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Join Date: Mar 2010
Location: Toronto
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3.2SS cam grind advice

I am looking to get some cam experts to weigh in on a new cam grind for my 3.2SS twin plug engine.

I bought this engine part way through a build for a race car and finished it for my 1970 RSR street/DE car 4 years ago and have enjoyed it on and of the track since. I have since broken a valve spring and had to pull it apart and since i am now vintage racing a 914 i no longer use this car that much on the track. While it is apart i would like to re-grind it to something more street friendly.

The current build was based on a big port 3.0 using 98mm JE pistons with a 10.5 to 1 CR. PMO 46mm throttle bodies and Electromotive tech3r EFI. Cam is way too big at GE80 for what i use the car for. As i live in Toronto and often drive when it is pretty cold heat is a must so i am running SSI's and a M&K 2 in and 2 out sport muffler. As you can see by the dyno this exhaust is very restrictive for this cam and there is no power beyond 6800 RPM. With this cam there is no power until at least 5000 either and since i have the motor open i would like to send the cam to be reground to something more user friendly or purchase a new cam. I know i should be able to get more HP than the 240 at the wheels with a more open exhaust but i would like to keep this setup and hope for more low end torque and power before 5000 rpm for the street.

I would grateful to hear what the Pelican cam and engine experts might suggest.

Paul


Old 08-18-2018, 04:07 PM
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DC40 with a wider lobe If John can regrind the 80's ....He may suggest an alternate grind, but should be in the neighborhood.

Dougherty Racing Cams

That grind will compliment everything you have currently fitted.
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Old 08-18-2018, 04:46 PM
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Ge-80

GE-80 CAM with SSI exhaust will not flow any air.Too small.
Old 08-18-2018, 05:27 PM
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To know what you need, you first should understand the physics behind this.

Two factors are hurting you probably. The lobe separation and the exhaust system.

With the durations you have with the existing cam, although not huge in crank degrees, it’s the lobe center (overlap) that is creating the lobe durations to become the limitation. You are effectively lowering the depression in the Intake system at the lower engine speeds by limiting the scavenging effect within the intake system. Add to this the restrictive exhaust system, you have a of a package that needs RPM to work. As you have seen the exhaust needs to be larger to help the engine extract the gases more efficiently.

In carburetor terms, you’re not pulling or generating enough vacuum in the combustion chamber with the exiting exhaust gasses to overcome the reversion that is happening with the restrictive exhaust. When you run the engine at the higher speeds, the overlap that occurs with these durations and LC’s, creates a higher scavenging effect, pulling in air faster at a greater volume, generating a higher differential pressure across the piston. This helps overcome the reversion pulse in the exhaust system. This is the theory but your exhaust system is not allowing the theory to prove itself. At the lower engine speeds, this effect is less (lower VE) and the reversion not only restricts the incoming air volume and speeds, but also contaminates the air entering the cylinder.

Whatever you choose to do, it must be done with consideration to the exhaust system and the inlet system. By reading what you have, the intake system is a lot larger in capability than the exhaust. Some compromise in engine performance will have to be expected because of this.
Aim to decrease the duration and try to leave as much lift as possible. Ramp speeds will increase with shorter durations with the same lift, so there will be some compromise here too. Lift is generally a mechanical disadvantage more than anything. With the high lift you have now, piston contact should not be an issue and if the engine ran fine before, you will have good spring clearances.

Shortening the duration will help build higher differential pressure across the piston, higher vacuum in the chamber at the lower engine speeds. This will help with scavenging at the lower engine speeds and help push air out of the cylinder against the reversion pulse in your exhaust. How much to change is very hard to say? I would widen the LC some, maybe upwards to 5. Without knowing how good the heads flow, etc., it all becomes a best guess at best. I think the LC will have a greater effect than shortening the duration a lot. I think I would error on the conservative side and take no more than 12 intake and maybe a little more on the exhaust.

In this DIY arena, you typically don’t have the option to try different cams to find the best one, so trying a proven profile if the basic engine parameters are close is your best bet. If someone else has an engine close and has good results, go with their choice. But knowing what is going on will help you make the best choice with the less compromise.

On a separate note, change out the Ignition system for a proven CDI system. I guarantee you this will help and make a huge difference too. You just might end up with a “hot rod”.
Old 08-18-2018, 08:12 PM
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DC40-108. Works great.

Have the same build and it doesn't feel restrictive at all.
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Old 08-18-2018, 10:32 PM
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Thanks guys for your input!!

I will have to call Dougherty Cams later this week to discus but good to hear that DC40 seems to be the way to go!
Old 08-19-2018, 07:00 PM
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Concur with DC40 or would like to add perhaps a DC44.
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Old 08-20-2018, 05:27 AM
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My mechanic usually deals with Webcam and because of timing and availability was suggesting either the 464/465 grind (which i believe is very similar to the DC-40?) or perhaps the slightly more aggressive 120/104 grind.

I really know very little on this front so wondering about your thoughts again. They can also do a custom grind on the 464/465 with a 108 lobe similar to jpnovak's DC40-108.

Because of my slot in the shops workflow i can't afford to wait too long to order one from DC unfortunately.
Old 08-21-2018, 04:45 PM
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Ok thanks for the advice again.

I spoke to John and managed my mechanics time to work and John is making me a set of DC40-108's.

Very excited to get the new cams!!
Old 08-22-2018, 05:18 PM
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Would love to know how this turned out for you?

Old 11-18-2018, 03:26 AM
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