Is 9.7:1 compression on iron heads too much for 91 octane
Read a little closer. I said it can be done. Easily. If you want to. This decades old bullshit of building low CR pump,gas engines is ignorant. 10:1 is stupid simple IF you don’t look through a catalog for a cam. That is the number one killer right there. Not a big cam, the RIGHT cam, which aren’t found in catalogs.
You have to get the cooling system correct. If you think your high CR pump gas engine should operate at the same engine temp as your jap car, you’ll fail. 180 degrees is as hot as they should get. Period. If you can’t or won’t do it, then run the 8.5:1 CR you deserve, and the crap performance you get for it.
It’s that simple. BTW, what the head is made of means ZERO when choosing CR. Nothing.
I wasn’t blowing sunshine up any skirts. I was pointing out the fallacy of low CR and pump gas. It’s stupid. It was stupid in 1980 and it’s just as stupid today.
The OP shouldn’t even be close to concerned over 9.7:1. In fact, he should be looking for more.
And yes, I run those CR numbers on pump gas at sea level. I’ve raced at below sea level with those CR’s, all on pump gas.
All well and good for a race only car. You retune every day at the track and shut down after every race.
180° all the time is great if you're drag racing, it's easy to maintain that if you're only doing a 1/4 mile at a time. If you're driving on the street that's not very realistic, you will have temperature variation no matter how great your cooling system is. And if you're driving on the street when it's over 100°F outside, well, yeah it's going to be even harder to maintain. Why did I pull timing on my car? Well, because I frequently drive in temperatures exceeding 100°F, and when I do that my coolant temperatures get closer to 200°F. Which is still great for a street car. But not great if you're running right on the edge of detonation. And there's the fuel, the blend changes twice a year here. I don't retune. Street car. So basically my tune has to work from below freezing to over 100°F, because I don't re-jet every month as the temperature changes and they mess with the fuel blend. Or even between when I start the car at 0' dark thirty and it's 50° out or when I take it out later that afternoon at 100°F+.
If temperature matters so much like you claim, so does the material of the head. Aluminum and cast iron dissipate heat differently, that's physics and materials science. Is it worth a full point of compression like some folks claim? Probably not. But it does make a difference.
And it's not 1980. And 91 octane isn't what it used to be. And given the cam the OP is running, that's not a drag race only car.
I’ll let you in on a little secret. Those computer programs are full of ****. The programmer puts his data in, and it’s set up to make you run something you don’t need to.
I don’t build off some nonsense computer produced, cranking compression, voodoo crap. Rediculous.
I just did my combo and I come out at 9.00:1 and a cranking compression of 190 and mine has no detonation issues.
:rofl:
So after all whining and complaining that you still ran your numbers?! Hilarious.
And I love all the "computer produced" "Voodoo" "computer programs are full of ****" nonsense. Those calculators and computer programs just run the math on the geometry of the engine. It's all the same math as doing it long hand, it's not voodoo. They're just calculating volumes based on dimensions. Same for dynamic compression, it's just calculating the volume in the cylinder based on the position of the piston when the intake valve is closing. If you know how to do the math the calculator just makes it easier. If you don't understand the math, then you can get yourself in trouble pretty quick feeding in bad information. There's nothing magic about the calculators, the fact that you don't understand how they're coming up with those numbers is more telling about
your knowledge than anything else.
Dynamic compression calculating can be a little interesting, because ultimately you're calculating the cylinder volume when the intake valve is closing and depending on the ramp speed on the cam you may be building compression when the valve is "closed" but before it's fully seated. So the KB calculator uses ABDC @ .050" +15°. Others use intake duration -180 +15, others use .006" of tappet lift. You get the idea, it's an estimate because you're trying to calculate the moment the intake valve actually closes enough to build pressure and where the piston is at that point, and that's very specific to the individual cam.
So the KB calculator says my dynamic compression is 8.195:1. The Wallace calculator I don't like, you can feed it any number that makes you feel good, but it doesn't say what wants. Like if I feed in the ABDC number instead of ABDC +15, it tells me my dynamic compression is 8.9:1 and my cranking pressure is 188 psi. If I feed it ABDC +15 then I get the same 8.2:1, but it tells me my cranking pressure is only 168 psi.
Realistically I know that I haven't put in enough data for it to be very good at calculating a cranking pressure, it has to be assuming the starter motor speed and a bunch of other stuff that basically makes the cranking pressure a wild *** guess. But if I use a few different estimates, I get a ballpark to work with. So I know that when it tells me my dynamic compression is 8.2:1, but I actually get a cranking pressure of 180 psi and I have to back off a couple degrees of timing, that the faster ramps on my cam may be closing that valve a little faster than ABDC +15. But I also know that it runs on 91 just fine unless it's 100°F +, so I know I'm probably not much higher than 8.4:1. Like any tool, you have to know how to use the calculators.
Using the same calculator you used above my dynamic comes in at 8.385:1 way less then that other calculator. Cam is 224/224 .465/.465 110LSA.
I think on the street you're going to be kinda borderline on 91. If your tune is always really great then maybe it'll work. But as I've said, if this is a street car and you don't want to retune it every time you take it out because the weather changed you may need more wiggle room than that.
