer style flat slide carbs like off a 450-500cc wheeler. Any thoughts ? Cant find reed stuffers fer this. Is there an amount to take off the intake so the reeds are closer to crank.? Yes I know I’m getting into a temperamental boat but that’s wat I’m into. It’s tiny 800lb sugar sand heatxr2

 

[jbcurt00]

Closed

JBs been gone a while, can't reply to your question from the great beyond