Don Terrill’s Speed-Talk

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".

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.....

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.

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.

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.

grant6395 wrote: ↑Sat Sep 22, 2018 5:11 pm

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?

grant6395 wrote: ↑Sat Sep 22, 2018 5:11 pm

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: ↑Sun Sep 23, 2018 10:56 am

grant6395 wrote: ↑Sat Sep 22, 2018 5:11 pm

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.

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.

aMC manifold carb bore generalities .jpg