PMO Sizing & Cam Recommendations

[jpgroth]

Hello All - The motor for my '79 SC is wrapping up and I'm looking for some insight on two factors that still need to be pinned down. The motor will be stock internally aside from 9.5 JEs and cams. The car has SSIs, a Monty muffler, PMOs and EDIS wasted spark.

Question one, the engine builder was leaning towards Mod-S cams but from poking around it sounds like those are higher rpm based? Webcam 20/21s are looking like they might be better? This is 99.9% a street car and I don't want to have to spin it over 5k to get into the powerband. I'd like a cam that gives a nice power bump throughout the rev range if possible.

Two, based on motor specs and cams, are 40mm PMOs too small? I'm seeing mixed info.. I have a set of 40s that I found locally that I'm about to buy. I'm not looking for crazy power out of the car but don't want to choke the motor either. I'm assuming, 46s will also be worse on mpgs? I know, carbs and decent mpgs shouldn't be in the same sentence, but if 40s won't choke the motor and provide like 2-3 mpg better, that'd be cool. Carb noob, could the 40s be modified to 42s of 46s down the road? Thanks in advance for the thoughts.

[icarp]

The KR45 cam would be your best bet , the 40 mm PMO is perfect , 36mm choke,
Smooth like a tesla and fast like a tesla
Ian

[snbush67]

^ What Ian said. The right cams, PMO 40’s with 36 chokes and SSI’s will be a blast. I think that if you ever decide to increase engine displacement, then you could then consider larger carburetors. The 40 PMO’s will hold their value, I don’t think anyone could reasonably modify them to a larger size, it’s a lot of work and the throttle plates/shafts etc. would be very difficult to line up/calibrate.

With properly jetted and tuned carburetors your right foot can deliver very good fuel mileage and immediate acceleration.

[jpgroth]

Sweet. Thanks to both of you, I'll stick with the 40s. Anyone have any other recommends on cam choice?

[IndyAlex]

Josh - give William Knight (KNIGHTRACE) a call. I'm sure he'll have a great option for you.

[PeteKz]

snbush67 wrote: "With properly jetted and tuned carburetors your right foot can deliver very good fuel mileage and immediate acceleration."

Pick one.

[NY65912]

I had DC40 cams with 40mm PMS with 10:1 Compression JE Pistons. Also had SSI and a Monty 2 in 2 out. Nice power curve up to 7K

That combo was awesome.

My .02¢ Good luck

[mikedsilva]

Originally Posted by jpgroth

Hello All - The motor for my '79 SC is wrapping up and I'm looking for some insight on two factors that still need to be pinned down. The motor will be stock internally aside from 9.5 JEs and cams. The car has SSIs, a Monty muffler, PMOs and EDIS wasted spark.

Question one, the engine builder was leaning towards Mod-S cams but from poking around it sounds like those are higher rpm based? Webcam 20/21s are looking like they might be better? This is 99.9% a street car and I don't want to have to spin it over 5k to get into the powerband. I'd like a cam that gives a nice power bump throughout the rev range if possible.

Two, based on motor specs and cams, are 40mm PMOs too small? I'm seeing mixed info.. I have a set of 40s that I found locally that I'm about to buy. I'm not looking for crazy power out of the car but don't want to choke the motor either. I'm assuming, 46s will also be worse on mpgs? I know, carbs and decent mpgs shouldn't be in the same sentence, but if 40s won't choke the motor and provide like 2-3 mpg better, that'd be cool. Carb noob, could the 40s be modified to 42s of 46s down the road? Thanks in advance for the thoughts.

I've limited experience but I used basically a Mod S cam.. 3.0... big port heads... 42mm itbs, 1& 5/8 headers, compression 10.2:1, single plug.
It was a VERY streetable motor. Torque from 2500.. strong from 3k and just nuts from 3500 all the way to redline... In hindsight, I should have used 45mm itbs.. but not sure if I would have lost my bottom end torque. New owner had it fine tuned and ended with 248 wheel hp.

I used a GT3 rear muffler... speaking to some of the experienced members here, they said an "off the shelf muffler" wouldn't give me what I was after. I did try one "off the shelf" well known muffler, and it robbed a LOT of hp.. there is a thread on here somewhere...

[snbush67]

Originally Posted by PeteKz

snbush67 wrote: "With properly jetted and tuned carburetors your right foot can deliver very good fuel mileage and immediate acceleration."

Pick one.

Oops, I guess I should have used ‘or instead of ‘and.

[jpgroth]

Originally Posted by NY65912

I had DC40 cams with 40mm PMS with 10:1 Compression JE Pistons. Also had SSI and a Monty 2 in 2 out. Nice power curve up to 7K

That combo was awesome.

My .02¢ Good luck

Cool, sold. This is what the engine builder recommended.

For those that are/were running PMOs, we’re any of you running electronic ignition? I have ignition set up with MS, EDIS and the MAP sensor plumbed to a vacuum manifold. Other than the brake booster port in the manifold is there another way to pull vacuum from each manifold runner or am I okay using the brake booster port on each bank?

[snbush67]

I have discovered that there are vacuum ports on each throat of PMO’s. Until now only two people in the world knew of this.

[jpgroth]

Originally Posted by snbush67

I have discovered that there are vacuum ports on each throat of PMO’s. Until now only two people in the world knew of this.

That's awesome. Any more pictures of that and your set up? I'm assuming there's just a bolt there plugging the hole when they ship?

[jpgroth]

Looks like it would be these little guys circled in yellow?

[snbush67]

Yep you got it.

[NY65912]

Mine had MSD doing the firing with recurved dizzy. Also went with heavier valve springs and Ti retainers. The sound was sublime.

[gamin]

My '70 has installed '79 3.0 big port, 9.5 JE's, solex cams, PMO 46's w/ 36 vents, SSI's, recurved dist, and MSD. Plenty of power. Love it.

[snbush67]

I’m using the 123 distributor for ignition. I initially used just the MSD but recently added the MSD and have better performance with the 123 and MSD combo.

[jpgroth]

Follow up question..sounds like tall manifolds are preferred to the short but is the difference noticeable or something I should be concerned about?

[winders]

Originally Posted by mikedsilva

...42mm itbs...

In hindsight, I should have used 45mm itbs.. but not sure if I would have lost my bottom end torque. New owner had it fine tuned and ended with 248 wheel hp.

Well, you can get better than 45mm air volume and 42mm air velocity if you go with the AT Power 42mm ITBs. Best of both worlds....more power up top with virtually the same torque down low.