Flames out exhaust on blown gas

[Robbie Racer]

We recently rebuilt then dynoed my 621" BBC a with a 14-71 with 3 Divinci/Holley 850's on top. We are experiencing a weird problem on the dyno during a normal 4500 to 6500 rpm sweep. It starts blowing flames out the exhaust headers around 6,000 rpm which looks really cool but at the same time the fuel usage goes way up and the power curve flattens out or even dips until around 6300 rpm, then starts to climb back up again. This happened to us after a rebuild last year too so we changed the cam with less overlap and that didn't seem to help the problem. It seems to get worse with more boost. We added 5 lbs of boost (went from 9 lbs to 14 lbs) and only picked up around 100 hp. Has anyone else ever seen flames like this under acceleration load on the dyno with a blown gas application?

[Infomaniac]

Honestly it sounds like valve spring flutter (harmonics). Are you using the same springs?
Is it BSFC that goes up or actual fuel flow? If the fuel is the same and power goes down during the evet. It will make BSFC look rich.

[hulshot]

seems like an extream rich condition and hot pipes igniting it
I am still amazed that a high helix delta blower works with 3 carbs on them. maybe fuel is puddling in the manifold.
I want to follow you at dusk with a camera, that would be a great shot.

[Robbie Racer]

hulshot, yeah it felt like an extreme rich condition. I don't think the Hi-helix blower made any difference as we saw this same problem with my standard roots blower last year. It does seem to be happening at a lower rpm though than last year. Maybe that could be because the hi-helix builds boost quicker in the pull than the other blower did. I will post the dyno numbers below so you can see what I mean. That would make a pretty cool picture at dusk. I would just have to figure a way to shut the water off to the headers. :D
Info, Yes we are using the same springs because they tested good. The same thing happened last year on the dyno when they were brand new. The dip in power seems to usually happen at a relatively low rpm (5600 to 6000) range then the problem seemed to get better as the rpm climbed above that range. If it were springs, wouldn't the problem get worse at the higher rpms? Also, I would think you could feel the boat "lay over" if this were actually happening in the boat, but I don't "feel" it happen in the boat. I tried to copy and paste the dyno numbers but it won't let me. Can you read a Super Flow .csv file if I email you a few sample runs?
For example one of the better runs looked like this where it really struggled between 5800 and 6100 rpm before starting to come back again around 6200:
5400 rpm - 1063 hp - 469 lb/hr fuel - .479 bsfp - LAM A/F 12.5
5500 rpm - 1074 hp - 474 lb/hr fuel - .479 bsfp - 12.5 A/F
5600 rpm - 1080 hp - 476 lb/hr fuel - .479 bsfp - 12.5 A/F
5700 rpm - 1101 hp - 479 lb/hr fuel - .473 bsfp - 12.5 A/F
5800 rpm - 1116 hp - 482 lb/hr fuel - .470 bsfp - 12.5 A/F
5900 rpm - 1079 hp - 497 lb/hr fuel - .501 bsfp - 12.4 A/F
6000 rpm - 1075 hp - 499 lb/hr fuel - .506 bsfp - 12.3 A/F
6100 rpm - 1076 hp - 499 lb/hr fuel - .505 bsfp - 12.2 A/F
6200 rpm - 1088 hp - 500 lb/hr fuel - .502 bsfp - 12.1 A/F
6300 rpm - 1105 hp - 505 lb/hr fuel - .500 bsfp - 12.1 A/F
6400 rpm - 1127 hp - 519 lb/hr fuel - .503 bsfp - 12.1 A/F
6500 rpm - 1143 hp - 528 lb/hr fuel - .504 bsfp - 12.2 A/F
6600 rpm - 1157 hp - 542 lb/hr fuel - .512 bsfp - 12.3 A/F
Whatcha' think?
Keith

[tbanzer]

Are you useing a camshaft with the stock 18436572 firing order?

[Kindsvater Flat]

So it looks like this?
http://www.v-drivevideo.com/jerrypic/5.JPG

[Robbie Racer]

tbanzer, hummmmmmm.............. good question. No, we are using a firing order of 18726543. Do you suspect that might have something to do with the problem?
KF, yeah......... something like that. :D
Keith

[bigkatboat]

With the cam change you gained no power? It sounds like the spark timing is wrong at those speeds. You said it "lays down", and the flames show that the power is going out the tail pipe. Your , mixture, overlap, flame travel speed, are causing the 'flat power' and the "fire power" is too late and going out the pipe. At 2500 your A/F ratio and spark advance are correct (or just better) and then as RPMs come up, #1, the mixture fattens up too much (slow flame travel). #2 spark timing is too late (lays down on powerband and EGT goes up). #3 intake port problem, (a wet spot in the air flow) the wet spot will cause uneven burning and slow "push" (lay down in power) at the piston. Then the carbs start to 'lean out' and the motor starts to pull again, or the late timing causes so much EGT that the motor starts to 'burn back' up the exhaust into the cylinder again. Please compare your EGT reading thruout the pull.

[Robbie Racer]

BigCat, thanks for your thoughts. Here are the average EGT's numbers on that particular run that you asked for.
5400 - 1041
5500 - 1049
5600 - 1060
5700 - 1066
5800 - 1074
5900 - 1099
6000 - 1106
6100 - 1108
6200 - 1114
6300 - 1118
6400 - 1125
6500 - 1128
6600 - 1139
The new cam has something like 4 degrees less of overlap. It looks like we actually lost somewhere around 20 hp over last year's pull (which had a peak of 1180 hp at the same boost level). We swung the cam timing from +4 to -4 and it didn't seem to affect the problem. We also moved the ignition timing from the normal 32 degrees to 28 then 34 and that didn't seem to make a difference with this problem either. The one thing that did seem to help was that we changed from annular boosters to drop leg boosters in the carbs and that helped flatten out the fuel usage curve to what you see in the numbers that I posted above. On earlier runs the fuel usage curve really climbed much faster and higher as the rpm's increased. The thought was that the carbs were seeing too much of a signal cusing way too much fuel usage as the rpm's increased. Another note: during last years dyno pulls the curve looked normal until a higher rpm (like about 6,000 or so). The new cam or the new hi-helix blower maybe causing the problem to happen at a lower rpm now.
Thanks again.

[Speedin' Ian]

I don't have any advice but I would like to see a picture of the motor:D . Three carbs, that sounds interesting. Why not run hilborn injection?

[Fiat48]

I say timing has to be an issue along with piggy fat.

[tbanzer]

Sorry I cant offer any solid advice. I would be wondering if the firing order is causing some strange intake manifold turbulance under boost at those rpm levels. Maybe a resonance thing, Have you leaned it out to see if there are any changes?

[bigkatboat]

1000/1100 degrees! Open up the choke! (all 3) A safe temp would be around 1250/1400 depending on your, load, altitude, intercooler, and fuel. Turbo motors (and NASCAR) run safely at 1550/1600 with NO FAILURES! "Thats the facts Jack" The flame you see may be unburnt fuel in the tailpipe. Way too much fuel. Is your spark STABLE at those RPMs?

[bigkatboat]

It just hit me "like two big boobs in the back of my head". Leave the jets as they are, PLUG THE POWER VALVES! That blower does not PULL evenly across the top (inlet), and your power valves are open, causing a too rich condition. Guys that run "hats" know that you always need at least one bigger nozzle in a corner, because the 'roots' is uneven. The "highhelix", twists the air to a greater degree. With the carbs at WOT, you may be getting a 'wave' of low vacuum, moving across the blower top (carb bases). This would make the power valves 'open or flutter', and create a "too rich" condition prior to having the blower OVERCOME the carbs and create VACUUM evenly under the carbs. If this starts to correct the problem, you will end up with "staggered jetting" to compensate for the fuel mix problems. Try putting a vacuum gauge on the carb bases! (all 3)

[Robbie Racer]

Fiat, we tried swinging the ignition timing from 28 to 34 and found no noticable improvement in this particular problem. It seemed to like 33 the best (with pump gas).
tbanzer, we did lean it some but maybe we didn't lean it enough. When we did pull some fuel out of it, we did see the bsfp numbers drop into the mid .4's with A/F numbers in the 13's. We were thinking that might be too close to the edge on a pleasure boat application where we would be at WOT for 60 seconds (or longer) at a time. When we did that, we still didn't see an improvement in the flames shooting out the exhaust. Also the dip in power at certain rpm's was still there.
Big Kat, yeah you are right about the EGT's being on the conservative side and we weren't worried about that at all. But we were concerned about the bsfp numbers and the A/F numbers when we pulled fuel away from it. (See note above to tbanzer). We think the spark was stable. We put a timing light on it during one run and it was stable. We also switched from a MSD digital 7 box to a standard MSD-6AL analog box to see if that had any affect and it didn't. We replace the plug wires last year when we suspected that was a problem at that time. I suppose it's possible that we could still have wires that are cross talking.
We were thinking along the same lines on the power valve's possibily causing the problem so we pulled them out and rejetted accordingly and it didn't seem to make any difference. Since then, now that the motor is back in the boat, I have left the power valves out of the secondaries and have just put them back into the primaries to try and get half way decent fuel economy. It works pretty well. Last weekend at Lake Berryessa I noticed the knock sensor start to move up on the gauge around 80 mph (somewhere around 3,500 rpm) at part throttle. But as I gave it slightly more throttle, it created enough of a vacuum to open the 2.5 power valves and cause the knock sensor to drop back down to zero. Since I normally cruise at 60 or so, that should allow half way decent fuel economy with the power valves closed just running on the primary jets.
Since we saw bsfp numbers drop in the mid .4's and A/F numbers climb into the mid 13's when we pulled fuel away, do you guys think I should try to duplicate that set up in the boat to see if it helps?
Great suggestions guys. This forum is great for brain storming. Thanks, Keith

[Fiat48]

As bigkatboat asked " Is your spark STABLE at those RPMs?" I'm thinking something changing in timing combined with the rich mixture equals flame out the pipe. No advance, locked out and at 33. Right?
Is there a chance the timing mark is off? And not really 33 degrees? Just a thought.
Saw a small block chevy on alcohol melt the headers right to the ground with an incorrect timing mark.

[Robbie Racer]

Fiat, I suppose that it's possible that the timing mark may not be in the right place if my motor builder made an error. It is locked in place with full advance (using a MSD crank trigger). We did move it around from 28 to 34 and it didn't seem to have any affect on this particular problem. We held a timing light on it during one pull and the ignition timing was rock steady through out the pull.
One thing that I forgot to mention and it may have some bearing on this problem. The dyno headers we were using had removable/adjustable length collectors. We pulled the collectors completely off during one pull and that seemed to have eliminated the flames but the power drop was still there. Maybe the scavenging affect of the collectors has something to do with pulling fuel out the exhaust. Unfortunately we couldn't fit my standard Lightening headers on the motor while it was on the dyno to see what affect they had.

[bigkatboat]

If the camshaft is "in" at 0 to 4 degrees advanced, set the spark timing at 32 degrees. Then plug the powervalves and LEAN IT OUT! Your EGT's are not high enough to get ANY thermal exchange. My god, do you know that B&M blowers (177's) on a 355 SM Chevy have in inlet temp of around 550 degrees F at 8 LBS? That means that with your 1000 EGT - 550 inlet temp = over HALF of the BTU's of thermal exchange LOST! If you have inlet temps under 100 degrees F, then you are PERFECT for a diesel work truck. (900EGT) Inlet temps DOWN, EGT's UP!!!! Make some power!!! Get that thing up around 1400 F and it will CURE itself!

[RACER52]

bigkatboat.
if the cam is in 0-4 degress retarded where would you set the timing??
for 13lbs boost and an inlet temp 110 or so
Racer

[Robbie Racer]

Big Kat, pardon my ignorance but how did you figure inlet temperature? Is that the same as bsfp? Are you saying that my bsfp numbers should be below the mid 4's? What A/F numbers do you recommend in my application?
Thanks again for the feedback.

[bigkatboat]

I tested some stuff for B&M back in the mid 80's. I used thermocouples inside the intake manifold. The thing ran HOT at 5/7 lbs, and was 'all ready to blow up' at 8 lbs of boost. I've got some guys who "must" run on 1580/1600 EGT (turbo) "that's where the power is". 950/1050 is actually diesel performance #'s As far as a 'retarded' cam timing, WHY did you do it??? Zero and back give top end, but advancing makes it PULL! I think if you must retard the cam, it's too small! I am not "the monster cam guy" either, I like them on the conservative side, but never retarded. Always advanced! Please go back to basics, your A/F is throwing the whole deal off. Are your O2 sensors before or after the flames? Or right in the middle!

[acatitude]

damn keith that would look cool running thru bridge to bridge, course you could sell me your back up and really put some power to that motor lol. then you could keep up with rim lol tak at ya

[Robbie Racer]

Yeah Ken, I think I will shut the water off to the headers when I make that run this year just so it will look cool with the flames coming out the back. I still have my 1160 hp Larry Peto back up motor sitting in my garage. Want to borrow it so you can run the Sacramento Bridge to Bridge event too? It ought to put your AO in the 125 mph range. :D
Big Cat, we had a thermocouple inside the intake manifold on the dyno. I don't remember the numbers off the top of my head. We just retarded the cam on one pull to see if it had any affect on the problem we were having then moved it back to zero. The O2 sensor was in the middle of the collector and we could only see a bluish flame coming out of the collector. I don't know how far (or if) the flames were back in the collector. Do you think they might be affecting my A/F numbers so that we were actually runninig a lot richer than the A/F number show? Do you think the bsfp numbers should be lower than the upper 4's?
Thanks again for the feedback.

[RACER52]

bigkat=
do you think it will make much of a difference on my application if I put the cam at zero. We decided to try the retarded cam strictly for outdrive
longgevity. THe engine still pulls super hard?? Harder than any cat I've been in, but could it be affecting the top rpm???
Racer

[RACER52]

I wonder what 1160 hp would do to my light little boat??
hmmmm.
the next engine will have some c.i.'s
I think I'm limited with a 454 at 7:5 to 1.
I would really like to see my boat close to 120. What do I have to do??
I spun a 26p prop to 6100rpms
what ###'s could I expect if I can spin a 28p the same rrr's???
I think that is a possiblity.
Racer

[Robbie Racer]

Racer, how much does your boat weigh? Do you know how much horsepower you have? At what RPM does your motor make peak power? My guess is that it's somewhere just over 6,000. If peak power is around 6100, then the 26 may be the optimum prop for max speed with your current power level. With my Carrera (4500 lbs with motor and a little fuel), I found that it takes about 1100 hp (measured at the crank) to push it to 120 mph with decent air and water conditions as well as a light load of people. A lot of hulls can go faster with less power than this particular hull so your results will vary.

[RACER52]

we'll its not like I can pull the legs on you guys, so I would have to guess my motor when the fuel mapping is done corretly to make peak power somewhere around 6500 but now it feels like its peaking at 6100ish.
I would have to say my motor is in the 850-950 range at peak power.
(i hope):confused:
Everyone tells me its more like 1000hp but the ##'s arent there.
I have not weighed the boat yet but would bet on less than 4000 :D
I'll still be happy with a 110mph boat.
ya right!!!! were never happy with what we have. I have room for 2 more fuel injectors and may just have to install for shasta and leave unplugged and try top speeds with and without.
Also will definately run the boat without 600plus lbs of buddies in the boat.
Racer.

[RACER52]

I will weigh it before shasta so we can run the ####'s

[hulshot]

I think I set the cam at 2 back and if I keep it I will set it to 2 advanced. I know my cam is small, but then againg it wont accellerate anyway because it is too damn heavy. What do think the power is on mine is if the speeds are int the high teens to low 20's and I run it to 6400??? 750-775 or so??

[Robbie Racer]

I would guess you have somewhere in the 1050-1100 hp area to be able to run those speeds with the weight you are pushing. Too bad none of us has a desk top dyno program to try and get some better guesses on the power numbers. :confused:

[RACER52]

I would have guessed DOn was pushin the 1200h.p mark with his new set up and the ###'s he's turnin without the juice. With the new set up he ran the same as he did with the big shot.
And his boat is almost Military edition with the weight:D :D :D
All that fancy interior and stereo add up.
Not to mention his 400lb engine hatch.
Either way, you guys are haulin ass!!!1
Racer

[bigkatboat]

Racer, 6100 RPM's with 1.50 gears and a 26" prop = 90 to 100 MPH at 90% to 100%. If you have 1.32 gears, = 102 to 113 MPH, 90% to 100%. If you have less than 90%, try some different props. Also, if you have a Bravo, you are throwing the oil off of the upper gears and bearings above 5400 RPM's