Camshaft profiles for Carbs vs Fuel injecton

[midnightrider36]

I've heard it stated that when considering fuel systems, cam profiles need to be considered. The specific example I remember was being told that when converting 2.7L CIS engine to carbs to go into a 914 that the CIS cam profile wasn't great for carbs. I also understand that CIS dictates a mild cam profile, and not just due to piston to valve clearance issues when using the CIS pistons.

Dougherty's cam profile sheet shows, for example, different versions of the DC 35 profile for carbs versus EFI.

My question is what about the cam profile indicates its suitability for carbs? Lobe separation angle seems to be closer on carb profiles, meaning more overlap. If i understand correctly, that would give lower vacuum at idle but more scavenging in the higher rpms. I would think that carbs, especially individual throats, would be more sensitive to vacuum issues.

Thanks for any insight.

Jesse

[stownsen914]

EFI with single throttle body (like Porsche CIS and Motronic) is sensitive to overlap in aggressive cam profiles. You'll see that the DC profiles for EFI / single throttle body have wider lobe center values, essentially meaning less overlap. The most aggressive profiles aren't really suited to single throttle EFI even with wider lobe centers - they're just too lumpy.

[MichaelSJackson]

The explanation I was taught is that overlap allows pulses back thru the induction system. In the case of CIS, there is a large airflow plate blocking/measuring the airflow and mechanically adjusting the fuel flow (in the fuel distributor). Larger pulses from aggressive cams push back on this airflow plate, causing it to misinform the fuel distributor of the required fuel.

I assume that these pulses through a single throttle body are no worse than a V8 with a single 4-barrel with a lumpy cam? Sure individual throttle bodies don't suffer this interference, but how significant is the problem with a single throttle body?

I'm running the BITZ EFI through the single throttle body of the old CIS. It still feeds the airbox which acts as a cache for the individual runners. However, the old airflow metering plate and fuel distributor are gone.

I've been planning to upgrade my '78 3.0 to a 3.2ss ala Max Moritz with wedge pistons (single plug). And upgrading the cams to the 993s, from Mr Dougherty. I did ask him about this combo and didn't get any pushback.

Michael J.
'78 SC w/BITZ EFI

[Emo993]

I have John's cams ( DC-30, Mod-solex). This is on a 2.2 E case/heads but with S pistons. Love this engine, just rips from 5,000 to 7,000. Also Ben's header and sport mufflers, MSD, 40 PMO's. Best, Mark

[PeteKz]

Jesse, start with your planned use for the car and your expectations. Street car? Race car? Something in-between? And be honest about it. Most of us think we want a car that can serve as a street car and and track car, but we end up doing 95+% of our miles on the street. That's reality. A car that is excellent on the track requires a lot of tradeoffs that make it very poor on the street. Not just the engine, but the suspension, tires, and removal of sound deadening make it noisy, nervous, jarring and tiring on less-than-perfect roads. Unless you are building a real race car, you are FAR better off building a street car and staying with reasonable cam profiles and lifts. It will still be lots of fun on the track, it just won't win races.

That said, what you understand about cams and CIS vs. carbs or ITBs is generally correct. But IME, the CIS will take much more cam than the OE CIS cams deliver. Remember those OE cams had to meet tightening emissions standards, which also required less overlap to prevent incoming fresh air/fuel charge from going out the exhaust valve before it gets burned, thus reducing high-RPM scavenging efficiency. The OE cams had to make multiple tradeoffs. I highly recommend that you consult with several engine builders about the cam and piston combinations they use. Unless you want to do your own R&D, you have a high chance of running into valve/piston interference problems, and having to machine the pistons or try different cam grinds, etc. If you are a competitive racer, well, that's part of the game; but if you don't want to do that, stick to known good combinations.

ITBs versus carbs: The only reason to go with carbs is that you like the old school tech and look. The flexibility of programming a modern fuel injection system at any load and RPM using ITBs is so much better, that if I was going to swap from CIS, I would go to ITBs, not carbs. A cam for carbs will work just as well or better with ITBs. I don't know if the cam manufacturers offer ITB-specific cams. I haven't seen them advertised that way.

As you can see in my signature line, I run William Knight's M1 cam with the SC CIS system in a 1979 SC engine that I stroked to 3.2. It pulls hard from 4000 on up. Since I have a 3.2 crank and rods, I hold the RPM to about 6000, but the cam has plenty of power left at that RPM; and William says it's good to 7000+. Ian Carpenter has M1 cams in his SC and runs it to 7000. Part of what leaned me towards the M1 cam is that it works well with CIS or with ITBs or carbs, so you can leave the CIS on it, and then if you want to later, switch to another induction without changing the cams. Given how well my engine runs with CIS, I can't see doing that unless I just want "the look" of ITBs. Since I do road trips with it, the fact that it gets close to 30MPG at a constant 65-70 MPH would further dissuade me from changing over.

Give William Knight "Knightrace" a call to discuss. His contact info is in this forum. Discuss with a couple other builders too.