219 replies to this topic

[jd lj]

From David Vizard's carburetor book his formula for choosing the correct size chokes for an IR set up is as follows :
Single cylinder capacity (550 for a 202) multiplied by the the max rpm
(let's say 6000),divided by 2600 and then the square root of this answer is the preferred choke size, but drop the decimal places.

This is 35mm from memory which just happens to be a sweet spot on my engine.
The correct size of carb to suit the choke size is the choke should be approximately 80% of the throttle plate size.

You need to subtract the restriction caused by the spindle from the throttle plate size and not fit chokes any bigger than that. For example on a 40mm dcoe if you fit a choke bigger than 34mm there will be only very minimal increase in airflow for the above reason and there's no point going bigger than 40mm chokes in a 45mm dcoe.

[LC-GTR-1969]

Don't think for a minute that if the gauge says 11.8 then that's what it's actually running. Remember that all you are reading is an average across six cyinders, and that out of those six only two are fed at regular intervals. The gauge is great to get you quickly into the ballpark but ultimately it'll be trap speed that determines the jet size.

 

Exactly...

 

This is why I mentioned earlier in either this thread or another one that I would like to check EGTs of each primary to see whether there is any major variation amongst the 6 pots...

 

I have suspected some overfueling from the front weber but have not been able to put my finger on it.

 

In contrast, I previously had an issue where the webers made the engine run like crap- AFR was 12.9. It turned out that some muppet had put the aux. venturi in back to front, which restricted the fuel flow when on the main jet in that cylinder. Subsequently, the engine would have read quite lean, so the same dyno muppet continued to richen everything else up until he reached the magical AFR number. D*ckhead did not have any idea. When I found the issue, AFR dropped back down to 10.8 (as the effected cylinder was now receiving fuel), and we proceeded to lean off the air bleeds until we got to about 11.8 or so, where it was the happiest. Also gained 56hp in the process.

 

JD- My tests were run on the same day, same conditions etc... and the jetting as determined on the dyno, produced the highest trap speed.

 

In saying that, when leaning off the main jet one step, the engine revs and transitions cleaner but it didn't do the numbers.

[SA EH]

Be very interesting what you come up with Jono.

So in the next couple of months I should have some flow figures of my own to post up, that being my well modified 1 3/4" su's, standard 2", then a modified 2". I'm tired of relying on someone else's dubious figures.

Purely for shits & giggles....

[jd lj]

What are the modifications you're making to the su's? I assume reducing the protrusion off the throttle plate screws out each side of the spindle???

[SA EH]

It'll cop that too, maybe reduce the spindle size(take off one side or machine down both sides), they'll get the piston chamfer, hit them with the die grinder internally to open up the flow path over the jet opening & sidewalls, smooth & deburr, descent ram tubes.....
Just depends on how much time you want to put into them really.

[SA EH]

Yep. But isn't that just the snowball effect for us idiots that want to modify everything as far as we can?

[warrenm]

I'd be interested in the results of this these tests oldjohnno, if you get "enthusiastic" enough.

[LC-GTR-1969]

A little off-topic, but I was wondering whether anyone here has tried any of the new oxygenated race fuels? They aren't cheap but with SUs it would seem very, very easy to adjust the mix to compensate for the richer mix needed with these fuels.

 

With webers I have never bothered, due mainly to the cost of the jets etc needed to re-jet it all with the race fuels, it would be a hassle, and costly. It could be worth the time and money, but it could also have minimal gains.

 

I have heard that 20hp gain is achievable using the right oxygenated fuel, like MS109 from VP racing.

 

The guy who flowed my Iast head has an engine dyno, and mentioned that the last 202 they build had 260hp at the fly on BP98, and then on MS109 made 280hp at the fly.

 

Not sure of induction setup they were using, but 20hp seams very good. They had no reason to lie as they weren't selling me anything. The conversation came up as to how hard it is to make 300hp N/A in a holden 6.

 

I will be giving this a go when I have the SUs on. Not sure exactly which brand fuel I'll use but it'll be oxygenated.

[warrenm]

Contact these blokes, they have numerous unleaded high octane fuels.

http://powerplusfuel.com.au/

[LC-GTR-1969]

Ok, so I got my kit and parts from SUMidel today, great bunch of blokes...

 

Got .125 jet and UP Needles... They gave me all the gear to change the angle of the fuel bowls etc...

 

Turns out the bushes were great, so no need to re-bush. In fact, the guys all commented that they are a very good set (condition wise). 

[N/A-PWR]

Great Man Oldjohnno,

 

your expert engineering, flow testing facility is top level thanks. 

Well I got all enthusiastic and tested a couple of carbs this arvo. I wasn't really sure what test pressure to use seeing as there are so many different "standards" but settled on the most commonly used one for carburetors of 1.5"Hg. I calibrated the bench with a 1.5" orifice before testing so the figures are good. The carb adaptors basically just dumped straight into a 3.625" bore so the flow numbers are about what you'd get dumping into a plenum manifold. A runner with a smoother transition would give a few percent more flow but at any rate they were all tested the same way so the numbers are comparable.

A Mikuni VM40 was the first one tested, it did 218cfm and ran quietly. These are a traditional round slide design and I think one of the best all-round performers ever.

Next was a flat-slide Mikuni VMX38, it's claimed that they flow a little better than the VM for a given size and that indeed turned out to be the case. It did almost as well as the VM40 despite being 2mm smaller, with 211cfm. It was very very quiet. The flow path for these is basically the same as the HSR series, so you could use this as a guide for selecting HSRs.

I also tested a 32mm round-slide which did 125cfm.

Next I chucked on a 1-3/4" SU, and was careful to mount it at about the same angle as it would be in a car so as not to affect the slide operation. This did 188cfm but keep in mind that all the other carbs had bellmouthed entries while the SU had neither a bellmouth or an air cleaner base. I couldn't put a radiussed entry on it (my tin of playdoh had gone hard!) but I'd expect that a bit better entry would get it just over 200cfm. I might grab some fresh playdoh tomorrow and check it to be sure. I don't have any 2" SUs here to test but going by the figures for the 1-3/4" I'd expect around 250cfm.

What did surprise me was just how little it took to open the slide on the SU - I was expecting it to be more restrictive. The slide was 90% open with 7"H2O pressure drop and at 10.7" it was fully open. Keep in mind that the last little bit of slide lift adds very little area to the throat, so for all practical purposes it could be considered fully open with about 7 - 7.5" of suction. Drag racers using Holley style carbs have found best results when the carbs are sized to give between 0.5" and 0.75"Hg across the line, and this works out to about 7 to 10" of water, exactly where the SU runs at WOT. I've heard reports of SU slides not fully opening - or even closing a little - at very high rpms. I saw no sign of this behaviour even with 28" or more suction and have to wonder if these misbehaving carbs have been subjected to some of the half-arsed mods that are commonly carried out.

The other interesting thing about the SU was the action of the oil damper. Whenever the throttle was opened or the airflow increased the slide would initially lift fairly quickly but then get progressively slower and slower as the slide position reached a point of balance with the airflow. The last couple of mm was painfully slow and could take 5 seconds or so to complete even with a completely stable airflow. I can see now why some people sacrifice some initial enrichment by leaving out the damper. Perhaps some work on piston or damper rod design could give the best of both worlds? At any rate the old SU certainly did better than I expected. In circuit racing applications the Webers seem to have the upper hand over SU's but I can't help but wonder if this advantage would go away with adequately sized and set up SU's on a good manifold?

Finally I chucked on a 350 Holley, mainly just as a sanity check. This had the choke tower still intact but no choke flap or spindle. It did 178cfm (one throat only) which is about what you'd expect. It was noisier than the others and for comparisons sake has 38mm throttle bores and 30.5mm "chokes". It seems the Holley cfm ratings are accurate so you should be able to use them with confidence, just be aware that some of the "special" Holleys as well as many of the lookalikes rate their carbs at oddball pressure ratings. The DCOES look a little dirtier aerodynamically - I don't have any here to test - but I think the figures for identically dimensioned Holleys would be close enough to get in the ballpark.

Back to the Mikunis for a minute - these things flow amazingly well. So well in fact that fitting one to an appropriately sized manifold runner doesn't decrease the flow in the slightest. It actually increases flow. As far as the manifold is concerned the carb is nothing more than another couple of inches of runner length with a bellmouth. Looking at the flow rates now it would appear that the smallest of the HSRs, the 42mm, would be adequate for even a very quick race motor. They'd also be oversized for nearly any street/strip car, where a 36 or 38 TM or PWK might be more appropriate.

Like button for you.

[EunUCh]

So the pommy shit actually works to some degree ?