Cox Engines Forum
You are not logged in! Please login or register! Guests are limited to posting in the "General Questions (Guest Posting Allowed)" section only. Becoming a member is fast, easy and FREE!


Log in

I forgot my password

Search
 
 

Display results as :
 


Rechercher Advanced Search

Latest topics
» GSCB meet this Sunday
by GWILLIEFOX Today at 7:00 am

» The Breezy Hill Flyers "Black Fly-Day" Event !!!
by Kim Today at 6:46 am

» *** Double SIGH ***
by Kim Today at 5:15 am

» 1/2A Non-Contest Control Line Gathering
by Kim Today at 5:02 am

» Drill Press Question
by Jason_WI Today at 2:06 am

» Rebuild videos?
by Sunbird Today at 1:24 am

» Submarine ARA San Juan Lost in the sea ....
by batjac Today at 1:14 am

» testors mccoy running poorly video
by batjac Today at 1:10 am

» Phishing email from someone posing as you
by RknRusty Today at 12:51 am

» Recent engine acquisitions
by MauricioB Yesterday at 10:08 pm

Cox Engine of The Month
November-2017
MauricioB's

"Cox Tee Dee .010"



PAST WINNERS
Speed Contest 2018
CEF Traveling Engine

Gallery


Win This Engine!
The Traveling Prop
World of Aviation

Sub induction

Page 1 of 2 1, 2  Next

View previous topic View next topic Go down

Sub induction

Post  grampa jack on Mon Jan 02, 2012 7:55 pm

Can somebody 'splain this to me? I've seen it referenced on the COX engines site, & somewhere else that I can't remember. Something about location of exhaust ports relative to piston bottom at TDC? Why? Good thing or bad thing ?
avatar
grampa jack
Beginner Poster
Beginner Poster

Posts : 5
Join date : 2012-01-02

View user profile

Back to top Go down

Re: Sub induction

Post  nitroairplane on Mon Jan 02, 2012 8:00 pm

Well it is SPI where air can flow beneath the piston.
This lets the engine breathe more at high rpm.
It increases performance when there is no muffler but with a mufflit it does more bad than good.
avatar
nitroairplane
Top Poster
Top Poster

Posts : 6956
Join date : 2011-04-11
Location : London

View user profile http://modeldieselengines.forumstopic.com/

Back to top Go down

Re: Sub induction

Post  RknRusty on Mon Jan 02, 2012 8:47 pm

I referred to this while answering a question on Stunthangar, so decided to update it with my latest rewrite.
7/9/2016

RknRusty wrote:
How SPI(Sub Piston Induction) works

The upstroke: On the piston upstroke the crankcase pressure is relieved by the action of the piston moving up to compress and burn the previously introduced charge. In a reed valve engine, the drop in crankcase pressure releases the reed which finally opens fully drawn by the vacuum created by the displacement of the rising piston. Fuel/air is drawn into the crankcase through the venturi behind the reed. In a rotary valve engine such as a Tee Dee, the intake valve opens while the piston is rising and the vacuum it creates draws the air/fuel mix through the carburetor venturi into the crankcase.

TDC and the downstroke: As the piston goes through top dead center, the piston skirt clears the bottom of the exhaust port, opening a gap exposing the crankcase to cold air(Not recommended for engines with mufflers). The crankcase vacuum created by the rising piston causes fresh air to rush into the crankcase adding additional air as the now descending piston compresses and forces the freshly inducted air to combine with the existing fuel/air mix.** This gives the crankcase a belly full of extra pressurized f/a mix ready for the next charge. The increasing pressure of the descending piston forces the mixture to scoot up through the bypass ports until the crankcase pressure is diminished. It starts over again with the upstroke and Kaboom, it's a wild cycle. With SPI more fuel/air mix is available for combustion power. It only has its full effect at high RPM. Usually a small prop, less than 6"(for 1/2A), is needed to achieve the high RPMs necessary for SPI to have a beneficial effect.

**Obviously, the bonus air pressure added to the existing air from the venturi leans out the mixture, so that the needle valve must be opened more(richer) to let in extra fuel to balance the mixture.

In summary, the crankcase is charged with fuel/air mix from the venturi plus the SPI inducted air and then compressed by the descending piston. All that, plus the extra fuel from the richer needle setting to balance the A/F ratio, now you have a higher charge in the crankcase than if there had been no sub piston induction.

Rusty Knowlton
www.coxengineforum.com

Here's a good read by Paul Gibeault, a winner of many Mouse racing championships. You can download it here:
https://www.coxengineforum.com/t753-the-revised-2011-gibeault-mouse-race-program.


Last edited by RknRusty on Sat Jul 09, 2016 2:59 pm; edited 4 times in total

_________________
Don't Panic!
...and never Ever think about how good you are at something...
while you're doing it!


My Hot Rock & Blues Playlist
avatar
RknRusty
Moderator

Posts : 10436
Join date : 2011-08-10
Age : 61
Location : South Carolina, USA

View user profile

Back to top Go down

SPI... Yikes! It's only a little more complicated than string theory

Post  grampa jack on Mon Jan 02, 2012 10:01 pm

Thank you for the link! For my particular needs, I believe the non-spi approach will be just fine; I may have to muffle the engine.
avatar
grampa jack
Beginner Poster
Beginner Poster

Posts : 5
Join date : 2012-01-02

View user profile

Back to top Go down

Ooops... forgot to mention

Post  grampa jack on Mon Jan 02, 2012 10:04 pm

I did a search on the RCG site for "sub induction" & got nothing; I may need to brush up on my search skills.
avatar
grampa jack
Beginner Poster
Beginner Poster

Posts : 5
Join date : 2012-01-02

View user profile

Back to top Go down

Re: Sub induction

Post  nitroairplane on Mon Jan 02, 2012 10:10 pm

grampa jack wrote:Thank you for the link! For my particular needs, I believe the non-spi approach will be just fine; I may have to muffle the engine.

Ok if you want more performance you can lightn the piston open up the Venturi a bit.
Lots of stuff.
avatar
nitroairplane
Top Poster
Top Poster

Posts : 6956
Join date : 2011-04-11
Location : London

View user profile http://modeldieselengines.forumstopic.com/

Back to top Go down

Re: Sub induction

Post  andrew on Mon Jan 02, 2012 10:42 pm

grampa jack wrote:I did a search on the RCG site for "sub induction" & got nothing; I may need to brush up on my search skills.

Search for: sub piston induction in the 1/2A Planes forum. You can designate the forum using the advanced search. I got 44 hits.

andrew
avatar
andrew
Diamond Member
Diamond Member

Posts : 1223
Join date : 2011-08-11
Age : 70
Location : Western Kentucky

View user profile http://web2.murraystate.edu/andy.batts/ps/POWERSUPPLY.HTM

Back to top Go down

Re: Sub induction

Post  Surfer_kris on Tue Jan 03, 2012 3:07 pm

nitroairplane wrote:
It increases performance when there is no muffler but with a mufflit it does more bad than good.

Have you actually tested that?

I think the SPI will help on the reed engines at high rpm, with or without the muffler.
avatar
Surfer_kris
Diamond Member
Diamond Member

Posts : 1384
Join date : 2010-11-20
Location : Sweden

View user profile

Back to top Go down

Re: Sub induction

Post  microflitedude on Tue Jan 03, 2012 3:25 pm

Surfer_kris wrote:
nitroairplane wrote:
It increases performance when there is no muffler but with a mufflit it does more bad than good.

Have you actually tested that?

I think the SPI will help on the reed engines at high rpm, with or without the muffler.

From what I have heard, when the the gap opens between the piston and cylinder port at TDC, instead of sucking in fresh air it will suck in mostly it's own exhaust fumes.
avatar
microflitedude
Top Poster
Top Poster

Trusted Seller
I Support My Forum

Posts : 3417
Join date : 2011-08-17
Age : 19
Location : Columbia, SC

View user profile https://www.facebook.com/MiniCub?fref=ts

Back to top Go down

Re: Sub induction

Post  nitroairplane on Tue Jan 03, 2012 3:27 pm

Yes actually.
On a muffled babe bee.
With SPI was about 800 rpm higher with no muffler an with mufler was about the same lower than no SPI and muffler.
Also my pee wee .020 idled lower (throttle ring) with SPI than without.
avatar
nitroairplane
Top Poster
Top Poster

Posts : 6956
Join date : 2011-04-11
Location : London

View user profile http://modeldieselengines.forumstopic.com/

Back to top Go down

Re: Sub induction

Post  microflitedude on Tue Jan 03, 2012 3:52 pm

.020's had SPI?
avatar
microflitedude
Top Poster
Top Poster

Trusted Seller
I Support My Forum

Posts : 3417
Join date : 2011-08-17
Age : 19
Location : Columbia, SC

View user profile https://www.facebook.com/MiniCub?fref=ts

Back to top Go down

Re: Sub induction

Post  nitroairplane on Tue Jan 03, 2012 3:56 pm

Indra's .020 has spi Wink
avatar
nitroairplane
Top Poster
Top Poster

Posts : 6956
Join date : 2011-04-11
Location : London

View user profile http://modeldieselengines.forumstopic.com/

Back to top Go down

Re: Sub induction

Post  microflitedude on Tue Jan 03, 2012 8:21 pm

Did you shorten the piston yourself?
avatar
microflitedude
Top Poster
Top Poster

Trusted Seller
I Support My Forum

Posts : 3417
Join date : 2011-08-17
Age : 19
Location : Columbia, SC

View user profile https://www.facebook.com/MiniCub?fref=ts

Back to top Go down

Re: Sub induction

Post  nitroairplane on Wed Jan 04, 2012 1:56 am

Yes
I did.
avatar
nitroairplane
Top Poster
Top Poster

Posts : 6956
Join date : 2011-04-11
Location : London

View user profile http://modeldieselengines.forumstopic.com/

Back to top Go down

AND.........

Post  Jaspur_x on Wed Jan 04, 2012 3:02 am

AND for his next tricks, Nitroairplane will make his cox 010 run and build a 5 oz r/c plane lol!

Neanwhile I have a few tricks up my sleeve too..........are you watching closely?
avatar
Jaspur_x
Banned

Posts : 710
Join date : 2011-04-22
Age : 45
Location : Shanksville,Pa, yes that flight 93 place

View user profile

Back to top Go down

Excellent thread

Post  John Goddard on Wed Jan 04, 2012 3:04 am

Great thread.
I liked the RCG link although I think there may be 1 or 2 bods on there who perhaps
don't fully understand (who does?) infernal combustion.
When you've all got a spare evening take a look at,- http://www.pumaracing.co.uk/gentune.htm
This guy is a genius and explains a lot a what makes various (ok 4 stroke but the theory is the same)
engines tick. I particularly like his description of them as 'airpumps'.
Happy New Year
avatar
John Goddard
Diamond Member
Diamond Member

Trusted Seller
Posts : 2447
Join date : 2011-11-24
Age : 53
Location : Leyton North East London

View user profile

Back to top Go down

Re: Sub induction

Post  GermanBeez on Wed Jan 04, 2012 8:35 am

or...anyone can pm me, and i'll PERSONALLY take em through every step of what happens in their
little works of art! always love to!
avatar
GermanBeez
Platinum Member
Platinum Member

Posts : 1167
Join date : 2011-06-15
Location : Bavaria, Germany

View user profile

Back to top Go down

Re: Sub induction

Post  Megan Fox on Wed Jan 04, 2012 8:40 am

I don't know much about engines, but i had to pose on a custom chopper once!
does that count?
avatar
Megan Fox
Account Under Review

Posts : 5
Join date : 2012-01-02
Age : 31
Location : California

View user profile

Back to top Go down

Re: Sub induction

Post  RknRusty on Wed Jan 04, 2012 10:22 am

grampa jack wrote:I did a search on the RCG site for "sub induction" & got nothing; I may need to brush up on my search skills.
I search forums with Google advanced search. You type in the domain name of the site you want to search and then the search terms. All of your results will be from that site. It works x100 better than the lame site search boxes.


_________________
Don't Panic!
...and never Ever think about how good you are at something...
while you're doing it!


My Hot Rock & Blues Playlist
avatar
RknRusty
Moderator

Posts : 10436
Join date : 2011-08-10
Age : 61
Location : South Carolina, USA

View user profile

Back to top Go down

BUMP

Post  PlaidHatter on Thu Jul 16, 2015 9:50 am

I hope it's ok to bring this thread back from the dead. (Still a noob here)
I just learned about SPI, (did I say noob already?) and after learning about it's performance value, my question is, can you have TOO much SPI?
Like if you were to take this particularly modified cylinder:
(Damn, I forgot I can't post a link yet cus I'm still noobin groovin. But basically a modified cylinder with a lowered exhaust port)
Which has a lowered exhaust port, creating SPI, and then also shorten a piston slightly to give the bottom maximum gap, further increasing SPI during TDC and WOT RPM.  Huh...
Forgive me if I'm repeating something that has already been gone over. (Hi. Noob.) Sad Goodbye

Thanks in advance.
avatar
PlaidHatter
Silver Member
Silver Member

Posts : 72
Join date : 2015-07-14
Age : 33
Location : Newark, CA

View user profile

Back to top Go down

Re: Sub induction

Post  RknRusty on Thu Jul 16, 2015 11:32 am

PlaidHatter wrote:I hope it's ok to bring this thread back from the dead. (Still a noob here)
I just learned about SPI, (did I say noob already?) and after learning about it's performance value, my question is, can you have TOO much SPI?
Like if you were to take this particularly modified cylinder:
(Damn, I forgot I can't post a link yet cus I'm still noobin groovin. But basically a modified cylinder with a lowered exhaust port)
Which has a lowered exhaust port, creating SPI, and then also shorten a piston slightly to give the bottom maximum gap, further increasing SPI during TDC and WOT RPM.  Huh...
Forgive me if I'm repeating something that has already been gone over. (Hi. Noob.) Sad Goodbye

Thanks in advance.
Definitely. If you open the gap beyond a threshold, then it's open for too many degrees of rotation. That gives the crankcase time to equalize with the ambient air pressure or even start venting the F/A mixture back out.

Here's my usual boilerplate reply, which may be updated since this original thread was posted:
RknRusty wrote:
How SPI(Sub Piston Induction) works

The upstroke: On the piston upstroke the crankcase pressure is relieved by the action of the piston moving up to compress and burn the previously introduced charge. In a reed valve engine, the drop in crankcase pressure releases the reed which finally opens fully drawn by the vacuum created by the displacement of the rising piston. Fuel/air is drawn into the crankcase through the venturi behind the reed. In a rotary valve engine such as a Tee Dee, the intake valve opens while the piston is rising and the vacuum it creates draws the air/fuel mix through the carburetor venturi into the crankcase.

TDC and the downstroke: As the piston goes through top dead center, the piston skirt clears the bottom of the exhaust port, opening a gap exposing the crankcase to cold air(Not recommended for engines with mufflers). The crankcase vacuum created by the rising piston causes fresh air to rush into the crankcase adding additional air as the now descending piston compresses and forces the freshly inducted air to combine with the existing fuel/air mix.** This gives the crankcase a belly full of extra pressurized f/a mix ready for the next charge. The increasing pressure of the descending piston forces the mixture to scoot up through the bypass ports until the crankcase pressure is dimished. It starts over again with the upstroke and Kaboom, it's a wild cycle. With SPI more fuel/air mix is available for combustion power. It only has its full effect at high RPM. Usually a small prop, less than 6' is needed to get to useful RPMs for SPI do have its effect.

**Obviously, the bonus air pressure added to the existing air from the venturi leans out the mixture, so that the needle valve must be opened more(richer) to let in extra fuel to balance the mixture.

In summary, the crankcase is charged with fuel/air mix from the venturi plus the SPI inducted air and then compressed by the descending piston. All that, plus the extra fuel from the richer needle setting to balance the A/F ratio, now you have a higher charge in the crankcase than if there had been no sub piston induction.

Rusty Knowlton
www.coxengineforum.com

Here's a good read by Paul Gibeault, a winner of many Mouse racing championships. You can download it here:
https://www.coxengineforum.com/t753-the-revised-2011-gibeault-mouse-race-program.

_________________
Don't Panic!
...and never Ever think about how good you are at something...
while you're doing it!


My Hot Rock & Blues Playlist
avatar
RknRusty
Moderator

Posts : 10436
Join date : 2011-08-10
Age : 61
Location : South Carolina, USA

View user profile

Back to top Go down

Re: Sub induction

Post  PlaidHatter on Thu Jul 16, 2015 12:59 pm

Looks like you've got a lot of well researched info there, I definitely gotta get to reading. Thanks for the reply!
avatar
PlaidHatter
Silver Member
Silver Member

Posts : 72
Join date : 2015-07-14
Age : 33
Location : Newark, CA

View user profile

Back to top Go down

Re: Sub induction

Post  RknRusty on Thu Jul 16, 2015 4:24 pm

PlaidHatter wrote:Looks like you've got a lot of well researched info there, I definitely gotta get to reading. Thanks for the reply!
Hey Hatter
Actually I never found any documents on it, so I just made that stuff up, but so far neither Paul G., nor any of the other experts have corrected anything I wrote, so I'm thinking it's all about right. It seems right to me anyway.

One other thing that I didn't mention is that there are two ways to achieve SPI. The early pre-slit type .049s use the standard piston and create the gap by lowering the bottom of the exhaust port(as you already know). However when the slit exhaust cylinders came out, their porting is the same, and they were used on all engines until Cox went out of business. The standard piston did not clear the bottom of their ports, so no SPI. However, high performance engines that needed SPI with that cylinder use a short skirted piston so that it will open the gap at TDC.

You can see the difference in the two pistons side by side, and the short skirted one is an SPI piston. If you were to take a short SPI piston and install it in an older SPI cylinder, there would be your example of too much SPI gap. It's been done by CEF members and confirmed that the engine's performance fell on its face.
Rusty

_________________
Don't Panic!
...and never Ever think about how good you are at something...
while you're doing it!


My Hot Rock & Blues Playlist
avatar
RknRusty
Moderator

Posts : 10436
Join date : 2011-08-10
Age : 61
Location : South Carolina, USA

View user profile

Back to top Go down

Re: Sub induction

Post  PlaidHatter on Sat Jul 18, 2015 12:25 pm

Just for the sharing information and keeping record, cus I didn't find it anywhere, I just measure the amount of SPI on my completely stock, and supposedly near new Golden Bee and found the SPI measurement at TDC to be .015 gap.
it would be interesting to do some research as to what gap starts having drawbacks. I guess it would depend on supporting modifications, but could start with a bone stockie.
avatar
PlaidHatter
Silver Member
Silver Member

Posts : 72
Join date : 2015-07-14
Age : 33
Location : Newark, CA

View user profile

Back to top Go down

Re: Sub induction

Post  1/2A Nut on Sat Jul 18, 2015 2:22 pm

The correct gap is about the same distance as when you crack the front door to tell the mother in law the wife's not home. Thumbs Up
avatar
1/2A Nut
Diamond Member
Diamond Member

Posts : 1300
Join date : 2013-10-20
Age : 54
Location : Texas

View user profile

Back to top Go down

Re: Sub induction

Post  Sponsored content


Sponsored content


Back to top Go down

Page 1 of 2 1, 2  Next

View previous topic View next topic Back to top

- Similar topics

 
Permissions in this forum:
You cannot reply to topics in this forum