What a dissapointing day.

Here's the story: My 412ci small block has 12:1 static, and orginally had a 286* (at .0045" lift) advirtised cam. It produced 240-245PSI of cylinder pressure. I knew I had to run high octane gas, but the cam peaked too low and I wouldn't mind producing a little less cylinder pressure. I later degreed the cam to find out that the cam does in fact have 286* advertised, but also 218* @ .050 on 110LDA, with 6* advance ground in.
I didn't like those numbers, so I threw in a larger cam that I had on the shelf. My goal was to rase the powerband a bit, and to reduce some cylinder pressure. The cam is a 302* advertised, 240* @ .050, on 113LDA with 1* advance. I thought for sure this would reduce cylinder pressure, since it has more duration, more LDA, and less advance. I expected a 20PSI or so drop in pressure.
Now the cam is broken in, everything ran good. I checked the pressure: 270PSI!! :eek: :eek: What the heck happened??? Even on 100-105 octane it diesels a bit when I shut it off now. What do you guys think? How could this be?

Are we talkin solid lifter, or hydraulic?

Net increase of air/gas trapped in cylinder=net increase in cylinder pressure.
It trapped more in the cylinder than it bled off......

UDHarold

12:1 needs about 265 degrees at .050" and work from there.
If you want to run on lower octane gas, then change pistons, aiming at about 10:1

This is a hydraulic cam. I was thinking along the same lines as you Harold, but I wasn't sure.

Is 270PSI workable with race gas? I'm not worried about the head gaskets failing since it's O-ringed.

I've considered changing pistons, but the rotating assembly is balanced. Also, I've never changed pistons before w/o a full rebuild...

Matt, That's a fine looking car. 270psi cranking compression is unreal for a street car-WOW. If the block is Oringed are you running a copper headgasket? Could you use a block saver type shim or multiple gaskets to lower the compression. I'm sure you must have some dome on the pistons? The burn might not be quite as efficient with the wider piston to head clearance but Oh well I'm sure the car will do OK. Maybe run a cam with more overlap, 113LCA is kinda wide with not a big overlap window I imagine. How does it idle now with the current cam? Spark timing must be critical with this combo.

Thanks for the compliment, Bob.

The pistons have a .100" dome, which is about 0.5cc. The problem is that the overlap doesn't effect the cylinder pressure - it's the intake closing angle. So, the wider the LDA, the more pressure gets bled off. This is partially why I am using this cam.

I'm not around full-race cars too often, but I don't know of ANYBODY who runs 270PSI...kind of insane :eek: . Here's a before an after comparison:

Old cam: 230-245PSI, 9.6:1 Dynamic compression
New cam: 250-270PSI, 8.3:1 Dynamic compression

I just don't understand it. I'm gettin real tired of messing with this! :mad:

i know your dealing with a sbc........but my 468 bbc has 13 to 1 compression(huge domes .580 +) and with it current cam it cranks out 270 psi same as yours.......but im running a solid roller with 256/268 @ .050 cam......on 108 center line......for me i have a hei locked out at 36 degrees and i just rumble around town on 91 or 93 octane which ever station im at.....but at the track i run 114 fuel in it........i dont have my combo dialed in yet.......but i would say you should shoot for a bigger cam........it makes for an interesting street ride with such big cams and compression and stall........lol ....but if this is to be any sort of a street car it should be detuned some.........other wise it will be like what i got.... lets take the race car up town for the heck of it...........no real purpose on the street other then to turn heads and rumble down the block......

I’m having this trouble with my 383 also. I don't really care if I have too much dynamic compression; the problem is my timing curve. I can only run so much total before it pings on WOT. I am thinking about getting one of those MSD boost retard controls...run 45* or so total timing while puttin around town, then if I want to get on it, just take 8* or whatever out of the timing curve with the flick of a switch. Think it will work?

Does the increased cylinder pressure mandate higher octane than before, even though my DCR is lower?

that 113 LSA is what is killing the cranking compression. You need more overlap.

Overlap doesn't have any effect on cranking compression. It's the intake valve closing angle. As the LDA is increased, the intake valve closes later. For instance, with cam the intake valve closes at 84* ATDC. If I reduce the LDA to 110*, the intake valve then closes at 81*, thereby allowing less pressure to bleed out.

I think you need a bigger cam, more duration everywhere if you want to run less than race fuel. UDHarold should be able to help here. Looking at the specs for both cams they look pretty conservative like driver cams/older designs. I ran a similar 302 duration cam in a 9-1 motor years ago, it was a PAW grind and made plenty of low end, I can see why you have so much pressure with 12-1! Although intake closing point has the most effect on cranking compression,Overlap does indeed affect low rpm compression, the reversion and pollution of the intake charge is in direct relation to non cylinder filling early in the intake cycle at low rpm. The way I understand it is when the exhaust velocity increases the intake pollution decreases and the intake velocity increases. In all reality when this combo is figured out for drivability on lower grade fuel (compromizes) the performance results will probably be similar to a good combo with 10-1 compression I would think.

You bring up a good point about the low RPM cylinder filling. Also, I want to keep the power peak at 6000RPM or below.

I should say that I don't really mind running higher octane fuel, but what I don't want to do is run 100% Sunoco 110 or something...that would just cost too much. I'd like to mix 25-50% race fuel to keep costs conservative.

I'm worried that with this much compression, I would indeed have to run straight 110 octane all the time. I don't want to rely on my ear to listen for pre-ignition, because this cam is fairly loud through the mufflers. Hopefully this sheds some light on my problems, and a big thanks to everybody who has offered advice graemlins/beers.gif ...keep 'em coming! graemlins/thumbsup.gif