Don Terrill’s Speed-Talk

Erland Cox wrote: ↑Wed Sep 09, 2020 1:55 pm From what I have seen on dyno testing with carbs and tri-Y headers I get a flat spot around 3500 RPM.
Around this RPM the lambda reads rich from double carburating.
From my understanding this is because of the second reflection from the first junction in the tri-Y header.
A positive pressure wave goes out and reflects at the junction as a rarefaction wave.
Back at the cylinder head it reflects back as a rarefaction wave and changes sign at the first junction to a compression wave.
This compression wave goes up through the intake port during overlap and causes double carburetion.
One of the reasons of tri-Y headers should be that the negative wave from the second junction arrives together with
the positive second wave from the first junction at the valve and cancel each other out.
I can not measure pressure, I can only see what happens at the carburetor.

Erland

Off topic, but what happens to this problem if you fit one of those anti reversion plates under the carb?

vannik wrote: ↑Wed Sep 09, 2020 1:42 pm You are both partially right.

Would it be possible/easy for you to quantify this, how right each of us is, using your simulator? I would suggest a single cylinder four valve turbo engine with primary/up pipe length such that the first exhaust pulse returns from the turbo close to max lift and then second time during the overlap. What happens according your simulator?

BLSTIC wrote: ↑Wed Sep 09, 2020 9:13 pm

Off topic, but what happens to this problem if you fit one of those anti reversion plates under the carb?

images (25).jpeg

If I had a plenum I do mot think that the problem would show as much but this is with 2 Weber 50DCO carbs on a 4 cylinder engine.

Erland

This questions is aimed more at Larry M-
Do the “header tube and collector trends” for a two barrel engine just follow similar to having low compression? Is it more complicated than that ——?? I have been thinking for quite awhile that one area to make a fair gain on these 4412 type circle track engines that I like to play with is in the header design. I’ll also ask- does pipe max work to predict what will work for a stepped header design for restricted engines like 2bbl?

Thanks-Jason

BigBro74 wrote: ↑Sat Sep 12, 2020 11:34 am This questions is aimed more at Larry M-
Do the “header tube and collector trends” for a two barrel engine just follow similar to having low compression?
Is it more complicated than that ——?? I have been thinking for quite awhile that one area to make a fair gain
on these 4412 type circle track engines that I like to play with is in the header design.
I’ll also ask- does pipe max work to predict what will work for a stepped header design for restricted engines like 2bbl?
Thanks-Jason

hi Jason , i've Dyno tested quite a few Chevy + Ford engines ( 1 Chrysler too ) with 2-Barrells + have actual Dirt Track tested results !

here's some Pics + Data :

Jason , this is what Exhaust Ports looked liked :
actual Air / Fuel Ratio = 16.917323 : 1
for a BSFC= 0.4061 , HPCFF= 1.067564375 , and Fuel Consumed= 170.4 Lbs/Hour
... Engine Builder used local Carb Guru .. had large BallBearing Needle/Seat , Mech Fuel Pump in Dirt Car, not on Dyno
a very restricted hi-plenum vacuum Carb likes a ton of Compression Ratio if Rules allow + it likes as much CFM Air as possible ,
even if has to run ragged-edge Lean to do it

this particular Engine , Rick reported back almost had 1 full Lap ahead in a local Dirt Track Race
other times , his closest Competion were half a Lap behind him .
Some of the other Guys were too afraid to run Lean Air/Fuel Ratios like we were doing , so we had a huge adavantage on them !

However , i don't recommend "right-off-the-bat" that anyone try running this Lean... take it safe at first , like 12.5:1 to 13.2 ranges

Jason , this is what Exhaust Ports looked liked : ( Spark Plugs looked like they came out the box or looked liked Methanol Plugs )
actual Air / Fuel Ratio = 16.917323 : 1 for a BSFC= 0.4061 , HPCFF= 1.067564375 , and Fuel Consumed= 170.4 Lbs/Hour

i forgot to Post the Fuel Data :

Larry M - thank you! for all of the pics and details! It sounds like I need to get pipemax (Needed to 8-10 years ago haha) when you get your new update done.

Here is another question about the “trends” — do try- y’s need the primary and secondaries together to be as long as 4 into one primaries? I’ve been trying to read about it but not successful yet....

What about with wide variances in tube length due to fitment issues? On my driver side header, the shortest tube is 26" and longest is 40".

RDY4WAR wrote: ↑Mon Sep 21, 2020 6:03 pm What about with wide variances in tube length due to fitment issues? On my driver side header, the shortest tube is 26" and longest is 40".

Many times I have read by different people that the primaries being different lengths will Broaden the power band by tuning different cylinders to different rpm ranges

BigBro74 wrote: ↑Tue Sep 22, 2020 7:20 pm Larry M - thank you! for all of the pics and details! It sounds like I need to get pipemax (Needed to 8-10 years ago haha)
when you get your new update done.

Here is another question about the “trends” — do try- y’s need the primary and secondaries together to be as long as 4 into one primaries?
I’ve been trying to read about it but not successful yet....

Many times I have read by different people that the primaries being different lengths will Broaden the power band by tuning different cylinders to different rpm ranges

Yes , TRI-Y's need the primary and secondaries together to be as long as 4 into 1 primaries .

On very many 4-1 Header Dyno tests , that's the "Trend" i've definetly see is a slightly broader Range with different length Primary Tubes on V8's engines
You can make more Peak TQ if Primary Tubes are "all" the same correct lengths for RPM Range , than unequal length Primary Tubes.

so Trend is : slightly wider TQ/HP Curve , with less Peak TQ with unequal length Primary Tubes on V8's , as long as , the Collector Length is correct !
slightly narrower TQ/HP Curve , with greater Peak TQ with equal length Primary Tubes on V8's , as long as , the Collector Length is correct !
Peak HP seems to close in each situation on V8's , again , only if Collector is correct length + diameter

.... just look at Dyno tests i Posted recently ... adding to Collector length gained huge amount of TQ and HP ,
if that added length gets the overall Collector to the correct Harmonic length, or very close to it . ( usually the 4th Harmonic Length )
--- Collector's Harmonics --- ( Both-Ends-Open Tube = Odd and Even Numbered Harmonics )

RDY4WAR wrote: ↑Mon Sep 21, 2020 6:03 pm What about with wide variances in tube length due to fitment issues? On my driver side header, the shortest tube is 26" and longest is 40".

same results as i just posted above your question .... the correct Collectors on V8's help out a lot with unequal lengths !

i haven't tested this on V6's or InLine 4 Cyls yet ... just V8's so far