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Valve overlap on 4 strokes

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Old 09-13-2012, 09:32 AM
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SJN
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Default Valve overlap on 4 strokes

Hi

When exactly should the valve overlap happen in 4 stroke model engines ?

I bought a few used engines, and have a few new ones too.
I ofcourse inspect the used engines before using them, and and I was checking the timing marks and noticed different valve overlaps at TDC

When my ASP 120FS is at TDC, both valves are equaly open at TDC. Very easy too see, as the exhaust and intake rocker arms are both at the same angle.
Timing mark is exactly in line with the lifters.

My OS 120FS II P which looks almost identicaly to the ASP copy both valves are equaly open a few degrees before TDC.
Again, the timing mark is exactly in line with the lifters.

My YS-120 Has both valves equaly open exactly at TDC like the ASP engine
Timing mark is directly up/down at TDC, as the manual states......which is actualy also in line with the lfters as they are 90 degrees to the crank.

My little ASP .30 like the OS 120 has the valves equaly open a few degrees before TDC, and is also times inline with the lifters.



Whats up with this?
Is there no general rule for valve overlap ?









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Old 09-13-2012, 10:12 AM
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Default RE: Valve overlap on 4 strokes

Valve lash to tight maybe. you can change the lash to slightly adjust when the lifter is starting to get on the "ramp" of the cam. i wouldnt worry about it as far as i know no one makes aftermarket cams. The OS 30 FS is the only engine that had two different cam profiles made for the engine.
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Old 09-13-2012, 10:25 AM
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Default RE: Valve overlap on 4 strokes

ORIGINAL: SJN

Hi

When exactly should the valve overlap happen in 4 stroke model engines ?

I bought a few used engines, and have a few new ones too.
I ofcourse inspect the used engines before using them, and and I was checking the timing marks and noticed different valve overlaps at TDC

When my ASP 120FS is at TDC, both valves are equaly open at TDC. Very easy too see, as the exhaust and intake rocker arms are both at the same angle.
Timing mark is exactly in line with the lifters.

My OS 120FS II P which looks almost identicaly to the ASP copy both valves are equaly open a few degrees before TDC.
Again, the timing mark is exactly in line with the lifters.

My YS-120 Has both valves equaly open exactly at TDC like the ASP engine
Timing mark is directly up/down at TDC, as the manual states......which is actualy also in line with the lfters as they are 90 degrees to the crank.

My little ASP .30 like the OS 120 has the valves equaly open a few degrees before TDC, and is also times inline with the lifters.



Whats up with this?
Is there no general rule for valve overlap ?










Generally speaking, a shift in valve timing will also shift the RPM where peak torque is produced in the "power curve" even though the cam profile, duration & ICL (intake center line) remain the same. Advancing the cam timing will tend to fatten the TQ curve @ lower RPM, retarding the timing will shift the TQ curve towards the higher end of the RPM range.

After-market timing sets for high performance automotive applications have 3 keyways in the engine sprocket spaced around the ID. They are usually sspaced for 4* advanced timing, "straight up" (valve overlap equal @ TDC) or 4* reatarded.

Chrysler's VCT (Variable Cam Timing) utilized on '09 up 5.7 Gen III Hemis shifts the advance value of the cam shaft to take advantage of the best of both worlds. At low RPM the cam timing is advanced to give the best TQ for acceleration from idle speed. As oil pressure increases fro increased RPM, the cam timing is retarded to make more power in the higher revs.

This is a simplistic version of VVT (Variable Valve Timing)

Evidently the engines that show a bit of advance in the overlap have the cam profiles ground for some cam timing advance @ TDC to enhance TQ @ the lower RPM ranges. To be honest, that seems a bit odd in an aircraft application.
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Old 09-13-2012, 10:37 AM
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Default RE: Valve overlap on 4 strokes


ORIGINAL: airraptor

Valve lash to tight maybe. you can change the lash to slightly adjust when the lifter is starting to get on the ''ramp'' of the cam. i wouldnt worry about it as far as i know no one makes aftermarket cams. The OS 30 FS is the only engine that had two different cam profiles made for the engine.

While minimal valve lash would advance the opening of the valve, it also retards the closing so while it would have an affect on duration, it has no affect on overlap or ICL.

Miniamal valve lash is a good thing as long as the valves are allowed to close completely. Generally speaking, minimal valve lash not only produces more power, it reduces cam & lifter wear.
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Old 09-13-2012, 10:38 AM
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Default RE: Valve overlap on 4 strokes

I always set mine so the center of the overlap interval center line is about 5 degrees before top dead center. This varies slightly on the different models.

The important thing is that the last part of the overlap does go through TDC.

A Magnum with the timing mark straight up and down at top dead center will complete the valve overlap sequence just before TDC. It will start and idle great but will be sick at high rpm compared with a OS or Magnum with the overlap centerline just slightly before top dead center.
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Old 09-13-2012, 10:59 AM
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Default RE: Valve overlap on 4 strokes


ORIGINAL: w8ye

I always set mine so the center of the overlap interval center line is about 5 degrees before top dead center. This varies slightly on the different models.

The important thing is that the last part of the overlap does go through TDC.

A Magnum with the timing mark straight up and down at top dead center will complete the valve overlap sequence just before TDC. It will start and idle great but will be sick at high rpm compared with a OS or Magnum with the overlap centerline just slightly before top dead center.

Problem is, W/our gear drive valve train, a shift of 1 tooth shifts cam timing 15* on a Saito. W/a 48 tooth can drive gear. I would imagine the Magnum & OS valve drive would also result in excess shifts in valve timing W/a 1 tooth change.

We are pretty much stuck W/OEM valve timing.
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Old 09-13-2012, 11:03 AM
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Default RE: Valve overlap on 4 strokes

There is something of a whole lot of art and science involved with valve overlaps and how much is enough.
Generally you adjust the overlap timing to coincide with the exhaust system and its scavenging effect at a specific RPM range, which can be quite narrow in some cases. To take full advantage of it, one needs to have a exhaust system designed for it. But normally as the overlap increases the power developed moves higher up the RPM band. But with a small 4 stroke engine that is revving over 10,000 RPMs already, the valve overlap can be quite extreme anyway.

Now with model engine the timings or degrees of overlap can vary from larger engines as there is a scale effect that occurs as the engine gets smaller and smaller. The air and fuel don't change size as the engine gets smaller, so that remains constant.

There are some valve overlap calculators for determining the camshaft to use or to make.
http://www.wallaceracing.com/camcalc.php
http://www.rbracing-rsr.com/camshaft.html

These valve overlap numbers can give you an idea as to hot an engine is setup too.
ref http://speedtalk.com/forum/viewtopic.php?f=1&t=23156
APPROXIMATE adv overlap "operational" reference chart:

10* - 40*….towing
30* - 60*….ordinary street
50* - 75*….street performance
70* - 90*….street/strip
85* - 100*...race
95* - 115*...Pro race

The thing is, his operational reference chart right above, only seems to fit Big Blocks properly. If you take percentages and scale things down for small blocks, it ends up looking like this:

9* - 35*….towing
26* - 53*….ordinary street
44* - 66*….street performance
61* - 79*….street/strip
74* - 87*….race
83* - 100*...Pro race


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Old 09-13-2012, 11:47 AM
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Default RE: Valve overlap on 4 strokes


ORIGINAL: earlwb

There is something of a whole lot of art and science involved with valve overlaps and how much is enough.
Generally you adjust the overlap timing to coincide with the exhaust system and its scavenging effect at a specific RPM range, which can be quite narrow in some cases. To take full advantage of it, one needs to have a exhaust system designed for it. But normally as the overlap increases the power developed moves higher up the RPM band. But with a small 4 stroke engine that is revving over 10,000 RPMs already, the valve overlap can be quite extreme anyway.

Now with model engine the timings or degrees of overlap can vary from larger engines as there is a scale effect that occurs as the engine gets smaller and smaller. The air and fuel don't change size as the engine gets smaller, so that remains constant.

There are some valve overlap calculators for determining the camshaft to use or to make.
http://www.wallaceracing.com/camcalc.php
http://www.rbracing-rsr.com/camshaft.html

These valve overlap numbers can give you an idea as to hot an engine is setup too.
ref http://speedtalk.com/forum/viewtopic.php?f=1&t=23156
APPROXIMATE adv overlap ''operational'' reference chart:

10* - 40*….towing
30* - 60*….ordinary street
50* - 75*….street performance
70* - 90*….street/strip
85* - 100*...race
95* - 115*...Pro race

The thing is, his operational reference chart right above, only seems to fit Big Blocks properly. If you take percentages and scale things down for small blocks, it ends up looking like this:

9* - 35*….towing
26* - 53*….ordinary street
44* - 66*….street performance
61* - 79*….street/strip
74* - 87*….race
83* - 100*...Pro race



I was in on pioneering performance cams for the Gen II Hemis. While old school performance cams has LSAs (lobe separation angles) of 110* to as little as 109*, the EFI & computer controled parameters on the new generation engines performed best W/LSA of 114* to 112*.

As you probably know less LSA = more overlap, all things being equal. In the heavy 4200+# LX cars, 112* LSA was a bit peaky while the 114* LSA tended to not pull on top. I had my last custom ground cam done by Crower W/a LSA of 113* W/.050" lift durations of 215/219 ground straight up. The cam lift profile was such that I could retain MDS (Multiple Displacemnt System) I had a very aggressive opening ramp on the lobes to give me a long closed cycle yet the quick opening of the valves allowed it to breath on top.

The stock rotating assembly W/a good set of heads & other breathing enhancements made 426RWHP & 460RWTQ. Crank HP was estimated @ 510 t0 530 HP.

It ran pretty good & still got 2 MPG BETTER fuel economy than stock, getting 26 MPG on the hiway after the mods!

It did pretty good the 1st time out @ 4300# race weight.

Here are my best N/A passes 2 my 1stbtrack session after the cam insallation & a swap to an 8.8 Ford rear from a Ford Explorer W 3.23 gears. In 1st through 3rd gear, the 3.23 was equivelant to a 4.55 W/a TH400 transmission.

[youtube]http://www.youtube.com/watch?v=T-Sbz2Etl_A[/youtube]

This was W/a 125HP shot of nitrous.

[youtube]http://www.youtube.com/watch?v=KhG0KlXEOnU[/youtube]







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Old 09-13-2012, 12:58 PM
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Default RE: Valve overlap on 4 strokes

Clarence Lee did a couple of reviews of the Enya R1.20 engine which is one of my favorites. ref http://www.rcmplans.com/issues/reque...011988-1-1.pdf
He details his valve timing measurements which shows how Enya got the engine to rev up high like it does.

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Old 09-13-2012, 05:00 PM
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Default RE: Valve overlap on 4 strokes

those LSA's applie to NA engines. add a form of mechanical super charging and you want very little overlap.

You can build a "tuned" exhaust for the engines also. a mega phone style with a 7 degree taper a good starting point. thing is they are very very loud.
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Old 09-13-2012, 05:29 PM
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Default RE: Valve overlap on 4 strokes


ORIGINAL: SrTelemaster150


ORIGINAL: w8ye

I always set mine so the center of the overlap interval center line is about 5 degrees before top dead center. This varies slightly on the different models.

The important thing is that the last part of the overlap does go through TDC.

A Magnum with the timing mark straight up and down at top dead center will complete the valve overlap sequence just before TDC. It will start and idle great but will be sick at high rpm compared with a OS or Magnum with the overlap centerline just slightly before top dead center.

Problem is, W/our gear drive valve train, a shift of 1 tooth shifts cam timing 15* on a Saito. W/a 48 tooth can drive gear. I would imagine the Magnum & OS valve drive would also result in excess shifts in valve timing W/a 1 tooth change.

We are pretty much stuck W/OEM valve timing.
One tooth off makes a considerable difference on the OS, Magnum, & Y-S engines.

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Old 09-13-2012, 05:35 PM
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Default RE: Valve overlap on 4 strokes

Yeah a Supercharger, that is the way to go.


Roots supercharger on a Saito engine, NOTE, they have a tuned exhaust too.



This cute supercharged Saito 82 dragster rail is really neat too.
It sounds more like a two stroke when it is wound up.

http://www.youtube.com/watch?v=eeyp5P-TmDE




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Old 09-14-2012, 04:09 AM
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Default RE: Valve overlap on 4 strokes

Hey earl that looks interesting and do you fly it??? i'm over bent eights and horsepower tricky things to do with that,does the engine sound different and how does it throttle up?would like to hear it if you post a vid somewhere for us.
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Old 09-14-2012, 04:15 AM
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Default RE: Valve overlap on 4 strokes

The Topic here is model airplane engines, please.
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Old 09-14-2012, 04:25 AM
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Default RE: Valve overlap on 4 strokes


ORIGINAL: earlwb

Yeah a Supercharger, that is the way to go.


Roots supercharger on a Saito engine, NOTE, they have a tuned exhaust too.






Where can I get one of those?
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Old 09-14-2012, 04:39 AM
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Default RE: Valve overlap on 4 strokes

Dan, that thing has been shown around for a good 10 years, I don't think it caught on. There was some Turbo talk also but that fidn't fly because of the oil in the fuel no letting the turbine rev high enough.
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Old 09-14-2012, 04:47 AM
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Default RE: Valve overlap on 4 strokes

ORIGINAL: Hobbsy

Dan, that thing has been shown around for a good 10 years, I don't think it caught on. There was some Turbo talk also but that fidn't fly because of the oil in the fuel no letting the turbine rev high enough.

The fact that it "didn't catch on" doesn't bother me @ all. Scarcity only adds to the cool factor.

I was out of the hobby for 14 yars so that explains why I never saw it.

Turbocharging doesn't do it for me, but a roots style blower? That's gnarly!

I would mount the engine horizontally W/the blower sticking out the top of the cowl. Make an old school scoop for the carby.

Put a 180 pot on a 150 crank in a 150 case for 1.71 cu in @ 7.8:1 CR & throw the boost to it!
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Old 09-14-2012, 05:26 AM
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Default RE: Valve overlap on 4 strokes

Yup! and then put the engine in an os46 sized scanner,it's exactly what they deserve
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Old 09-14-2012, 06:05 AM
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Default RE: Valve overlap on 4 strokes

A tuned pipe will not work on a four cycle engine as it would on a two stroke. You would have to alter the exhaust valve timing.
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Old 09-14-2012, 06:51 AM
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Default RE: Valve overlap on 4 strokes

ORIGINAL: controlliner
A tuned pipe will not work on a four cycle engine as it would on a two stroke. You would have to alter the exhaust valve timing.
The newer more current 4 stroke glow engines have the valve timing already for supporting a tuned pipe pretty well. They have a lot of valve overlap and higher lift cams.The engines over timed improved and power increased. The 4 stroke engines used commonly for pattern or aerobatics are setup to use a tuined pipe even more. The early 4 stroke engines were quite mildly tuned in comparison to the newer ones. So those engines don't do well with tuned pipes.



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Old 09-14-2012, 06:55 AM
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Default RE: Valve overlap on 4 strokes

ORIGINAL: Old Fart
Hey earl that looks interesting and do you fly it??? i'm over bent eights and horsepower tricky things to do with that,does the engine sound different and how does it throttle up?would like to hear it if you post a vid somewhere for us.
I wish. The miniature roots blower was made by a machining firm inCalifornia (if I remember right), to promote their business. They didn't make any more, other than a few for their own purposes. No one outside of OS with their 1.20 supercharged roots blower pattern engine has made any that I know of. I think there were some videos made, I will post them if I find them.

There is a small company that makes superchargers for RC cars, but I honestly don't know how well they work. I have thought about getting one to see how it performs, but I haven't yet. That same business also offers various nitrous oxide injection systems for RC cars too. http://www.rbinnovations.com/default.asp

Here is the original thread about it here : http://www.rcuniverse.com/forum/m_15.../anchor/tm.htm

Their problem is that the 4 stroke engine needs to be modified for the supercharger, plus the supercharger has to be made for a specific engine displacement too. Glow engines tend to be more finicky in this way. So they never went into production with it.




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Old 09-14-2012, 07:08 AM
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Default RE: Valve overlap on 4 strokes

ORIGINAL: earlwb

Their problem is that the 4 stroke engine needs to be modified for the supercharger, plus the supercharger has to be made for a specific engine displacement too. Glow engines tend to be more finicky in this way. So they never went into production with it.




ORIGINAL: SrTelemaster150

I would mount the engine horizontally W/the blower sticking out the top of the cowl. Make an old school scoop for the carby.

Put a 180 pot on a 150 crank in a 150 case for 1.71 cu in @ 7.8:1 CR & throw the boost to it!


Hey Earl, a low compression FA171 like I metioned above would work & I don't run glow ignition anyway if i can help it.

Also, as long as the design displacement is close, couldn't the drive pulleys can be altered for a little more volume?
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Old 09-14-2012, 07:57 AM
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Default RE: Valve overlap on 4 strokes

Yes that would work.  I don't think they thought about it at the time.

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Old 09-14-2012, 12:53 PM
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Default RE: Valve overlap on 4 strokes

Looking at that old thread, led to his current Facebook page:

http://www.facebook.com/media/set/?s...9086457&type=1

On it he says all the photos of his engine were lost in a hard drive crash. The photos that are up now were taken recently, and he is still flying this airplane!
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Old 09-15-2012, 02:11 AM
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Default RE: Valve overlap on 4 strokes

Easy explanation of cam LSA: http://www.compcams.com/Pages/413/ca...ion-angle.aspx
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