How do you determine what the max compression ratio that you can use for a small block chevy and still run on 93 octane. I know the easy number is a static compression of 9.5:1 with iron heads but I know guys who run higher than that and reportedly do not suffer detonation what is the reason for this? Is the dynamic compression ratio such that it allows the engine to actually see less compression. Do longer duration camshafts produce less dynamic compression and where does the benefit of compression and longer duration end on a street small block running 93 octane

 

There are many varibles that determine how much compression the engine can take. Head design and material has a great influence. Aluminum heads typically can tolerate an extra point due to the heat absorption of aluminum. Fast burn heads can tolerate more compression as they produce more swirl and require less spark lead. Long duration cams lower the dynamic compression at low RPM, but increase it when running around the torque peak. Spark timing and A/F ratio also come into play. Reverse cooling schemes such as in the LT1 also allow for more compression. Modern engines using knock sensors can tolerate much higher compression because they have the ability to retard the sparkto the knocking cylinder. The best way is to see what others are getting away with and proceed from there.

 

Do dynamic compression ratios change? I think cylinder pressures would change with rpm due to the cylinder ram charging effect caused by the rpm and valve timing overlap makes me wonder if I should base the question on cylinder pressure and not compression ratio but I have to read more before it actually makes sense. I did find a interesting Vizard chart that uses dynamic compression and water temp to determine octane what I dont know is if it is based on a iron of aluminum head(more reading)
Looks like at 94 octane and 180 degee temp the dynamic compression could be like 8.4:1. So many variables though

 

The dynamic compression and volumetric efficiency (V/E ratio is what really affects things. The static compression is just a starting point. Usually the dynamic compression is less than the static compression and the V/E)is less than 1due to valve overlap and the fact that the cylinders do not get fully filled. The dynamic compression should peak out at the torque peak when everything comes toghether to create maximum cylinder fill. This is also when the engine comes on cam and high speed detonation sets off. At the torque peak, some engines, depending on design, the intake system will produce a ram effect and the V/E will go above 1 and actually cause the dynamic compression to exceed the static compression. It really all depends on engine design.
As shown on your graph, the colder the engine the less chance for detonation, but also reduces efficiency because the heat energy is being taken away by the cooling system. Everything is a compromise.

 

I am slowly catching on what I dont quite get is how the dynamic compression can exceed the static compression from the formulas I have been running the dynamic compression is not dependent on rpm and I am now kind of wondering if it takes into account ram effect of cylinder filling(in the case I am looking at more related to exhaust overlap creating a cylinder vacuum allowing the cylinder filling to create greater than 100% effeciency in effect supercharging, really though I doubt we would see a whole lot of ram effect in a street cammed engine where a 90%VE is considered good) I attached the link I am using for the DCR http://cochise.uia.net/pkelley2/DynamicCR.html.The chart was from a Vizard artical http://www.popularhotrodding.com/tec...on_ratio_tech/. I ran the dynamic compression formula on a 327 and then with a 350 because I only had to change the stroke exactly the same cam and everything else turned out to be two differnt animals , The 327 could run over 1 full point of static compression and still be at the magical 8.4 DCR(it was nearly 11:1) whereas the 350 needed to run around 10:1 to not exceed the 8.4 DCR... interesting stuff.

 

A VE of over 1 is not common on street engines, but is the norm on race engines. The old Dodge ramchargers with the 30" long intake runners were getting VE ratios around 1.3 when the air column resonated with the engine RPM.

 

Ithink my grandfather had the crossram setup on a dart he drag raced in the 60s. What I have come up with for the 350 we are building is that to run the 270 duration cam wewould havea static compression of 10:1 it would give us a dynamic compression of around 8.3:1. I would actually like to run less compression to be safe but to maintain a acceptable quench distance of 0.035 we need to deck the block and that raises compression. To get the dynamic compression past 7.5 we need to run a domed piston having a 3.25cc capacity a flat top having a -5cc cant do it unless we negative deck the block which I dont want to get into. The head we are using is a 993 gm casting with a 74cc chamber which is really the root of the compression problems(ie domed piston). I think I am going to run some other cam profiles with shorter durations and see what happens I would rather not run domed pistons. This combo works out to be 400 hp with the desktop dyno at about 5600 rpm but it looks like a peaky curve.

 

I don't particlurly likr domed pistons as they interfere with flame travel, and wathever you cain in compression you will probably loose in efficiency.

 

I agree I ran a few other things andto get away with using a flat top piston I either need to sacrifice compression and have a dynamic compression in the lower 7s at (0 deck) which I think is low for the cam, I tried going -0.005 on the deck height and it puts me into the danger zone with a quench distance of .025. Deciding against the dome I am left withthree practiable choices 1)the first is to ditch the smog heads and get a set with a smaller chamber or 2)increase the stroke to 3.75 using a scat or eagle cast crank or 3) opt for a different camshaft with a shorter duration (so far I havent found one that creates the 400hp target) current cam :http://www.compcams.com/Technical/Se...umber=12-211-2. Being I have a 350 crank and the 993 heads need rebuilt I am leaning towards the cheaper summit heads??????? $650.00 unless someone knows of ones cheaper. I am guessing Id have $450 in rebuilding the 993s and 150 in the scat crank but Ill have a 383. Any opinions?

 

Check out this site for your cam:

http://store.heartbeatcity.net/store...yword=camshaft