[firechicken]

Having now switched from iron heads to aluminum KRE Ds I have major changes to the timing requirements of the new engine. I realize that it IS a different engine so all parameters are not the same but in general these heads obviously do not need as much timing lead as the iron heads.
The question is, because the KRE heads have a quicker burn rate, would they be prone to get into detonation sooner if using similar total timing as was required by the iron heads? I've got the engine running good now at 8 psi on 92 octane pump fuel but had to pull the timing back to 20 deg. total where I was running 28-30 deg. total with the old iron and no detonation. I originally started at what I thought was a safe 26 deg. assuming the KREs would be less detonation prone but now am wondering if this may not be the case.

[70 bird]

Need more info?? cam, compression, what changes were made? a/f ratio's?

[71firebird400]

71 bird- I don't believe his question is specific to his application; he's simply asking if with a better chamber is less timing necessary?

Sorry firechicken, I'm unsure myself.

[firechicken]

Sorry if unclear.
Basically, I'm asking if a more efficient chamber in an aluminum head, which requires less timing lead for max performance would start to detonate with less timing lead than an iron head which requires more timing for max perf.
I hope that makes sense.

[Ron H]

The aluminum is more forgiving than iron to start with. The fast burn chamber is more efficient and needs less timing over an open type chamber. I'd think the KRE head would be more forgiving over an iron head. But to know how prone you are to detonation means your combo has to be shown. N/A or turbo? All engines are different though the norm seems to be 30 to 34 total timing for the KRE heads as posted by quite a few owners. Advancing the timing beyond that yielded no more power or they made less.

You say you had 28-30 with the iron heads and now 20 with the KRE's? Total timing, no vacuum advance connected and it is coming in at what rpm?
Are you running vac advance? Ported or Manifold source?
Stock distributor?
Base timing? at what idle rpm?

List your engine specs, bore, stroke, piston, chamber size, head gasket thickness, cam specs.

[Tom Vaught]

A moderate Tumble Head is a good thing. A High Swirl Head is not necessarily a good thing as more heat goes into the liner vs on the piston top. High Swirl heads tend to have holes in the Mixture profile whereas tumble typically has a nice "burn" as the mixture travels past the plug and is ignited.

Has the engine temp gage reading increased or decreased (water temperature)?

Less timing is not necessarily a bad thing if the sweet spot for the ideal "Push" is determined. Push too late = Lost work capability.
Push too early = Typically wounded parts over time.

Tom V.

[firechicken]

I was really just looking for a generalized answer on the timing requirements regarding fast burn aluminum vs. iron heads and their relationship at the limit of detonation but my engine in question specs are:

turbocharged
4.180 bore
4.210 stroke
90 cc KRE D port Al heads
30 cc dish pistons
8.2:1 C.R.
Crane hyd. roller 224/224 114 LS installed at 114
Balancer was checked for timing mark accuracy while degreeing cam
Distributor is aftermarket HEI. Vacuum advance at ported source.

The old iron headed 400 would take 28 deg. total and 10 psi boost without detonation in good cool air and 92 octane pump gas.
The new combo will ping with any more than 20 deg total and 8 psi under same conditions.
I know the new heads should require less timing in general but should they in turn also tend to cause ping at a lower total timing than the iron head.

[firechicken]

Thinking out loud, perhaps the superior flow of the new heads is boosting v.e. by such a large margin over the old iron that it is actually the increased cylinder fill making the engine more detonation sensitive and has nothing to do with chamber efficiency.

[punisher#1]

your problem may be more about the quench area with the piston design and the shape of the chamber than the swirl characteristics of the chamber itself. is this a reflective dish or a spherical one?

[firechicken]

Pistons are Ross with a D shaped dish. Sould have good quench IMO.

Forgot to add for Tom - Water temps are 160 with the new setup. Slightly cooler than the iron was which I attribute to the bottom filled block. Oil temps are the same as I'm using a good front mounted oil cooler.

[Travis Q]

The KRE heads DEFINITELY require less timing than with other designs. This is typical of combustion chambers that atomize/vaporize the fuel efficiently on the compression stroke, as well as chamber designs that have good flame front propagation characteristics. In my opinon, the KRE heads have both of these nailed.

It would not suprise me at all if your combination makes peak power with less than 20 degrees of timing, depending on the fuel (some pump gas has oxygenates, which require less timing and more fuel than other types). I have seen some other makes of pump gas turbocharged engines make peak power at air fuel ratios richer than 10:1 and total timing around 13 degrees.

Combustion chamber design has EVERYTHING to do with timing requirements. Hope this helps answer your questions.

TQ

[firechicken]

Thanks TQ
I guess I may have to go with a locked out timing setup. I'm having to remove so much initial to keep the total inline that I can't keep the idle up without opening the throttles quite a bit. Idle vacuum is suffering and giving me a weak signal at the carbs.
I suppose I could use a manifold source for the vacuum advance instead.
Does this sound like I'm on the right track?

[bobzdar]

Originally Posted by firechicken

I was really just looking for a generalized answer on the timing requirements regarding fast burn aluminum vs. iron heads and their relationship at the limit of detonation but my engine in question specs are:

turbocharged
4.180 bore
4.210 stroke
90 cc KRE D port Al heads
30 cc dish pistons
8.2:1 C.R.
Crane hyd. roller 224/224 114 LS installed at 114
Balancer was checked for timing mark accuracy while degreeing cam
Distributor is aftermarket HEI. Vacuum advance at ported source.

The old iron headed 400 would take 28 deg. total and 10 psi boost without detonation in good cool air and 92 octane pump gas.
The new combo will ping with any more than 20 deg total and 8 psi under same conditions.
I know the new heads should require less timing in general but should they in turn also tend to cause ping at a lower total timing than the iron head.

Are you trying to compare an iron headed 400 to alum headed 455 now? Timing requirements will be wildly different, 4.21" stroke engines like 2-4 deg. less timing than 4" due to higher piston acceleration. Couple that with alum KRE heads and 10 deg total less timing would not be completely out of whack, especially considering with the bigger engine you're craming more air into the cylinder each time and compressing it more...

[Motor Daddy]

Let's say the peak pressure should occur at 14 ATDC for optimum performance. A chamber that burns slower and more inefficiently requires more spark lead to keep the peak pressure at 14 ATDC. Inefficient chambers have a slower burn rate, so you need to start the burn earlier, which means more spark lead is required. The problem with more spark lead is that it starts the burn earlier BTDC. That means there is more TIME for detonation to occur after the spark plug has ignited the mixture BTDC, which increases the risk of detonation, which occurs after the plug has fired and the mixture has been ignited.

In other words, an inefficient chamber with its required increased spark lead is more at risk to detonation, because the burn occurs over a longer time, which increases the risk of detonation. The optimum situation is to have a fast burn chamber, that requires less spark lead, so that the spark lead requirements are reduced to keep the peak pressure at 14 ATDC. Reduced spark lead with a faster burn chamber means a smaller amount of time of burn, which is more optimum for performance due to less negative work done BTDC after the mixture is ignited and pressure starts to build. It also means less risk for detonation while keeping the peak pressure at 14 ATDC.

Lean mixtures take longer to burn than richer mixtures. Lean mixtures require more spark lead to keep the peak pressure at 14 ATDC. Richer mixtures burn faster, require less spark lead to maintain the 14 ATDC peak pressure.

Higher cylinder pressures due to increased DCR's or increased VE% also required less spark lead, as higher cylinder pressure makes for a quicker burn.

Also, let's not forget, an intake port that flows a greater CFM will flow the same air at a lower boost than an intake port that flows less at a higher boost.

You may actually be flowing more air/fuel mixture into the cylinder (higher VE%) at a lower boost with the KRE's than you did with the iron heads at a higher boost PSI.

Just food for thought.