Maybe spool isn't improved, but I'd like to see recovery time compared. In-between shifts I'm sure the higher compression motor can get up in rpm off boost quicker vs. a lower compression motor. Off boost it's also making more power, making it more livable to cruise around with a big turbo off boost. Also if it's spooling the same, but making more power doing it you just did what you intended to do but at lower boost. Make more power earlier then make even more power later.

On the T70 I switched from 8.0 to 1 to 9.5 to 1. I went from 681 awhp to 703 awhp with 4 degrees less timing and 1.5 a/f ratio points richer at the same boost level. The only thing I changed was cams from FP2's to BC 280's (not a big jump) and the pistons. More power with a stupid consertive tune, vs. pushing it harder.

I also think bigger turbo cars with low back pressure have more to gain from higher compression than stock turbo and small turbo cars. If your running E85 or straight meth then you can see a gain on any size turbo. An average person with a stock turbo car wanting to run pump gas all of the time wouldn't see much of any gain past 9.0 to 1.

 

So if I used a .043" head gasket with "0" deck height I should be good? Should there be more or less clearance at top dead center for an 88mm stroke vs. a 106mm stroke? More specifically, if I am running a 106mm stroke with 550g rods, 300g pistons and 106g pins would the longer stroke necessitate more clearance at TDC due to the longer stroke?

 

Whens it gonna be done?

 

can't see why higher compression would effect spool as in the "compression stroke" its exactly that and the contents of the cylinder are not released to the turbo until the exhaust stroke where compression is not happening.
yes / no ?

 

More compression means more cylinder pressure, more cylinder pressure means more power, more power means quicker acceleration thru the rpm's, quicker acceleration thru rpm means you arrive at your spool rpm sooner.

 

not sold on your points after the power stroke sorry .anyone else?
is this going OT?

 

If you were asking about mine, I am still waiting on some options for the crank I am going to use to materialise. When I know more about that then I will take everything I have to the machine shop for bore/fit/etc. and start assembly. I am hoping to be done and have it installed by the middle of march. Turbo kit to follow.

This is what I have always thought about the spool, its not the compression helps spool but the all motor power is improved and that is why its snappier. Cylinder pressure is up which is power all around.

Ted, I am sure you have posted it before but why is this increase in efficiency detrimental to spool in some cases?

aaron

 

Yes

No, it makes no difference. The only time you'd purposely put the piston lower in the bore at TDC is if you used an aluminum rod.

 

Greater dynamic pressure in the cylinder increases torque, because torque is a direct reflection of cylinder pressure. The greater the cylinder pressure, the greater the force (torque) pushing the piston downward. Increasing the dynamic pressure can be done by increasing the compression ratio. It can also be done by using different cam timing (although these two techniques work very differently).

When dynamic pressure is increased by way of the static compression ratio, this generally increases torque by ~5% if moving upward of one full point (e.g. from 8:1 to 9:1, or 9:1 to 10:1). The increase in torque is most noticeable as an improvement in off-boost response.

But don't get torque and spool confused, as they are somewhat disconnected.

What spools the turbo is exhaust gas energy. When we increase torque by increasing the compression ratio, we are not increasing the mass or energy of exhaust gas. Actually, we are forcing a greater percentage of energy to be released as pressure (torque), and not heat. Because we are generating more work and wasting less energy as heat into the exhaust, we are making the engine more thermally efficient. There is nothing here to improve spool, although with the engine making a little more torque, it should accelerate a little faster while off boost.

If we want to improve spool, we can't realistically increase the exhaust gas energy by increasing its mass, because that requires more rpm, which requires more torque to get there (chicken and egg conundrum). What we can do is increase exhaust gas energy by raising the temperature, and WRC cars do exactly that with the antilag system. This system effectively burns fuel in the exhaust instead of the cylinder, and while that doesn't increase power in itself, it drives up the heat in the exhaust, which spools the turbo, and that increases power.

The reason why twinscroll turbos work so well for 4-cyl engines is because the TS design keeps more exhaust gas energy in the system, which keeps the turbine spinning faster during the long periods between exhaust pulses (180 deg) in a 4-cyl.

If we reduce the static compression ratio, we reduce thermal efficiency, which increases exhaust gas energy (and should increase spool) at the expense of some off-boost torque.

WRC engines use 98 RON and 10:1 SCR with 2.0L, run 35+ psi boost, use antilag to keep the turbo spooled, and generate some 500+ ft/lbs torque at 3500 rpm with a turbo that peters out around 6000 rpm.

BMW's turbo F1 engine was a stock-block unit that used 8.5:1 SCR on straight toluene, ran upwards of 70 psi boost in qualifying trim, and generated 1300bhp from 1.5L with a large, twinscroll turbo, and topped out around 10,000 rpm.

What this demonstrates is there is no magic compression ratio. It has to be factored into the big picture, like everything else. But like I've reiterated previously, I've seen nothing that indicates increasing static compression improves spool characteristics. If anything, the effect seems to be the opposite. But then again, we are probably speaking in small differences one way or the other.

 

Great post!

-Jalal

 

I just saw one of these engines last week. BMW actually produced several of these engines from used street car blocks...ahh, the good old days.

 

That motor can be seen here:

BMW F1 Turbo

There is a short video of some lucky soul who found one and got it running.

Here are more pics and specs:
More BMW F1 Turbo

Notice the 11,800 rpm redline on the tach in the cockpit, and the exceptionally smart CF intake manifold. Incredible that they built these with used production car blocks.


Anyway, if one is after best possible spool, that hinges largely upon keep exhaust energy managment and exhaust design. That is definitely discussionworthy within itself.

 

The SCR vs "spool" conundrum, to elaborate on Ted B's post, is further muddied by this: increasing SCR reduces the clearance volume in the chamber. This tends to improve VE since there's less dilution of the incoming fresh charge with old, stale unburn-able exhaust.

Higher VE means there's more mass flow for the turbine at a given RPM. This, in isolation, would tend to improve "spool."

Unfortunately, nothing is in isolation in an engine. This bump in VE may or may not be offset by the reduction in EGT that occurs as you increase SCR. Ted B already mentioned this part (e.g. SCR improves thermal efficiency and hence reduces EGT).

 

Note the single gate used on the twin scroll manifold...

 

I always tell people to read threads. You'll learn A LOT!!!

I'm glad I have an iPhone that makes this all possible considering we just lost out cable, home phone, and Internet.

Keep this going!!!

Ted, do you do consulting work for setup design?

-Jalal