Planning to replace the heads on our 350. The compression ratio is coming out to 10.02 : 1. The gas we have here in Phoenix is pretty bad, and for the most part the best I can get is 89 octane.

Wondering if this will work without fighting ping? I thought about using a thicker head gasket, but I know this will change the quench area and that can also increase the chances of pinging.

Are there any of you running a similar compression ratio and the gas we have here in Phoenix, and if so, what have your experiences been?

It this helps - the motor is a mild street 350 with flat tops and a Summit Racing cam (.466 lift / 298 duration).

Thanks!
Adam

 

I'm running a 454" with a 9.95 to 1 compression ratio and I get by using 91 octane as long as I run a 8 degree initial and about 32-33 degrees total timing. It'll lightly ping when I'm chugging up a 6% grade in 4th at 2000 rpm but otherwise it does just fine. So I would think you'll have no problem in normal flat-ground driving.

 

Was thinking exactly what you said. I'm guessing I'll have to back off on the timing. Currently running it at 10 initial. Also thinking I'll need to find a station that has better gas.

 

There isn't any big gain by running a lot of initial timing for a street driven car and running a lot of initial timing just makes it harder for the gasoline to resist pinging. When I encountered some light pinging when pulling the 6% grade coming home I just reduced the timing from the 12 degrees that I was running to the stock 8 degrees and that took care of it.

 

Iron Heads? 89 octane and back off on timing. I have a roller cam .525./.525 with AFR heads with 10.2:1 compression and it runs perfect wit 32 total timing and 89 octane with no ping...

 

Yes, sorry I should have said that - iron heads.

 

Thanks guys. jb, correct me if I'm wrong but a larger cam can actually help the situation out right? My meaning is that you're running a more aggressive cam, and that might be a benefit for you to avoid some ping.

Wondering if running one step cooler on the plugs might be a good idea as well.

 

I am no cam expert so I would rather not make comments about things I am not sure about but I do know that the aluminum heads have MUCH better detonation resistance than iron heads...

 

Agreed. I found quite a bit of really good information on that myself. Cost comes into play in that area.

Not to get too far off track here, but those of you who have aluminum heads on your street motor, what are the advantages outside of the obvious weight savings and the detonation resistance?

 

With that big of a cam u want to calculate the dynamic compression anyways. There are plenty of information on how to calc this and online calculator/spread sheets for this and it doesnt cost a penny - just some patience. Then u can decide what dynamic compression ratio is needed for your local pump gas.

Have u accurately measured compression for your engine. If want to run different cams with limited octane choices this is something required. If u dont have the time or resources to do this i cant help u as only u can do this correctly rather than guesses over the internet. U need some small pieces of plexiglass, red axle grease and a liquid like alcohol to do this. If u need more specifics just ask but it your choice to make an accurate build or a guesswork build.

 

I just experienced this very thing as I drove my car from Canada to Phoenix. I was running 34 degrees total @2800, and had to back it off to 28 to get rid of most of the ping.

We have no ethonol in the premium fuel at home, not so sure about down here. I still have to get an adjustable vacuum advance unit to be able to fine tune the ignition. Still have a slight ping on inital go from part throttle, and under heavy load.

I'm running vortec heads, and about 10.3 to 1 compression. Also running a very mild roller cam (xr276hr) for a 406, which is contributing to my problems. If I was running a longer duration cam, my cylinder pressure would be less due to the valves closing later in the cycle.

Here is my thread;
https://www.corvetteforum.com/forums...n-arizona.html

 

Aluminum vs iron allows you .5 points in compression generally speaking. However many things come into play when trying to avoid detonation.
Water temp,
intake charge temp (this means not pulling air from under the hood for your engine),
splash shield on the intake manifold,
blocking the crossover heat on the manifold,
Jetting,
Exhaust back pressure or lack there of for detonation prevention,
overall cylinder pressure, meaning cam dynamics and CR combined,
spark plug heat range,
gearing, steeper gears make the engine not work as hard and not detonate as easily,
Altitude and just or maybe even more important Density Altitude
Outside air temperature

So you need to figure when and how you are going to operate this car and rebuild it to meet that need.

My aluminum Dart SHP headed 350 is running 9.9 CR and 8.25 DCR. I run 85 octane with no ping this is with A/C a Th350 and a 3.08 diff.
I also run 34* of advance all in by 2570 RPM. Dyno'd at 406 HP at the crank yesterday.
On the hottest days I put in a bit of 91 to get up around 86 octane.

I'm also at 4000 ft. Density altitude runs up to 8000 feet on hot days.

So cylinder pressure is a biggie. That is indirectly indicated by your DCR. Higher DCR higher cylinder pressures. Too high and it detonates.
Your intake closing is an indirect indicator of your cylinder pressure as well.
My cam closes the intake valve at 59*. That's pretty early for a performance cam.
Close it later and you get a lower DCR. You also loose bottom end torque.

As you mentioned quench is also a biggie. Don't give it up. But your piston needs to have a quench pad. The stock pistons provide no quench pad.
Tons of stuff to learn. This is an excellent book to do just that if you take the time to read and absorb the information.


I found it fun to build the engine while learning all the in's and out's of building a performance engine. Your 10.02 can be just fine on 89 octane if done well.

 

Those are very important considerations Reel'r mentioned. My 2 cents is the DCR spreed sheets are worse case situation. Using the basic spreadsheets will keep u safe w/o improvements and that is where most new enthusiests are. Many want argue DCR calcs are not totally accurate and I have to say they don't compensate for every configuration - they don't need to as long as they keep u on the conservative side of detonation. Don't try and fly the parachute, just land w/o getting hurt.

Good night.