The good news ... if any, is this not a daily driver. The reason for selecting a domed piston was the original GM piston 3942545 had a dome and this at the time seemed like a good alternative. It came up in the Jegs selector as a viable choice for a 70 350 corvette. I guess if I could have purchased 0.030" over 3942545 pistons I would have done so. I did check the head gasket per a previous request. Best I can tell it's 0.033". Changing the heads is not an option as I wish to keep numbers matching wherever possible. I need to think this through some more but should I pull the engine again and replace the pistons, any thoughts on what they should be considering crappy fuel etc? Going back this morning looking at the piston specs I had assumed that 11.4:1 related to a stock piston and going to an oversize bore of 0.030" would reduce the compression ratio some what. In fact, on reflection I think it's saying the 0.030" over size piston is 11.4:1 cr.
Thank you for your insight.

 

The pistons sweet70 used are a 2.4cc dome. I'm not familiar with how much dome is in the stock motor.

http://www.jegs.com/i/Sealed+Power/8...04F30/10002/-1

Boring the cylinder oversize would result in a greater CR than the stock bore given the same piston top cc size. It increases the swept volume of the piston. So a greater volume of intake charge is squeezed into the same size head chamber giving you greater pressure, or a greater CR.
I believe that the 11.4:1 is referring to the approx CR using 58 cc heads. They cannot account for how far down the hole the pistons are or the gasket used so the CR can vary depending on how you adjust those two items.

Use this DCR calculator to play with the numbers.
http://www.jeepstrokers.com/calculator/

If I put in the following;

4.030" bore
3.48" stroke
58cc heads
.025" deck clearence
.033" gasket thickness
4.1" gasket bore
-2.4cc piston volume
5.7" conrod length
300* duration cam
112* lobe seperation
4* advance

as an example for your engine I get a CR of 11.7:1 and a DCR of
8.26:1.
I just pulled those numbers out of the air to give you an example. Put in your specifics (cam etc.) to get a more accurate #.
Note what happens to the CR if you adjust just the bore size.

 

Let's say you have 10:1 or a bit more, but your quench are 0.05 or 0.06 will that still give detonation problems?
If yes, if you then lower to 0.030, will this end the problems?

 

Don't forget to factor in your elevation :-)

 

Squish and quench are actually two different things. They are often used interchangeably and the meaning is understood so no big deal really.
It's like taking a drink to quench your thirst. Taking a drink is the action and quenching your thirst is the result.

Squish is the distance from the piston to the head squish pad and the action of "squishing" the intake charge.

Quench is the result of squish by heat being transferred to the head. Then the head transfers that heat to the water jacket etc.
Greater squish distance less heat transfer to the head.
Heat = power.
Too much heat = detonation.

One purpose of squish distance is to add some turbulence to the air fuel mixture in an effort to create a even distribution of air molecules to fuel molecules for a good, even, and quick burn.

So less squish action, ie greater distance between piston and head, can lead to less turbulance which can lead to a less even distribution of air to fuel in your mixture. This leads to hot spots than may spontaneously combust rather than burn progressively once lit by the spark plug. This is detonation. Detonation becomes less likely as RPM increases and/or load is reduced.

Ignition by some hot surface in the combustion chamber other than the spark plug prior to the spark is called pre-ignition. Ignition prior to the spark ignition.

So yes more squish distance will most likely lead to greater possibility of detonation even at a lower CR. My 7.66:1 OEM engine detonated on hot days under a heavy load.
Experience shows that .040 is a good distance that provides good turbulence and at the same time a safe distance from the head to prevent impact with the piston.

.030 squish is possible and has been done, but you run the risk of impacting the head with the piston. Rod stretch, length of stroke, expansion of components max RPM, etc. must be taken into consideration to prevent damage to the head and piston top.

 

Thanks indeed for your time and trouble. Since you shared the calculator I've done a couple of things. My 3927186 heads are rated at 64cc and not 58cc as previously stated. This change significantly reduces the comp/ratio as does the change to a slightly thicker head gasket. I know this isn't really ideal, but it's in the right direction. What do you think?

L46 Engine 11:1 comp/ratio fitted with KL2304F30 pistons
Cylinders 8
Bore 4.030”
Stroke 3.48”
Combustion Chamber 64 cc
Deck Clearance 0.025”
Gasket thickness 0.033” 0.045”*
Gasket bore 4.1” 4.2”*
Dome/dish relief -2.4cc
Con Rod length 5.7”
Intake valve closing angle 64 ABDC
Altitude At sea level
Static comp/ratio 10:83:1 10.44
Dynamic comp/ratio 8.69:1 8.38
Quench 0.058 0.07
*Gasket set 555-210042 from Jegs

 

Tough situation. The intake closing point is so early on that cam your DCR is too high. You need more like 8.0:1 or less for the iron heads.
I'm a pretty firm believer in squish distance to reduce deto. Either of those two head gaskets give you too much space.
Is a cam change an option?
Possibly the heads could be gasket matched or maybe that advertised 64 cc is more, either would help. Gasket matching the head may gain you a
cc or two.
An intake closing of 76* and a 66 cc head with the .015 head gasket would give you a 11.09:1 CR and a 8.01 DCR. Still highish but with good cooling system and short gearing probably doable.
In either case remove and cc the heads before you buy any parts if that's the direction you want to go. At least you'll know exactly what you have then.

 

Thanks for the comments, I would prefer going the way of replacing the cam to something less radical rather than chase a cc or two in the heads. I'm confident the 64cc chamber is correct for these heads. I would prefer to change the cam as step 1, it's far less disruptive plus I'd rather make one change at a time. The stock cam is part # 3896962, it is available from Jegs, GM Direct and others. Tomorrow we head north for some cooler weather so not much is going to happen until most likely October when my garage temp levels drop below 80*. But I will gather the parts necessary to change the cam. What do you think about going to the stock cam? I'll need gaskets and seals cam and lifters, my timing chain was new about 3500 miles ago anything else while I'm in there? I'll check the thermostat, I did check it before and reduced it, it may be at 180* but I'll drop it to 160*. Any thoughts about blocking the exhaust flow in the intake manifold to reduce temps in that area? I have already removed the thermostatically controlled valve from the right side exhaust manifold. It would be easy to block the passage while the intake manifold is off.

Many thanks,

 

The stock cam is 222 @ .050 and a 114 LSA. That still sound too short to me. It has long opening ramps for a long advertised duration of 312/312. I don't really know how to equate that to a modern cam with much shorter ramps for the purpose of bleeding off pressure until a later RPM. 114 LSA sounds good though for building less cylinder pressure.
Chris Straub is a CF member and his expertise in cams might help you for this situation or he may even have a better idea to reduce CR. Try sending him a PM.
I wonder if restricting intake lift at the valve with a lower rocker ratio would reduce the cylinder pressure enough to help reduce detonation.
As far as restricting the crossover heat goes, I would say unless you drive this in the winter do it. I blocked mine off to reduce intake charge temps. I also installed an oil shield to help keep oil splash off the bottom of the intake manifold to reduce manifold temps.

 

I read an interesting article comparing three cams from yesteryear in L79 vs L46 vs L82 engines. My original cam (3896962) rated very well, it made reference to the domed pistons cast iron heads and cast iron inlet manifolds along with the 11:1 static compression ratio. If I understood the article it also described my 186 heads as, small chamber, big valve fuelie heads. If you are not familiar with it, if you google L79 vs L46 vs L82 it should come up or if you send your e-mail address as a pm I can forward a copy as an attachment. I am getting the picture now re quench, static and dynamic etc. The trouble is I don't have the experience to gage the difference in performance 0.04,
0.45, 0.58 except 0.04 is better. The same with lobe separation in the cam I have read that reducing lobe separation can increase knock along with lowering inlet manifold vacuum, I have both of these situations, so going from a stock cam with 114* to a cam with 110* isn't good unless offset by something else. I'm starting to think my cr is "fine tuning" and my cam the first place to start? I'll contact Chris Straub as you suggest, not sure how you do that, but I really thank you for your information.

 

The best way to reduce CR without changing the heads is with the piston. However if that is not on the table for discussion then the cam is your next best option to reduce DCR.
Dynamic compression is an attempt to estimate the effective CR under running conditions.
If the intake valve closes later in the compression stroke there is less stroke left to compress and this reduces the pressure in the cylinder. At least at lower rpms. Other valve events are changed as well that bleed off pressure at low RPMs.
Changing the cam to a later intake closing point delays the higher cylinder pressures until a higher RPM. Detonation needs time to happen. At low RPMs it has that time. Not to say it can't happen at higher RPMs as well, but is less likely than lower RPM high load conditions for a given CR if cylinder pressure is too high for the given fuel and conditions at that time.

To find Chris go to the top of the page click on "other", click on "select member list" then once the member list comes up go to the right top of the page where you''ll see "search members" click on that and type in Straub. Then find him in the list and you'll have the option to send PM

 

I tried the Mr gasket kit of springs, they didn't work for me, I found as et of what were called stock springs and used them, all working okay now!

 

were

Thank you for the tip. When I last ran the engine I switched to the two gold springs in the MR G kit. Like you I suspected the springs that were in the distributor were allowing "all in " advance at or around 2000 rpm. I'm not sure if they were original GM springs or not, they would have been at least 25 years old based on the time the previous owner had it and basically did nothing to it ... including drive it. I seem to recall there being another recommendation to Mr G. I'll see if I can find it. However I suspect my troubles could rest in the cam/piston combination I have. I'll try some stiffer springs and who knows!

 

This is great stof.....thanks.
So of a good squish is 0.040, what is then a good quench??

So squish = distance
and quench = area

That is easy to remember.......

 

Quench is the result due to squish. It's the transfer of heat from the combustion chamber to the head.
Aluminum heads have better quench properties since aluminum transfers heat better than cast iron.
This is what allows aluminum heads to run under higher cylinder pressures, given everything else is the same, and not detonate.

The area of quench is the flat part of the combustion chamber that the piston approaches.
I don't know any rule of thumb for the square area of the quench, the amount of surface area required, just the distance of .040" which gives better transfer of heat to the head than a greater distance away would.
The piston top design would dictate the area of quench. Ie an open dish design has no quench area.
A flat top has the most quench area.
A d-dish would be somewhere in between, but may have as much as the flat top depending on the size of the squish pad on the piston and the size of the dish.

Here is some more reading on quench/squish.

http://www.crankshaftcoalition.com/wiki/Quench

 

I'm not sure if you will receive this but I'll give it a try. We have been out of town for the last month, so today I tried your suggestion of plugging the vacuum advance. At an ambient temperature of around 90* F I ran for about 10 miles with the ignition timing as per Lars set-up. As before when hot it knocked in first and second under load. I removed the vacuum line from the B28 vac/can and plugged it with a golf tee. A miracle ... no more knocking! I pushed it in first and second gears, knock-less. When comparing the stock setup for the L46 engine the combined advance of mechanical and vacuum below 1000 rpm is 20* (8* mechanical + 12* vacuum). When compared with Lars paper below 1000 rpm I have 28* (12* mechanical + 16* vacuum) this is 8* over stock. By blocking off the vacuum for a test effectively removed 16* from the advance. I will now need to optimize the advance based on these results but now I feel I'm going in the right direction. This is way easier than changing the pistons and cam combination! Thanks for the suggestion.

 

Thanks, I did send a detailed request to Chris per your suggestion without reply. Based on my testing today I am going to work on the ignition timing first. Thank you for the time, I have learned a great deal from our conversations.

 

You welcome. Maybe you'll hear from Chris. Trying to reduce the DCR with the cam is less than ideal than a properly planned CR and cam, but sometimes you do what you gotta do with what you got.

Retarding the timing to eliminate ping to the degree you have is also less than ideal, but if it works for you then use it.
Keep an eye on your water temps and plug wires. The manifolds and the engine will run a bit hot. You could experience run-on or dieseling, engine continuing to run after ignition key turned off.