Weber selection and initial jet tuning

[240260280]

2019 Wish:

 

Hi @Sean Dezart, would you be interested in taking your header to the next level?  If so, read the details below.  I think it could be thoroughly validated by @blodiafter he completes his initial dyno testing.

If all works out, you will have one of the best performing headers and we will have conquered the mid-range rich-dip. The solution is to add a simple piece in the hottest spot of each runner and transition to slightly larger pipe.

 

  

 

 

Improving Header Design (content from Speed Talk):

1. So, what does an anti-reversion valve do?
It is simply a one way resistor to delay the reversion effect and give time for the exhaust valve to close.

2. How does it work?
Since hot exhaust wants to expand, we give it the opportunity to, but only in the reverse direction.

3. Where to put it?
At the tail end of the largest pulse of exhaust, where the exhaust slows down. If your header is port matched, it is approximately 7-8 inches from the header flange. To be more accurate, you can map out the temperatures of the header pipe and there will be a temperature spike where the exhaust slows down. This spot will have the greatest affect. This spot is relative to engine displacement and not rpm. So, moving it higher or lower just makes it less effective.

 

Top

[Sean Dezart]

Hi and thanks to the introduction to this thread.

Interesting but nothing proven yet.

Also, those reversion 'valves' would fit in just where the headers' primary pipes are closest to the carbs ie a much wider diameter to interfere with the lower edge of some carb versions.

I might suppy a header for local back to back tersting but like should be compared with like - no ?

[blodi]

Spring is here! Lets get back to it! 

I switched to 34 chokes over winter to try to get a bit more drive-ablilty back to it. Same exact setup as before other than that.   Did a pull last night. It was high 40's or so here.....so results may vary once it gets a bit warmer.....

 

[240260280]

Looks similar to Sept. but ~ half a point richer.  I'll plot later today.

[240260280]

~ 0.5 to 1 point richer.  Probably due to the choke and colder weather.  Shape is similar.  I think the exhaust reversion may be causing the rich dip that seems to be in all EFI and Carb runs with 45mm DCOEs.

 

The only big difference is right after going to WOT, the rich dip now starts earlier.

 

A smaller main should bring the A/F up a point.

 

[blodi]

Thanks.  Well, lets wait until I can test in a bit warmer weather and see what the results are before I change up anything just yet. 

[240260280]

After all the testing, we could never tune out the rich dip at WOT onset.  The only time I saw it tuned out was when exhaust reversion was dealt with.  Also, it does not seem to be as significant on 40mm carbs.

It would be great if someone could fab a beat-off old header into an anti-reversion header and see if it can help the cause.  We have done a lot of work on this but it seems it will need a look at the full breathing system to optimize further.

Edited March 26, 2019 by 240260280

[blodi]

Did a couple more logs...same as before.  I'm going to put the 35 chokes back in since we were happy with the results there.

I think my low speed chugging issue is just more related to idle jet.  The idle is rich ~12.5 but just off idle and driving around low speed like 25mph the car chugs a bit..and afr's are high 15's or so. Pulling up on the choke lever smooths it right out. 

 

[240260280]

The trick is to find  an idle jet for a cruise of ~ 14.7 then go back to idle and adjust the enrichment screws to taste.  13 works nice for a smooth idle but 14-15 is cleaner but not as smooth.

I wonder if you could do a run with the timing retarded 5 degrees and at normal  to see if the rich spot is affected.

 

 

 

Edited April 13, 2019 by 240260280

[blodi]

Cruise is nice and lean...I ordered 65F12 jets to try out.  The 70F9's ran too rich and cruise. Idle is currently 12.5 or so. 

I can try adjusting the timing. Got partway though installing the 35 chokes.  I'll report back. 

 

[blodi]

Some logs from the other day....

51819-1 by blodi, on Flickr

51819-2 by blodi, on Flickr

[blodi]

Going to the dyno on Monday finally. I'll be sure to post the results. Car has been running great!