vettenuts

iTrader: (4)

I have been trying to read some of these threads to learn as much as I can and understand some of the things my car is doing. One of the topics discussed is reversion. How would this show itself, for instance on a dyno graph would it be something that is detectable? Can you tune around it? Do you need a huge cam to have this occur or do even smaller cams run into this? How much does the header design play a role, for instance pipe size, Tri-Y vs. 4 to 1 design, etc?

ddnspider

iTrader: (26)

from what i know about this it is mainly a problem for forced induction applications although it can happen for some NA applications as well.Its when exhaust gas goes back into the combustion chamber instead of out the exhaust.1 main cause of this is backpressure or a poor exhaust setup.or when the intake valve opens too soon and the exhaust valve closes too early some gases can be sucked back in.1 way people have found to counteract that is with a reverse split camshaft which helps to get more of the fresh air in and keep out the exhaust gas.someone could add to this cause i dont know about it extensively but thats what ive gathered.for my particular turbo app im using a reverse split custom cam.

SuperC1

iTrader: (4)

Negative, a reverse split cam is the main cause of this type of problem. When using forced induction, normally the intake duration of the cam is much smaller than the exhaust duration. This gives the exhaust gas time to escape before the intake valve opens again. A reverse split will usually work better on a normally aspirated setup to allow more air into the intake to give simular results as having a forced induction setup. But, all vehicles aren't the same each individual setup is diffrent. There are so many variables to consider when choosing the right type of cam its almost impossible to determine which is best unless the setup is thoroughly planned out from start to finish.

ddnspider

iTrader: (26)

actually your wrong read the FI sticky in the FI section and go to www.forcedinductions.com and read turbo 101 ill quote it for you....."Duration when using a manifold or log design on most turbo cams is usually about 6 degrees more intake duration than exhaust duration (226/220, 240/234). This is mainly because a manifold/log design will typically see higher then a 2:1 pressure ratio in the exhaust ( as high as 4:1 with some logs). By using a reverse split duration this will somewhat help prevent from getting exhaust gas reversion. " i asked everyone from HPE, comp cams, Futral motorsports, Forced Inductions, and member when specing out my custom cam....and its a reverse split for that reason.
you may be thinking of a traditional split such as a 207/220 LPE GT2-3 camshaft.

Adrenaline_Z

Actually, it's more an issue with naturally aspirated motors, but it happens in any motor.


It has more to do with engine tuning as a whole. It happens throughout the
rev range and at different intensities at every instance.

Reversion wont appear on a dyno graph as some sort of anomaly.
Contamination of the intake charge will result in loss of power; it happens so
gradually in and out of the tuned areas that it can't be measured on a dyno graph.

The only thing you need to know is that it will happen on any engine, and
different points of time. The engine tuner will setup the motor to reduce, or
hopefully eliminate any contamination in the area most used by the gearing
in a race application.

If you search for key words like Pulse tuning, or resonance, you might pick up
more points on this topic.

SuperC1

iTrader: (4)

Oh well, I haven't read it but I think your cam should be tailored to fit your car. Only the engine builder himself or your tuner can tell you what type of split or how much duration you should need. Things change from car to car and who knows what will cause problems on one may not on another.

vettenuts

iTrader: (4)

How much does header design, restrictions, etc. play into this?

ddnspider

iTrader: (26)

go to the website i posted and read the turbo 101 because it discusses how the camshaft relates to headers and such.it should help you out alot.a better flowing headers and exhaust will change your cam idea,if youre exhaust and intake match well then youre better off with a single pattern cam ie 224/224...if you have a poor exhaust a reverse split will benefit more.and if you have a better exhaust than an intake then a traditional split will suit your needs well.these are just general rules of course as its been said all motors and setups are different.
superC1....you said you havent read that stuff you may want to so you dont give wrong advice.no flaming just trying to help.

Adrenaline_Z

Of the few books I've read, and some real world dyno data, header and intake
tuning play a big role.

Collector design and secondary exhaust tuning is huge in this department.

The primary design of an exhaust is pretty tricky. The length, diameter,
steps, port isolation, etc. are just a short list of things I've read about.

When moving onto the secondary points - mainly the collector design and length,
there are massive gains to come. Simple things such as moving a reflective
surface back six inches (IE: catalytic), or removing it from the system entirely
can change tuning at a noticeable level.

This is not to be confused with enlarging the catback from 2.5" to 4" pipe diameter.

More complex reading comes with cross-overs pipes, and interference tuning,
primary cross-over designs for pulse tuning, and usage of thicker gauge metal,
and/or coatings.

Intake is the compliment, and as mentioned above, the cam timing also must
match to tune a specific window.

MadBill

Adrenaline, you're exactly right regarding one of the biggest causes of reversion! Here is a great example:
I participated in a dyno session on a Toyota twin cam 4 valve 1600 cc Sports (road racer) fitted with a pair of dual throat Weber carbs. The plan was to dial in the cam centerlines and do some jetting and spark work. The thing was a slug from the start, giving ~115 WHP @ 7,000 RPM instead of the expected 160 @ 8,500. We gained a bit with cam timing but with our limited jet supply couldn't get the AFR leaner than ~12.0:1. At one point we pulled off the air box, and found that there was a huge cloud of fuel standing off the carbs all the way up to over 6,000 RPM. We checked and re-checked cam timing, etc. without finding the cause and basically wasted the whole day.
Back at the shop, I noticed that the Supertrapp muffler (don't get me started on them...) was joined to the header primaries with a 2 -1/4" pipe, which seemed way too big. We stuck on an 18" length of 1 -3/4" pipe instead, and the engine was transformed! The car lapped 2 -1/2 seconds faster with great throttle response at all speeds and no more fuel draining out of the airbox. Unfortunately, we never did get it back on the dyno, but we later learned that the car had never been raced with that %@$# sewer pipe, it was just stuck on before the current owner picked up the car.
So what exactly happened? This:The cams were selected to coordinate with a great scavenging header but the big collector killed the extraction effect, so that when the intake valves opened about 45 degrees BTDC, there was massive blow back out the intake.
If just one instrument is out of tune, the whole orchestra (engine) sounds like hell!

Adrenaline_Z

No doubt, it's amazing how simple adjustments can make a world of difference.

I wish I knew all of this about 15 years ago. I could have saved quite a bit of
time and money.

treyZ28

story of my life