It occured to me a while back that there is no limit to how much power can be made with pump fuel. You simply keep lowering the compression ratio and adding more boost. Each little hit you take in off boost power comes with a huge gain in on boost power. Here are some rough calculations I did for a typical small block turbocharged intercooled street engine at various static compression ratios. I attempted to see at what boost the auto ignition limit would be for each engine and then what the power would be at that boost. I used 4% loss in power for each compression point drop. They are likely grossly inaccurate but demonstrate the point.

10:1 432Hp N/A 638Hp@7psi
9:1 414Hp N/A 780Hp@13psi
8:1 396Hp N/A 962Hp@21psi
7:1 378Hp N/A 1201Hp@32psi

My question is with so many trying to extract the most power from pump fuel why do we never see below 8:1? You see a few import cars but I can find very few examples. Your thoughts please.

 

It may just be the challenge that cooling the intake charge enough takes to run 32 PSI on the street. Much less a turbo that can push that without dropping a decent amount of coin.

I have an 8:1 408, we will see what it makes with just 10lbs. It made 400 RWHP N/A. If I had a good enough block I would test your theory of 21 PSI.

 

For me I spend 99% of the time out of boost. I would think driving around a 7:1 motor would be sluggish, get bad mileage and be slow to spool a turbo capable of over 1200hp. I could be totally wrong on this though. I know that desktop dynos are not exact, but my 9:1 motor would loose 40 ft/lbs of torque at peak dropping to 7:1. I can't see building a very low cr motor for the street or building a pump gas motor for the strip. The only reason I can see to build an all out pump gas motor would be for pump gas drags, and I don't think they build motors with that low of CR. I would rather just add race gas, timing and more boost if I want to go fast at the track. What would be the purpose of such motor? Why not make E85 your pump gas?

 

cuz at 8:1 who needs more then 962hp on the street?

 

And at 9:1 you can roast the tires on the interstate on street tires and have better street manners.

 

We recently ran a "Pump Gas engine on Steve Morris dyno at 9 to 1 Compression ratio.

I think your calculations are pretty close. You show 9:1 414Hp N/A 780Hp@13psi

We made 436 hp NA and made 861 hp at 13 psi boost (inter-cooled) Blow-Thru carb.

Your multiplier for 13 psi boost was 1.88 Our multiplier was 1.97

8 to 1 your multiplier is 2.42 so if we used 2.52 for our engine 1098 hp at 21 psi. Similar
engines at the same boost level have made the same HP as calculated but on race fuel.

A Merlin aircraft engine had a 6 to 1 compression ratio and made over 2030 hp @ 18 psi in a 130/131 version of the engine. No, I would not be afraid of lower compression ratio, but
would you rather have quick transient response or "eye sucking" HP?

http://en.wikipedia.org/wiki/Rolls-Royce_Merlin

Tom Vaught

 

My nissan engine came factory turbocharged with 7.4:1 compression in the 280zx, with a normal felpro head gasket like most people run this falls .1 or.2. Most people report upper 20's for highway mpg while making ~400rwhp in a light car with horrible aerodynamics(the Cd is about .46 for the 240/260/280z).


If the turbine is sized so you never drop out of boost on a shift, waiting the extra few hundred rpm(if that) at the beginning of the run is worth the increase in power. If I had a much bigger engine, it probably wouldn't be necessary, you can only use so much power on the street. However, the wait for 500rpm isn't a hell of a lot of time in a 400rwhp N/A car, 8:1 compression or not.

 

I had both a 1987 Chrysler Conquest with 7.6:1 and a 1992 Eclipse with 7.8:1. Both were dogs out of boost with lazy response. But in boost, both felt very good and it hit really hard, enough so to break the tires loose. The Eclipse got a best of 33mpg at about 250 FTWHP with 17PSI. But I can only imagine during highway speeds, there was increased volumetric efficiency due to the turbo spining just enough to help fill the cylinders.

That same engine in the Eclipse in N/A form produced 135hp with virtually no torque and the same MPG.

 

This could be another reason. Maybe space constraints only allow an undersized intercooler or even no intercooler and a lower compression ratio is needed to compensate. There is no reason why you couldn't run 40psi with no intercooler on 87 octane althought whatever the safe compression for that would be rediculous.

 

Modern turbocharger designs can do PRs of 3.0 absolute no problem these days. While there is additional heat from this you can get into the "sweet spot" of a lot of units if they are matched correctly at this sort of pressure ratio

 

We looked at a Bonneville salt flats engine one time that needed a 4+ pressure ratio to make the needed HP at altitude. There are turbos out there that can do that too. That is 45 psi gage.

Tom Vaught

 

Where are the actual trade offs? It seems like such a small thing to give up 1 point in compression to achieve so much hp. With out changing timing I'm not sure I really understand why the increase is so dramatic for such a small reduction in compression. Could someone explain please?

Sean

 

Auto ignition of fuel is almost completely dependant on temperature and temperature rise from the engine's compression far exceeds the temperature rise from a supercharger. If the engine has a compression ratio of 10 then the pressure ratio from the engine's compression is basically 10:1. You can see how going from an engine compression pressure ratio of 10 to 8 is huge and allows gobs of "pre compression" from a supercharger especially if intercooled.

A compression ratio of 11:1 brings the temperature of the air almost to the auto ignition point all on its own because an 11:1 pressure ratio is a lot.

The trade off really ends up being a little off boost performance (torque and efficiency) for a huge amount of on boost torque.

 

Basically preignition temperature. Thanks.

 

The other advantage of a lower compression ratio is that the combustion chamber is larger therefore once the sparkplug ignites the mixture, the is more dense mixture to burn during the expansion cycle. Think "M-80" firecracker vs "lady Finger" firecracker.

Tom Vaught

 

^^
Nice analogy, that brought it home for me. I get it now.

 

So, if I have a 10:1 compression motor (Not counting blow by and cam bleed off) wouldn't this mean that I could only achieve 147psia (14.7psi standard day X pressure ratio 10)? Then subtract the 14.7psi gauge and we're at about 132psig on a static compression tester. Maybe I'm reading that the wrong way? Your statement really makes a ton of sense and helps me understand how much lowering compression helps combat auto ignition but when I do the basic math (that pops into my mind) it tells me that a 10:1 motor is incapable of showing over 140psi with a compression tester. Am I way off with my thinking? I've gotta be off somewhere because i've seen 9:1 motors read 200psi on a comp gauge.

 

Everything your saying makes sense to me except for getting 200psi compression from a 9:1 motor.

 

I could see that much cranking compression if the cam was made that way.

 

No lie my friend. My old suzi samurai was 8.8 to 1 and put out 190's. Buddy just built a 9.xx 460 with a wide lsa lunati and he told me he's up in the high 190's