944Willie

What is the method of comparing low compression high boost engines ( like ours) with the new cars that have high compression and low boost? Which puts the most "load" if you will on the engine? That is the advantage/ disadvantage of each? Many of the new cars have 4-7 PSI boost but also have 10.5 and higher compression ratios.
Thanks for the insight.

Willard Bridgham 3

The advantage of a turbocharger is that compression ratio rises only when you need it; more economical of fuel and engine parts.

High compression engines w/0 turbos use more fuel and wear out quicker.

Kesepton

but dosent higher boost put alot of strain on the parts? even if it were for short bursts of time?

toddk911

The advantage to high compression/low boost is you have more low end torque and driveability, i.e. you don't have to get on the boost as much for accelleration.

macnewma

And one disadvantage to high comp/low boost is a limit on the power levels when on boost.

I think that newer turbos like use lower boost because people have complained for years about turbo lag and engine power sub 3k rpms on turbo cars. They want a more tractable car for traffic and the average customers.

BTW, what cars are you referring to exactly? I don't keep on alot of newer cars.

Max

daniel951

i am using higher compression pistons unlike the lower compression of a 951 piston. I made more power at lower boost. I made 351rwhp 361tq at around 16-17psi. These were with messed up A/f which will be fixed and i can gain some extra horsepower at those levels

hosrom_951

Daniel: Yeah, i remember you posted your dyno results, you are using the NA pistons or was that Danno?

TRACKIN951

Daniel951: What compression is it that you are running? Also how much earlier are you reaching full boost because of these pistons?

pk951

I would think that running higher CR you would have a tendency to detonate much easyer if proper octane is not used.

will951

I am not sure what car he is talking about....but I know the VW 1.8T motor runs high compression.....I think over 9.5:1....not positive though. Isn't that what the stock 8V NA runs for compression?

hosrom_951

944 NA models:

Euro models (asuming ROW as well) CR: 10.6:1

USA Models CR: 9.5:1

944Willie

I think stock CR for the 951 was about 8.8. The 2003-2005 cars with turbos are running above 10 some above 11:1. I see the Supras running in the 11:1 range and the Hondas are runnig in the 11:1 range. The new 997 turbo is in the 11.5:1 range I was told.
I could see where a high compression- low boost would be good for low end tq. and would decrease lag. The current trend seems to be higher CR and low boost on new factory cars so I figured there must be a reason.
A friend is finnishing the process of turboing a 2004 Z350. First cut is showing about 425HP at the rear wheels. Tq readings were not available. I am not sure what the stock CR is for that car. it is the sport model with the big Brembo brakes and it is very impressive. He is using 4 PSI boost at the moment and working his way up- keping safe AF ratios, inj cycle time, EGT, ETC. His car is very easy to drive in city and on the track. VERY little lag time noticed. Yes you can still feel the build of the boost though.

hosrom_951

944Willie: The stock CR for the 944 Turbo models is 8.0:1

as for turbocharging a Z350, SFR have a turbocharger kit running at low boost as well (website is down at the moment)

Dark Lightning

I feel the need to rant.
I have NEVER heard of a turbo Supra running a compression ratio higher than 9.5:1. Hondas, yes, but the high (11+) compression ratios are usually in normally aspirated motors. The new 911T has the highest factory static compression ratio for a turbocharged car I've ever seen.

7.8:1 - 1990 Eagle Talon Turbo
8.0:1 - 1988 944 Turbo
8.1:1 - 2004 Dodge Neon SRT-4 (turbo)
8.5:1 - 1995 Eagle Talon Turbo
8.8:1 - 2004 Mitsubishi Lancer Evolution 8 (turbo)
9.4:1 - 2004 911 Turbo

Most other new factory turbocharged / supercharged platforms use somewhere between 8.0:1 and 9.0:1.

I agree but there's no such thing as a free lunch.

Consider this. Higher compression ratios increase the effect of boost. More static compression requires less boost to achieve the same horsepower, but it becomes very difficult to control the boost, maintain proper fuel ratios, deal with increases in knock and other issues. This requires more fuel, makes fuel emissions harder to reach, and reduces overall reliability, things to be generally avoided by manufacturers.

But yes, there is more off-boost power. The car will have more low end power and all that stuff, but the vast majority of aftermarket high performance turbo engine builders stay between 8:1 and 9:1 for ease of tuning.

Again, many of the new factory normally aspirated cars have 10+ CR because that's one of the very few ways they can go to make more power. Talk to the 944 N/A guys, what options do they have? Higher compression, bigger cams, or more displacement. The reason compression ratios are slowly creeping up is because auto manufacturers are taking advantage of more sophisticated fuel injection systems (multi-port injection, direct-port injection, etc.) and ignition systems; as a result the consumer has enjoyed more powerful engines from the same displacement.

Here we have a normally aspirated motor with an unknown sized turbo bolted on. Unfortunately the 350Z doesn't have a stout bottom end from the factory like the 300Z did, and aftermarket tuners are forced to (more often than not) to either run low boost or completely replace the factory equipment.

Bottom line: Higher compression is good up to a certain point, usually up to around 9.0:1 and after that generally becomes counter-productive to a forced-induction motor with very few exceptions.

macnewma

Dark Lightning-

I was about to chime in with the same comments about stock cars using high compression like a N/A car. I don't know of any doing that myself. I do know of stock cars like the 350Z and E36 M3 adding a turbo to their high compression motors and running very low boost.

A stock 93-98 Supra Turbo has a 8.5:1 compression ratio.

Daniel, are you certain that you have N/A pistons? 9.5:1 is pretty damn high. I would be surprised if you could run 16psi safely. But maybe I am wrong.

Max