Small Block Engine Builders

[Eric Perez]

I finally got the good old protractor out and started mapping out my small block .12- .15 engines.

Got lots of good info.

Found out that the exhaust timming was WAY lower than what I've read on some of the rc boat literature that I've got.

I was brushing up on some numbers and I recall John Finch considered 168-172 deg of exhaust timming a good range for heat racing.

Some of my small block engines have exhaust timmings in the 150-deg range.

Is this why folks say that off-road buggy engines don't perform well in boats?

Just currious -any comments?

 

[anthony_marquart]

On these little engines the top of the exhaust port (sleeve) usually lines of pretty well with the top exhaust outlet in the case. If I was to open the the top of the exhaust port on the sleeve would it still be effective even though the exhaust path is not even close to straight in a situation like this?

 

[Eric Perez]

On these little engines the top of the exhaust port (sleeve) usually lines of pretty well with the top exhaust outlet in the case. If I was to open the the top of the exhaust port on the sleeve would it still be effective even though the exhaust path is not even close to straight in a situation like this?

82614[/snapback]

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I'm raising the exhaust timming on one of my TRX2.5 engines. I'm just increasing the timming on the sleeve and then I install the sleeve on the case and grind some material on the inside roof of the exhaust port until it lines up with the new location of the exhaust port. It's actually not to hard to do -the dremel eats the aluminum case in no time

I've got some polishing up to do but I've basically raised my timming on the TRX 2.5 with an eye-brow cut to the sleeve to 160-deg.

I will test on Sunday if the weather cooperates.

The funny part is that to get the exhaust timming to 168-172-deg which is the numbers recommended by john Finch in his book -The opening would be HUGE.

I will test with it a couple of times to see what happens with different pipes and props.

Then I will repeat the experiment with 10 more degrees on the port to see what the effect is.

 

[anthony_marquart]

This may be a dumb question but I'll ask anyway. It seems like whatever you do with the exhaust port it has to open before the intake ports. Is this true? What is the relation between exhaust opening and intake opening?

I have a 1/8 endmill that pretty much chews anything up in seconds with a dremel.

You just have to be able to hold one to it or it can really do some damage!

On these little engines the top of the exhaust port (sleeve) usually lines of pretty well with the top exhaust outlet in the case. If I was to open the the top of the exhaust port on the sleeve would it still be effective even though the exhaust path is not even close to straight in a situation like this?

82614[/snapback]

​

I'm raising the exhaust timming on one of my TRX2.5 engines. I'm just increasing the timming on the sleeve and then I install the sleeve on the case and grind some material on the inside roof of the exhaust port until it lines up with the new location of the exhaust port. It's actually not to hard to do -the dremel eats the aluminum case in no time

I've got some polishing up to do but I've basically raised my timming on the TRX 2.5 with an eye-brow cut to the sleeve to 160-deg.

I will test on Sunday if the weather cooperates.

The funny part is that to get the exhaust timming to 168-172-deg which is the numbers recommended by john Finch in his book -The opening would be HUGE.

I will test with it a couple of times to see what happens with different pipes and props.

Then I will repeat the experiment with 10 more degrees on the port to see what the effect is.

82633[/snapback]

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[J Solinger]

Perhaps the exhaust port can be cut in this way. That way the case mods are minnemised.

View attachment 3818

 

[Eric Perez]

Perhaps the exhaust port can be cut in this way. That way the case mods are minnemised.

82637[/snapback]

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Yea, many engine are cut this way to allow larger exhaust timming and reducing the volume of the exhaust port.

Question: "What is the relation between exhaust opening and intake opening?"

Answer: On the engines that I have measured starting from BDC the schernle ports open at the same time as the exhaust port (sometimes the boost port opens earlier)

The schnerle ports range from 114 to 117-deg and the exhaust ranges from 150 to 155 deg. So we are really taking about 37-deg difference total. Note: These are values avereaged from an Ofna .12 red-head and a Traxxas TRX 2.5

Note: I've got a bout half a dozen engines :Novarossi (1), Picco's (2) and OS (2) Ofna (3) engines that I have not measured the numbers on yet.

 

[anthony_marquart]

What keeps the exhaust from blowing into the crankcase?

Perhaps the exhaust port can be cut in this way. That way the case mods are minnemised.

82637[/snapback]

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Yea, many engine are cut this way to allow larger exhaust timming and reducing the volume of the exhaust port.

Question: "What is the relation between exhaust opening and intake opening?"

Answer: On the engines that I have measured starting from BDC the schernle ports open at the same time as the exhaust port (sometimes the boost port opens earlier)

The schnerle ports range from 114 to 117-deg and the exhaust ranges from 150 to 155 deg. So we are really taking about 37-deg difference total. Note: These are values avereaged from an Ofna .12 red-head and a Traxxas TRX 2.5

Note: I've got a bout half a dozen engines :Novarossi (1), Picco's (2) and OS (2) Ofna (3) engines that I have not measured the numbers on yet.

82640[/snapback]

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[Eric Perez]

What keeps the exhaust from blowing into the crankcase?

After TDC the mixture ignites and the piston starts to move down the first opening that the hot gases see is the top of the exaust port. The hot exhaust gases are now flowing rapidly into the tuned pipe.

The tuned pipe is actually sucking the exhaust gases from the combustion chamber

Which by now is opening the scherle and boost port (This is where the exhaust manifold becomes charged with air and fuel)

At this time the piston is going down and occuping space in the crankcase, effectively displacing it's volume into the case further helping to evacute the content of the crankcase into the side ports in the sleeve.

When the piston makes it way back up the sleeve it closes the schnerle and the boost port. At this time the back pressure wave from the pipe super charges the combustion chamber ( This is where the extra kick of a longer exhaust duration kicks in!)

shortly thereafter the exhaust port closes and the process starts all over again.

 

[anthony_marquart]

I think what I was missing is that the timing on the intake rotor / crank has sealed the crankcase at this point, the exhaust can't blow back into the case because it is sealed. Dumb question I guess. I'm very interested in hearing the results of you exaust timing experiments on these engines. I've opened up the ports on mine but haven't changed the timing. Car guys I've talked to are afraid to touch the inside of their motors and wouldn't dream of changing the timing of their engines.

What keeps the exhaust from blowing into the crankcase?
After TDC the mixture ignites and the piston starts to move down the first opening that the hot gases see is the top of the exaust port. The hot exhaust gases are now flowing rapidly into the tuned pipe.

The tuned pipe is actually sucking the exhaust gases from the combustion chamber

Which by now is opening the scherle and boost port (This is where the exhaust  manifold becomes charged with air and fuel)

At this time the piston is going down and occuping space in the crankcase, effectively displacing it's volume into the case further helping to evacute the content of the crankcase into the side ports in the sleeve.

When the piston makes it way back up the sleeve it closes the schnerle and the boost port. At this time the back pressure wave from the pipe super charges the combustion chamber ( This is where the extra kick of a longer exhaust duration kicks in!)

shortly thereafter the exhaust port closes and the process starts all over again.

82712[/snapback]

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[Preston_Hall]

Keep cutting. 160 is too low. You should buy Marty's engine software so you know exactly what is happening. As the saying goes, "there is more than one way to skin a cat". Well, there is more than one way to adjust timings. Some methods effect other timings and it can be in a positive way. By entering different numbers you can see what happens when you raise/lower the sleeve or deck the piston. You can also measure and cut once.

 

[Eric Perez]

Had a very good time testing the modified TRX 2.5 engine last weekend. Temp was in the lower 60’s I was well dressed for the weather and my hands did not hurt until latter in the day...

I was running the TRX 2.5 with the modified exhaust timing. Following Prestons suggestions I increased the exh another 5-deg to bump up from 150-deg to 165-deg. It’s my understanding that 165-deg exhaust timing on a nitro boat engine is still very low, but my intent was to cut and do some testing and to continue cutting until I started to notice a drop on the lower-end of the engine. My next mod will be to increase it to 175-deg.

The whole thing that started this was after I put a highly cut-back Prather S215 SS prop on my Nitro Aggressor cat hull. The engine was turning higher RPM’s –but the speeds did not seem to improve by much.

I put a wire tie in cooling line to cut some of the water cooling from the engine. I noticed that after I started reducing the water cooling the engine would continue to accelerate hard after it left the shore. Before that the RPM would go down after it got into the open water.

I moved the strut up and boy the engine looked like it got a second wind. The rooster tail was way up, the nose of the boat went down and it just flat out hauled a$$. I’ve never seen the boat go that fast. The engine was operating in a total different RPM range it was finally starting to sound like a real heat racing boat! The funny thing was that I was making very sharp corners to see if the engine would sag or slow down like before, but it just kept on humming like it was going straight!

I kept shortening the pipe a couple of times by 1/8”. I did not notice much change in RPM and it was going around the corners without missing a beat. Then I slid the pipe all the way down, till the coupler was on the elbow of my exhaust manifold and still it was running fast and it did not loose a beat in the corners. I ran out of fuel on that last run ( I sort of spaced out cause it was going so freaking fast! ). When I got the boat back to shore the plug looked brand new.

I’m running 33% Nitro fuel with 165-deg exhaust duration. I got a set of thin feeler gauges, still need to grind on them some more to check what my deck height is. It makes sense that I’m going to end up with a very short tuned pipe. Low- nitro and low exhaust numbers will tend to go that way…

I cut half and inch from the header last night to continue to shorten the exhaust to see if I can go over the edge. I’m still looking for my piece of soft silver solder that I use to measure my pipe length with, I will give out the numbers the next time out.

One interesting thing was that I swapped out my flex-shaft with another that has a 1/8 step-down stub-shaft. I had a Y535 prop on it and the engine was loaded more than with my hacked down Prather S215. I’m going to do some testing with some 1/8 props to see what happens with smaller higher-pitched propellers in the 32-35mm range.

I know that some of you guys are pushing 38-40mm props with your small-block riggers. This is a 24” cat and the hull does not ride as light as a rigger, remember that if you are comparing my numbers to yours. As a matter of fact I don’t see any development occurring on racing small-block boats for the exception of riggers and the couple of .12 monos that grim adapted from a .21 John Finch wild thing design. I’m sort of a lone-star in my quest to development a small block tunnel hull that can handle race water and break the 40 mph barrier. I hope that this will be the year that I reach my goal!

Thanks to all for the tips and tricks that have made me get much closer to my goal