WHAT IS THE BEST COMPRESSION FOR BOOST? (8.5:1 VS 10:1 540 BBC STROKER TEST)

HOW MUCH POWER DO WE GAIN WITH HIGHER COMPRESSION? WE HAVE ALL BEEN TOLD THAT WE NEED LOW COMPRESSION WHEN BUILDING POWER ADDER (SUPERCHARGED, NITROUS AND TURBOCHARGED) MOTORS, BUT IS THAT STILL TRUE? IF YOU LOOK AT THE VARIOUS PRODUCTION TURBO AND SUPERCHARGED MOTORS, ALL OF THEM HAVE LOW COMPRESSION, BUT WHAT HAPPENS IF WE RUN HIGH COMPRESSION WITH BOOST? CHECK OUT THIS VIDEO COMPARING A PAIR OF 540 BBC STROKERS (8.5:1 AND 10.0:1) TO SEE HOW THEY DIFFER IN POWER. GOOD STUFF!

[Greaseland]

I thought we were going see what the blower on the 10 to 1 motor did.

[deanmoser]

Richard I think you're videos are fantastic my only issue is you rarely use pump fuel i understand the limitations and possible engine damage when testing but I think its what a large percentage of viewers run. I would love to see more of it. The power you get on hi octane would be unobtainable for a lot of us.

[rotorr]

The issue we had back in the day with high static CR's was timing, the quality of the head gaskets and the carb technology. Today, you can pull timing out of the motor exactly where you want it, tunable closed loop MPI with knock sensors and MLS head gaskets, so yes, I feel you can run higher static ratio's and pick up torque down in the lower RPM band, especially with turbo's.

[DerrangedGadgeteer]

I don't think this comparison is terribly helpful. The question I think we're all wanting answered is: if you're OCTANE LIMITED, say building a pump gas motor, which is a better approach, low-compression/high-boost, or higher-compression/low-boost.

[mjcmustang]

The only reason to run lower compression in my mind is if you don't have access to quality high octane fuels. Otherwise, more compression, the better, assuming you can time it/tune it correctly

[RecklessEnterainment]

Before even watching it I know the high compression one will make more power with boost because boost just enhances the NA curve of that engine. The better NA the better with boost.

[bdugle]

You said it, the roots application is probably best off with the 8.5:1 long block. By my rough calculation, the 11 psi boost would have given you well over 1100 hp if not for the power needed to spin the blower, about 195 hp if the formula held true. That’s basically lots of heat added to the intake charge. If I had a couple GT-45s making that same 11 psi, I’d want the 10:1 or even more, EFI, and E85, too! That 540 would be a beast either way, I think.

[joshstovall]

I like stock compression + ring gap + boost because well, pistons are expensive and junkyard engines are cheap!

[chrisgoodnow]

Man, this is a timely video. I am looking at these exact motors and wondering the same things discussed here. Thanks Richard for the comparisons.

[EricErnst]

If you have race gas, high compression and high boost is best. Lol

[oldwheels]

The noise you made for each engine at the beginning made my day! Well done sir! 👏

[Faolan]

If one wants to run high rpm and 30-40 psi boost, then 9:1 would be as low as I would ever go... Tuning becomes critical with higher compression, but will always make more power, and efficiency on daily drivers... The way the compression is made is also important, like having a dome piston with it down in the hole 60 thou would be asking for detonation and/or pre-ignition. Proper design combustion chamber is imperative for the proper platform.

[evanc]

Imo for street usage lower CR sounds nice to run regular pump gas. Go on the track and pump up the boost with better gas.

[Cliff_Anderson]

For running on pump gas, I think it makes a difference how much boost you can run without running into detonation. The dynamic compression ratio based on cam specs is a big factor, too. Even though the static compression may be 10:1, the dynamic compression may be a lot lower with a big cam, so lots of variables there, tough to do a true apples to apples. Good stuff to talk about, though!

[johnmortimer]

Higher compression with boost increases the likelihood of detonation. Lower compression gives more wiggle room to withstand that issue. If the tuning is spot on and the fuel has really high octane (91+), then the higher compression will not cause an issue. Lower compression is often recommended for boosted engines because it just allows for more error.

[timothyevans]

Finally some quality content coming across my notifications…

[carllattimore]

I think this is an excellent question. Most people are going to be limited by the fuel octane availability, and I also dont see anyone else providing any data on this subject.

[kkmfcs]

the test that needed to be done would be to boost the both motors to the same cylinder pressure at TDC, the lower compression motor motor is in theory larger then. All compression done in the motor creates heat in the charge, compression done outside the motor with boost and intercoolers, results in more mass flow rate.

[bbkr]

To answer the CR question just look at diesel tractor pulling. They use lower CR pistons so that more boost can be put into the engine. So yes, depending on the extra air to be added, lower static compression can be a benefit.

[azreeal]

When you factor in limited octane, it comes down to the detonation limit of the compressed charge. 8.5 to 1 at 14lb of boost will have 250psi compressed where 10 to 1 will have that same compression pressure at 10lb of boost. the difference is the lower compression engine will have a larger combustion chamber with a larger charge at the same pressure. obviously at the same boost the more efficient higher compression ratio will make more power. But that only works with fuel that will tolerate it. Run the same engine on pump gas boosted to the detonation limit and I think the lower compression combo will come out on top.