How do you guys with high compression motors get by on todays pump gas?

 

Depends on if you are running aluminum heads or not. Also, how well the engine is tuned.

 

Alum heads, tight quench, cold range plugs, and back off the total timing and don't use a real aggressive curve while it's on pump gas (93) and in summertime I still get some light pinging occasionally on a real hot day under a heavy load. 11.94:1

 

Yikes! That c/r is up there. What are your cam specs?

 

277, 289 duration .521 lift 110 lsa 106 centerline

 

110 ls - what are the specs at .050:?

Thanks

 

230 intake and 236 exhaust. Thats with 1.5 rockers, I'm running 1.6. I think they are supposed to increase the duration something like 3* total. Don't know exactly it would be at .050

I believe the biggest thing that helps stop my detonation is I'm running a .037 total quench. It's pretty tight but I have heard of some folks going tighter, some successful, some not.

But it all has to work together heads, quench, timing, how rich you run your carb, intake charge temp, cam overlap.

What I'm trying to say is if you build a 12:1 motor without thinking everything thru you'll never get it to run on 93. It will detonate itself apart.

Kinda like an old dog I used to have, always wanted can dog food... but it would eat dry if thats all I gave him. My motor is the same way it doesn't like 93 but it will eat it. Now it loves VP 110, but at $6 a gal., he only gets to eat that on weekends

 

Aluminum heads, tight quench, big cam and proper advance curve will help. Flat tops with better designed chambers work better than the domes and old school chambers in general.

I run an 11:1 302 engine with stock iron heads and the factory "30-30" cam. It runs fine on pump gas.

-Mark.

 

Multi Port Fuel Injection also helps if you are leaning that way.

 

11.75:1 iron headed long rod 427...32 deg total mech adv with a vac sec to get milage on the highway and smooth out the idle and .035" quench. runs great on 93. Light ping at some light throttle postions between shifts. Pistons are ceramic coated as well.

 

I run 11.2 with ceramic thermal coated pistons. 236/244 solid roller cam .041 quench. Dart aluminum heads. It's right on the edge because I got some cheap gas out of the super unleaded pump and it started rattling within a mile of the station.

 

Quench? Engine has a fairly new rebuild edlerbrock heads intake 10.1 compression pistons,comp cams (268) detonion comes in about 3400 someone spent some money here but I'm looking for some tips to make it a little more driveable

 

Quench - It's the distance from the top of the piston to the non valve, flat side of the heads combustion chamber.

So in my case the resurfaced block leaves my flat top pistons @ .020 down in the hole. + the thickness of the head gasket of .021 = a quench of .041 inches.

As the piston comes up this squish area shoots all the fual air mixture towards the spark plug. The tighter this squish area the faster the flame travel. This makes a motor with higher compression more knock or ping resistant. .040 is generally the ideal clearance.

I had an iron headed 355 ci with a true 10.74 compression with a Crane powerMax 272 cam and later a Crane PM 278. The 272 would rattle under light load like going down a level freeway at 70 mph on 92 super unleaded.

My fix was I bought a Mallory Unilite vacuum advance ignition. I had Mallory recourve the timing to 22 degree mechanical all in at 3000 rpm. So 14 initial and 22 mech. makes for only 36 total. I also bought the Crane adjustable Vacuum advance can. I limited it to about 4-6 degrees where a normal vacuum advance might be as much as 12. which will cause the rattle at off the throttle high vacuum situations like my cruising down the level freeway.

 

More c/r is good.



I suppose this chart needs some explanation. It shows the increase in power in % of a C/R increase. EXAMPLE: If the original CR is 9:1 on right of chart & it is increased (A line) to 12:1 (B line) on left of chart, then the increase in power is about 4.5%. If the power is increased from 9.5 to 10, the increase is about .08%. Careful on overdoing it.

 

Ok, short of tearing the motor apart and having the block decked, you might try a thinner head gasket. You'll need a coated one (alum on steel thing)the thinner they go the more expensive they get.

If your not up to that try backing off the total timing to around 32* to see if that helps. Not sure what dist you have but you can play with diffrent combinations of springs on the mech weights(lite-lite, lite-medium, medium-medium, so on etc.) and run your carb a little on the rich side (more gas in the chamber cools things off some) Get a good fuel press gauge to make sure your pump is keeping up at 3400 RPM and above, if its not no matter what you do it will run lean and rattle. Run a colder range plug (in running #5 rapid fires in mine)

It all depends on what you are willing to do vs what your willing to sacriffice performance wise.

The first option I gave you will more than likley control it without giving up any perfmance at 10:1.Te rest of the options are kinda like a bandaid but will help to control it although it gives up some performance

 

It's been in the garage since Feb with only a few test runs it's time for a little road time now that it's painted

 

I also run 11:1 on pump gas - premium...

 

whats your timing set at? mines @12

 

Your initial timing doesn't matter a whole lot less you are having problems starting it. What does matter is the total timing. If you have an adjustable timing light dial it to the number you want to try (32,33,34 etc.) rev your motor to about 3000 rpms or untill the mark doesn't move any more and check the mark. It should be on zero, if not adjust your dist till it is.

 

Thats gonna be my first move when it's back from the painter