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| Joined: | 05-01-2005 |
| Location: | Minneapolis, Minnesota USA |
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The 104^ spacing isn't as critical on the 104 cams since they have the same overlap as the stock JH cam. You can run them 110/110 no problem. The 107 cam has less duration, hence less overlap on a 110/110 timed engine.
The hot ticket is the 104/107 combo which you can run on stock 110/110 sprockets. Heck, the stock cam/107 combo is pretty good as well on 110/110 sprockets.
If you run a 107/107 combo you'll probably want the 'green dot' to get maximum top end breathing. Running them on a 110/110 timed engine will produce more low-end grunt and better around town drivability.
W-e-l-l... I'm not sure I agree across the board.
The 104 cam does have the same duration as the stock C-cam, but the profile is revised to pop the valve open much more quickly, has significantly more lift and is generally more aggressive. It hammers the valves open such that the full-open time is actually longer than for many cams with much longer seat to seat duration. To complete that aggressive personality profile, the overlap is increased 12° by setting the lobe centers (MOP) at 104°.
You can run the 104 cams at 110° MOP and they will function... the engine will run. However, they will not deliver the high end performance for which they are known. At 110° MOP, the 104 cams are more like high-lift versions of the stock cam. Okay. But no longer really "hi performance" cams, and not very good as low-torque drivability cams either. Why bother. If you don't want the high end power, then there are other cams that are better for producing low end torque... like the 107.
What's meant by "hot ticket"? The 104 int/ 107 exh set up is popular for the street, and in some circles "hot ticket" describes popularity. However, the 104/107 combination is a compromise between more high end power and low end torque/ driveability and not a "hot" set up as in hotrod/ high performance. Two 104's properly timed and complimented by appropriate compression and carburetion will produce more horsepower than the 104/107 combination. However, some folk find the loss of low end torque to be unacceptable (I have a pair of 104's in a Lotus Eclat and think it drives in traffic just fine).
Intake cam timing has more impact on the engine's personality than exhaust timing. The exhaust has the advantage of "Critical Flow". With sufficiently high pressure, when you open the hole the exhaust gas WILL flow. However, the intake is only working with atmospheric pressure and needs all the help it can get.
The 104/107 combo provides much of the benefit of having two 104's with little power loss due to the milder timing on the exhaust. The low duration 107 cam on the exhaust reduces the overlap which makes for a more mild mannered engine (compared to two 104's), and the higher lift (compared to the stock cam) improves breathing despite the shorter duration. As a result, you get most of the power of two 104's (but not all) while maintaining acceptable low-end torque. It's a good compromise for a massaged street engine, but not a "hot" set up in terms of max power.
The 104/107 combination was Lotus' stock factory set-up for the 2.2 912 used in the Esprit S3 HC and the Excel SE & SA. It met Euro emissions, gave good low end driveability and made reasonable top end power -- 180 in the Excels and 175 in the Esprit (more contorted exhaust lost 5 Hp).
If you run either the 104 or the 107 cams, I strongly suggest timing them properly at 104° MOP. A pair of 107 cams with 104° MOP is pretty mild mannered. They give better power than stock due primarily to the higher lift, but the personality is pretty milquetoast. If someone needs the engine to be even more drivable at low rpm to the point of running 107's at 110° MOP... ??... why bother driving a sportscar? Life begins at 3500 rpm... it doesn't end there.
Regards -- Tim Engel
Last edited on 06-14-2009 06:21 pm by Esprit2