| | | quote | Originally posted by wiccantoy:
ok this is the fun part the compression is 10.5 to 1 cam is the e303 mustang cam engine has a 100 complete rebuild done on it with ported and polished heads and intake. not really looking to go to too much boost and blow anything alot has to do with the high compression. also has the buds outback northstar pistons. pretty much everything in my sig. redline is the same as the stock v6 was
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As you already know,you will get a variety of opinions,so I again figuer,I might as well add my two cents.I would never tell anyone what to do with their car,because I never liked someone telling me what they thought I should do.In my experience the builder always makes the right decision for themselves.If I post something as a fact,it is only things that are common knowlege,and can be easily referanced to reliable sources.All other info,I will make very clear,that it is only my opinion.I make this post to stimulate your mind,but read carefully,It may seem contradicting at first,but read carefully,and It makes sense.
Someone is going to look at your 10.5:1 compression ratio,then say,"you will blow your engine" maybe blow the heads off,break the head bolts,or at least blow the head gaskets.Why?,because the cylinder pressure is so darn high,you raised the compression ratio from 9.5:1 to 10.5:1,and obviously raised the pressure inside the cylinders.
Well,you may already know that is not true.Even though you raised the static compression from 9.5:1 to 10.5:1,by increasing the the cylinder volume with the .040 overbore and decressed the combustion area at TDC with the flat top piston(I don't which N* piston you have,but none have a real dish like the 4.9).You then changed the duration on the cam,I think your cam is around 220 degrees at .050 lift,thats about 20 degrees more than the stock profile.This then lowered the Dynamic compression,which is the actual cylinder pressure.In my experiance,for most engines,20 degrees added duration is more than enough to offset an increase of 1 compression point.
You probably know why,but just to refresh everyones memory.As the piston is beginning to upward from BDC, the intake valve is beginning to close. The intake valve is not completely shut until the piston is near TDC. There is a connection between the combustion chamber and the intake runner, as the intake valve is still partially open. As the piston is squeezing and approaching TDC, some cylinder pressure bleeds into the intake port,which reduces overall cylinder pressure.So the longer you hold the valve open,the more cylinder pressure you lose.knowing this also makes it easier to understand why cam grinds for turbos have very low duration.
Like I said before,on most all other engines,20 degrees added duration is more than enough to offset an increase of 1 compression point.Thats fairly basic knowlege,and if it applied to the 4.9,this would mean Your engine,as it is now,has less Dynamic compression than a stock 4.9 with 9.5:1 compression.
Is the above statement accurate? In my opinion "no" It is in my opinion that the 4.9s volumetric efficiency is very low,lower than anyone would expect.like I said before in another thread,the rpm range does not change much with this cam and all the port work anyone has so far been able to do.Everyone knows this because there are a couple dyno reports of this engine with the e303 cam,and they are fairly consistant.
What does this have to do with Dynamic compression?Why would I bring up the discussion about the rpm range?well,using a 350 chevy as an example,this cam with even the worst heads avaliable for the 350,you would see a considerable loss of low end tourqe.This is because there was a considerable amount of cylinder pressure lost,as it bleed into the intake."220 degrees@.050 ramp is not a small cam,however looking at the E303 equiped 4.9 dyno,I don't see the dramatic loss of low end tourqe.I suspect there was also no dramatic loss of cylinder pressure.
Why do I think the loss of Dynamic compression is not consistant with other engines?Not only do I think the volumetric efficiency is much lower than expected,causing poor flow at higher rpms,I also suspect the cylinder pressure does not bleed back into the intake as well as it does in other engines.
Do I think the size and design of the intake and exhaust runners is the only thing causing this effect?No I suspect there is some type of weird pulse going on inside the engine,I can't fully describe it,and do not have a full understanding of what is going on.I suspect the square bore may have something to do with it,but I do not have the knowlege to diagnosis what is happening.I have witnessed the dyno of several 4.9 cars with wide band and have asked for advice from several experienced dyno operators.I am more than convinced there is something funky going on inside the 4.9,maybe pulses canceling out eachother or God only knows what.I think this is an issue that someone definitely needs to be looked into.
I want to tell a story that may stimulate your thinking about turbo selection.It's about a 301 turbo TA,which may be very related to this subject,because it is also a 4.9 with small valves and heads that are proven to flow like dog doo doo.
Back in 1999,while atteneding UTI in Houston,TX .as some of you my know,the school is broke down into phases,with different instructors.I had a instructor,that on the first day of the phase,gave kind of like a "mini orientation"It was a 100 percent true story that ended as a moral story"
Back in the early 80s,The guy had been some kind of tech/writer/editor for a main stream magizine,even though he had the magizines their for us to read,and I did read it,I forget which one it was,but I'm pretty sure it was one of the pontiac mags.He had done one of those Tech/buid-up articles that are broke down into several parts and come out in different issues.This is done,obvoisly to get you to buy more mags and keep reading.The thing is,sometimes,or at least in this case,the project is not completed,before the first issue hits the news stand. (LOL) You aready know where this is headed!!
The article was the build-up of a turbo TA,which was near brand new at the time.The heads for the big boy pontiac engines,that did flow well,are the same as the 301,but will not work with the 301,because it has a shorter deck height.The 301 turbo head had a large combustion chamber,so they fitted it with huge pontiac valves,I think they were 2.11 -1.60,of coarse they did not what to raise the cam duration,so they added a very high lift cam and other pontiac valve train parts,along with massive port work to the intake and exhaust.They then had a pretty decent flowing set of heads.They did exactly what they wanted to do.They wanted to improve the volumetric efficiency.Which was sure to show a remarkably lower ET at the drag strip.
The project was completed after one issue of the article was already sold,The end result was the turbo would now,not even make enough boost to show on the gauge,only showing 0 boost"no vacuum" when it should have been at full boost.Normally,getting a different size turbo would not be a big deal,but at that time,there was no other draw through turbo to work with the engine.This was a early end to his career
The moral of the story was for us to learn,while we were there,and not leave as sheep in wolves clothing,then ruin our life,as he then made less money,than he did when he was a younger man in 1982.
Thanks...
Chad
