Boost with High Compression

gene cooley

gene cooley

Autocrosser
Considering adding boost to the 2200 Ecotec in my 600 Autocrosser.
Before I pull it down to change pistons I was wondering if I could add low boost (5 lbs?) to the current configuration.
Specs as follows:
GM L61/2200cc Ecotec.
Aftermarket rods, cams, pistons, valve springs, keepers.
Approx 14:1 compression.
Megasquirt
110 leaded race gas.
Boost would be from supercharger.

Any thoughts?
 
gene cooley said:
Approx 14:1 compression.
Click to expand...
I seriously doubt you could add even as little as 5lb boost without getting detonation at that CR. Even with severely retarded timing, huge intercooler, ethanol fuel, rich mixture, etc, you will likely suffer damage before long. From everything I've heard (which might be wrong) around 9.5:1 is considered the upper range, with 10 to 10.5:1 pretty much the most you can go without damage from pretty much any level boost. The added charge pressure raises the chamber temps too much. However I'm sure there are race engines that have been designed from scratch to run higher CR's with boost. But it sounds like you aren't talking about that level of build.

Consider at least a much thicker MLS head gasket or even a head spacer to lower the CR. Then you can go with more boost and get MUCH greater results - both in terms of power and longevity. But I don't know how much the CR will drop without some fairly serious changes - like pistons? You can change to shorter rods to lower the CR, but if you replace the pistons instead then not only can you reduce the CR, but also get boost spec items (different design from NA pistons) - that will improve longevity even further.

I've considered this with other engines. Changing the CR isn't easy without replacing internal components.
 
Dr.Jeff said:
I seriously doubt you could add even as little as 5lb boost without getting detonation at that CR. Even with severely retarded timing, huge intercooler, ethanol fuel, rich mixture, etc, you will likely suffer damage before long. From everything I've heard (which might be wrong) around 9.5:1 is considered the upper range, with 10 to 10.5:1 pretty much the most you can go without damage from pretty much any level boost. The added charge pressure raises the chamber temps too much. However I'm sure there are race engines that have been designed from scratch to run higher CR's with boost. But it sounds like you aren't talking about that level of build.

Consider at least a much thicker MLS head gasket or even a head spacer to lower the CR. Then you can go with more boost and get MUCH greater results - both in terms of power and longevity. But I don't know how much the CR will drop without some fairly serious changes - like pistons? You can change to shorter rods to lower the CR, but if you replace the pistons instead then not only can you reduce the CR, but also get boost spec items (different design from NA pistons) - that will improve longevity even further.

I've considered this with other engines. Changing the CR isn't easy without replacing internal components.
Click to expand...

Wiseco has pistons, I am/was hoping that I wouldn't have to pull the engine down.
Some sources say that the limit for 92 octane is "the equivalent of" 12:1 compression (compression ratio + boost) when 92 octane fuel is used. I can't seem to find anything that expands on that for octane levels above 92. We use 110 now.
I also am waiting for a possible rule change that would increase NA engine size without weight penalty, if that doesn't happen I'm back to boost.
Probably better off at the moment to just sit until the rules do/don't change. I will have to make a decision when that happens.
 
I assume you were referring to 14:1 static compression (i.e. no boost)? What's the calculated CR with adding 5 lb boost to 14:1? Then see if there are any engines running that level of CR (NA or boosted) and what they do to enable it.
 
Dr.Jeff said:
I assume you were referring to 14:1 static compression (i.e. no boost)? What's the calculated CR with adding 5 lb boost to 14:1? Then see if there are any engines running that level of CR (NA or boosted) and what they do to enable it.
Click to expand...

Unfortunately, I don't think I have enough info to calculate the compression ratio with boost. Doesn't seem to be a simple calculation.
 
I'm running 24lb boost on my 2.5l T5 (10.5:1), with a fairly small turbo (K16). I'd say it's all about the EMS. These motors crack the liner sleeves at much lower boost than I'm running. I'd say even 10.5:1 compression would be pushing it unless you can really fine tune the timing under load. Not sure how refined MS is in that regard.
 
At the moment the early/updated Megasquirt ECU is not capable of a high degree of control of a boosted (will be supercharged in this case) engine.
If all the "boost" mods that I need to run the supercharger are a $600 set of pistons and a new head gasket, then I'll do that and a ECU upgrade. There are too many mods already to this block to scatter it with an iffy boost/compression setup. Guess I'll stay away from that.
I need about 350hp to be competitive with the 600.
 
Lots more than detonation control and what the engine parts will tolerate. Have a look at Saab-Scania's Tronic engine management system to get some idea of what is needed.. which is FAR more than detonation control. As compression and boost goes up, far greater demands are made on the ignition system.
Trionic 5 - 4SAAB.COM4saab.com › Trionic5

Had a conversation with Dave Coleman (chassis engineer Mazda USA) during one of the 24 Hours of LeMons races years ago. They had to reduce the spark plug gap as the amount of boost they were running was causing the stock spark plug gap was severely mis-sparking. The spark plug gap (hem = 0.050" to 0.055") got down to 0.20"- 0.025" to have reasonable ignition per cylinder at the boost levels they were using at the time. Essentially the stock Miata ignition system did not have enough sparky for a significantly "boosted" Mazda motor.


Bernice
 
Thanks Bernice. We have a 6 hour tow to the SCCA autocross in Bristol, TN later today I should have a couple hours to try and understand at least some of this.
"Hopefully"
 
With ignition maps being so critical for high boost and/or high compression, the MegaSquirt may fall short. I'm not sure about the latest, greatest version of MS, but the ones I'm more familiar with really focus mainly on fuel and do very little with ignition. Yes they "control" the ignition, but not to the level we are talking about here. You are pretty much on your own to tune the ignition maps, so unless you really know what you are doing it could be quite questionable. However maybe they are improving in this regard?

I think you are on the right track here: "If all the "boost" mods that I need to run the supercharger are a $600 set of pistons and a new head gasket, then I'll do that and a ECU upgrade."
 
Right now I am sitting at Bristol
20200904_132115.jpg
20200904_133917.jpg
in the shade beside the creek in a spot saved for us by our competition. It's "run what you brung" at least for the weekend.
 
Broke a drive axle leaving the line on my first run. Loaded up the car, worked the course, went back to the hotel and checked out at 4PM. Hit interstate 81, home about 9:30.
Mod cars are always fun.
Ride back in the Denali made it all better...
 
Rupunzell said:
Picture of the broken axle when possible.


Bernice
Click to expand...

We are pulling it out tomorrow and I'll post some pics. It was a shortened Olds Alero axle from a 6 cylinder car. The shaft is 7 3/4" long, I had to shorten the inner CV housing to keep the "heel" of the strut from hitting it when the wheel went through it's 2" of travel
(1" up, 1" down).
I should have some pictures by Noon tomorrow.
 
Looks like the axle was a weld up job...

Original axle cut to length, then one end of the axle was bored with a hole, the other end was turned down to fit into the other end for alignment. Two ends of the axle matched together and welded at the seam..

As mentioned countless times, welds unless done in a very highly controlled condition with proper post welding heat treatment will have different hardness across the weld, cracks, stress risers, gaps, gas bubbles and numerous other defects waiting to cause a weld failure. This is why welds are the most difficult and complex method of making a joint as there are SO many items that can go so wrong.

The axle failed at the welded seam.. absolutely predictable and why welded axles are a hazard and can be lethal. Image for a moment if that axle were to come apart at 100+ Mph, full throttle (which is the moment they are most likely to fail.

Do this properly, get an axle made with the the proper lengths of the proper material (8640 alloy steel), heat treated to the hardness required (Surface hardness not higher than 50RC with a soft core for impact and load reversal endurance), splines ends with precision fit. The stock snap ring location is sort of ok, but not ideal.

Or

Get another one of these axles that were modified by welding, cut to required length, then sent the axle out to a speciality shop that can cut the proper splines and snap ring groove on the cut off end. We did this on the rotary LeMons racer, never had a problem.
BTW, axles are considered a consumable item on real race cars as they twist, take a set and will fail due to fatigue with race hours. Indicator for this is a painted line on axis with the axle to indicate axle twist and logging of race hours per axle. Once the number of race hours is up, the axle is replaced with a proven (test-certified) good new one.

On a power train with any significant amount of torque, axles are a serious failure point. Keep in mind it is not hp alone that snaps axles, torque and tire grip is what often snaps axles.


Bernice
 
Rupunzell said:
Looks like the axle was a weld up job...

Original axle cut to length, then one end of the axle was bored with a hole, the other end was turned down to fit into the other end for alignment. Two ends of the axle matched together and welded at the seam..

As mentioned countless times, welds unless done in a very highly controlled condition with proper post welding heat treatment will have different hardness across the weld, cracks, stress risers, gaps, gas bubbles and numerous other defects waiting to cause a weld failure. This is why welds are the most difficult and complex method of making a joint as there are SO many items that can go so wrong.

The axle failed at the welded seam.. absolutely predictable and why welded axles are a hazard and can be lethal. Image for a moment if that axle were to come apart at 100+ Mph, full throttle (which is the moment they are most likely to fail.

Do this properly, get an axle made with the the proper lengths of the proper material (8640 alloy steel), heat treated to the hardness required (Surface hardness not higher than 50RC with a soft core for impact and load reversal endurance), splines ends with precision fit. The stock snap ring location is sort of ok, but not ideal.

Or

Get another one of these axles that were modified by welding, cut to required length, then sent the axle out to a speciality shop that can cut the proper splines and snap ring groove on the cut off end. We did this on the rotary LeMons racer, never had a problem.
BTW, axles are considered a consumable item on real race cars as they twist, take a set and will fail due to fatigue with race hours. Indicator for this is a painted line on axis with the axle to indicate axle twist and logging of race hours per axle. Once the number of race hours is up, the axle is replaced with a proven (test-certified) good new one.

On a power train with any significant amount of torque, axles are a serious failure point. Keep in mind it is not hp alone that snaps axles, torque and tire grip is what often snaps axles.


Bernice
Click to expand...

Some background; axle has been run for about 8 years of autocrossing. It broke about 4 years ago leaving the line and car was pushed to trailer. This time it had to be driven slowly back to grid, about a thousand feet. (One wheel driving, tried not to destroy the limited slip.) I don't intend to repair this one. I am going to temporarily replace it with another cut/welded shaft for the remaining 3 or 4 events of the year. I intend to lengthen the stub so it goes further into the other end.
At the moment thae shaft cannot get out of the housing unless I wipe out the rear suspension. Welds are done by professional welders, I didn't do them.
After we supercharge the engine I will replace both shafts with custom parts.
What should it cost to make 2 similar shafts? One is 7 3/4", other is 11".
 
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