This is a "theoretical" question, but I have a real world problem related to it.

Here's the question......

Let's say you have a N/A engine and you find that it makes best power at, say, 34* total advance.

Now, take that same engine, identical in every way, and put 6 PSI of boost into the intake (of course matching it with an appropriate amount of fuel to hold the same identical A/F ratio as in the N/A application). Assuming no possibility of detonation, will it still make best power with the same 34* of advance?

I'm guessing this would probably deal with a question about whether the speed of the flame front in the combustion chamber would be any faster in this boosted scenario than the N/A one.

 

No, it won't need the same advance. I don't know why, though I could speculate. In fact, I was just thinking about this the other day. I have a fair amount of experience with small block Chevys and Fords with blowers. Neither seem to need much advance at all. Somehwere in the 24-26 degree range.

Rich

 

Rich, you just hit the "real world" part of my situation on the head. 26* seems to be the limit before detonation on my combo (383 SBC with AFR aluminum heads). I was thinking that 8.7:1 with a roots blower @ 6 PSI was too high to allow for maximum power- producing spark advance (I hit detonation if I go 28* or above). I guess not. I was wondering if lowering the compression ratio would allow more spark advance and, therefore, more overall power. Sounds like the answer would be 'no' to that question.

 

Many of the dyno runs were with race gas. We have a huge number of runs on my buddys Mustang which is a low compression/high boost setup. We run C16 (117 motor octane) and it makes max hp with ~26 degrees. I am just guessing, but wonder if all the tubulence created when the cylinder fills so rapidly under boost decreases the need for ignition advance by promoting rapid combustion.

Rich

 

Or maybe it's because the flame front can travel away from the plug more quickly throught the very densely packed air/fuel mix that exists in a boosted cylinder? All the molecules mashed up real tight against eachother.

 

No, I know that's not the reason. It's harder to ignite/burn a mixture under high pressure. That's why blower cars need a higher voltage ignition system.

Rich

 

Roots blowers are also notorious for adding heat, as opposed to centrifugal or turbos. As you raise the air temp, your octane requirements go up. Also, by forcing more air into the cylinder, you raise the compression, so your octane and timing requirements change.
You see the same thing with nitrous, you have to pull the timing back as the levels go up.

I think of it kinda like a diesel. The more compression you have, the easier it is to light off early and cause preignition or detonation. So you pull the timing back closer to TDC to alleviate it.

 

If it's running anything like a diesel, it's blowing up! A diesel is a compression ignition engine, it is SUPPOSED to "detonate". A spark ignition motor is not. Higher combustion chamber pressure make it MORE difficult to ignite the air-fuel ratio. However, as Damon suggested, a denser mixture does burn faster. I was wrong when I responded to that post - not sure what I was thinking.

Rich

 

OK, cool. So I guess it could be said that a boost retard system doesn't just exist to stay out of detonation, but becuase the more air and fuel you pack in there the less spark advance it needs for best power.

 

this may be a little off the topic but i thought it was intresting


Fuel Octane vs. HP

03/13/98

In turbocharged engines there is a fine balancing act when it comes to making a lot of power on low octane fuel. In most cases, ignition timing must be retarded as the boost pressure rises above a critical point and finally there reaches a further point where the engine simply loses power. If the timing was not retarded with increasing boost, destructive preignition or detonation would occur. Normal combustion is characterized by smooth, even burning of the fuel/air mixture. Detonation is characterized by rapid, uncontrolled temperature and pressure rises more closely akin to an explosion. It's effects are similar to taking a hammer to the top of your pistons.

Most engines make maximum power when peak cylinder pressures are obtained with the crankshaft around 15 degrees after TDC. Experimentation with increasing boost and decreasing timing basically alters where and how much force is produced on the crankshaft. Severely retarded timing causes high exhaust gas temperatures which can lead to preignition and exhaust valve and turbo damage.

We have a hypothetical engine. It's a 2.0L, 4 valve per cylinder, 4 cylinder type with a 9.0 to 1 compression ratio and it's turbocharged. On the dyno, the motor puts out 200hp at 4psi boost with the timing at the stock setting of 35 degrees on 92 octane pump gas with an air/fuel ratio of 14 to 1. We retard the timing to 30 degrees and can now run 7psi and make 225hp before detonation occurs. Now we richen the mixture to 12 to 1 AFR and find we can get 8psi and 235 hp before detonation occurs. The last thing we can consider is to lower the compression ratio to 7 to1. Back on the dyno, we can now run 10psi with 33 degrees of timing with an AFR of 12 to 1 and we get 270 hp on the best pull.

We decide to do a test with our 9 to 1 compression ratio using some 118 octane leaded race gas. The best pull is 490 hp with 35 degrees of timing at 21 psi. On the 7 to 1 engine, we manage 560 hp with 35 degrees of timing at 25psi. To get totally stupid, we fit some larger injectors and remap the EFI system for126 octane methanol. At 30psi we get 700hp with 35 degrees of timing!

While all of these figures are hypothetical, they are very representative of the gains to be had using high octane fuel. Simply by changing fuel we took the 7 to 1 engine from 270 to 700 hp.

From all of the changes made, we can deduce the effect certain changes on hp;

Retarding the ignition timing allows slightly more boost to be run and gain of 12.5%.

Richening the mixture allows slightly more boost to be run for a small hp gain however, past about 11.5 to 1 AFR most engines will start to lose power and even encounter rich misfire.

Lowering the compression ratio allows more boost to be run with less retard for a substantial hp gain.

Increasing the octane rating of the fuel has a massive effect on maximum obtainable hp.

We have seen that there are limits on what can be done running pump gas on an engine with a relatively high compression ratio. High compression engines are therefore poor candidates for high boost pressures on pump fuel. On high octane fuels, the compression ratio becomes relatively unimportant. Ultimate hp levels on high octane fuel are mainly determined by the physical strength of the engine. This was clearly demonstrated in the turbo Formula 1 era of a decade ago where 1.5L engines were producing up to 1100 hp at 60psi on a witches brew of aromatics. Most fully prepared street engines of this displacement would have trouble producing half of this power for a short time, even with many racing parts fitted.

Most factory turbocharged engines rely on a mix of relatively low compression ratios, mild boost and a dose of ignition retard under boost to avoid detonation. Power outputs on these engines are not stellar but these motors can usually be seriously thrashed without damage. Trying to exceed the factory outputs by any appreciable margins without higher octane fuel usually results in some type of engine failure. Remember, the factory spent many millions engineering a reasonable compromise in power, emissions, fuel economy and reliability for the readily available pump fuel. Despite what many people think, they probably don't know as much about this topic as the engineers do.

One last method of increasing power on turbo engines running on low octane fuel is water injection. This method was evaluated scientifically by H. Ricardo in the 1930s on a dyno and showed considerable promise. He was able to double power output on the same fuel with the aid of water injection.

First widespread use of water injection was in WW2 on supercharged and turbocharged aircraft engines for takeoff and emergency power increases. The water was usually mixed with 50% methanol and enough was on hand for 10-20 minutes use. Water/methanol injection was widely used on the mighty turbocompound engines of the '50s and '60s before the advent of the jet engine. In the automotive world, it was used in the '70s and '80s when turbos suddenly became cool again and where EFI and computer controlled ignitions were still a bit crude. Some Formula 1 teams experimented with water injection for qualifying with success until banned.

My personal experience with water injection is considerable. I had several turbo cars fitted with it. One 2.2 liter Celica with a Rajay turbo, Weber carb and no intercooler or internal engine mods ran 13.3 at 103 on street rubber on pump gas back in 1987. This was accomplished at 15psi. With the water injection switched off, I could only run about 5 psi before the engine started to ping. I think you might see water injection controlled by microchips, catch on again in the coming years on aftermarket street turbo installations. It works.