R/C Tech Forums

Go Back   R/C Tech Forums > General Forums > Nitro Off-Road > Offroad Nitro Engine Forum

Like Tree29Likes
Reply
 
Thread Tools Search this Thread
Old 09-20-2017, 03:04 AM   #91
Tech Elite
 
Join Date: Sep 2010
Location: Beneath a rock down by the river. Don't have money for van
Posts: 3,043
Trader Rating: 1 (100%+)
Default

Quote:
Originally Posted by Roelof View Post
The compression does not come from the grooves but from the bended material arround it.
Correct, I mean, the grooves were packed with debris (carbon from combustion, dirt or metal dust I don't know) so they didn't hold any oil. Now they do. About the debris i think it isn't dirt because the con rod oil hole is clean.

What do you think of making more grooves or deeper ones? Will it help gaining compression?
__________________
( ͡ ͜ʖ ͡) https://www.facebook.com/becomeasetupguru/
30Tooth is online now   Reply With Quote
Old 09-20-2017, 04:59 AM   #92
Tech Champion
 
Roelof's Avatar
 
Join Date: Aug 2007
Location: Holland
Posts: 6,150
Send a message via ICQ to Roelof
Default

Many years ago I read something about a guy who used a pipe cutter on the grooves to resize the piston.

We have seen that with a worn piston w/o grooves running crap on a synthetic oil based fuel can run perfect with a thick oil based fuel (10% castor oil) so yes, it can help to have a good layer of oil, the grooves do hold some oil acting as a hydraulic piston ring. Sometimes you see more than 2 rings, I do not know why but it will have its reasons.
__________________
The quality of an answer comes with the quality of the question.
Roelof is offline   Reply With Quote
Old 09-20-2017, 07:03 AM   #93
Tech Adept
 
Join Date: Aug 2017
Posts: 194
Default

Cutting grooves or cutting deeper grooves basically displaces some material and pushes it out giving the impression the piston is sealing better, but the effects do not last. Tapered bore engines rely on the tight fit and hydrodynamic oil pressure to seal the piston. These engines do not run metal to metal but rather have that thin oil film between the piston and liner. Turning the engine over slowly displaces the oil (no hydrodynamic oil pressure), whereas when the engine is running, the piston moves so fast the oil doesn't get displaced and keeps the piston and liner from contacting one another. This is one reason why good fuel with the proper oil package is important just as much as keeping the mixture a bit rich.

Those oil grooves do help promote hydrodynamic oil pressure by trapping oil, but deepening the grooves only has so much benefit and often requires running the engine somewhat hot to get the castor oil to polymerize in those grooves to promote sealing. Castor varnish is what keeps ferrous technology engines alive as long as they tend to last. The varnish essentially makes the piston a few microns larger in diameter and the wet oil film between piston and liner serve the same purpose.

Ferrous engines use a meehanite piston (a kind of fine grained cast iron)and steel liner with minimal taper in the liner. Very old technology that was the basis for today's ABC and AAC ringless tapered bore engines.
stanleyw808 likes this.
SlowLST2 is offline   Reply With Quote
Old 09-20-2017, 04:58 PM   #94
Tech Elite
 
Join Date: Sep 2010
Location: Beneath a rock down by the river. Don't have money for van
Posts: 3,043
Trader Rating: 1 (100%+)
Default

Quote:
Originally Posted by Roelof View Post
Many years ago I read something about a guy who used a pipe cutter on the grooves to resize the piston.

We have seen that with a worn piston w/o grooves running crap on a synthetic oil based fuel can run perfect with a thick oil based fuel (10% castor oil) so yes, it can help to have a good layer of oil, the grooves do hold some oil acting as a hydraulic piston ring. Sometimes you see more than 2 rings, I do not know why but it will have its reasons.
Well the RB piston only has one groove so I'll add one more just to see.

Quote:
Originally Posted by SlowLST2 View Post
Cutting grooves or cutting deeper grooves basically displaces some material and pushes it out giving the impression the piston is sealing better, but the effects do not last. Tapered bore engines rely on the tight fit and hydrodynamic oil pressure to seal the piston. These engines do not run metal to metal but rather have that thin oil film between the piston and liner. Turning the engine over slowly displaces the oil (no hydrodynamic oil pressure), whereas when the engine is running, the piston moves so fast the oil doesn't get displaced and keeps the piston and liner from contacting one another. This is one reason why good fuel with the proper oil package is important just as much as keeping the mixture a bit rich.

Those oil grooves do help promote hydrodynamic oil pressure by trapping oil, but deepening the grooves only has so much benefit and often requires running the engine somewhat hot to get the castor oil to polymerize in those grooves to promote sealing. Castor varnish is what keeps ferrous technology engines alive as long as they tend to last. The varnish essentially makes the piston a few microns larger in diameter and the wet oil film between piston and liner serve the same purpose.

Ferrous engines use a meehanite piston (a kind of fine grained cast iron)and steel liner with minimal taper in the liner. Very old technology that was the basis for today's ABC and AAC ringless tapered bore engines.
That is a very good post, thank you.
__________________
( ͡ ͜ʖ ͡) https://www.facebook.com/becomeasetupguru/
30Tooth is online now   Reply With Quote
Old 09-20-2017, 05:19 PM   #95
Tech Adept
 
Join Date: Aug 2017
Posts: 194
Default

I'm An engine nerd. What can I say? LoL.
SlowLST2 is offline   Reply With Quote
Old 09-21-2017, 03:26 AM   #96
Tech Regular
 
Join Date: Dec 2010
Location: Sweden
Posts: 378
Default

Cutting the groove without a lathe is probably just a great way to create a stress raiser.

They use grooves on many racing pistons etc as an "accumulator groove", a labyrinth seal that also adds ring stability by decreasing the pressure.
But as they are very thin here, my guess would be that it's more of an oil retention purpose, or maybe a little of both.
NitroVein is offline   Reply With Quote
Old 09-21-2017, 04:05 AM   #97
Tech Elite
 
Join Date: Sep 2010
Location: Beneath a rock down by the river. Don't have money for van
Posts: 3,043
Trader Rating: 1 (100%+)
Default

Quote:
Originally Posted by SlowLST2 View Post
I'm An engine nerd. What can I say? LoL.
Which off road engines do you have?



Another thing i found, the Sirio EVO4 STI (not even going to put the full name) I have has lots of compression still, never used as I'm still on the EVO3. One thing I found is that piston has three grooves.

Another thing is that most rods I have wear very close to the oil hole.
__________________
( ͡ ͜ʖ ͡) https://www.facebook.com/becomeasetupguru/
30Tooth is online now   Reply With Quote
Old 09-21-2017, 04:22 AM   #98
Tech Adept
 
Join Date: Aug 2017
Posts: 194
Default

Quote:
Originally Posted by 30Tooth View Post
Which off road engines do you have?



Another thing i found, the Sirio EVO4 STI (not even going to put the full name) I have has lots of compression still, never used as I'm still on the EVO3. One thing I found is that piston has three grooves.

Another thing is that most rods I have wear very close to the oil hole.
Most of what I have are RTR engines as that's all I'd used for about 17 years. Now I am using better engines in my monster trucks (maybe too much better) and once my Hobby budget rebounds (got twin infants born recently), I'll replace my stadium truck engines with good ones.

Current collection of off-road stuff;
(3) Traxxas 3.3s - one has 9 gallons on it, another 5 gallons, one is new/out of box. 1 oil groove.
(1) SH .28 P6 - 5 gallons - still a powerhouse for a cheapie. 3 oil grooves in piston.
(1) Mach 427 - 1.5 gallons - runs good but was abused by previous owner who also "ported" it. 2 oil grooves.
(1) Novarossi BX21 SBK01 .21 modified buggy engine - Ron Paris Racing (NIB)
(1) Novarossi Legend 28-8RT
(1) Picco P3 .28
(3) Traxxas 2.5 engines - all in parts - worn out. I repinched a liner in one of them - going to convert for airplane use. 1 oil groove.
(1) HPI .15FE - has one run on it - ABN piston/liner has poor fit. Going to do work on and convert to airplane use. NO oil grooves I believe.
SlowLST2 is offline   Reply With Quote
Old 09-21-2017, 05:07 AM   #99
Tech Regular
 
Join Date: Dec 2010
Location: Sweden
Posts: 378
Default

Quote:
Originally Posted by 30Tooth View Post
Another thing i found, the Sirio EVO4 STI (not even going to put the full name) I have has lots of compression still, never used as I'm still on the EVO3. One thing I found is that piston has three grooves.
If it's as many as three my guess would be that it's to reduce pressure to help seal better.
NitroVein is offline   Reply With Quote
Old 09-21-2017, 06:32 AM   #100
Tech Elite
 
Join Date: Sep 2010
Location: Beneath a rock down by the river. Don't have money for van
Posts: 3,043
Trader Rating: 1 (100%+)
Default

Quote:
Originally Posted by SlowLST2 View Post
Most of what I have are RTR engines as that's all I'd used for about 17 years. Now I am using better engines in my monster trucks (maybe too much better) and once my Hobby budget rebounds (got twin infants born recently), I'll replace my stadium truck engines with good ones.

Current collection of off-road stuff;
(3) Traxxas 3.3s - one has 9 gallons on it, another 5 gallons, one is new/out of box. 1 oil groove.
(1) SH .28 P6 - 5 gallons - still a powerhouse for a cheapie. 3 oil grooves in piston.
(1) Mach 427 - 1.5 gallons - runs good but was abused by previous owner who also "ported" it. 2 oil grooves.
(1) Novarossi BX21 SBK01 .21 modified buggy engine - Ron Paris Racing (NIB)
(1) Novarossi Legend 28-8RT
(1) Picco P3 .28
(3) Traxxas 2.5 engines - all in parts - worn out. I repinched a liner in one of them - going to convert for airplane use. 1 oil groove.
(1) HPI .15FE - has one run on it - ABN piston/liner has poor fit. Going to do work on and convert to airplane use. NO oil grooves I believe.
Which one you feel as been the best at keeping compression? Can you find a pattern that supports that claim?
Sorry about the lack of manners, congrats with the twins!

Quote:
Originally Posted by NitroVein View Post
If it's as many as three my guess would be that it's to reduce pressure to help seal better.
I see, and even if the top groove is too hot to keep oil in it the piston still has two to keep compression.
__________________
( ͡ ͜ʖ ͡) https://www.facebook.com/becomeasetupguru/

Last edited by 30Tooth; 09-21-2017 at 02:47 PM.
30Tooth is online now   Reply With Quote
Old 09-21-2017, 07:07 AM   #101
Tech Adept
 
Join Date: Aug 2017
Posts: 194
Default

Quote:
Originally Posted by 30Tooth View Post
Which one you feel as been the best at keeping compression? Can you find a pattern that supports that claim?



I see, and even if the top groove is too hot to keep oil in it the piston still has two to keep compression.
In 20 years of enjoying this Hobby (nitro the whole time), I've never worn an engine out. I had some that didn't run hat great or strong, but it wasn't due to a worn piston/liner fit. Bad carburetors are usually the biggest culprit, and poor fuel usage a close second. In my early days I ran Traxxas engines (sport .15, and the successor to the sport .15 was the Pro .15. I used Traxxas fuel at that time, and could never get the engines to run consistently. Changing to a better fuel made a significant impact on the engines for the better.

After running different engines (both surface and air) and experimenting with different fuel blends and different glow plugs did I find a much better method to run the engines and adjust them better. When it comes down to brass tacks, the best thing that promotes a long lasting engine is a nice tight pinch when new. When an engine still has pinch (piston will get stuck at TDC turning by hand) after a good length of time (2+ gallons), piston sealing is still at its best. Once the engine loses its pinch, they do run faster and make their best power, but they are also at the peak of their life. Once the pinch is gone, the engine will gradually lose its power. How quickly it loses its power is dependent on a lot of things; with fuel and how the operator treats the engine being the biggest two IMO. I don't have any scientific data to back this up, but I do trust the information I've been given from a very reputable and world renowned aircraft racing engine builder. In his words; "A tight piston fit, good fuel & plugs, and proper needle settings make these engines run the best and the longest." "A tight engine is a good engine". (Dub Jett)

And FWIW - Dub Jett's engines do not use any grooves in the pistons. He uses a specially formulated alloy for the pistons (of which he will not divulge) that promotes long life and very low failure rates. Also, all of his engines are AAC constructed due to Aluminum having better stability; when the piston/liner fit is worn to an undesirable level, he fits a new piston to the existing liner. Aluminum liners do not bellmouth when overstressed (overheated) like brass does. This is why Brass liners often need replacing or repinching when the fit is lost.

In this day and age of people wanting instant gratification, many engine manufacturers are setting their engines up with a looser fit so they break-in quicker and give peak power quicker at the expense of longevity. Compare the fit of an engine from 25-30 years ago to an engine of today and you will find a pretty significant difference in how tight they are. The two Novas I have are a good example of this; the Ron Paris BX21 pinch is pretty intense - the piston starts to stick just above the exhaust port whereas my Legend 28-8 didn't stick until it was just barely below TDC. This comparison was made before the engines were run.

So this is all mostly my anecdotal experience. I have preference to older engines due to their better fits and tolerances, but this isn't to say that modern engines aren't powerful or well made. My Nova Legend is a brutally powerful engine that is a blast to run/drive.
SlowLST2 is offline   Reply With Quote
Old 09-21-2017, 07:10 AM   #102
Tech Adept
 
Join Date: Aug 2017
Posts: 194
Default

And just to add - besides the HPI 15FE - all of my engines will hold compression without any leakdown if you turn the engine in normal operating direction and just hold the flywheel against compression. If there is any leakdown and the crankshaft slowly rotates to and through TDC, the piston fit is compromised in some way or the piston and/or liner may be out of round. If the fit has gotten too loose - to the point it affects engine behavior, the first thing one will notice is the idle quality will slip. It will be harder to tune the bottom end to get a good idle.
SlowLST2 is offline   Reply With Quote
Old 09-21-2017, 07:27 AM   #103
Tech Regular
 
Join Date: Dec 2010
Location: Sweden
Posts: 378
Default

Quote:
Originally Posted by 30Tooth View Post

I see, and even if the top groove is too hot to keep oil in it the piston still has two to keep compression.
When you get blow-by the gas expands in the grooves and therefor (hopefully) stops, if not the next groove will do the same again.
It has been used for a long time and is a proven method. But I suspect that in this case (ABC engines), that it doesn't hurt that it can hold some extra oil as well.
NitroVein is offline   Reply With Quote
Old 09-21-2017, 08:20 AM   #104
Tech Adept
 
Join Date: Aug 2017
Posts: 194
Default

One thing I forgot about IRT deepening the oil retention grooves in pistons - take caution when doing this - if the piston is cast (NOT machined from extruded barstock), you run a strong risk of cracking or chipping the piston. I tried this on a piston only to find it was cast and half of the piston crown broke off with only minimal pressure.

I don't see a large benefit to these retention grooves only because so many engines do not have them. It seems common on car engines, but not on aircraft engines. I suspect the principle has some merit as Nitrovein mentioned, but how efficient they are remains to be seen IMO.

Last edited by SlowLST2; 09-21-2017 at 08:59 AM. Reason: Fixed typo.
SlowLST2 is offline   Reply With Quote
Old 09-21-2017, 03:13 PM   #105
Tech Elite
 
Join Date: Sep 2010
Location: Beneath a rock down by the river. Don't have money for van
Posts: 3,043
Trader Rating: 1 (100%+)
Default

Quote:
Originally Posted by NitroVein View Post
When you get blow-by the gas expands in the grooves and therefor (hopefully) stops, if not the next groove will do the same again.
It has been used for a long time and is a proven method. But I suspect that in this case (ABC engines), that it doesn't hurt that it can hold some extra oil as well.
See what I can do, in the mean time I will be thinking of other engine improvements.

Quote:
Originally Posted by SlowLST2 View Post
One thing I forgot about IRT deepening the oil retention grooves in pistons - take caution when doing this - if the piston is cast (NOT machined from extruded barstock), you run a strong risk of cracking or chipping the piston. I tried this on a piston only to find it was cast and half of the piston crown broke off with only minimal pressure.

I don't see a large benefit to these retention grooves only because so many engines do not have them. It seems common on car engines, but not on aircraft engines. I suspect the principle has some merit as Nitrovein mentioned, but how efficient they are remains to be seen IMO.
Aircraft engines turn lower RPM and have way less load on them, maybe car engines should have a lower RPM ceiling and we could just correct that with gearing?


Also, I have been thinking about this or something like it.



A replaceable con rod bearing.
__________________
( ͡ ͜ʖ ͡) https://www.facebook.com/becomeasetupguru/
30Tooth is online now   Reply With Quote
Reply

Similar Threads
Thread Thread Starter Forum Replies Last Post
JQ RACING - The Black Edition ozziii Nitro Off-Road 430 10-16-2017 02:51 AM



Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off



All times are GMT -7. It is currently 02:09 PM.


Powered By: vBulletin v3.9.2.1
Privacy Policy | Terms of Use | Advertise Content © 2001-2011 RCTech.net