I'm trying to find an answer, if one exists, to the following: Assume your ports are configured to give max power at higher rpms, and you will sacrifice some power at lower rpms as a result. For overall street performance would you be better off with long runners on an intake manifold to compensate for the poor low rpm power or would it be better to use short intake runners to maximize the benefit of your ports even though there will be a further penalty at low rpms?

 

Short runners. High-rpm ports have more overlap, so you'll just get reversion at low revs. It's never going to idle well or pull down low, even if it's breathing through a tuba. There's a reason that P-port intakes are never more elaborate than just getting the runner vertical and capping with a carb or TB ASAP.

 

Depending on port size/type, longer runners for streetablity, shorter for max hp. Max power, Lake Cities intake mani. Best streetability on a 1/2 bp, RE's dual DCDs.

Answer to the question lies in what porting are planning/have and what are the goals/pupose for the car?

 

The porting is stock 6 port with the rotating valves removed. Before 50 people tell me to just put the stock actuators and valves back, understand that I have clearance problems in a unique installation that prevent using the stock intake. Intended use is primarily bombing around town with a few autoX events each year. It would be nice to move the power band down a little, but I don't want to kill the advantage of the extra ports. I'll be using a sidedraft dellorto 48 instead of the current RB holley 650. Would smaller chokes accomplish my purpose better than long runners?

 

Crap, I read right past your name Don, without even thinking. The wrap around intake should work the best for your Sprite for overall performance. Then you have the option of running a Weber, Mikuni or Dellorto 2 barrel carb.

For those that don't know, Don is working on an immaculately restored 59 Austin Healey Bug Eye Sprite, rotary of course. Hopefully he wil post pics or a link to his photo galley. It's an amazing car.

 

Regardless of intake porting, longer runners still give you more low end power. Yes peak power goes down but it should. Here's a dyno chart of a peripheral port engine with 28" long intake runners. Notice it doesn't make near the peak power of a shorter intake runner setup and the peak is much lower but the average is nice with a peak around 230 hp. This engine has much more left in it with shorter intake runners and a higher redline.

http://rotaryeng.net/Weber58-28inch-...dyno-curve.jpg

Here's the same engine with 24.5" runners. The power peak goes up and so does redline. Now it's up around 245 hp or so.

http://rotaryeng.net/Weber58-24.5inc...yno-curve2.jpg

If you put a short runner setup on these engines and spin them up to 10K, they can get as high as 350 hp. Intake runner length is important to tuning no matter what your porting style. You just need to understand the effects on peak power and average power.

What rev limit are you going to run? For best results with the best average power where you can use it, you'll want your horsepower to peak somewhere between 500-1000 rpm's below the rev limit. If it peaks any lower, you won't use all the available rpm range and if it peaks any higher, you'll lose average power. Both results would make you slower.

 

Thanks for the kind words Trochoid. Pictures are at
www.spritespot.com/gallery/My-Rotary-Bugeye

The internals on this motor are stock '86 NA with about 60K miles on it. Don't know what RPM to shift at or what it will take without coming apart. Of course if it blows that'll give me the excuse to have a hot 4 port built. Probably should dyno the thing after its back together and see where it peaks. Stock '86 trans. Just blew the dinky Sprite rear end and am in the process of having a Chevy S10 rear narrowed. Will use a 3:73 gear. Tires are 175/70/13.

 

I would think the RB wraparound or lake cities wrap around with the Dell should make it pretty streetable. I use the Dell on my stock port 12a and it works very well. I am thinking about putting some longer velocity stacks on it to see how that affects the low end. I also anticipate building a street ported 13B 6 port and using this intake with modified jetting to run it as well. I'd really be interested in seeing your dyno results once you get it tuned and running.

Nice bugeye and that garage looks great. I think I see a 914 and a Caddy plus others. I'm jealous.

 

The long wrap around intake with a side draft 2 barrel Del'Orto over the motor made torque about 500 rpm sooner than a short intake with vertical 2 barrel Weber. The long intake was much easier for me to drive on the street since the torque came in earlier. The shorter intake makes more max power for me, and suffers with later torque on the street.

 

Part of the reason for Don's question is he's having clearance problems with the Holley set up that's on it. Sprites have rather narrow engine bays. Options to consider are the stock LIM with the RB upper, a wrap around, or an IDA style.

Since the Sprite is so light, and he's in the process of beefing up the drivetrain, I'd opt for the stock LIM, with working aux ports, RB UIM and any of the 44-48 mm 2 barrels that mount. It's all going to come down to test fitting for what works.

Don, since you're going with the 3.73 gears. I wouldn't worry too much about high rpm perfomance, the gearing will get you to a frightful top speed for that Sprite. With a tire size similiar to our 1st gens, you should see 130+ with no problem. Shift by 7k rpm. At this point, there's no need to push the engine, unless you want to rebuild sooner.

 

I didn't build the Sprite, I bought it pretty much as it sits. The pictures are from Riter Restoration in NY. The wrap around one piece intake looks like the way I should go. Problem is I can't find one for a 6 port. Atkins/American Rotary makes one that they say can be used on either a 4 or 6 port. Looks like they opened up the outer ports of a 4 port manifold to cover the two outer ports on a 6 port. My concern is that going from one to 2 ports will mess up the flow too much. I would prefer to find a manifold with 6 ports on the intake like my Holley unit from RB. The 650 was prepped by RB and has center hung floats. The Dellorto looks cooler and it should perform better in hard turns. My last car was a Lotus Super 7 clone with a street ported 12a in it. Bought it with a Holley and switched to a Mikuni. Absolutely better in the turns and the butt dyno reported more power.

 

If there is any chance you can fit the stock LIM in there, that's where I would start. If you could get your hands on one and mount, post some pics. Someone may have an extra LIM you can have for shipping costs. Eventually, the 4-port engine will be your best choice.

 

Probably worth the effort to get a LIM and see if I can make it fit. Maybe grind on it a little. It'll be cold and snowy in Detroit for a while so I've got some time to mess around. Thanks again.

 

All you need is the stock bottom intake for the 13B and the RB top intake for weber dcoe and clones to get the Dell hooked up.

http://www.racingbeat.com/resultset....rtNumber=18100