Question about Comp Cam XE256H, low compression 383, and valve spring shims

[moper]

You need at least a true 10 to 1 to get decent torque/horsepower.

Nah. It just makes it easier.

 

[Triple Black 73]

If you use the , you can input all your data, and get to the C/R you want. I'm getting you at 8.6 to 1, at best. Those heads may be closer to 88 ccs. And if there are valve reliefs in the piston, you have to add that to your compressed volume. Some KB pistons can have 4 to 7 cc of valve relief. You need at least a true 10 to 1 to get decent torque/horsepower.

Thanks man! They have a nice calculator for static and dynamic compression ratio. I didn't figure in the gasket bore size when I gave the 8:75:1 number. I got a static pressure of 8.3 with 88cc heads and 8.6:1 with 79.5 CC heads. The dynamic compression with the XE256 should be right around 7.3:1 since the intake valve closes at 53 ABDC. The CS-661 dynamic compression was around 4.5:1 as it close at 70+ ABDC(!!) No wonder why it didn't start and ran horrible when it did! Not enough compression!

I tell you one thing I learned from this experience. Be careful when making piston choices! You're basically stuck with them once they're installed.

 

[Garys1969RR]

Yes, that's important. And you can increase the dynamic C/Ratio with any cam by advancing it 4 degrees. That closes the intake valve earlier, which builds more compression. Any motor that runs good at 8 to 1 will produce more power at 10 to 1, other factors being equal, as long as it doesn't start pinging.

 

[Triple Black 73]

I installed the cam.today. I measured between 1.810 to 1.840 installed height across the heads after installing .060 shims. I don't want to double stack the shims. Ill let you know how it runs tomorrow.

 

[dvw]

Nothing wrong with stacking shims. Check any race head and I'm sure you'll find spring shims. Mine runs 320lbs on the seat, 820lbs open with stacked shims. What is most important is that you have the correct spring for the cam. Do you have new springs? Or have you pressure checked the ones you intend to use? How did you measure the head volume? The biggest issue you will see with unequalled stem heights is the ability to get proper lifter preload without resorting to adjustable valve train. Stem height difference of .030" should be ok, but I'd check it. A bent piece of Mig welding wire between the lifter snap ring and lifter piston will tell you. Test with .023" and .045" . It should ideally be in between those 2 sizes.
Doug

 

[Triple Black 73]

Nothing wrong with stacking shims. Check any race head and I'm sure you'll find spring shims. Mine runs 320lbs on the seat, 820lbs open with stacked shims. What is most important is that you have the correct spring for the cam. Do you have new springs? Or have you pressure checked the ones you intend to use? How did you measure the head volume? The biggest issue you will see with unequalled stem heights is the ability to get proper lifter preload without resorting to adjustable valve train. Stem height difference of .030" should be ok, but I'd check it. A bent piece of Mig welding wire between the lifter snap ring and lifter piston will tell you. Test with .023" and .045" . It should ideally be in between those 2 sizes.
Doug

The CompCam springs are rated at 110 pounds at 1.800" and 450 pounds at the spring bind of 1.100". The springs are new ones that came in the cam kit. I think it will be fine. We had 160 to 165 psi across all cylinders. Compared to 140 to 150 and 120 on #5 before.

We got the engine running today. The XE256H has a smooth idle with 20 inches of vacuum(!). Idles nicely at 650 rpm. And it starts on the second rotation. Turns out the carb was the issue. We replaced the early 80's themoquad with a Summit Racing reman unit, which is a 800 CFM '71 unit that was before they started putting all that HP kiling emission crap on it.

I'm going to feel a lot better knowing the spring aren't 400k miles worn out junk. I'm kinda wishing I went for the XE268 but I was turned on by the thought of 460 pounds of torque at 2500 rpm compared to 430 at 3500. Yeah... I lost top end but these pistons wouldn't live to see life past 5000 rpm anyways. Test drive coming soon!!

 

[Mebsuta]

Have you driven it? Interested in what happens since I use a cam identical to CS-661.

 

[RT K_line]

I have used the 260 comp cams unit back in the day on a 72 low compression 400 engine. I added a duel plane intake from wiand with a 780 vacuum secondary carb and headers. It woke up that car. It was a 4 door Coronet sleeper with 294 gears. It pulled to 6000 rpm strong and had bottom end that most others could only watch as I pulled away. I recall that car would stay with anything until I had to shift to 3rd because I didn't have a shift kit back then and the engine would flash between the 2nd and 3rd shift and I had to lift momentarily. So I would recommend this cam to any stock type build.

 

[Triple Black 73]

I drove the car today 5 miles to the store and back. The car has a TON of torque but the carb is running very rich. The engine floods and stumbles at WOT. I can't tune it at the house because of the HOA bylaws. The A-Hole neighbor to the west of me already called the cops on me! (Didn't help the car backfired like a bomb going off in front of his house) I mean damn, I had it home for just two days and I'm already getting in trouble with the law! lol! Anyways, I'll have it running like a top very soon. There's a Mopar shop that will do the alignment, tune it, and even dyno it.

I have used the 260 comp cams unit back in the day on a 72 low compression 400 engine.

I considered the XE260 and the XE268, but I went with the smaller XE256 cam because the 383 is my dad's motor and he wanted low end torque. I felt like it was a good comprise between top end horsepower and low end torque. According the CompCam calculator, I gave up 20hp at 5,000 rpms for 30 pounds of torque at one thousands less rpms.

 

[Mebsuta]

The A-Hole neighbor to the west of me already called the cops on me! (Didn't help the car backfired like a bomb going off in front of his house) I mean damn, I had it home for just two days and I'm already getting in trouble with the law! lol!

I love it! lol Neighbor deserved it.

 

[Garys1969RR]

Sounds like the secondaries are running way rich. What kind of carb? And don't worry about the neighbor, sounds like he has no sense of humor, or knows how to have fun! Prob thought you were doing a drive by! LOL

 

[Triple Black 73]

Sounds like the secondaries are running way rich. What kind of carb? And don't worry about the neighbor, sounds like he has no sense of humor, or knows how to have fun! Prob thought you were doing a drive by! LOL

You mean VERY RICH! Like Trump rich. The great thing is the black soot is making all the pin-hole sized exhaust leaks easy to find, lol!

Looks like Summit Racing sent me a 6090S Thermo-Quad, which is for a 1972 400 auto.

I got the car to idle much better yesterday. I can now drive through the neighborhood at 650 rpm to 800 rpm with no problems. Which is a lot better than 1200 to 1500 rpms like it was doing. Sure, I can still hear the exhaust note echoing off the houses but some people are still waving to me... so **** the Flanders, **** the noise police, **** anyone that thinks my car is too load, I didn't spend half my life and a small fortune restoring this car not to hear the engine roar. The load *** mufflers stays!!! End of story.

I hit 1st gear WOT today. OH MY GOD!! 20 years in the making! What an amazing sound! Love the Thermo-Quad sound. The cam makes a ton of torque, TONS!, from 2000 to 4500. Hard not to spin the tires! The engine doesn't sound mean at idle, but you can tell it's not stock. Still need to tune it. I think there's still a lot of power left to be found in the engine. Any second thought I had about buying the XE256 cam are GONE! CompCams FTW!