Differential cam grinds

[Kendo]

In that fascinating NLC post about a Ferrari engine swap, on page 19 someone talks about doing something about siamesed intake ports by having two different grinds for adjacent inputs. https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/frankenferrari-v12-ferrari-308/148959/page19/ near the bottom.

Since I have a Stromberg TC, is that at all a reasonable idea? Has anyone heard of doing that? Does any of the usual sources sell such a cam? On one hand, different grinds would be hard to calculate. On the other, the existing system leans the second cylinder of the pair.

Thoughts? I'll go look for my Nomex suit now.

[BDA]

I have not heard of such a thing. One of the many things I am not is a cam designer but I would think that different cam profiles for siamesed inlet ports would 1) be very expensive to develop, 2) would yield very little, and 3) not at all be worth it on a street engine. I think it would only make sense when trying to squeeze the last 1/4 hp from the engine and there are a lot of well trodden routes to get extra power and torque from your TC motor that are more practical. For example, I would sooner convert my Stromberg head to a Weber head than pay for the development of an asymmetric cam.

But, as I say, I'd never heard of such a thing before you pointed it out (I must have skimmed over that part when I read those posts) and I'm not a cam designer.

[TurboFource]

I know of a motor with a Stromberg head and carbs that recently made 123hp and 112 ft lbs at the wheels…..
Not sure the Stromberg head is as bad as people think…..

[Pfreen]

I designed engines and the problem I think with the siamese ports on the tc is the firing order 1-3-4-2.   The siamese ports are only 180 degrees apart in the firing order.   With a new crank and cams, they could be 360 degrees apart, allowing more time for port filling and improvement of volumetric efficiency.

I have no idea what that would do to engine balance.

Other than that, I can't see an advantage to changing just the cam timing to improve siamese ports.

[jbcollier]

A four with siamese intakes is completely different from a 12 with them.

Pfreen, sounds like you are reinventing the big bang engine.

[Kendo]

So, thinking as a non-cam, non-engine building person, the problem with the Siamesed ports is that the first gets air-fuel at some ratio, say too rich, and the second gets a slightly starved ratio, assuming a carb setting that averages them. A cam lobe for the first port could have a profile that took some amount of the flow from the carb so the more aggressive grind from the second lobe took an equal amount of air-fuel mixture. I’m sure that would only work perfectly at one rpm.

I’m just thinking about this from the standpoint of minimal changes to a TC. I could change cams, or carbs (Mikuni) pretty easily. A new Weber head would be another level of $$$. Boring out the cylinders, longer stroke crank, those would be more major changes. The BDA engine would be a simpler approach.  So, what simpler changes could one make? Seeing that dual grind cam approach to the siamesed ports looked like a simpler one. And maybe just too impractical.

[GavinT]

Lynx manifolds were a popular choice for the siamesed ports issue with their 'crossover' manifold.
Can still get 'em but it doesn't help the Stromberg TC situation.

[MRN I J]

BMC A seies cams can be ground with a 'scatter' profile which supposedly helps with the siamesed ports, doesnt work so well for other engines, APT David Vizard sells them for a B series BMC though   

[BDA]

…I’m just thinking about this from the standpoint of minimal changes to a TC. I could change cams, or carbs (Mikuni) pretty easily. A new Weber head would be another level of $$$. Boring out the cylinders, longer stroke crank, those would be more major changes. The BDA engine would be a simpler approach.  So, what simpler changes could one make? Seeing that dual grind cam approach to the siamesed ports looked like a simpler one. And maybe just too impractical.

What are you aiming for? A fast street motor? A race motor? How much are you willing to spend? Are you set on keeping your TC? Are you amenable to an engine swap?

A BDA would be a very expensive proposition. If you’re amenable to an engine swap, I think the best is a Zetec. Any engine swap is going to require some fabrication. There are lots of go fast parts for a TC. Your motor will probably spend a bit of time in a machine shop.

I think you need to sit down and define your performance and monetary parameters.

[Pfreen]

As far as hp, my car dynoed at 130 hp at the rear wheels.  Over 150 bhp.
It has 10.2:1 cr, 46 mm hitachi su carbs, ported and polished head, headers, balanced and blue-printed, stock lowe end, aluminum flywheel and a Kent cams L14 cam.

The carbs are off of a 240z and these su's are good for over 200bhp on a 6 cylinder z car.

The car is very drivable and revs strongly to the redline.

I think the radiusing of the divider on a Stromberg head is one of the keys to making power.  The idea is for the air to enter the port through a bell mouth and not a sharp edge, as it is as-cast.

I only tell you this in that the Stromberg head can make a lots of hp.

[jbcollier]

The original post referenced a Ferrari article on fitting a V12 to a 308.  The reference to different cam grinds for different cylinders was a passing reference to A-series engines in the Midget and Mini.  These engines have a single intake port feeding two cylinders.  The length of the runners to each cylinder is very, very short.

On a Lotus TC with a Stromberg head, the runners to each cylinder are much, much longer.  I really think this is a non-issue.  On all but the most highly tuned engines, a Stromberg head TC will put out similar HP, especially if you fit carbs that are lower in restriction than the stock Strombergs.

[Kendo]

Thanks all for your thoughts on this. I thought the differential cam grind idea was interesting and might apply to a Stromberg head. I'm glad it doesn't, actually. Cheaper that way. I want to keep the TC, mostly stock, but get more horsepower. I've been looking at the Mikuni option for a while. But all of this will have to wait until the car is done being painted. So for now I'm just kicking ideas around.