Snrub

iTrader: (1)

I have been thinking about doing a semi-perpheral or full peripheral port engine with a bit of a different philosophy. Rather than shooting for full out power it would aim for slightly lower performance but good streetability (eg. fuel economy, etc). I have a few questions and thoughts I was hoping to get some comments on.

1. If I had the peripheral port opening at around 75 degrees ATDC it produce zero overlap with the exhaust port, correct? Are there any down sides to doing this?

2. My understanding is that at lower RPM the flow of gases starts to go into the opposite direction at around the time that the stock intake ports close ~40-45ABDC. If the engine is operating at a reasonably high RPM how much longer could one expect positive intake flow? I realize this is during the compression cycle.

3. To reduce air flow going back into the peripheral intake port I was thinking about using a reed valve. They're used in lot of snomobile and other off road vehicle applications and basically shut if flow occurs out of the intake port rather than into the port.

4. Initially I was thinking about a semi-PP with the PP opened only at a certain RPM, but if a reed valve allowed the engine to flow air later into the compression cycle without the the compression cycle squeezing air into the intake, would a full PP be viable? Perhaps a good sized port to further compesate for the 32-75ATDC portion where the port wouldn't be open? I'm not the most knowledgable person on this topic so any input would be greatly appreciated.

Bob's #45

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I think it's a really cool idea, becouse the reeds would stay shut untill the side housing ports shut. Then your only overlap would be stock, seems like it would still idle good too. But I bet your biggest power increase will be above 5000 R.P.M.
I dont think you would have to wait so long before the begining of the port opening, the reeds should stop any overlap into the Pport. From what I can tell anything over 80 deg ATDC you will start loosing displacement.

Bob's #45

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Delta V-Force makes a reed block that you can adjust the tension of the reeds if needed.

peejay

Yes, the down side is you'd have to have a port window of negative area. When that rotor face is at TDC, the previous face is at 90ABDC, which is well after even the most hugestful ports close. Intake port closing for the most part is what defines where the powerband is. To make a peripheral port with the timing you are describing is impossible, physically and geometrically.

I have some really "short" ports in my engine. They close at roughly 55-60ABDC. They open at 80BTDC. I defined the powerband with closing timing, figured the potential HP I could make at the top of that powerband, and the port area required for that, then opened the port as late as possible and still maintain that area.

RandomHero

iTrader: (2)

Wouldnt that reed valve be a nasty restriction with the cfm that a peripheral port rotary pushes, as compared to a snowmobile engine?

peejay

Oh yes it would.

With a wide open exhaust mine does barely 5 inches of vacuum, which is almost enough to draw fuel through a carb. Played hell with tuning, since the old "hold your hand over the carb to see if it speeds up" trick to see if it's running lean will not work. The restriction will just kill the engine, even at 1500rpm.

And even with a leafblower like ability for the exhaust to make stray leaves go far, far away at idle, capping the exhaust pipe has the same effect. Buuhhh, dead.

I am not sure what a reed valve would accomplish, anyway. When the seal passes over the intake port, the (relatively) high vacuum in the chamber finishing its intake cycle can draw exhaust gases up from the chamber just starting the intake cycle. (Which is why when you boot it it runs smoothly... little vacuum in the chamber at that point)

Two cycle engines can use reed valves because they first draw the air/fuel mix into the crankcase as the piston is going up, then when the piston comes back down it forces it into the cylinder. The reed valve is a necessary check valve between the crankcase and the carb, although some of the more exotic engines used a spinning plate to control airflow (GACK, cannot think of the term! But it's been about 15 years since I paid any attention to 2 cycle tech)

Whizbang

really? i still have to come by and marvel...

peejay

As soon as I get a carb calibrated on it, sure. And then it's gonna come apart so I can see where I forgot to put sealant, pesky external coolant leak

With the 750, I found that the #72 jets were too much. It likes 58's much better and it still wants less. On the road test it started blubbering again at about 5000-5500, and my first thought was "aw crap, I have to play with air bleeds?" But after some thought, and a quick check of my reference book and some math, and I realized that it was blubbering at that point because the *secondaries* were opening. Holy crap! My old 460 couldn't get the secondaries on this carb to open...

Anyway, driveability is right out with the 750, you need to baby it up to 3000 before you get on it otherwise the engine just *cuts out*, so I need to get the 450 working right, I'm going to start off with some #45 jets (hope it doesn't need too much less, they don't make them any smaller than 40!) but the real trick will be the secondary side, since even though my 450 is a mechanical secondary, it's a 4160 carb meaning it has a plate on the secondaries, no metering block with replaceable jets. And they don't *make* secondary plates small enough for what I'll need. I'm looking into modifying the plate to take standard Holley jets.

All this work and I plan on having ther stockport back in the car by the 15th because I still don't trust the engine enough to drive it 250 miles away to a *race*....

Whizbang

seeing carbs are a world unfamiliar to me....it sounds like you have a project. but your the uber genesis anyhow. some time i do want to pick your mind about how you determined where to set the ports to determine the power band.

Marcel Burkett

So why not go F.I.

mwatson184

They do make a metering block conversion kit Peejay. I think that would be a wise investment. 60 dollars i believe.

Snrub

iTrader: (1)

But wouldn't a reed valve close the port once duration of the intake was into the "negative area?" I don't understand why a hole couldn't be drilled towards the top of the housing?

Could you share a little of that math (port size vs. timing) so that I can wrap my head around the ideas you're talking about?

It stands to reason that it would in a typical application, but assuming you had reed valves that were in an engine making the same amount of power as the proposed engine, would it be that bad?

peejay

$$$$$.

The cheapest route to EFI is going to be about $250 plus fuel injectors, I already have enough bits and pieces to make a fuel system.

That's about $240 plus fuel injectors more than I have. And that's if I just adapt a throttle body (again, have plenty) to the existing manifold, instead of utilizing the space advantages of EFI and making something a bit better (a LOT better).

It's something to do over the winter.

Snrub

iTrader: (1)

Any chance anyone wants to answer any of my questions or point me in the right direction? Thanks.

rotarygod

iTrader: (1)

Well you really can't have no port overlap with the exhaust using a peripheral port. Not even using a small one. That just means you have less port overlap because you have less port. It would be theoretically possible to drill a peripheral port very high up in the housing so there isn't any overlap. This poses a very serious problem though. It would be placed so high in the intake cycle that the chamber would be physically be getting smaller as the port opened and moved across it's cycle. You couldn't get any air in. Eventually you could reach an rpm (in theory) that would be high enough that intake velocity would overcome this to a point and you would get some intake. At what rpm this would be is anyone's guess but it is probably far too high to ever be of any use at any point whatsoever.

As peejay described, a p-port engine is very sensitive to intake and exhaust pressure. Way much more so than other types of porting which is why a peripheral port engine can't necessarily use the same types of intakes and exhausts as other porting styles. You don't want to use anything like a reed valve in the intake as all you'd be doing is closing off incoming air and it's velocity and allowing exhaust gasses to carry over into the intake at this time. Bad idea. Dont' even worry about trying to make a reed valve work. Although I am a big fan of trying new things, this is one time I can definitely say don't waste your time.

A semi peripheral port could work just fine. Don't do it like most others do it though. That is basically do not add in a small p-port to an existing runner/port. Too many people will retain their secondary ports and runners and then just add in the small p-port runners into these existing ones. This effectively cuts velocity in half after the junction. That is a loss in power. They'd probably gain even more power by completely blocking off the secondaries altogether and just running the small p-ports as the secondaries. They would still ahve the same cross section area as the intake runners which means they would still flow the same amount of air, just at a higher velocity which is benefical. Alot of people are using boost to mask the effects of doing it this way. Think of it this way, airflow is airflow. It doesn't matter at what pressure it is at. It still behaves the same way in terms of flow. To get a nice setup like this that would make great power, think differently. Different is good.

Use only the primary ports and completely block off the secondaries altogether. You won't need them. Then cut some small 1.5"-1.625" p-ports. These will be your new secondaries. Don't be too tempted to good too large. Velocity before volume is key to making power. Now fab up a custom manifold. You will need a valve inline to the new p-ports. You basically want to copy the Renesis intake in this regards as you want the secondaries to be completely blocked off until a certain rpm. It is best to make sure each runner has it's own valve like the Renesis intake does. Do not do it the same way the 2nd gens do by having each secondary runner join after the throttlebody. This allows some pulses to go back and forth. There's a reason Mazda did away with that. Another option you can use is to take an S5 n/a intake manifold and cut out the VDI actuator. You will need to enlarge one of the holes. Then weld this inline in your new manifold. You can now use a single plate throttlebody with no issues as only the primaries are open until you decide to open the secondaries. If you do it right you can even set them up to open at any rpm you want or if you are running forced induction you can even have them open only when in boost. As long as you have a standlone ecu, this setup will work great.

I know some people will be inclined to say this will flow less as you will have less total port area, but you will see that it can't as it will have the same total intake runner area. You will just have the advantage with increased velocity. Someone will probably disagree and say this is a bad idea but nice idling, decent drivability, and added power where you can use it and when you need it sure don't seem like downsides to me. Make up your own mind on this but it makes the most sense when you think about it. Just remember that bigger and more isn't always better and you'll start seeing alot of flaws in traditional thinking.