Anyone running 11:1 compression on pump gas?

Larry is sooo correct, You can run all the static you want ( good thing ) and compensate with valve timing to make it work great on pump gas. That's the whole reason EVERY cam manufacturer says " needs 10-1 " to work because they have done their homework. My motor is at 8.85 dynamic but I have have to retard my timing to get away with pump gas when I don't add race gas. In my opinion I believe that I can control the detonation better with timing than camshaft...as well as believing in ve for the motor.

 

Per my earlier comments regarding being on the "ragged edge" when it came to detonation with iron heads, don't forget that the reason the rule of thumb came about that one can run another point of static compression with aluminum is because of the difference between aluminum and cast iron's coefficient of thermal conductivity.

In other words, if I had iron heads with no detonation issues that flowed exactly the same as a set of aluminum heads, I'd get more output from the iron, because they keep the heat in the combustion chamber longer!

I grew very tired of constantly managing ignition timing due to pump gas quality and ambient temps while I ran the iron heads, and this was on the street! With aluminum heads and the same static and similar dynamic ratios, I have no problems, and I'm glad to be off the edge.

Now, I'm going to experiment with timing bit more to push the envelope again when I get the chance.

Bottom line, the feedback you get from a dynamic ratio calculation is useful, but it's not so simple as that. Your choice to live with the engine and enjoy it depends on how much you're willing to dive deep and get the best out of it without sacrificing durability. If you play on the "ragged edge" of detonation, learn to watch spark plugs. When I first started out 30 years ago, I didn't know any better until the time I took it apart and saw hammered bearings and scuffed pistons, too late.

Devon

 

I agree with you 100 %...I read my plugs every time I shut it off.....You have to remember though...you big block guys have aluminum heads...we don't....And we have to work with what we have, and we make pretty decent hp and tq for the little small blocks we have.

 

I would love to see what your car does at the track. It should run very well!:TU:

 

you know sean...every time I pull a plug I think it's going to show rich...but it just is perfect even though I add more off idle fuel. I am now using 80/85 jets and 31/35 squirters 5.5 pv in the front and none in the back and it still seems like it could use more fuel in the squirters....all in the 830 annular. But for the most part it goes very well, it chirps the tires in second gear with just 1/16 throttle and 3.92 gears. Every time I go to the car meets on saturday most think it,s a big block. There is a guy with a gs and a crate 572 chevy motor :spank:and he thought it was a big block driving in....

 

Understood, I'm very much aware that you don't have aluminum available...yet. And I'm also very much aware of what the small blocks are capable of having built a few. The reason I brought up aluminum vs iron was to reiterate that calculating dynamic compression ratio using the closing point of the intake valve is well and good, but that it is certainly not the only end all, be all variable to consider when looking for compatibility for a combo using a specific octane fuel.

The hard part is getting your arms around all the variables to consider when trying to get max power out of a given fuel while avoiding detonation. Here are a few I can think of, and this is just considering wide open throttle:

Static compression ratio
Dynamic compression ratio
Combustion chamber surface finish
Piston crown surface finish
Quench (or lack of)
Ignition timing including curve and total
Carb jetting
Ambient pressures, temps, humidity
Thermal conductivity in the combustion chamber
Coolant thermal efficiency

The good news, is that's the short list for WOT dragstrip driving.

If you're going for drivability, and part-throttle acceleration and cruising is the biggest part of that, it gets more complicated. Take the above and add

Vacuum advance
Carb power circuit and rate
Carb power circuit as vacuum advance affects it
Carb power circuit as vacuum advance and ignition curve affects it
Transitions between idle, power and main carb circuits

All of this can be a serious handful if you choose to make the most out of an engine on a given fuel's octane. Sure, you can go overly rich for protection, but that's not where the power is found.

Does that help? That's what I went through while running "on the edge".

Again, DCR is just one tool to help you get an idea of peak cylinder pressure, then you're "advised" as to what number is acceptable on "pump gas". There's so much more going on it's almost a tragic comedy.

Devon

 

Very good discussion!

When we were on the dyno with the Buick 350 at 551 HP NA it loved lots of fuel with a 1000 CFM carb. That was a 10.5:1 compression engine with a huge solid cam. Using the roller cams available now we could get that HP with 9.5:1 static compression I bet!

 

Devon brings all good points which are for real issues and solutions to the problem of detonation. My whole original point was given the list of items that were just brought up ( of which there are potentially even more ) , would it be better to stay within the bounds of mechanical efficiency or manipulated events ( timing ) to control the problem of detonation with a given fuel.
I have already polished the chambers ( 52cc ) large valve ported heads, polished the piston surface, closed the heat transfer in the manifold ( single plane ) and heads, I keep the motor at more or less 165 degrees of coolant temp, I run zero vac advance and have my timing locked to start at 22 degrees and mech advance to 31 total, the local ambient temp is +/- 78 degrees most of the time and I am at an altitude of 50 ft. above sea level, the fuel settings are much as I described plus a bit, but you get the jest....still just learning this motor.

 

It is a balancing act between them both but staying within the bounds of mechanical efficiency should be primary.