11:1 CR+pump gas+iron heads = detonation.

[62 Dart Convertible]

Hey all

Some of you may remember an engine build thread I posted early last year - http://www.forbbodiesonly.com/moparforum/showthread.php?52492-426-pistons-in-a-060-quot-over-413-WTF-!

CR measures at right on 11:1 (old school forged MW pistons, closed chamber 324 heads, felpro 0.040" gasket) with Voodoo 271deg Hyd cam.

When timing is set where it should be, it detonates like all hell, even on 98 octane. So timing is backed right off and this naturally just spoils the torque party.

I am planning on installing thicker head gaskets to lower CR.

(Yes, I could change pistons, increase quench, install aluminium heads, put in a bigger cam, run racing fuel/AV gas etc, but there is no budget or inclination for any of that.)

A 0.080" Cometic gasket will give me right on 10:1. They are available up to 0.120".

IQ52 has pointed out before how little effect high CR has on power output, and my feeling is to play it safe, as I would also like to run 91 octane if I could.

Is 10:1 a good target or do you think I should play it safe and aim even lower?

 

[lewtot184]

i tried the thicker gasket thing a couple of decades ago on a 10:1 compression engine. i didn't help, could make things worse,

 

[Cranky]

I'm not a fan of increasing the quench distance either. What is your compression PSI? If you cylinder PSI is over around 195 or so, a cam change may be a better choice. Had a 340 a few years ago where just one gallon of av gas on top of 93 did the trick. And IQ knows what he's doing but in my experiences, upping compression does increase power output but the rest of the combination needs to be considered too but if your cam is producing too much cylinder pressure, 10-1 will rattle too.

 

[62 Dart Convertible]

I hear ya Cranky. I will do a compression test on the weekend if it drops below 50 degrees C (120 deg F) in the garage. Expecting it to be real high as I know the Voodoo cam closes intake early.

Nobody doubts increasing CR will make more power with correct timing, but IQ's point is it isn't as pivotal to good numbers as some believe.

I'm not prepared to change the cam though, no way, uhuh, not doing it, even if I get sent to the naughty corner...

 

[snakeoil24]

62 Crank is telling you the facts if Compression pressure is to high everything else is not going to matter. Shoot for 150 PSI. Detonating will rage on with 110 Octane with 195 PSI.

 

[mopar 3 B]

X2 with cranky. If you can find some 120 octange what you have may or may not work.

 

[khryslerkid]

Have you had the detonation from the beginning or has it been steadily getting worse? How do the plugs look? How's the operating temperature been? Running two four bll carbs, are they a little rich for the cam, head combo?
What size valves are in the 324 heads? They were small from the factory, (1.60 exhaust)
Just throwing some simple things out there.

 

[Roadbird]

I'm planning on using .30 over 383, 11.5:1 CR dome pistons on my 383 but machined original 906 open chamber heads. This is what my machinist friend recommended and I probably stick with the original Roadrunner cam and burn 92 octane or higher.

 

[Meep-Meep]

You also have to consider fuel distribution. Pinging will happen on the worst cylinder - can be on one or more than one - so playing it safe isn't a bad idea. My own 440 has 10.1:1 CR with 190 PSI and it would ping on pump 91. A splash of race gas (110) made it run great with the resultant octane being about 96.

In my experience you want the proper balance of compression ratio and cranking pressure with an upper limit for pump 91 being 9.5:1 and 175 PSI. 87 seems to work well with about 8.5:1 and 160 PSI. You can probably get away with more CR and pressure but will have less margin to work with - meaning no tolerance for a hot day, etc..

Now here is an account of further experience with my 440. With the factory iron square bore intake and a 750 AFB jetted 4% richer in the secondary, the car ran like a scalded dog, but only with a few glns of race gas. Install the factory 6 BBL and it just killed it. Clearly there were issues. But what surprised me the most was the mixture, as measured under load on a dyno, was very lean but the engine didn't ping. This has to be the result of the 6 BBL offering improved fuel distribution over the iron 4 BBL or perhaps some cooling effect when all 6 BBLs were open.

So really what I'm saying is there are a few factors that need to be in the balance for all this to work. We are stuck with the bore size and chamber design so that leaves timing, fuel distribution and static ratio. Quench is a good thing but IMO you won't notice a difference between .040" and .090". Keep in mind that the open chamber heads still have quench. Just not as much as the closed. The opening up of the chamber was primarily an emissions design, which was to ensure that all the fuel got burned in the quench space. That little space around the top of the piston above the top ring is also of great concern and is why the hypereutectic piston was designed. The stronger material made it possible to put the top ring higher on the piston, which reduced that annular space.

 

[62 Dart Convertible]

Thanks for the edumacation fellas. When I do the compression test, I'll post results!

Another question - how does mixture affect detonation? i.e. does running rich or lean make detonation better or worse?

KK - engine used to ping before rebuild, but not as bad likely due to bigger MP solid cam. (I had forgotten just how little timing I had in it before I rebuilt it, and realised after that I should have done something about CR then...) Plugs look fine, stock valves in heads, cooling system copes with engine barely, as in, not too hot a day and steady speed - no probs. Hot day and stop start driving, engine temp climbs rapidly. Detonates worse as temp rises above 180 (no surprise there!)

Yet another question - my timing at idle is around TDC and total around 24 - any more and it pings constantly. Would this contribute significantly to my high engine temps?

 

[khryslerkid]

I'm going to wait until I hear what your cylinder psi is....

 

[Garys1969RR]

Just as an experiment, try running with out a thermostat for a minute and see if the pinging stops. Sounds like you may have a heating problem. Maybe get a bigger radiator. And what are the 324 heads? Are those 361/413 heads from the early 60s? 4 Bolts on each valve cover?

- - - Updated - - -

I really believe that good quench, like .035 to .040" is very helpful not only in reducing ping, but in promoting combustion chamber mixing to ensure a more complete and rapid burn. The throttle response with tight quench is great! The motors I have built with tight quench tolerances have always performed really well.

- - - Updated - - -

My 451 with 13 to 1 C/R, .037" quench, 915 heads, and Ross dome pistons runs super strong. The 69 RR has posted a best ET of 12.33@110 MPH at 5600 feet. I"m sure it would run mid 11s at sea level. And I have found through experimenting that a mix of 2 gallons of 91 octane premium, 2 gallons of 100 Octane Sunoco race fuel, with 1 gallon of E 85 eliminates any pinging. And that also reduces operating costs.

 

[Meep-Meep]

A lean engine will ping, or if you have a cylinder or more going lean due to poor fuel distribution. What's most important is the fuel must enter each cylinder equally, in a fog state, in order to burn properly. Atomization and temperature control is the key as the inlet charge temp has a lot to do with overall performance. The hotter the engine gets so does the intake and so does the inlet charge and pinging can result. Unless the inlet charge temps are consistent it will be impossible to optimize the mixture. Or get the engine to thermal equilibrium and tune for that and live with the results when it's cold. Gasoline also has several components to aid in atomization and changes for summer and winter. And modern fuel isn't even designed for a carb so we're pretty much screwed!

As far as running hot I'm going to blame the higher compression. That crappy 426 hemi in my 66 Charger is miserable and is the best example I have seen that converts gasoline to noise and heat without the side effect of horsepower. I have no idea what's in the mystery motor but can say for sure it has 225 PSI of cranking pressure with what sounds like a stock cam.

Regarding quench.....My 440 has 10.1 CR and will ping with about a .040" quench (depending on intake and fuel combo), so not holding my breath that increasing the quench (smaller gap) by .020" or? will help. It won't. What is guaranteed to solve the pinging is adjusting the compression ratio and cylinder pressure for the available fuel. I'll say again we are stuck with the architecture of the late 50's designed engine running on fuel designed for EFI so 9.5:1 and 175 PSI is about it for 91 gas. 4 valve pentroof chamber with a small bore over 10:1 is reality but we're not there and have to play the hand that is dealt.

 

[lewtot184]

considering that 62 dart convertible doesn't want to do anymore than head gaskets, then i believe there is no solution other than race gas.

 

[khryslerkid]

I'm thinking that if you go to the trouble to change head gaskets, go ahead and throw a set of open chambered heads on it to lower the compression. Other than that, add some racing gas $$$

 

[Meep-Meep]

I'm thinking that if you go to the trouble to change head gaskets, go ahead and throw a set of open chambered heads on it to lower the compression. Other than that, add some racing gas $$$

Yes, a set of 906's with thicker gaskets would help for sure. .....Or race gas.

 

[1962polara]

Anyone see a water injection system helping out here?

 

[62 Dart Convertible]

Spent lots of $$ on these 324s (yep, they're early 361/413 heads with 4 bolt covers) in machining, so I ain't changing them for 906s. (I reckon a set of 906s and my domed pistons will still have too high CR with 0.040" gasket anyway, if I was to go that way, which I'm not.)

Race gas is the right solution to the problem, just ain't gonna happen either. I guess I could attach wings to the side and drive into the local airport for Avgas..

The weather has cooled here today, so I will go and pull plugs....

 

[HT413]

Have you inquired about avgas at the local airport? Here in the States, many airports will gladly sell avgas to anyone with a gas can, though I'm not sure they'll pump into a car directly.

 

[1962polara]

In melbourne we have this 44 gal .......expensive but maybe it could be used as a 50/50 with regular gas ( maybe even less)
They have distributors in Adelaide

http://powerplusfuel.com.au/shop_product.html?product=67