Are the gaskets really that fragile? Headers and free-flowing pipes actually reduce compression? News to me... I thought these engines needed a proper balance of back-pressure to run right.
It seems you are thinking in N/A terms... This is a forced induction engine. We're cramming air into the cylinders while it fights to get it out. When exhaust backs up in the cylinder, dynamic compression ratio rises, and soo does cylinder pressure during the cycle.
Cylinder pressure is cylinder pressure and until the exhaust valve opens, it shouldn't make any demonstrable difference to the HG's whether or not the exhaust scavenges better. Unless of course the cats are plugged, but that's another story
Once again, this motor is forced induction. Scavenging is irrelivent.
On a worn engine, I'd think that ring blow-by, valve stem leakage and carbon buildup would reduce compression to a point where increased scavenging would be a wash, if any effect at all.
I'm just quoting this because I'm an a$$hole, and know that the valve seals aren't going to do anything but possibly bleed off a small amount of pressure while the next charge waits.
I'd study the engine a bit before taking steps towards mods. Do a compression test, etc. and make sure you have a good platform to work from. Might save some headaches in the long run.
I back this statement completely, though the spark plugs aren't very accessable on these motors to remove for a propper compression test.
I'm all for custom exhaust (if the engine is otherwise healthy), but this HG tip is news to me - please explain, thanks.
The spent charges from these motors will stack in the exhaust system as it is not adequate enough in the velocity department. It backs up all of the way back to the collectors/heads. Eventually, you can't expell as much spent gasses as you are pushing in with that little nifty supercharger of yours. Cylinder pressure rises, putting stress on the gaskets and causing the motor to be more knock sensative; heat rises, also contributing to the issue. Want proof? Look at the readings on a boost gauge before and after a GOOD exhaust. Less pressure, eh? Remember, you're metering it on the intake side. HMMM, Now what was getting in the way of that air which was previosly stacking in the intake???? Could it be exhaust backup?
Detination, Detination, Detination, BLAH BLAH FRIGGEN BLAH
Once again, I'm being an ~~~~. Yes, detination can be hard on the headgaskets. That's obvious. The flamefront contacts the piston before the crankshaft is in a position allowing enough mechanical advantage to relieve the pressure by converting it to mechanical energy... If too extreme, it can put a considerable amount of pressure on the cylinder walls, rotating assembly, AND headgaskets. This isn't an "OMG Shez gunna BLOW! (and not in the good way)" type thing thought (EDIT: For the head gaskets... You still need to stop the knock as it still overstresses the internals)... The surface area of the head gasket that are exposed to the burn is very minimal, as well as the overall design of it. As long as the pressure rises moderately evenly on all 'sides' (its round) of the cylinder, this isn't that bad of a thing. The only thing the gasket can TRY to do is expand to a larger radius; though the tensile strength of a good gasket won't allow much expansion.
'my experiences are that a poor exhaust system results in high boost not blown headgaskets'
I'm with you all the way. Thanks for the reply & helping make my point.
More boost = more backpressure in the intake caused by restrictions infront of it. I've covered this. If you don't understand by now, you probably won't get it.
Increasing exhaust size will see a nice increase of HP and a drop in boost. I went from 13.5lbs of boost to about 10lbs of boost when I did my exhaust but felt a large increase in acceleration. Thats when it becomes necessary to put a smaller pulley on to gain some of that boost back.
Sorry Ken, I have to comment on this. The horsepower gain is from you getting more burnable fuel into the chamber rather than spent exhaust gasses. Its like running a car at WOT with EGR wide open... It fills the cylinder, but not all with burnable gasses
Obviously, the fresh air is going to make more power, and a noticeable increase in acceleration. The boost decrease.... "Boost" is backpressure in the intake side. Your boost went down when you did your exhaust because you could now fit more in the combustion chamber (and possibly lost a little bit out of the exhaust depending on your cam (NOTE: NOT A BAD THING!!)), and it didn't stack in the intake and build up in heat. The blower still output just as much as it did before, and you technically shouldn't need to put a smaller pulley on. There is no need to gain boost back.
'ppl have blown HG's even WITH a full exhaust.'
Right - so why would someone say that custom exhaust helps to not do that?
People blow their HG's N/A. What's your point? There are MANY possible reasons to blow headgaskets. You can't assume that the exhaust doesn't greatly help simply because people can still blow the headgaskets with a good exhaust system.
NOW, I'm done being a douche for a little while atleast.... So here's my thoughts on the original post... Around KC, We've got quite a few SCs with 10%s, gutted cats, and good exhauts. The cars do tend to heat soak after a few runs, and pull timing from charge temps, but we haven't had any with issues with detination. It's definately do-able, but a double intercoolelr or something to that effect would help.