I was wondering what would having less exhaust result in the powerband?

Is it totally dependant on the exhaust flow???

What are the advantages or when do you choose a reverse split cam?

I dont think they are good for LT1s but they are fairly common on ls1s...

Thanks

 

I'll get back to this more later.

The main thing I see is that a reverse split is a band aid for a intake restriction.

Do a search on this, I think we have already talked about it.

Bret

 

The reverse split....

I dunno, you'll see some engines pick up power on a dyno with a simple change to lower ratio rockers on the exhaust. "Intake restriction" or an exhaust port that's too good? I'd say that it might be a combination of a few things. An exhaust port that's really good combined with a cam that doesn't time the engine properly at a certain rpm range may over-scavenge a bit. That'd definitely lose you some of the volumetric efficiency you could have had. It may also be an issue of too large an intake port.... from talking with some really good head porters I've come to the understanding that too large an intake port is for one.... poor for intake charge ramming (getting that last little bit in after BDC). The large port still has ram capabilities, just not as broad a range of ramming as would go on with the "right" sized port. I've also been told by Don Lucido, Chuck Riddeck and a few other porters that an overly large port for the application is quick to revert the intake charge...... reversion, which dilutes the charge mixture, costing horsepower and lowering ve.

I don't claim to know all the factors at work but I can buy these explanations considering their source. Seems logical when you really think about it but it's most likely a combination of things. It's really as easy as a rocker swap on the exhaust to find out.

You also never get 100% of what these guys know out of them. So you have to peice things together as you go. That's just the way it goes though.

-Mindgame

 

O.k.

The best example of this is the LS1 with the 230/224 cams.

What is happening is that we have a pretty good E/I Ratio at about 75-80% on most LS1 style ported heads. I have found that you get away from the traditional split with that kind of ratio, more of a single pattern cam.

I personally like the 224/224 cams for the LS1, or for high RPM a 230/230 cam. The point of the 230 is not just teh good exhaust flow but the fact that the LS6 intake does not support 300cfm. Once you see the E/I Ratio rise up to above 80% then you get why the revearse split cam works so well. As soon as you get a intake on there that supports the 300cfm then you revert back to the single pattern cam. In fact going to a 230/230 is just as good as a 230/224, and is probably better for a 7000rpm engine.

The whole intake tract of a LS motor is odd. From the 8-9" long runner that has a 2-3 deg tapper angle to a 215-225cc intake port on a 346 cube engine. The power rises early and then stays on for a while, so the long runner is not taking alot of power away on the top, or the intake pulse tuning effects psi is not falling off drastically, mostly due to the larger tapper angle.

Mindgame mentioned the reversion in a big port, one good way to prevent that is to design anti-reversion into the intake system, which will allow one to have a better power number below peak because it prevents reversion. That and designing a proper exhaust system to go with the engien combo will allow you to not overscavenge the cylinder, since the tuning pulses of the exhaust system are timed with the cam correctly, which in most cases means that you are not using a reverse split cam.

Bret

 

Bret,

I know what some of the other guys are doing to reduce reversion but I'm curious about how YOU go about designing anti-reversion into the intake system? Also, what is the flow bench showing with these modifications?

Thanks,
Mg

 

I'm probably doing what everyone else is doing. I just use it to move the TQ peak around and add average power. Yes the flow bench does tell the results. I found that reed valves in a 4 stroke seem to not add any benefit since they hurt forward flow more than the 100% anti-reversion effect they add.

As to the second part.......

You hit the nail on the head. Cam timing and the right port size has alot more to do with it than anit-reversion. You use anti-reversion after you get everything else right to make the system work even better. One of the things I like to do is use a larger port (say it's larger than the one you feel is optimum) which will have more "ramming"/intake port tuning at high rpms and a smaller cam with a smaller amount of overlap @ .050 and the right intake/exhaust system. This will let you have all the good effects of a larger cam, because the intake tuning pressures will be greater at high rpm and that will compensate for the small cam. The small overlap will prevent the reversion from happening since it does not allow the intake tract to be poluted due to less exposure time. The small cam duration will increase TQ and power below HP peak. The right intake tuning will help keep the TQ where you want it and the proper exhaust system will have the exhaust tuning pulses working at the right time to cause a low pressure in the exhaust to scavange exahust to prevent reversion also, having it do this at the rpm's where the rest of the system is week is helpfull. Then after you get all of this right you concentrate on helping the anit-reversion characteristics that you already have from the good cylinder head design you are using.

So anti-reversion is not the "new" thing to use for HP (actually TQ), because most people don't get the 10 steps before that right.

Bret

 

Cool info.
But, as mindgame said above. You don't always get 100% info from these guys.