Compression ratio too high for 91 octane?

[yellow rose]

I'm with RustyRatRod on not jumping to conclusions without more details. Without the dynamic CR, how can anyone answer the question with certainty? One crucial factor that I haven't seen mentioned, unless I missed something reading through the thread, is the OP's elevation, which impacts cranking compression and therefore dynamic CR.

I take it that people claiming that 11.8:1, iron heads, on 91 octane is impossible have never bummed around the pits at Bandimere Speedway. Here in Denver at 5,600', the elevation robs us of approx. 15% cranking compression versus sea level, and that's with a less aggressive cam. Add more cam overlap and the dynamic CR gets skewed further. 11.8:1 here would have a dynamic compression ratio that acts more like 10:1 at sea level, which is certainly doable on 91 octane. I beat on a 383 with 11:1 compression, iron heads, 91 octane. No issues. I wouldn't have any hesitation trying for 11.8:1 if someone wants to buy me the pistons, cam, and converter At sea level, heck no, but not everyone lives at sea level.

I pay no attention to DCR. It’s not hard to do if you pay attention to the details.

 

[RustyRatRod]

Thee's a guy over on The Hamb who has an engine with a blueprinted 13.5:1 That runs on 91 pump premium non ethanol. Of course it doesn't hurt that it's a 354 Hemi either. Their chambers are very efficient.

Plus, like yellow rose always says, it's all in the tune and he's 100% right about that.

 

[yellow rose]

Thee's a guy over on The Hamb who has an engine with a blueprinted 13.5:1 That runs on 91 pump premium non ethanol. Of course it doesn't hurt that it's a 354 Hemi either. Their chambers are very efficient.

Plus, like yellow rose always says, it's all in the tune and he's 100% right about that.

Do you have a link to that??? I’d love to read about his build.

 

[Krooser]

I saw a question on another forum asking how much compression this guys engine will gain with a "hotter" cam... we are doomed.

 

[Brooks James]

I’ve never seen hard

Yep. It can be done. You just have to get all your ducks in one row and happy.

I’ve seen it fail miserably because the cam was compromised. It’s not about getting a “big” cam, it’s about getting the RIGHT cam. It’s about getting the cooling system in shape so you can control engine temp and lower it. It’s about getting the ignition system tuned up for the application. It’s about tuning it once you get the engine done.

The tune up window gets smaller as the CR goes up. By the time you get to about 11:1 on pump gas the tuning window is about the size of a pencil eraser. Past 11.5:1 it’s about the size of the tip of a needle.

I almost forgot...converter/clutch, gear ratios and driving style make a huge difference. If you aren’t willing to get the RIGHT converter then you have to keep dropping the CR to get it out of detonation. If you aren’t willing to gear the chassis accordingly, forget about compression ratios that high. And, if you think it’s cool to be in high gear at 10 MPH and stomp the throttle you’ll never keep it out of detonation.

I can make anything detonate if I load it hard enough. You just can’t drive like that and expect to not make it rattle.

And lastly, for the umpteenth time, CR doesn’t know what the head is cast from. The engine has no idea it has cast iron or aluminum heads. This was clearly proven on an episode of Engine Masters. Comparable iron and aluminum heads made comparable power (very close in TQ and HP...well within the margin of error) and the only way to make the iron head have issues was to heat load the engine to the point it would run on at shut off.

No one in their right mind would heat load any engine like that.

The ability of the head to transfer heat to the coolant in the time you go from steady throttle position to the point of load detonation is so quick that it’s impossible for the cooling system to ever respond. IOW’s, what is happening is happening so fast no head or cooling system can reject the heat that fast.

One last time because it matters. If you think 200-205 degree engine coolant temps are the ****, then plan on dropping your CR about .5 for every 10 degrees of coolant temp. I can’t remember where I read those numbers, or how they came up with them but you have to run the engine coolant much lower (and then tune for it) than that to run the CR up that high. Plus, lower coolant temps make more horsepower. FWIW, if you run the engine coolant down to 160 and the cooling system can maintain that you need to run a lower grade of oil. I’m going to use 0W20 this time around instead of the 5W30 I was using.

Oh yeah, I always forget. Running the CR that high with an intake manifold that has a heat crossover is a guaranteed detonation maker. Again, you have to tune for a cold intake. You can’t just bolt it on and go. A cold air intake manifold will take a different tune up than a heated one.

If you are concerned with atomization (we all should be) it’s better to do that with the correctly designed booster and port surface finish rather than heating the intake with hot exhaust gasses.

My dad bought a brand new 3/4 ton Ford pickup in 1985. The last year Ford used a carb on their trucks. What a steaming pile of whale dung that thing was.

It was probably less than 8:1 compression and towing the race car you couldn’t keep it from rattling. I fought it like crazy but couldn’t get a handle on it. The cam timing was atrocious. Ignition timing was the same. I tune the distributor some but I wasn’t going to change the cam in that pig.

One day I was working on that junker with the air cleaner off and I noticed the fuel vapors coming out of the vent tubes. You could smell them. That’s what made me start looking at it. If you stood in the right place you could see the vapor coming off the vents.

So I pull the carb to see what’s going on and I look into the manifold. It is coked like crazy. So I pull that huge pig iron intake off so I can clean it, and the bottom of it at the heat crossover was bright blue. It was getting THAT hot.

No way on planet earth would any carb, with any tune tolerate that. Just garbage. I cleaned it up, put it back together with a gasket that blocked the heat off and it ran a bit better, but it wasn’t happy. So my brother and I waited for my mom to go on vacation to Hawaii to visit her sister and we took my dad to the Dodge dealership.

We traded that junker off on a Shelby Dakota (my dad should have kept that) and called my mom to tell her he was getting a new truck right before he signed the papers. Not much she could do that far across the Pacific. You have to pick your battles right.

Anyway, I said all that to say you have to be systems oriented when you want to run what has become unorthodox compression ratios on pump gas. You can’t just buy a big cam. Or drop the coolant temp. It’s the right cam, the right tuning, the right gearing and all that.

If you aren’t willing to do the full meal deal, drop the compression ratio. If you are, you can be rewarded with an engine that makes gobs of torque, will run cleaner and drive better and most likely be much more fuel efficient (BSFC).

I have never seen hard evidence of lower engine temps producing more hp, only anecdotal. You may be able to lower detonation and raise hp cool,
But your durability in the cylinder will be compromised and you will definitely have more atmospheric damaging pollutants.

 

[yellow rose]

I’ve never seen hard

I have never seen hard evidence of lower engine temps producing more hp, only anecdotal. You may be able to lower detonation and raise hp cool,
But your durability in the cylinder will be compromised and you will definitely have more atmospheric damaging pollutants.

Ever been to a drag strip? Even been on a dyno? It’s a FACT. Do some research. Cooler engine temps make more power and reduce detonation. Ain’t nothing anecdotal about those facts.

EDIT: coolant temps have ZERO affect on durability. If you think your exhaust pipe emissions are killing the planet to ride a bicycle. You have no clue about what you are saying.

 

[K.O. SWINGER]

I for one don't understand why you would build a motor on the razor's edge of detonation? Can it be done possibly, maybe, certainly, (why)it seems like way too much to lose for the gain. Unless possibly you're building the motor for competition and are being dictated by class regulations? Overheating, lean condition, a change in timing it seems all of this could lead to catastrophic engine failure.

 

[yellow rose]

I for one don't understand why you would build a motor on the razor's edge of detonation? Can it be done possibly, maybe, certainly, (why)it seems like way too much to lose for the gain. Unless possibly you're building the motor for competition and are being dictated by class regulations? Overheating, lean condition, a change in timing it seems all of this could lead to catastrophic engine failure.

It’s not on the edge of detonation if you plan for it and do it right. You can’t just slap a bunch of **** together and expect it to work. It’s about PLANNING. And it’s not HARD.

We have been brainwashed since the planned “arab oil embargo” that we are wasting oil, polluting the globe and that we can’t run compression ratios that make power. It’s all bullshit.

Running a lower compression ratio because of fear is a bad thing. It’s a waste of money, power and drivability.

And what looks like a small gain (or no gain) on the dyno shows up in mid range power, which is right where all the “street” guys snivel about where they want power.

 

[Bewy]

This time I agree with YR, post 31. There have been tests done over the years that show that a cooler running engine makes more HP. My guess as to why would be that the intake manifold being cooler keeps the oxygen in the air denser for more power.