We've got alot of Weber people here that have tried many different setups so I'm hoping that this could get answered here. My friend's GSL-SE has a Weber DCOE carb installed on a stock port 6-port 13B. That car has fantastic low end power and gets off the line great but it starts to lose it on the top end. It is all tuned in properly and everything works great. Contrast this to my stock manifold GSL-SE 6 port engine. He'll kill me off the line but on the freeway he doesn't stand a chance. He is also slower on the freeway than my n/a 2nd gen but again faster off the line. We can't figure out why. It all appears to be tuned in properly and is great for autocrosses. It just doesn't have the top end we like.

I have also read a thread on here before where someone went from a Weber style setup to an EFI setup using the stock n/a S5 intake manifold and gained tons of top end power. Does this have to do with the front and rear rotor intake pulses not interacting with each other since it is essentially a true dual intake? It works the same way on exhausts and seems to make sense but I'd have sworn that the Weber's should be faster everywhere. Has anyone tried installing a plenum in front of the carb with a 3" or so intake pipe feeding it? At full throttle, the location of the plenum in relation to the throttle plates is irrelevant. I may experiment with this to see what kind of effect it has. Any insight or personal experiences would be appreciated.

FWIW: We've played with all sorts of jettings and it is running better now than it ever has. Unfortunately he's the one who tuned it so I don't know which jets they are. It still lacks the top end of the other cars but has more low end. It doesn't even have the 6 port sleeves anymore (removed) and it still has more low end. I am confused.

 

First off, have you tried a wideband on it to see how the fuel curve goes?

If this is all OK, then you may have an inlet that is too long. I had a setup on my car once that used a T2 LIM and weber adaptor and was great to 5500 and then fell flat (by great I mean a stock and tired NA 4 port (possibly repu?) with lousy port matching to the LIM giving 140 HP at the rollers). The combined inlet was very long and the carb just didn't like it.

When I fitted EFI I gained another 1000 RPM. Now I never got to the bottom of why the carb was falling off. Could have been fuelling (accel jet too small to fill in at high RPM), could have been the choke being too small. I'll never know now.

I'd start buy looking at your AFRs. If they are OK you may need to up the chokes.

 

The intake is the stock 6 port lower manifold with the Racing Beat Weber upper manifold. Lots of people have this same setup. Unfortunately I don't have a wideband. It would be nice. We'll probably just have to do it the old way. Take it to the track, run it, change a jet size, and then rerun it to see the difference.

On a side note, when I changed my stock ignition system over to the MSD direct fire, I gained top end on my car. Before there was no point in shifting over about 6500 rpm or so since the power just fell off but now I can take it right up to 8000 with no proplem. I'm wondering how it would work on my friend's car.

 

Grab 3 or 4 mates and get yourself a wideband. Innovate is only $349, which isn't much shared a few ways. It will the THE BEST tuning tool you have ever found. Cuts out a load of the guesswork and allows you to actually map out the fuel curve so you can target your improvements. Heck, if you buy one and don't like it I'll buy it off you at full retail. I am that sure. (aside: I rent out units to people. In 99% of cases they then buy them).

Looking at my scan of the dragsport article on carbs vs FI You shouldn't fall flat at high RPM unless you have a silly small choke, as even the mikuni 44 did 207HP on a ported 6-port and revved to 8000.

A good spark always helps get the best out the engine, especially if you are running too rich or lean. how you get that spark is up to you...

 

it's 6 port.

 

Your point being?