Don Bad

So here's something I've been wondering about and haven't found an answer. I'm currently running a stock '86 6 port fed by a RB 650 Holley with an RB header. The 5th and 6th port rotating valves have been removed. From what I hear, this set up will give less than perfect low end performance, but should come on strong at higher rpms. Again from what I hear, longer intake runner lengths show move a power band down to lower rpms. My question is this. In my set up would I be better off with longer runner lengths to try and compensate for my poor low end performance or should I opt for short runners to maximize my advantage at higher revs? I've got this engine in an Austin Healey Sprite that weighs about 1600 lbs. I'm thinking of switching to a side draft Dellorto and I'm trying to decide whether to use a longer runner one piece manifold that would lay the carb on top of the engine or a very short one that just sticks the carb out the side of the block. My primary use for the car is just cruising around but I'm sure I'll autoX now and then and I might get out to the strip to see what'll happen.

BurlesonRX7

I think getting your ports working properly would be the best option for you. Then tackle the longer runners if you want more low end performance.

rotarygod

iTrader: (1)

A side draft makes it easy to experiment in this area. You can effectively change tuned lengths will different length bell mouths on the carb. This is easy to do. On your current setup, if you are curious, why don't you try spacers of different thicknesses between the carb and manifold to see what you like better? At the very least, it'll give you an idea what to expect.

Some people claim that they don't lose much lose end with the aux ports open full time. I noticed a difference on mine but at the time didn't care. My current car has them open but it's a play car anyways and about to get a turbo. This is a subjective area but playing around a little bit with runner lengths to see what happens should be fairly easy for you.

Don Bad

There is a clearance problem with the Sprite that prevents using a stock manifold with port actuators. I'd really like to avoid cutting the sheet metal.

Don Bad

The experimenting with spacers and intake horn lengths is a good idea, but in my case it puts the cart before the horse. I haven't bought an intake for the side draft yet and I wanted to choose one based on the answer to my original question. There is a considerable difference in runner length between the 2 types I'm considering. Something like 3" vs 10".

rotarygod

iTrader: (1)

Does your Holley have vacuum secondaries or mechanical? Vacuum secondaries work quite well in this scenario. There are some people that like mechanical better but this is just my personal opinion.

I agree that there are big differences in runner length but runner configuration is also very different between the two. My best friend's car had a Weber carb on it (unported 6 port) and it had great low end power. It also didn't have working aux ports. The top end suffered but that was a result of far too little timing advance.

Don Bad

Mechanical secondaries. I've thought about trying to adapt the dellorto to the RB holley intake but figured that would really screw up the flow.

3x dope

I have the RB Holley 650 and manifold on my '73 rx2 which wieghs a considerable amount more than your Sprite and its plenty quick. Although mine is a streetported 4 port 13b, I don't feel that the car is lacking on low end. Rotary god makes a good point suggesting you get your aux ports working first as a properly funtioning 6 port system should make even better low end. In such a light car, once its sorted out, it should be a blast to drive. BTW, I'm very happy with the Holley set up both for streetability and milage. Tom

Tom93R1

iTrader: (2)

My REPU runs a stock-port 6-port motor with a Weber side draft and wrap around manifold, so same setup you are considering. I run just a filter, no velocity stacks and no spacers. I never put it on a dyno but at upper RPM's it really moves! I cant imagine the top end is suffering much compared to if I had the Lake Cities (or whatever it is) short manifold.

diabolical1

iTrader: (3)

i would have to second the thought of trying a spacer. there was a thread in the Gen I section a while back where i found that rotaries don't like the straight shot into the 4 runners. a reliable source (i count peejay as a reliable source) said that a spacer would help. i may not be an authority on Holley-fed rotaries, but i have plenty experience. if your Holley is a true RB unit, then i think you're better off working with what you have than trying to switch to a side-draught system now.

you should be able to get spacers from Summit.

Don Bad

Thanks for the input guys.

bighroder

you are welcome

j9fd3s

iTrader: (3)

i think in a 1600lbs car, non working 6 ports arent going to be a big deal....

you loose low end without the 6 ports, but if everything is setup right, its not too bad.

i'm kinda thinking exhaust as i type this, had a gsl-se and with the short header and the 6 ports wired open, it was like having huge turbo lag, with the long header and the 6 ports open, it had about the same low end as with the 6 ports working and the short header.

nBOOSTe

first of all mr don bad, before u turn to DON RIDICULOUS, what manifold r u running right now? racing beat does not make holleys, they sell them in kits w/ their manifolds. who told u all of this runner lenght crap? a mag? forum? come on

Don Bad

The current manifold is a RB 6 port one piece unit. The Holley carb was prepped and sold by RB. The most obvious modification is the fitting of an oil line to the carb. I first became aware of the runner length/power band issue in about 1960 when Chrysler Corp. came out with Sonoramic dual quad induction systems. This forum has provided a wealth of interesting and helpful information on the subject.

dj55b

iTrader: (2)

Sorry runner length crap? Maybe you need a little more info

Don, what you can do with your holley is just add a spacer on the bottom of the carb if you wish. Not sure who makes them but I'm sure various companies would as it is a holley bolt style (4150 i believe).

I have ditched my holley for a sidedraft ITB setup, and once I hit the dyno, I plan on getting various air horn length and spacers to try out. Just to get some confusion out of the way spacers and longer airhorns even if you add 10mm of either will not exactly produce the same difference. The reason being is that with spacers you increase the length of the fuel spray to travel, usually resulting in better fuel atomization, and also a cooler air charge (this is only from me reading magazine articles so correct me if I'm wrong), and air horns as you guessed it only increases the path needed for the air to travel. The one magazine that I was reading about v8 engine performance, from moving the injector from the bottom of the bodies to the top they gained about 20Flywheel HP (All engine dyno).

rotarygod

iTrader: (1)

There is one thing that can be tried here to see the effect on performance. I know there isn't much space available. If you can get a spacer to fit between teh carb and manifold, and it doesn't have to be thick, you can play around. The way the stock Holley manifold comes, all 4 runners stay separate. Use a spacer that bridges 2 of them together. Basically 2 ovals. Try 2 different configurations each 90 degrees off. Try it with both primary ports bridged together along with the secondaries connecting. See how it works. Then find a way to do the same thing but instead it connects the primarie and secondaries from each rotor together. You'll find drastic performance differences between each of them. The carb will have to be rejetted for this though and might not idle well if at all if it isn't.

Keep in mind that if for some reason you were to do this with a tall enough spacer so there is more area joining the runners, the effect will be a torque peak shifted higher with less low end. Try this but keep it small. It's easy to do and shows different powerbands very well. You never know. You just might like one of them.

Don Bad

I'm curious about the difference between adjusting air horn lengths and adding spacers between the carb and manifold. There are a variety of undercarb spacers in various materials and configurations available in Jegs and other catalogues ranging from .5 to 2.0 inches thick. The thinner ones can be stacked. Playing with them and the Holley would probably provide some helpful data, but keep in mind that my ultimate goal is to switch to a sidedraft Dellorto 48. The runner length difference between a carb that lays on top of the engine and one that just sticks out the side is a lot greater than using spacers. I think it looks cooler, is less likely to flat out in hard corners, and is more tunable. My last car was a Lotus Super 7 clone running a street ported 12a with an Edelbrock on an RB intake. I switched to a sidedraft Mikuni and felt performance was better all around. As an aside, the Dellorto I have has two holes in it for a nitrous set up. I've never used nitrous, so comments on that are certainly welcome.

SPiN Racing

Something else to consider those of you who are going to hit a dyno.

DO NOT FORGET TO RE-JET.

If you are adding spacers, and or changing runner lengths.. you may actually loose horsepower because you are not jetted correctly. So a setup that may gain HP will actually lose because of incorrect jetting.

Or you may not see much difference but you are a bit too fat.. or are a bit too lean.

Dont forget to re-jet.

If this was a fuel injected setup with a nice wideband.. you would be able to see the differences much easier, as you would see the engine lean our or fatten up slightly.. and could re-tune the map accordingly, and get the real peak HP from the changes in the intake system.

One of my pet peeves with Carbs.
Its a 80 degree day with low humidity.. you make X horsepower.
Its a 60 degree day with high humidity... do you make the same?
Or more?
Whats the density of the air?

If you are running fuel injection (not factory) and you have it tuned properly to compensate for temperature.. you should be making X.. plus or minus depending on the density.. but it should always be very close to the proper max amount if tuned correctly.

SO..
If you are making spacer changes.. you need to re-jet, and or adjust the AF ratio to compensate for the change.
If you are slightly fat without it.. and you space it just right.. you may make more power.
If you space it too much you will err on the lean side and not see the additional power.

All this will also be changing your torque curve as well. You may not see much HP increase, but will have a change in your torque curve. Is it better? Or worse? Do you need the torque down low, or up high? What do you do with the car?

The 787 etc used the variable intake runner length to give more torque down low coming off corners.. and then after moving them make more high end power for the long runs through the gears.

Don Bad

Certainly can't argue with the advice to properly jet the carb - regardless of intake length. I envy those who are up on FI because its obviously a more efficient system. Converting to a stand alone setup at this point seems an expensive proposition that would require a lot of learning on my part. On the other hand, hooking up a bottle ....?