5.9, max safe compression?
This calculator is TERRIBLY misleading as to how overall engine performance is effected with CR, and I wish people would stop pointing to this, as they are doing a great disservice to new engine builders. This ONLY reflects the PEAK HP under certain LIMITED circumstances. It tells you NOTHING about how much added CR effects low RPM torque, widens the torque band at the low end, and improves engine efficiency.
If we did not care about this, we would also never change cam timing to make changes in peak combustion compression numbers.... etc. And we all KNOW that running too big a cam in a low SCR engine kills the low RPM torque. So why then turn around and say that CR is not important? Pure nonsense.
If CR was not important, we would all run 6:1 SCR and never worry about detonation. Let see some dyno curves from that....
I second that. My Hughes 2330 was a tractor, yet reved to over 7000. I think the power peak was around 5200, but it was sorta long and flat up there. And 7200 got me over 60 mph, in first-over so I went there cuz The engine could.And that cam also made super fantastic gas mileage, that nobody believes so mostly I keep it to myself.
The current HE3037 is a tractor too, and peaks perhaps 200rpm higher.But It can't make the the same mpgs. Although I have never paid attention to what the rpms are exactly, I think it will take secondaries at just off idle. Try that with your stock teener, 268*, 3.55s, 295s,and clutch . Just dump it and floor it,Hah! I can see the embarassment on your face from here.
But that's not fair you say, you have a 360. Yes I do.
And if you wanna try that on your 7.5Dcr 360, go ahead. Dump it and floor it,Hah.
Ok wait, I'll give you a headstart. I'll wait until I see your car moving. Got your earplugs in? I will be coming by at Seventy-two hundred revolutions per minute. There is no dyno for measuring that.
On the next engine, I will try a Dcr of 9 to 9.2,still with the aluminum heads, but back to a 270ish cam, and Somebody is gonna have the task of porting my Eddies, and I won't tell you who it won't be. And it's not gonna be a race port.
And I'm gonna run 3.23s. And I will fly by you again, at 7200, cuz that is what the rev-limiter is set to. When I floor it, it is foot-in-the-carburetor-WOT.And the tires don't quit spinning until I tell them to, or until 65mph, whichever comes first.
Ok wait, it's a 367 How tractor-like could it be? Well it's not the tractor like a 440/727 in your 68 Chrysler. You know, the one you can watch the speed-O winding up to 100 mph at a ferocious pace. No tire spin. No exhaust screaming. No drama of any kind, outside of that speed-O and a bit of wind noise starting up. No my 367 isn't like that at all.
But, it will idle down to 550 or less with a little clutch,and pull itself around on flat pavement. And from the stop-lite it's a blip, dump and go deal. Now I admit it, I have a 10.97 starter gear; that's a 3.09x3.55=10.97. Your 2.66low gear car would need 4.12s to match it.You might say I have excessive starter gear, and I think you might be right. I am very sure I could run 3.23s and give up nothing. But I am too lazy to go and swap them in. I think I could even get away with 2.94s, but then my GVOD would be kindof useless........
In a streeter, if you can tune it,High-compression RULES!!
OK hang on. I have an old Vizard book that says going from 9.5 to 11.5 SCr will be about 3% power increase, in a street engine with a "relatively short cam duration". He has a real nice chart and everything. So if you have a 360 hp street stormer, According to the chart, you might gain 11hp. Big deal you say. Well it is a big deal! It is almost 1 full cam size in the "relatively short duration cam" that the book talks about. But unlike the cam which makes that power by trading away low-rpm torque, the extra compression makes the extra torque throughout the rpm range! That IS a big deal. Only a similar increase in gear ratio can touch it off the line. But again, gears can only take you so far, and then horsepower has to take over.
But I like what
nm9 says.
The extra compression at lower rpm and at part-throttle, means you don't have to drive as deep into the carburetor to get the job done.
It's all related to effective compression ratio.
If at WOT and a certain rpm,your VE was 100 % and your 9.5 SCr360 could pull in 360/8=45 cubic inches of air per cylinder, and compress it to say 1/9.5 of that,and during that, the pressure might rise to 9.5 times atmospheric, or say 143 psi for simplicity sake.
Now lets say at 2000 rpm throttled and cruizing, your VE might be I'm guessing 30%, so that same cylinder pulls in just 13.5 cubic inches, and compresses that to .3x143 =43psi.
But if the Scr was 11.5, and the cylinder pressure was 170 psi@ WOT, then the same cruize pressure might be 51psi. If your car only needed 43psi to cruise at, then you could back out of the throttle, and be burning less fuel to maintain the cruize.
But, and this is the important part, More pressure is available at all times with the higher Scr!
And even more importantly this is why a low-Scr/low-Dcr, small displacement engine with too-big a camshaft, gets soft on the bottom ; namely, a major loss of cylinder pressure.
If you have been following along, it might have crossed your mind that with just 43 psi at PT, the cylinder pressure is just 43/143= 30% of what the gas will support. In other words the current cylinder pressure is less than 1/3 of what the gas will support.!! So why are you still buying 93gas? You won't be needing it until nearing 100% VE.
So you can see that the true Scr could be tripled under these theoretical conditions, to 3x43=129 and the actual cylinder pressure would still not exceed the ability of the gas . What is 3 times 9.5? You guessed it 28.5/1.
Now some of you will try and muddy things up, saying the cylinder is always gonna be 100% full of air. You will get no argument from me on that. 100% full of air that is 30% as dense as 1 atmosphere. Go suck an egg, my math saves a lot of figuring. Besides it's all theoretical. The exercise is just to show a little about what often goes on when guys put 268* cams in sub 8/1 stock teeners, and wonder why they have to spend another wheelbarrow full of money to actually be able to drive the thing in traffic. I woulda left the engine stock,except pumped it up to something like 12/1, maybe more. Then watch that 230cfm 2bbl roar....BTW, that's only 6.5% more power,lol, according to the book,like 14hp,lol,about the same as going up one gear size. But I guarantee you, around town, it will feel like a lot more than that, cuz the extra power is everywhere, not just at max VE.The entire power curve moves up; the average power increases across the board.
Some of you might remember my big-bore teener experiment of about 1974. Where I put the entire top end and cam from a teener onto a junkyard 340.I was only 21 at the time so don't ask me what parts I used, that was forever ago. But that engine was phenomenal. I dropped it into a 65 Valiant wagon with the 904 from a 273,and fenderwell headers,that is all. I drove it for 2 years until the tranny gave up. That was one of the funnest streeters I ever had. And easy on gas,lol.