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Question: Compression versus LOP

CFI1513840

CFI1513840

Well Known Member
So far I have not been able to run LOP due to what I consider to be unacceptable engine roughness at LOP mixture settings. I typically see anywhere from .5 to.7 GPH GAMI spread when leaning. I was wondering if there could be any relationship between rated compression and LOP operations. Specifically, would mixture distribution be more critical in a higher compression engine? Or to put it another way, would an engine with 9:1 compression be more susceptable to roughness running LOP than an 8:5 engine, given the same mixture distribution? This is obviously just an uneducated, unfounded hunch on my part, but if it was true, for higher compression engines, the GAMI Spread might have to be less than the maximum suggested .5 GPH specified by GAMI for smooth operation.
 
Ken a little more information would be helpful. Is your setup Carb or Fuel injected. What ignition system are you using.
Regards
 
Ken's signature says IO-360, so presumably injected. I have a 9:1 IO-540 in my -10. GAMI spread is 0.1 gal (actually AFP) and I have no problem running LOP at 10.5 gph. From my sample of 1, I assume compression is not the issue.

Jim Berry
RV-10
 
Engine Details

tadsargent said:
Ken a little more information would be helpful. Is your setup Carb or Fuel injected. What ignition system are you using.
Regards
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Tad,

It's a TMX-IO360 with 9:1 pistons. Superior hoizontal induction. Precision (Bendix) FI. LSE EI fires the top set of automotive plugs, Slick Mag fires the bottom set of fine-wire aviation plugs. Mag timing set at 25 BTC as specified (and confirmed) by Mahlon at Mattituck. LSE installed by Mattituck and set to high-compression specs per Klaus' installation instructions.
 
Ken, have you balanced your injectors. You should be able to get no more than a .1 or .2 difference in peak fuel flow.

Sounds like you may need to work on that to smooth out you engine LOP.
 
Great GAMI Spread

N15JB said:
Ken's signature says IO-360, so presumably injected. I have a 9:1 IO-540 in my -10. GAMI spread is 0.1 gal (actually AFP) and I have no problem running LOP at 10.5 gph. From my sample of 1, I assume compression is not the issue.

Jim Berry
RV-10
Click to expand...

Jim, Your GAMI spead of .1 gph is near-perfect mixture distribution and I agree compression is not an issue for you.
 
Balance the injectors and you'll be fine, it's not the compression that causes roughness, it's the mixture distribution. I've got a .1gph spread on my IO360 (matched GAMI injectors on Bendix injection) in the C-172 and I can pull the mixture back to about 2000 rpm at WOT before the fires go out - never gets noticeably rough.
 
.2 would be very nice

RV8iator said:
Ken, have you balanced your injectors. You should be able to get no more than a .1 or .2 difference in peak fuel flow.

Sounds like you may need to work on that to smooth out you engine LOP.
Click to expand...

It just occurred to me that I wouldn't have to worry about any other factors such as compression ratio if my spread was in the order of .1 to .2 as you suggested. I guess I was leaning on the .5 GAMI spec too much. I'm going to discuss this with the GAMI guys to see if I can make it happen.

PS Our new home is in Evans, GA. with N92LT based at Thomson-Mcduffie (HQU). We were really looking forward to your recent (cancelled) performance at DNL, but will be sure to catch up with you next time.
 
Thinking out loud here, but lean mixtures and are hard to light and high cylinder pressures makes it harder for the spark to jump the gap, so you do have the most stressfull combination available with your setup. That said, I would sure think that the single LSE would be acceptable, if not optimum to get the fire lit. But it has been my (admittedly limited) experience that ignition is the biggest key to LOP. I have a 200HP angle valve with zero injector tuning and it will go to 100LOP and still be smooth as silk. That's with dual Pmags and auto plugs. With the old magnetos, the same engine would not even run at peak EGT without stumbling.

You just might need a second LSE.
 
You will save a lot of money if you just get your restrictors from Don Rivera at Airflow Performance. There's no need to used GAMI unless you're flying certified. Don helped me get my spread to .2gph even though I don't have one of his FI systems. My next airplane will certainly use AFP fuel injection though.
 
No LOP but it looks pretty good

IMG_1514.JPG
 
Thanks for all the good advice

Guy Prevost said:
You will save a lot of money if you just get your restrictors from Don Rivera at Airflow Performance. There's no need to used GAMI unless you're flying certified. Don helped me get my spread to .2gph even though I don't have one of his FI systems. My next airplane will certainly use AFP fuel injection though.
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I really appreciate all of the inputs. I definitely have my marching orders and I'll be sure to check with Don at AFP before I make any moves.
 
Looks pretty.

While you're out running around go over to Greenville Spartanburg SC and have Don balance them, or just go up and fly and take the time to write down all the numbers and send them to Don an he will get you running smooth LOP. It's worth the time and effort. You'll save a bunch of money on fuel.

Hope you can come see us next year. I think we'll be back in Augusta the end of April, and hopefully Sun n Fun early April.

Come on over and say hello..
 
While Michael is right about lean mixtures and compression affecting spark, I would still look to the injector balancing (to deal with too lean to fire mixture) before ignition because it is a known item to be improved.

After 2 tries, my GAMI spread is also 0.1 gallons and my LOP operation WITH MAGS is so smooth that the only indications are the EGTs and ASI drop.
 
.5 to.7 GPH GAMI spread
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Problem is right there, and if you do the test too quick it may actually be worse and you do not know it.

Also check for induction leaks. Check plug resistance is under 5000 ohms.

The rule of thumb 0.5GPH is relative to a typical 6 cylinder engine. I believe that as a relative terms thing you want better than or equal to 0.3GPH spread.
 
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