SPOautos

I'm trying to wrap my brain around overlap. It seems like with each rotor face being sealed off there wouldnt be any overlap unless the ports were HUGE, like HUGE HUGE HUGE. Some RiceRacing exhaust ports huge.

Is this true or not? I've heard people talk about overlap with just street ports and I just dont see how its possible. It seems like overlap would be impossible because each cycle stage is seperated by the apex seals.

Someone please school me on this.

STEPHEN

RICE RACING

Stephen, you are right you need some big exhaust ports to get any "real" overlap, but this is only one side of the equation.

The exhaust closing timiming is defined by the trailing apex seal as you mentioned. The other side is the intake ports and in a non bridge the opening timing is defined by the outer edge of the rotor and side seal and edge of the intake port running near the water seal. the port can be take back quite a bit on the primary ports and a little way on the secondary ports (I make these both the same in opening timing) i.e. as far back as you can take them witho ut the leading edge of the side seal droping into the port.

When you do this to the intake (move it back to the water seal) and lift the exhaust (remove the sleeve and port it as high as you dare "without causing a water leak !) you will have the maximum overlap possible for a street port. The other little trick you can do is chamfer the leading edges of the rotor sides down to the side seals (bit like a bridge "eyebrow mod" but on both sides on all three faces of each rotor. This will make the intake port slightly sooner again and will improve initial flow.

With all of these things you can generate a bit of overlap, not enough to make the engine undrivable or overly sensitive to exhaust back pressure (i.e you can run it far quieter than a partial bridge) and still make realy good horsepower, best compromise for the street, that is why it's called a street port

P.S. I was a bit scared when I tried my porting the first time as people just looked at it and laughed, but it worked pretty well for me Hope that helps ya some ?

SPOautos

Ok, but here is what I dont understand and maybe my terminology is just off or I'm not thinking about it right.

Overlap as I understand it is when your intake and exhaust ports are open at the same time. From what I understand the benifit of this is that when your exhaust is pump out it creates a vacume that suckes in air thru the intake. I think most of this relates to piston engines and I'm sure also carries over to rotary BUT I dont see how its possible since each combustion stage is self contained.

Thanks
STEPHEN

Kenku

Well, in theory each combustion stage is self contained. But in practice, with the way the ports work, that's not quite the case. What happens is that the trailing apex seal hasn't passed over the exhaust port yet (which would close it to that rotor face) by the time the rotor has moved far enough to open the intake ports. This is kinda hard to explain well without a visual aid; http://personal.riverusers.com/~yawpower/dectech.html is a place to look if you want specifics on when the opening and closing events are happening. But yes, overlap is possible on rotaries.

600HP CLUB

all these words! i can't think straight need diagram ....i'll be soon porting myself and this would be my first time, trying to learn best possible way to port a street port, all other template base portting is too small!



-mike

SPOautos

Kenku, the link you provided had some diagrams that helped very much. I think it was just hard to visualize cause I've never actually turned the engine with it opened up. For some reason I was thinking that when the intake port starts to open up the exhaust would be long since closed BUT that really isnt the case.

From the diagram when the leading seal opens up the intake the exhaust JUST closed so it seems you could gain quite a lot of overlap with minimal porting. Just pulling the intake down or the exh up (or both hehe) would really make quite a difference.

SO, what my explantion of what overlap really does correct???? How the exhaust leaving creats a vacume that causes air to be sucked in on the intake.

STEPHEN

Kenku

Yep, you were correct. Don't worry, it took me a while to get this stuff too.

The exhaust creates a vacuum through something called pressure wave tuning. Which... well, there's a bit going on to it. Simplest way of thinking of it is inertia. The exhaust port first starts opening when the gasses are still burning, so they're going really quickly down the exhaust system. This is a really strong pulse, with a lot of energy. Now... behind this sharp pulse is a bit of a vacuum that helps draw the rest of the exhaust out of the chamber. So, it's actually inertia from that initial pulse that's pulling most of the exhaust gasses out, and not the engine pumping them.

Well, that's all well and good for getting exhaust out. What does this have to do with overlap? Well, if the intake opens up, that same vacuum that draws exhaust gasses out will be pulling air / fuel charge in too. And it's that simple.

... sorry if that doesn't make much sense; I didn't get to sleep last night and am living dead. But I think that should have made sense.

SPOautos

It makes perfect sence....its exactly as I thought it was. For somereason I was just having a hard time grasping how you could have overlap with each section close off. I really didnt realize that the apex seals were that close to each port (intake and exh) at the same time.

STEPHEN

Dak

I have a question about this over lap.Though it is benificial at higher RPMs doesn't it hurt a bit on low end,because at low RPMs it has the opposite effect and sucks some exhaust into the intake charge before the exhaust closes?Or does this only happen if your exhaust has too much backpressure?

Kenku

Yes, it does hurt a bit on the low end! Even moreso if you have too much backpressure... yet another reason why engines with the port timing optimized for high RPM work don't do so well at lower RPM.

SPOautos

How does overlap affect your vacume readings and smoothness at idle. I'm asuming is lowers the vacume a considerable amount since the intake "chamber" is somewhat opened to the exhaust for a brief moment.

hmmm, could this have something to do with the fact that my car runs extremely rich at idle and cruise when ever I'm under 10 vacume? No matter how much fuel I pull it doesnt seem to lean out, could it be from the exhaust getting into the intake???

STEPHEN

Bucrx7

Stephen what are you idling at and what's your vacuum? I have a VERY BIG streetport with alot of overlap and I have 10 vacuum at 1000rpm.

SPOautos

Well, it depends on what gauge I go by the Power FC shows around 400 which is about 16 but the Autometer shows about 11-12.

My car runs extremely rich at idle which also makes the vacume lower. My a/f at idle AND cruising is around 10.8

Basically anytime my vacume get to about 10 on the Autometer no matter what rpm my a/f drops from like 14 down to 10.8. Its a VERY distinct line, at about 8 vacume I'm at 14ish a/f then at 10 vacume it drops to the 10.8

I've tried tuning it out, I've tried opening up the air screw to let in more air when the tb plates are closed and nothing seems to work.

I idle around 900 and it has a lope to it.....obviously due to the richness.

STEPHEN

fdracer

why can't you tune out the rich idle, you got big primaries?

SPOautos

Well, I'm running 850, its not huge but its pretty big for pri. At first I thought this was the problem but I've heard from other people that thiers was fine.

STEPHEN

RICE RACING

You can set your idle mixture to whatever you wish if you have a programable ECU regardless of porting. I have one of the largest street (inc exhaust) ports in existance and what I have found is that you do need a slightly rich mixture to get the engine to idle in a stable manner. My idle mixture is around 13.0 to 13.2:1 and leaner than this and is a has a slight lump and missing sound to it combined with a bit of hunting (searching up and down the rev range at varying loads). If you set it too rich (in my case 11.0 and richer) there is excessive fuel use at idle and also you have emmissions and plug fouling problems when going through cold start cycle when running racing type plugs. In regards to vacume I pull around 55kpa ABS at idle from memory @ 1100rpm (stable idle point for me) or around 12" on the autometer gauge. I lost about 2" of vacume when I went to the "extreme" side port exhaust port combination.

rotarygod

iTrader: (1)

Even a stock port rotary has a very small amount of port overlap. The new Renesis howver doesn't. Open an intake port earlier, close an exhaust port later, or any combo will increase the amount of overlap in an engine. Since Rice Racing revs his engines up to an absolutely stratospheric high rpm of around 10000 or so he needs his engines to breathe really good. Overlap is no longer a detriment to performance at high rpms since the gasses are going so fast in and out of the engine that inertia keeps them going in the proper direction. This is why at low rpms, engines with high overlap make little power. Exhaust gasses go back into the intake and dilute the incoming charge making it less efficient. There are certain ways to help aid this such as increasing intake runner length but this would hurt top end power. How crazy you go on port size and style should be determined by how YOU intend to use the car. If it is mostly a daily driver and will never see more than the factory redline then there should be no reason to go with anything larger than a streetport. Gotta love that bridgeport sound though! If you are going to race most of the time then maybe a bridge is what you want. Also if you are turbo or n/a could determine what you want to do. Remember the more overlap you have the less gas mileage you will get, unles you just don't care.

RICE RACING

rotarygod, my current "street port" is no 10k motor...I wish I would need shorter intake and exhaust manifolds to tune for this range.

But it does make power to 9k if need be (to hold a certain gear for a corner combination etc) but it does loose power in this higher range nad is not efficient, I run the engine mostly when racing from just below 5k to 8.5k with maximum power being generated from 7.5k to 8k rpms.

Yes if you plan not to exceed the factory redline of 7k or there abouts then you are wasting your time porting the engine much, as you realy do need to rev to 8 or 8.5k to get the maximum benifit of the shifted power/torque curves for maximum acceleration.

rotarygod

iTrader: (1)

Oh crap sorry Rice. I got your engine confused with crispeed and his rpm range!

crispeed

My porting alone does not have as much influence on my power band or redline. It's the rest of the combination that does. It's the same porting I used before with the original combination and power was good to about 8.5K. Now with the new combination power is good to about 10k with no lost in midrange and some increase in low end! It took a lot of trial and error to figure that out.
I'll say intake and exhaust tuning are responsible for about 50% of the power band.
One of the reasons why so many people don't have any luck or success with BP and PP turbo applications is because of the wrong combination of parts to do so. Why use the same combination of parts that were designed to operate at stock configuration.

crispeed
87 Rx-7 TII
9.204@150.47mph
2600lbs
un-tubbed

SPOautos

If you dont mind me asking what "parts" are you refering to Chris? You talking about different internals or externals such as manifolds? I know you've got your own intake manifold setup so is that what your talking about? Do you think there would be any gain on a street car (say less than 500rwhp) by switching to a custom intake manifold....provided it is a good design?

Just trying to pick your brain a little without asking you to cought up the spacifics cause I'm sure you dont want to get very spacific in this conversation.

Thanks man,
STEPHEN