What would be a good cylinder pressure (not C.R.) for a carbureted street engine running 93 octane fuel? At what pressures would I begin to see detonation?

Will Desktop Dyno model cylinder pressures?

Mark



[Modified by 81vette, 10:36 AM 6/17/2002]

 

Yes, DD2000 will estimate cylinder pressure for you.

 

It's been awhile since I've looked at those charts, but it seemed to me dynamic pressure was @ 9 to 1 on 91-93 octane. I only use Crane cams and they rate their cams with your intended static compression ratio. I have found that if they rate a cam for use with 9 -10.5 and you run 10.5 You get lite detonation going down the freeway at 70 mph. Because the Dynamic compression is so high that you have to compensate with retarded timing and or octane boosters. Their cam event numbers are made to build cylinder pressure. Low to mid 11-1 is easy to run on 91-93 octane without a problem in properly setup engine.

 

Monty, is 190 psi too high for 93 octane? I'm not at home to give you the correct variable name, but in DD2000 they have 3 variables listed in the right most columns under the Table Tab that I believe refers to cylinder pressure (I am not familiar with DD2000's terminology for cylinder pressure, and the help file was no help). Any idea which one of these variables I should be looking at to determine cylinder pressure for my cam and compression ratio?

Gkull, I'm modeling a Crane HR cam that I really like that has around 240 @ .050 and 24 degrees overlap. I'm trying to keep the overlap down for the street, but the cylinder pressures in DD2000 are showing 190+ psi at around 3500 rpm, and then fall off. It looks like a perfect cam for me....producing 535 lbs. at 3500-4000 rpm, and 500 h.p. at 5500 rpm in a 427 SB. I am concerned that the 190+ psi cylinder pressures may be too high.


Mark


[Modified by 81vette, 2:52 PM 6/17/2002]

 

Your better off with a hotter cam and higher compression. higher static compression gives you more power at every rpm. I also think that with some though you can come up with a better cam than the generic. I don't have my custom event specs here at work. But it's a fast ramps solid with 240/248 @.050 112 with .635/.644 with 1.65 intake rockers and 1.6 ex.

I was going to try a H-roller motor and every company talked me out of them. Solid rollers in low rpm motors would last a long time. I have 17K miles on mine with blasts to 7500 rpm. Some days I'd like a hotter cam other day I don't.


How did you get the weight down so low? I need to get on the scales again but that's really low. I have nearly everything out of my car.


[Modified by gkull, 3:04 PM 6/17/2002]

 

Are you asking about cranking compression pressure, brake mean effective pressure, or dynamic compression ratio?

Duke

 

I was trying to stay with a cam that had a fairly small overlap because I may have to pass emissions if I can't figure out a way around it. I have a few years to go before I can declare it an antique and not have to worry about emissions.

Let me know the specs on yours so I can plug it into DD2000, as it looks like the overlap may not be that large with a 112 LDA. Your cam really doesn't look that much different than the cam I was modeling except for the higher lift. The one I was looking at had a .558 lift I believe.

I posted some info on weight for 1981 models a few weeks back, and the weight of optional equipment. I also posted some info on how I plan to get my car down to around 2800 lbs in another post somewhere, and the weight of parts that I've removed from the car. What is the weight of your car, and what have you done to lighten it?

Actually my car is probably down in the 2500 lbs range now that the engine and tranny have been removed. :D

Mark

 

Duke,

I'm actually looking at BMEP.

Mark

 

There are a lot of variables in the detonation equation - inlet air temperature, spark advance, air-fuel ratio, combustion chamber turbulence, which affects the rate that the fresh charge burns, and others like volumetric efficiency.

The latter is very important because the VE and BMEP curves pretty much lay on top of one another. A high overlap cam will reduce BMEP at low revs because of exhaust gas dilution. Of course this kills torque and fuel efficiency too along with NOx, which is why some early emission control engines purposely had excess overlap.

A later closing inlet valve will also reduce low rev VE and reduce detonation because there is some reversion until higher up in the rev range. This is why OEM SHP engines of yore ran higher CRs than their mild cam brethren.

Engine Analyser outputs recommended spark advance for peak torque at all revs and also retards advance if it detects detonation, but in my experience it's not worth much because it seems too sensitive to detonation - more so than the real world.

Bottom line are that there are no magic formulas, but with 91-93 PON fuel a SB can typically take about 9.5 to 10:1 with a mild cam and about another half point with a typical factory SHP cam. When selecting CR your best to go conservative as a half point will not buy you a whole lot in terms of the torque curve, but if it makes the engine detonation happy to the point where you have to run less than optimum timing through most of the rev range, you're worse off.

Duke