over-pressure blow-off panel, how big?
Discussion
I've got a forced induction installation which occasionally backfires into the intake system producing some eye-watering explosions. The intake is quite long and I guess the total volume is around 15-20 liters, it's wet some of the time with the supercharger at the far end to contain the explosion and when this lot goes bang it can be quite spectacular. I've decided I need to fit a blow-off panel so the explosions don't dismantle the rest of the intake. The question is, how big does the blow-off panel need to be? I'm thinking of using a sprung flap valve with an area of about four square inches, is that big enough to do any good?
Instead of a blow off panel....
How about one of the overboost protection style devices ?? ie, a pop off valve or similar.
Just spring it high enough, that it will never release under normal use. But should vent during your problems ??
At least it wouldnt require maintenance.
Where do you intend mounting this panel ? If its on the intake..I'd make it as big as is reasonably practicable. I guess the bigger the better....
But how thick or secure do you need to make a burst panel, so that it actually bursts when you want it ?
How about one of the overboost protection style devices ?? ie, a pop off valve or similar.
Just spring it high enough, that it will never release under normal use. But should vent during your problems ??
At least it wouldnt require maintenance.
Where do you intend mounting this panel ? If its on the intake..I'd make it as big as is reasonably practicable. I guess the bigger the better....
But how thick or secure do you need to make a burst panel, so that it actually bursts when you want it ?
I've got a 2.5" transfer pipe between the blower and the intake manifold, and I'm thinking about cutting a hole in the side and then putting a sprung cover over the hole to keep it sealed. If I can get a good enough seal, it'd stay air tight until the manifold pressure exceeded the spring preload. Theory is that it should re-seal itself automatically afterwards.
Hello Pete
As stevie turbo says you can buy pop-off valves from any pneumatics supplier
http://uk.rs-online.com/web/search/searchBrowseAct...
Whether these would be enough to vent all of the pressure quickly enough would be another matter, still getting a large panel to seal properly under pressure would be awkward.
How about a burst disc?
http://www.visilume.co.uk/bursting-discs.shtml
Have you got the sequential injection working yet?
Is there any way of fitting any sort of trumpet into the very short ports?
As with your inlet manifold there is a large flat area under the the charger where fuel can collect and potentially 'wash' into a port. If you could have a small pipe leading into the 'chamber' with a 1.0 mm drain hole on each trumpet to slowly drain any excess fuel. A la RV8 plenum / trumpets
Alternatively lower the floor of the chamber so the ports are relatively higher.
Feel free to drop me a line
Matt
As stevie turbo says you can buy pop-off valves from any pneumatics supplier
http://uk.rs-online.com/web/search/searchBrowseAct...
Whether these would be enough to vent all of the pressure quickly enough would be another matter, still getting a large panel to seal properly under pressure would be awkward.
How about a burst disc?
http://www.visilume.co.uk/bursting-discs.shtml
Have you got the sequential injection working yet?
Is there any way of fitting any sort of trumpet into the very short ports?
As with your inlet manifold there is a large flat area under the the charger where fuel can collect and potentially 'wash' into a port. If you could have a small pipe leading into the 'chamber' with a 1.0 mm drain hole on each trumpet to slowly drain any excess fuel. A la RV8 plenum / trumpets
Alternatively lower the floor of the chamber so the ports are relatively higher.
Feel free to drop me a line
Matt
Looking at your pictures....Im sure I said last time about moving the throttle closer to the engine.
Id guess that could help with the backfire thing.
From a performance point of view...blowing thru it would also be better than sucking through it.
Id guess that could help with the backfire thing.
From a performance point of view...blowing thru it would also be better than sucking through it.
Edited by stevieturbo on Sunday 4th May 09:30
Thanks for the suggestions. I'd rather not use a burstable panel - the reason I'm considering the blow-off panel is so that it's self repairing and doesn't leave me with a dead engine afterwards. I don't know how hard it will be to get an adequate seal though.
I did run it with sequential injection using the Omex710 but it didn't cure the off-load stutter. There hasn't been much difference between the original hotwire setup, Omex/sequential, and MegaSquirt from that point of view. I've tried all sorts of ideas but eventually what fixed this problem was upstream fuelling. I fitted a throttle body injector upstream of the blower and the problem vanished - the engine idles as smooth as you like and I've seen the CO down to about 0.5% without cats.
The second problem I had happens under load at various revs. The symptoms are a total loss of power, indicated boost goes off the dial and smoke pours from the top of the engine. Again this has happened with all three management systems, and I think what's happening is the charge is burning in the manifold. I suspect it's caused by a misfire putting charge into the exhaust which is then burning back through the chamber during the overlap. If so, hopefully that'll go away once it's set up properly.
Third problem is a big bang occasionally while cranking. With port injection it's just a bit of a pop that stalls the engine. With upstream fuelling it is a lot more exciting and tends to dismantle things. Not so much of a problem in the garage but could be inconvenient if it happens at the traffic lights.
This is what I'm hoping to avoid with the pressure relief panel.
I did run it with sequential injection using the Omex710 but it didn't cure the off-load stutter. There hasn't been much difference between the original hotwire setup, Omex/sequential, and MegaSquirt from that point of view. I've tried all sorts of ideas but eventually what fixed this problem was upstream fuelling. I fitted a throttle body injector upstream of the blower and the problem vanished - the engine idles as smooth as you like and I've seen the CO down to about 0.5% without cats.
The second problem I had happens under load at various revs. The symptoms are a total loss of power, indicated boost goes off the dial and smoke pours from the top of the engine. Again this has happened with all three management systems, and I think what's happening is the charge is burning in the manifold. I suspect it's caused by a misfire putting charge into the exhaust which is then burning back through the chamber during the overlap. If so, hopefully that'll go away once it's set up properly.
Third problem is a big bang occasionally while cranking. With port injection it's just a bit of a pop that stalls the engine. With upstream fuelling it is a lot more exciting and tends to dismantle things. Not so much of a problem in the garage but could be inconvenient if it happens at the traffic lights.
This is what I'm hoping to avoid with the pressure relief panel.stevieturbo said:
Id go for port injection, sequential. I assume you played with injection timing ? That could have an affect on your stutter....
The stutter only appeared at low load and low revs but that was enough to play havoc with any emissions test. I played around with the sequential phasing and found there was a sweet spot but the effects were only small and the stutter was still quite prominent. My best theory is that I was getting fuel blown back out of the ports due to the poor port layout - the way the upstream fuelling fixed this seems to support that theory. Now I run upstream fuelling in the low load/low revs area of the map and switch to port injection at higher revs and higher loads. I've got the upstream and downstream fuelling on separate maps so I have a lot of control over the transition, but I still need to do a lot of tuning to get it right.I have found now that if I leave the engine idling on upstream fuelling for extended periods and then apply some load it runs mahoosively rich for thirty seconds or so, and I guess there's fuel puddling somewhere along the intake and being blown out when I open the throttle as Matt suggested. I'll get to the bottom of it one day. X-ray specs would come in very handy right about now ...
Peter not really followed previous exploits too closely, but if the symptoms have been present in one way or another for three different management systems then perhaps its more fundamental, Cam profile/overlap/timing perhaps 
.
The big boys spend a lot of time with variable valve timing to smooth out the blown delivery across all loads and revs. The fixed overlap profile for the single RV8 cam has to be jack of all trades, is yours a special grind.
Soz if this has been covered before..
. The big boys spend a lot of time with variable valve timing to smooth out the blown delivery across all loads and revs. The fixed overlap profile for the single RV8 cam has to be jack of all trades, is yours a special grind
.Soz if this has been covered before..
GreenV8S said:
HarryW said:
is yours a special grind .
Yes, it's a blower cam with almost no overlap. (That was one of the thing I tried to get rid of the stutter at idle. It didn't make any difference, and I might go back to a more conventional cam at some point.).HarryW said:
Sticking valve/lazy spring if its not consistent
It's been rebuilt from top to bottom, mechanically it's in A1 condition. The stutter has been completely cured by the upstream fuelling which supports my theory that it was a fuel distribution problem. The remaining problems are that occasionally it seems to light the inlet manifold up under power (hopefully this will be fixed once it's tuned properly) and a slight tendency to go 'bang' when it's cranking. The blow-off panel is intended to help with the bangs. OK, knew I would dig a hole as I've not been keeping up at the back 
.
re the blow back on cranking, can you add a delay to the injector firing to allow it to get up to speed a bit thus reducing the probability of getting the fuel in the intake in the first instance. I'm sure the original Lucas CUX system had this features as it was one of the things Mark put into his mapping. As you say once up and running and mapped properly then the blow back under load should be mapped out.
. re the blow back on cranking, can you add a delay to the injector firing to allow it to get up to speed a bit thus reducing the probability of getting the fuel in the intake in the first instance. I'm sure the original Lucas CUX system had this features as it was one of the things Mark put into his mapping. As you say once up and running and mapped properly then the blow back under load should be mapped out.
Pete
How about one of these? http://www.vortechsuperchargers.com/product.php?p=...
I use one on the 520SE to stop stalling the blower when the throttle shuts off. Very very effective and flows a lot of air which is an important part of the equation.
How about one of these? http://www.vortechsuperchargers.com/product.php?p=...
I use one on the 520SE to stop stalling the blower when the throttle shuts off. Very very effective and flows a lot of air which is an important part of the equation.
Pete
Is there any room under manifold in the Vee?
If there is, it might be worth putting in a fuel drain point which is then routed to the suction side of the charger? Admittedly you'll lose a little boost through it but a 3mm hole will stall pretty early so I doubt you'd notice much difference.
Matt
Is there any room under manifold in the Vee?
If there is, it might be worth putting in a fuel drain point which is then routed to the suction side of the charger? Admittedly you'll lose a little boost through it but a 3mm hole will stall pretty early so I doubt you'd notice much difference.
Matt
Thanks for the suggestions so far. I think the commercial dump valves would be more than adequate to prevent compressor stall but I'm less confident that it would flow enough to dissipate an explosion. Then there's the $300 price tag, which is fair bit more than I was hoping to spend on it. 
I guess I might need to do something to stop fuel sloshing around if that turns out to be a problem, but perhaps the answer will be to reduce the use of upstream fuelling. Plenty to play with and I think it's going to keep me out of trouble for the next few months...
I guess I might need to do something to stop fuel sloshing around if that turns out to be a problem, but perhaps the answer will be to reduce the use of upstream fuelling. Plenty to play with and I think it's going to keep me out of trouble for the next few months...
How much ignition advance are you running on cranking Peter? Surely if you have no overlap and run a fairly retarded cranking ignition value you won't get anything back into the intake? Also what's the ignition on idle/ light throttle? If you run a lot of advance you'll need the throttle very closed to get your idle speed down, so creating a very high inlet vacuum and reducing any puddling as the high vacuum will promote vapourisation?
trackcar said:
How much ignition advance are you running on cranking Peter? Surely if you have no overlap and run a fairly retarded cranking ignition value you won't get anything back into the intake? Also what's the ignition on idle/ light throttle? If you run a lot of advance you'll need the throttle very closed to get your idle speed down, so creating a very high inlet vacuum and reducing any puddling as the high vacuum will promote vapourisation?
Cranking advance? It's ... erm ... dunno, but not very much. Idle advance is almost nil, since it has no active idle control I've brought the advance back to almost nothing as it comes down to idle and then throw in a load of advance if the revs drop below idle. I'm hoping that having minimal advance will also reduce the emissions at idle by raising the in-cylinder pressure and giving a nice hot exhaust.GreenV8S said:
trackcar said:
How much ignition advance are you running on cranking Peter? Surely if you have no overlap and run a fairly retarded cranking ignition value you won't get anything back into the intake? Also what's the ignition on idle/ light throttle? If you run a lot of advance you'll need the throttle very closed to get your idle speed down, so creating a very high inlet vacuum and reducing any puddling as the high vacuum will promote vapourisation?
Cranking advance? It's ... erm ... dunno, but not very much. Idle advance is almost nil, since it has no active idle control I've brought the advance back to almost nothing as it comes down to idle and then throw in a load of advance if the revs drop below idle. I'm hoping that having minimal advance will also reduce the emissions at idle by raising the in-cylinder pressure and giving a nice hot exhaust.I find the transition onto light throttle (where you want to be running lots of advance for economy and it usually picks up better smoothness too) much better when you don't have a massive change in advance figures. For example at 1000rpm on idle you may have say 2 degrees but at throttle site 1 you might have 20 degrees and at throttle 2 you might be up to 30 -40 degrees already. The beauty of running high idle ignition values is you lose the massive change of hp as you open the throttle and move up the ignition sites and because you have a small initial idle airflow then even a small idle control valve adds a proportionately large extra airflow so stalling issues are a thing of the past.
I regularly run with 15-20 degrees at idle.
I know what you're saying about heating the exhaust keeping the advance low and it's a trick i have used to get through CAT tests, but I dont now send cars out like that. However that's not to say that your car will like lots of idle advance, it may be quite happy on low advance figures ramping up through the throttle sites. I just find that the more mapping I do the more I go for lots of advance at closed throttle/just off closed throttle.
How far upstream is your injector aswell? is it just before the charger or very close to the throttle?
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