How much ignition retard per pound of boost?

[In-Tech]

I still dont have much faith in compression charts and boost. A lot of combinations i have seen wouldnt work according to those

Same here all the methanol blown running 35lbs+ between 13:1 and 14.5:1 CR are at between 25 and 28 degrees at 9500 rpm. All Hemi's with a 6-71. They run in the high to low 6's.

Yep, more garbage charts. Clint could probably provide some real world info. This thread started asking about ignition retard per pound. It's already been stated the standard answer is 1 degree per pound. The truth is there is no magic number. The truth is it will want its' least amount of timing at peak torque and generally more after. af2 knows hemi chambers are pretty efficient and forgiving and like ~26 degrees. Shoot, I've had some junk BBC chambers that liked/lived forever with 38 degrees at 15lbs of boost at 9:1 SCR on 91 octane. To each his own

[F-BIRD'88]

I still dont have much faith in compression charts and boost. A lot of combinations i have seen wouldnt work according to those

Same here all the methanol blown running 35lbs+ between 13:1 and 14.5:1 CR are at between 25 and 28 degrees at 9500 rpm. All Hemi's with a 6-71. They run in the high to low 6's.

And this relates to this posters engine and pump gas how?

[af2]

I still dont have much faith in compression charts and boost. A lot of combinations i have seen wouldnt work according to those

Same here all the methanol blown running 35lbs+ between 13:1 and 14.5:1 CR are at between 25 and 28 degrees at 9500 rpm. All Hemi's with a 6-71. They run in the high to low 6's.

And this relates to this posters engine and pump gas how?

It relates to the charts and such that are not correct... That's all.

[plovett]

As you probably already know, car weight and gearing make a difference in how long the engine sees full boost on the street/strip vs. the dyno;. Also, a manual trans car behaves differently under boost than does an automatic.

As an example, a tall geared automatic in a heavy 4X4 will be likely be holding full boost a lot longer than a short geared Cobra replica with a top loader.

So yes, rule of thumb, one degree per pound of boost, but there are variables. The experts here could probably give a better guess with car weight, gear and tranny.

Then, there's application; any plans to run the Silver State Classic Challenge?

Sorry, I missed this post until now. I couldn't agree more. Gearing, weight, length of time at load, tranny type, etc. all make a real difference in detonation resistance (avoidance?). I would say that is true of naturally aspirated combos, as well. There are a lot of variables at play. Still worth talking about, even in general terms.

Not intending to do a Silver State type deal. Just short highway blasts and maybe some 1/4 miles. I can't imagine being at full throttle for more than 10-12 seconds.

[plovett]

I still dont have much faith in compression charts and boost. A lot of combinations i have seen wouldnt work according to those

Same here all the methanol blown running 35lbs+ between 13:1 and 14.5:1 CR are at between 25 and 28 degrees at 9500 rpm. All Hemi's with a 6-71. They run in the high to low 6's.

Yep, more garbage charts. Clint could probably provide some real world info. This thread started asking about ignition retard per pound. It's already been stated the standard answer is 1 degree per pound. The truth is there is no magic number. The truth is it will want its' least amount of timing at peak torque and generally more after. af2 knows hemi chambers are pretty efficient and forgiving and like ~26 degrees. Shoot, I've had some junk BBC chambers that liked/lived forever with 38 degrees at 15lbs of boost at 9:1 SCR on 91 octane. To each his own

All this stuff is related. Ignition retard is just one question, related to all the others. I think it is okay for posts to meander. I Agree there no one magic number. Any experience you want to share would be appreciated.

paul

[JohnP]

This reply contains SPAM. Feel free to skip over it.

How good is your hearing? Good enough to hear inaudible knock?

Even if you tune using a knock detector, you will be setting timing for the cylinder that knocks the easiest, robbing power from the other cylinders. Get on the throttle a bit and you may need to retune in a few minutes.

I encountered this problem in 1981, then spent ten years coming up with a solution. I called it the SafeGuard, and released it in 1991.

Thirty years later it's still the only stand alone individual cylinder knock controller. And it has boost retard with two knobs: Start, and Rate (when and how much).

It also has two rev limiters, nitrous retard, and cranking retard. And it's a high energy inductive ignition.

[David Redszus]

Thanks guys!

The timing controllers some of you are talking about are more sophisticated than I will use. I still appreciate hearing about them, though. I purchased a used MSD 6BTM. It is an older one that was made in the USA so hopefully a good unit.

It sounds like 1 degree per psi is good general area to be.

While I won't have an intercooler, the carb will cool the charge some. I am not ruling out Methanol/Water injection, but I don't want an intercooler.

I will try to make sure the blower has access to cool air at it's inlet. I will likely cut a hole in my fenderwell just in front of the shock tower.

I also may go slightly lower in static compression ratio. I was originally thinking around 9:1. I could go a little less, maybe 8.75:1 or 8.5:1. It's a small loss in power and worth it if it keeps the engine in one piece.

paul

Please remember that an engine ONLY responds to DCR, not SCR.

Actually, they only respond to actual temps and pressures, not ratios.

Agreed, but wouldn't you say that all those are related?

A higher SCR will have a higher DCR for a given intake valve closing point? A higher actual pressure will produce a higher actual temp?

And If I lower the SCR, my DCR will be lower too, right?

Yes, your assumptions regarding compression ratio and temperature/pressure trends is correct.
What is critically missing is the correct relationship between SCR and DCR, and the actual temps and pressures produced at various boost levels.

The posted graph has limited value unless the ratios can be converted to actual temps and pressures.

Years ago, Bosch indicated the heat range of their spark plugs by a three digit number, corresponding to the number of second that a given plug could be run at full load in their test engine. What that means is that, in addition to inlet air temp, boost and load, a time under load factor must be considered.

Considering the rapid changes in inlet pressure resulting from changes in turbine speed and throttle, one degree per pound of boost seems extremely simplistic and irrelevant.

But then again, even a stopped clock is correct twice a day.

[Orr89rocz]

Considering the rapid changes in inlet pressure resulting from changes in turbine speed and throttle, one degree per pound of boost seems extremely simplistic and irrelevant.

But then again, even a stopped clock is correct twice a day.

Well if boost control is good, in a drag race application, boost pressure is held somewhat constant after initial spool up, Intake air temperature slowly rises but will never reach steady state temps, as heat loss thru piping occurs for a few seconds. If it was intercooled then it takes alot longer to heat up as the cooler is doing its job absorbing heat. Given the lower rise in overall temp, 1 deg per lb has been a very good guideline for most drag short run time applications. On less than optimal gas...on good boost fuel like c16, methanol or e85, you may not need to pull timing until 5-10 psi boost depending on combo and then its maybe 0.5 deg per psi. I know on my bbc i dynod it wants no timing out til 10 psi

I dont know anything about ford FE stuff but typically if you know where max hp timing is for the given rpm range and naturally aspirated, 1 per lb is a good baseline to get a timing map in to start tuning

[F-BIRD'88]

The chart accounts for the carb fuel cooling etc
and realistic on road air inlet temps range. (real world in car)
The amount if boost retard has to be found by testing.
You want to sneek up on the tune..
Boost jetting timing.
91 octane is not the same as 93 octane gas..
good fuel is cheaper than more pistons.

[David Redszus]

The original topic appeared so easy to answer, but the more I examine actual data, the more I see a complex situation.

Following is actual data from a modern turbo four stroke drag race.

Throttle opens: 45:392sec
Boost pressure (psi): 3.32
Back pressure (psi): 4.37
Spark advance (deg): 35.5

Time: 45.997sec (elapsed time 0.625 sec)
Boost pressure (psi): 28.05
Back pressure (psi): 51.76
Spark advance (deg): 31.0

In a little more than one half second, boost increased from 3.32 to 28.05psi, or by 24.73psi (over 800%).
In the same time period, exhaust gas back pressure increased from 4.37 to 51.76psi, or by 47.39psi (over 1180%).
The exhaust gas back pressure now dramatically exceed boost pressure. Which way is mixture moving?

During this time, spark advanced was reduced from 35.5 deg to 31.0 deg.
That is not much ignition retardation in view of the boost increase, and the excessive back pressure.

By all reasoning, the heads should have visited the grandstand. And this all occurred in a mere 0.625 secs.

Why did the engine not hand grenade itself? What factors are missing?
Fuel quality, inlet air temperature, mixture ratio and preparation, chamber design, engine rpm, are all factors
needed to determine ignition timing, much less to postulate regarding boost vs timing relationships.

Inlet air temperature in the port, is not easily measured due to the inaccuracy and delayed response
of all thermocouples.

Neither boost nor back pressure are accurately measured due to extremely slow data sample rates
which mask numerous data peaks and valleys (and there were several).

If one is serious abut the matter, get a very good data logger and a proper knock sensor for your engine.

[Orr89rocz]

The exhaust gas back pressure now dramatically exceed boost pressure. Which way is mixture moving?

Still going from intake to exhaust like normal. High backpressure doesnt necessarily mean a real bad thing and it certainly doesnt mean a lot of reversion is happening in every case.

[Orr89rocz]

The chart accounts for the carb fuel cooling etc
and realistic on road air inlet temps range. (real world in car)
The amount if boost retard has to be found by testing.
You want to sneek up on the tune..
Boost jetting timing.
91 octane is not the same as 93 octane gas..
good fuel is cheaper than more pistons.

I would be curious to know how much difference there is in 91 vs 93 octane

I knew a bbc blow thru guy in the mid 9:1’s compression run 15+ psi on pump gas 93.

I myself have run 24 psi on intercooled efi sbc with 130-150 deg intake air temps on regular 93 oct gas, no adders. 9:1 comp
I have run 12-13 psi on 10:1 comp big block chevy on 93 as well. Guys have been pushing 93 pretty far. 91 cant be that far off but even then if i could only do 15 psi instead of 24, that chart didnt work.

[David Redszus]

The exhaust gas back pressure now dramatically exceed boost pressure. Which way is mixture moving?

Still going from intake to exhaust like normal. High backpressure doesnt necessarily mean a real bad thing and it certainly doesnt mean a lot of reversion is happening in every case.

It certainly does mean exactly that.
The only open issues are when and how frequently does it occur, and how does it affect the combustion process?

High exhaust gas pressures during blowdown are what drive the turbine. But high exhaust gas pressures during
overlap are a real problem and should be avoided. But how would we know when it occurs?

The topic focus was regarding the efficacy of ignition retardation as a function of boost pressure. Surely we
would not consider retarding ignition timing by 24 degs due to an increase in boost of 24 psi. There are other
critical factors involved in the mapping of ignition timing.

[Orr89rocz]

I agree. High exhaust gas pressure should be avoided but there are plenty of setups with higher backpressure ratios running just fine and making real good power. Most single turbo combos seem to get up into higher ratios when guys push the turbo to the max. It doesnt seem to be as detrimental but then again im talking less than 3:1 Ratio. 2:1 is quite common for a v8 turbo deal. More racey setups that arent class limited on turbo size and with aids to spool may be much close to 1:1-1.5:1. Maybe even less. Certainly more turbine to reduce pressure can help, ive seen higher hp gains there and some cases engine may take more timing or it may want same or less. Might be some reversion effect, might be pumping losses, might be better exhaust scavenging helping fill cyl on intake stroke. Or all those variables. Cyl pressure at evo is still higher than exhaust back pressure. So it does evacuate the cylinder. How much and how well will vary on whole system design and cam specs obviously. But for the sake of the original question its not as relevant i think...

Original Topic isnt all that complicated imo. Dyno and cylinder combustion pressure analysis will tell you what is optimal. But to begin the test, what timing values would you “guess” at to begin the pull? You gotta enter something into the distributor or efi timing table. Thats all the rule of 1 deg per psi has been used for. And it tends to be conservative on sbc/bbc in my setups. For instance 24 psi sbc made best power at 22 deg timing. 34-36 is typically sbc best timing Na so thats only 0.5 deg per psi. On e85 my bbc doesnt want anything pulled til like 10 psi. It dynod best power at 15 psi at like 29-30 deg. Thats 4-5 deg out from best na timing. So 1 deg per lb is good for a few psi like say 7 psi to start making pulls and then find optimal timing on a low boost value, then you can rescale your map maybe 1 deg per lb from the new found value or try .5-.75 deg per lb.

If there is a better way to estimate a beginning timing map, i’m all ears. It would save on dyno time or track passes

[F-BIRD'88]

This is neither a "I know a guy". or a with intercooler chart.. Do you have an example of a car tuned within this chart and 93 octane ,where the combo failed?

A 454 with flat top forged pistons and open chamber heads is under 8:1... A 496. same is less than 8.7:1.