I believe it may have been discussed here but I couldn't find it using the search feature.....
This comes up from time to time with the vintage bike racers.......Is this a valid theory today or just a 1950's relic of poor port design? Does anyone use port mismatch to actually i crease power?
The Goldstars mentioned are 350-500 cc single cylinder OHV hemi engines that made about 50 hp@7000 rpm back then..
Roland Pike:
"One of the strangest occurrences to do with carburettors was when one day Reg Wilkes sent an apprentice from the test shop into our, main shop for a 1 3/32 GP carburettor and he misread the size and gave Reg: a 1 3/16th carburettor and they immediately got quite a jump in power. They reported this to me and I went to the shop and the test was repeated and there was no doubt it was quite a gain in power.
I suggested to Reg to open the port to match, immediately we lost what we had gained plus a bit more, so we made a thin sleeve and pressed it in and the power was back. We repeated this on other engines always with the same results. A number of private owners of DBD Gold Star noticed the carb being bigger than the port and opened up the port thinking to gain power, but not having a dyno were not aware of the results.
Later on we fitted a venturi behind the carb and picked up even more power. The smallest diameter of the venturi could be 80% of area of the carburettor. We tested this on several engines and it always worked and seemed to improve carburation. The venturi had to have the classic included angles of 22 degree in and 7 degree out".
Motorcycle land speed racing... wearing animal hides and clinging to vibrating oily machines propelled by fire
I have read the description a few times now and if I understand correctly, I think it points to an improvement in mixture quality as the mixture leaves the carburetor and "adjusts itself" passing the sharp edge of the smaller manifold. Yes, I think it is a relic of the times and will not be found with most modern flatslide carburetors, maybe with roundslides.
Truckedup wrote: ↑Fri Sep 21, 2018 7:24 am
I believe it may have been discussed here but I couldn't find it using the search feature.....
You are correct that it has been discussed numerous times (over 140 hits). It is much more efficient to use Google to search the site. Note carefully the format for the search parameters in Google:
port mismatch site:speedtalk.com
I have done this so many time,s earlier in my career trying to find out why it worked. I did find why, or at least all the evidence points to the fact that I did. The ramp does three things.
(1) Increases (spikes) velocity at opening
(2) Re- Atomizes fuel into air stream
(3) Effects resonant tuning
If your ports are to LARGE and to SLOW the velocity spike at the mismatch causes the fuel to break up and atomize instead of flowing over the SSR as a non burnable stream of pure liquid. The added air speed helps prevent reversion and boost inertia plus the manifold tuning is effected slightly. In the late 80s early 90s just before we went to oval ports in Pro/Stock we used the ramp on the floor all the time. When we got the air speed up by oval porting and making the ports of correct size, it didn't work anymore.
If the ports are sized and shaped correctly and the tuned length is correct, you wont need a ramp in the intake. Its just a band-aid for slow air speed poor wet flow and to a much lesser degree, incorrect tuning.
Now, if for some reason you cant size and shape everything correctly (class restrictions), then I would use it. It does work.
Attached is an excerpt from the intriguing 1972 SAE paper by AMC, and specific to right angle turns common in auto engine carb > manifold junctions, but it seems related.
aMC manifold carb bore generalities .jpg
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Frankshaft wrote: ↑Fri Sep 21, 2018 8:46 am
In some cases, running a rectangle port intake on an oval port bbc will show a power gain. About as massive of a port mismatch you could have.
I know that this is true, but I don’t get why.
We strive for the best match as possible and this works... huh?
Frankshaft wrote: ↑Fri Sep 21, 2018 8:46 am
In some cases, running a rectangle port intake on an oval port bbc will show a power gain. About as massive of a port mismatch you could have.
I know that this is true, but I don’t get why.
We strive for the best match as possible and this works... huh?
Frankshaft wrote: ↑Fri Sep 21, 2018 8:46 am
In some cases, running a rectangle port intake on an oval port bbc will show a power gain. About as massive of a port mismatch you could have.
I know that this is true, but I don’t get why.
We strive for the best match as possible and this works... huh?
Early 2000 a local guy was testing the above BBC mismatch and finding power gains, more in TQ than HP but most discredited him, since then others have reported similar gains... Why? I don't know.
In a conversation with DV about cutting the spider off of an intake to perfectly match the ports I got an hour long story of what they did and why they were so specific about the process (runner length and size) and his conclusion was that the power gain if any was maybe 1.5 horse power or within test variation.
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
Frankshaft wrote: ↑Fri Sep 21, 2018 8:46 am
In some cases, running a rectangle port intake on an oval port bbc will show a power gain. About as massive of a port mismatch you could have.
I know that this is true, but I don’t get why.
We strive for the best match as possible and this works... huh?
My theory, is the engine is sucking through the head, through the intake. The intake port on the head, think of it as the hose from a shop vac. The air and fuel will be drawn to the vacuum source. On the flip side, if you had boost, and were forcing it from the other direction, it would be much worse. Just how I have visualized it. Stick a small shop vac hose to the flange of a BBC rectangle intake, it's all drawn to the vacuum source.
Frankshaft wrote: ↑Fri Sep 21, 2018 8:46 am
In some cases, running a rectangle port intake on an oval port bbc will show a power gain. About as massive of a port mismatch you could have.
I know that this is true, but I don’t get why.
We strive for the best match as possible and this works... huh?
My theory, is the engine is sucking through the head, through the intake. The intake port on the head, think of it as the hose from a shop vac. The air and fuel will be drawn to the vacuum source. On the flip side, if you had boost, and were forcing it from the other direction, it would be much worse. Just how I have visualized it. Stick a small shop vac hose to the flange of a BBC rectangle intake, it's all drawn to the vacuum source.
In both cases the air is moving from the intake to the cylinder because of a relative pressure differential. In the case of boost, the pressure differential is higher.
Take the risk of thinking for yourself, much more happiness, truth, beauty, and wisdom will come to you that way. -Hitchens
Dan Timberlake wrote: ↑Sat Sep 22, 2018 12:49 pm
Attached is an excerpt from the intriguing 1972 SAE paper by AMC, and specific to right angle turns common in auto engine carb > manifold junctions, but it seems related.
