VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics

  #1  
Old 08-14-2016, 09:47 AM
Randy Randy is offline
 
Join Date: Jan 2005
Location: Sedona Arizona
Posts: 349
Default Timing vs CHTs with PMAGs

First off, this is relating to my RV7A, with IO-375 low compression Aerosport engine with dual Pmags. I also run the EI Commander that lets me see and change the timing etc. I do like the setup I have, and I wold not want to have it without the EI Commander.

Recently I called Bill Repucci and he walked me through the steps to retard the timing in an effort to get CHTs more in control as they would go past 400F very easily. We reduced the timing advance by shifting the curve 2.8 degrees and this adjustment did make a difference. Now that I have flown this way awhile, I think maybe even more retardation is something to consider. I will post a savvy analysis plot of my recent light here:

https://www.savvyanalysis.com/flight...f-9cda52ffecd2

Too bad the EI Commander can't be part of the data log posted above. I did not take notes on the flight, but remember seeing 23 degrees advance at full power at 4,500' with 80F OAT. The curve will advance up to about 30 deg. up at altitude with lower MAP. 23 degrees does not seem like much advance but reducing it by the 2.8 degrees did help so maybe even less advance will help more? That is my question.

The 2.8 deg. adjustment made my CHTs more manageable for sure, but I still have to dump fuel through the engine to keep them below 400F in full power conditions down low as the logged data from the flight shows.

Running LOP brings the temps down very nicely of course.

Richening to 12 GPH will still get CHTs above 400F, take it up to 13GPH, and then they come into good control staying below 400F.

MY thought here is that pouring that much fuel into the engine is nearly the same thing as retarding the timing via slowing down the fuel burn. If I retard further, I should in theory be able to maintain the same performance at a lower fuel flow...

I wonder if my data is unusual or do others experience similar fuel flows to keep their CHTs under 400F?

I am interested in opinions on this. I was going to write this directly to Bill Repucci but realized others might benefit from the data and I am interested in others thoughts on this theory..


Randall Crothers
From Sedona, working in Northern California.
Reply With Quote
  #2  
Old 08-14-2016, 12:10 PM
gblanck gblanck is offline
 
Join Date: Nov 2010
Location: Redwood City, CA
Posts: 98
Default

I have the same setup- low compression IO-375 and dual pmags in my RV-8 and don't see CHTs nearly that high using the standard setup. Looks like you're spending some time in NorCal, stop by KSQL sometime and we can compare.
__________________
Greg Blanck
RV-8 Completed March 2014
Redwood City, CA
RV-8 Build Log
Reply With Quote
  #3  
Old 08-14-2016, 12:41 PM
Skyflyer Skyflyer is offline
 
Join Date: Mar 2007
Location: Sugar Land, TX
Posts: 144
Default Ditto

I also have the same setup, a low compression IO-375 with dual P-Mags and never see CHTs that high. ROP cruise burning 11.3 GPH at 7500 produces CHTs 340-350 max. CHTs in climb will stay below 380 and seldom get that high.
__________________
Chuck McCluggage
Sugar Land, TX
RV-8 IO-375 WW 200RV
Flying
Reply With Quote
  #4  
Old 08-14-2016, 01:26 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,597
Default

Something does not sound right. Perhaps a timing verification with a timing light would make sure you really have the timing set where you think it is.

Not saying you did, but are sure you set the pMag timing at TDC, not 25 BTDC?

Carl
Reply With Quote
  #5  
Old 08-14-2016, 02:26 PM
Toobuilder's Avatar
Toobuilder Toobuilder is offline
 
Join Date: Jul 2009
Location: Mojave
Posts: 4,652
Default

High CHT is often an indicator of an engine problem, but you should not be adjusting timing to "tune" your CHT. Ignition timing is adjusted to optimize the combustion event with the goal of achieving the highest possible cylinder pressure at the most advantageous crankshaft angle. The CHT is simply the byproduct of that process. If your engine is correctly timed and making maximum power but can't shed the heat, then you need to look at cooling.

Step one however is to rule out the base timing as a factor. Make sure its right before you start playing with anything else.
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
Reply With Quote
  #6  
Old 08-14-2016, 07:35 PM
mike newall's Avatar
mike newall mike newall is offline
 
Join Date: Jan 2005
Location: Yorkshire, England
Posts: 2,052
Default

We had issues with high CHT's on ours. TMX IO-360 dual P-Mags, MT prop, Silverhawk injection.

Ran with high temps for a while - put a dam on #1 cyl, adjusted the baffles and sealed them.

We then swapped to auto plugs and re timed the mags using the blow tube method - no retard, just standard timing.

Temps came down 50f.

Sadly, because we did multiple changes, we are no sure what made what difference.

Have another look, bench mark everything, see if it works.
__________________
"I add a little excitement, a little spice to your lives, and all you do is complain!" - Q

Donated in 2020
Reply With Quote
  #7  
Old 08-15-2016, 03:29 AM
Randy Randy is offline
 
Join Date: Jan 2005
Location: Sedona Arizona
Posts: 349
Default

Thank you all for the good advice and good data to compare with.

I will definitely double check the timing again and see what I find. It has been awhile since I looked there.

Randall
Reply With Quote
  #8  
Old 08-15-2016, 03:31 AM
Randy Randy is offline
 
Join Date: Jan 2005
Location: Sedona Arizona
Posts: 349
Default

Thank you all for the good advice and good data to compare with.

I will definitely double check the timing again and see what I find. It has been awhile since I looked there.

Randall
Reply With Quote
  #9  
Old 08-15-2016, 12:45 PM
nigelspeedy's Avatar
nigelspeedy nigelspeedy is offline
 
Join Date: Dec 2008
Location: Tehachapi, CA
Posts: 154
Default Leaning in initial climb

Hi Randy.

My stroker motor also has dual PMags with the EICommander, the difference is that it has 10:1 compression. I use 0 shift and max 35 deg advance.

I see in your initial climb after take off you lean a little and the resulting EGT is higher by about 100F than what I assume is the full rich take off. If you FI is set up right full rich will be about 300F ROP so leaning like this puts you about 200F ROP which is probably a bit lean for high power and low altitude. Try leaning in the climb so the EGT stays the same as the full power sea level EGT. This will give you faster climb on less gas (than full rich) but will be rich enough to help keep the CHT in check. Using this method I end up about 100F ROP when I get to 10k. The other thing I see is that when you level off at 4,000' you are at about 80% power when you lean which is a pretty high power setting unless you are a long way LOP.

One thing I have noticed now that we are in the middle of summer is that I have to either climb a little richer than normal (~50F EGT) to keep the CHT around 375 where I like to see them or reduce the timing from the normal 36.4 deg to 35 deg. As toolbuilder says making power is fine but you have to be able to reject the heat. Hard to do when the OAT is higher and the delta t is lower or if the baffling is not in top shape and you are leaking air.


Cheers

Nige
__________________
Nigel Speedy
RV-8
Flying
Tehachapi CA
I paid my -VAF- dues July 2018
Reply With Quote
  #10  
Old 08-15-2016, 03:03 PM
FasGlas's Avatar
FasGlas FasGlas is online now
 
Join Date: Mar 2014
Location: Prescott, AZ
Posts: 741
Default

You are not alone. I can only run 26 degs max on my dual pmags. There are plenty of threads and post about running pmags and having certain issues, high CHT's being one of those issues. Pmags have a very aggressive advance that's not adjustable, you can only adjust the max advance and the ramp shift. The best I can get to be within a usable CHT temp range is 0 shift and 26 degs max. If you have the EIC you can try changing the shift to -2 or -4 and that will pull the advance ramp back but not the max degrees. Also try bringing down the max advance some as well and see if you like the CHT temps. If you don't have an EIC you can remove a few degrees by retiming the pmags with the flywheel timing marks a few degrees retarded from TDC.
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 08:32 AM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.