[Skippy597]

Could someone take a stab at educating me on why a camshaft manufacturer will recommend a certain compression ratio for a given cam? Looking at ideas for a cam and found one that I like (crower 60244) however it states an 11:1 cr is required and I will be in the 9.5 range (I think I'll have to double check everything as I get it). I do plan on using rhoads lifters and hoping that it would be a good combo.

[Goatracer1]

The cam must have a lot of valve opening duration. With the valve open when the engine is trying to build compression at low rpm your compressed air is going right out the valve. Thats why the cam co. wants you to have more compression to start with. That cam with low compression will make your car a real dog at low rpm also requiring a stiff rear gear. Why don't you just go with the cam recommended by the cam manufacturer?

[Skippy597]

Ok that makes sense. Thank you.

I'm just engine building in my head for now so figured I'd try to gain some understanding on it.

[74Grandville]

11:1 looks to be based on a 370ci engine. that looks like a RAIV style cam with a little more lift.

what CI is your engine? A 455 with 9.5:1 and rhoads would work well with that cam.

[Skippy597]

It's a 400, I was wondering about that because I have seen on other threads that the ra iv cam is recommended a lot with rhoads lifters. I figured the additional lift would be nice without being too much.

I realize the cam companies are smarter than I am by a long shot but it also seems they are more strict on what they recommended versus what people run in the real world.

[67Fbird]

i'll stab at this.... A 400 with a taller RAIV cam would be FUN....just depends on the TOTAL COMBO
no stall and 3.23 rear = DOG
2600 stall and 3.73 rear = FUNNNN
small cam no stall, 3.23 gear = STOCK
small cam, 2600 stall, 3.73 gear = useless

There is never just 1 thing that MAKES everything work out.

[Cliff R]

They are missing one important part of the "equation", the CID of the engine.

The cam requirements for any engine are based on CID and compression ratio, not just the compression ratio.

I've never understood why cam companies list a cam as fitting the 326, 350, 389, 400, 421, 428 and 455 Pontiac engines, then go on to tell us that it needs 11 to 1 compression (for example). The needs for each one of those engines is slightly different simply based on CID alone, not compression alone......FWIW.

The RAIV cam in a 400 build needs at least 10 to 1 compression to be minimally effective. I've had numerous 400's in here over the years for custom tuning using the RAIV cam. One even showed up with 6X-8 heads on it down around 7.7 to 1 compression (complete TURD everyplace). At 10 to 1 compression they are still a little "soggy" below 3000rpms, so that would be the least amount of compression I'd want to see in one.

Once you get to about 10.6 and higher that cam really starts to show it's colors in a 400 build, but still requires a manual transmission, 3000-3500 converter if an auto trans, and at least 3.73 gears.......IMHO........

[Skippy597]

Thanks guys really appreciate the info.

After using one of those compression calculators I'm gonna be even lower than I thought, which is fine but I'll find a different cam.

On a related note installing the cam advanced can help build cylinder pressure correct. Can that help with a lower compression build?

[Skippy597]

Also car is getting a 4 speed and rear gear hasn't been finalized but will be either 3.50 or 3.70 ford 9"

[Steve C.]

My previous engine was 4.181" x 4.210" / 462 cid
My goal was to make 600 hp at or below 6000 RPM.

I had Edelbrock #6056 heads with a 90cc chamber volume and 9.7:1 static compression. This was not my intended compression ratio, it was a result of a mistake on my part by ordering incorrect JE custom pistons.

In Order to help with the inadequate compression ratio Chris Mays at Comp Cams recommended the following single pattern cam. The heads were ported with 313 cfm intake and 255 cfm exhaust at 0.600" lift.

Comp Cams Xtreme Energy Solid Street Roller lobes #P4876B/4876B with a 108 lobe separation. 254/254 @ 0.050", .389/.389 lobe lift, .622" gross lift with 1.6 rockers, set at 104 degree intake centerline.

Dyno results:

589.1 ft.lbs. Torque @ 4400 rpms and 580 HP @ 5800 rpm using a Performer RPM intake and a 1/2-open carb spacer. With a Victor intake and a 1/2-open carp spacer it made 600 HP at 6000 RPM.


Information provided in this post does not represent any endorsement and is offered for general interest only. Side note I was vey pleased with this build and the results despite the low compression.



.

[SD455DJ]

To echo Cliff's RAIV cammed 400 engine experiences, my RAIV 400 '70 Judge with the original RAIV engine only made 9.95 to 1 static compression ratio. It ran 13.50's @ 105 mph weighing 4050 lbs with 3.90 gears with a stock TH400/13" convertor. After rebuilding RAIV 400 (stock) with 11.0 to 1 compression being the only difference ran 12.90's @ 110 mph. The full point of compression made a huge difference everywhere from off idle to the 6000 rpm shift point it became a monster compared to before. The 9.95 engine wouldn't even spin the tires matting it from idle, where the 11.0 to 1 engine would fry the tires from a stand start or a roll. It felt like a high revving 455!

I have to admit the original 9.95 to 1 engine was a dream to race at the track since it wouldn't spin flooring it off the line and I was able to cut .005 lights consistently, not so easy or consistent with the 11.0 to 1 engine!

Dennis

[jhein]

So for any given cam, increasing the CID, decreases or increases the need for higher CR?

[SD455DJ]

Decreases...