Big Turbos: Lag vs Boost threshold

[scheides]

Clip from my evo discussion thread:

What it comes down to is trade-offs. More lag = more power. I LOVE a 16g'd car (dsm or evo). The car is sporty and fun! It makes mere mortals into rally drivers, makes city-scapes a playground of fun loopdy-loops and back roads. Then come the big turbos. Whether it's a simple 20g or a 600++ hp turbo, there will be more 'lag', and there will be more power. I was really wondering really how drivable a gt35r'd car would be in every day driving (I drive 50+ miles per day now). I have to say, I absolutly love it. Yes, it is a comprimise. But the dynamics of that big turbo are such that it is VERY streetable. The car has decent torque down low, as it makes 5-10 psi between 3000-4000rpms, but up top it *really* kicks ass.

Now, everyone talks about Lag vs Boost Threshold! Everyone talks about hating the 'lag' on bigger turbos, but is it really lag they are afraid of? Lag is the transient time between when you step on the gas and the turbo spools up to provide full boost. Whether you're inbetween shifts or cruising and then stomping on it, that is LAG.

Boost Threshold is the RPM at which a turbo (roughly) provides full boost on a given engine (based on its compression ratio, timing advance, and displacement). On a stock motor, stock 14b tends to be in full swing by about 2700rpms, a 16g at 3000-3500rpms, 20G and 50trims at around 4000rpms, and a 61mm turbo such as a SCM61 or GT35R tends to spool at around 4500rpms.

Personally, when I went from a 20G to a GT35R, I was afraid of the boost threshold, and super curious about the amount of LAG the bigger turbo would have. With the twin ball bearing setup, my transient time between shifts was actually LOWER with the gt35r than with the 20G. With the 20G, the car would pause inbetween shifts. With the gt35r, this actually dropped considerably, as the turbo spins more freely.

So, I'm super happy with my gt35r and am not afraid of the lag it has at all. The boost threshold however, is more of a concern, but with the extra power this bigger turbo provides, I am happy to live with. How about you?

[JET]

For a daily driver I like to stick with a moderate turbo. I really loved the 50 trim on a 2.0 and felt the SCM61 had too high of a boost threshold. Some people are fine with it, I am not. The SCM61 on my 2.4 is great and I am curious how a slightly bigger BB turbo would be on it.

For a weekend warrior things move quite a bit. A T67 or equivalent seems the norm and I would be alright with one on a 2.4, especially a BB version. I consider a T67 too big for a street driven 2.0.

Remember, these are just my opinions.

[rst95eclipse]

This is a very good discussion. It's very simple, you want power but don't want to sacrifice where you see it. Turbo selection should be thought out and supporting mods have alot to do with it. Also, the power curves that you want to see also play a role. Lastly and most important, if you're meaning to make power, then you'd consider flow numbers. Power isn't made with boost, it's made with how much air you're flowing. I'm excited to see what my BB will do in the future. Scheides - I'm really curious as to what your bore is and when you see full boost in your RPM range.

[Goat Blower]

They've had huge advances in turbo design and sizing in the last 7-8 years. And the now common big blocks, the 2.3 and 2.4's also make a big difference. My 3575 2.4 spools only a bit slower than my old 20G 2.0 setup. Good turbo design and extra displacement make a world of difference over the old setups. I wouldn't want a big turbo on a little 2.0, but the extra .4 displacement adds ~50 whp, ~70 ft/lbs of torque and spools a big turbo 400 rpm faster.

[carltalon]

I love My scm 61 on my 2.0. when starting from a stop the lag is almost unnoticable. it a little lagy when I punch it frum a roll. But if you know your car and what rpm it spools it comes on pretty fast. Haveing a good tune and the ability to rev past the stock rev limiter helps alot also. when it comes to daly driving I almost like the fact that I can shift at 3500 rpm's and stay out of the boost, works great in the rain etc. I dont think I would go mutch bigger though IM thinking I'll go with a true GT35r when the 61 bites the dust.

[YiNYaNg]

I love My scm 61 on my 2.0. when starting from a stop the lag is almost unnoticable. it a little lagy when I punch it frum a roll. But if you know your car and what rpm it spools it comes on pretty fast. Haveing a good tune and the ability to rev past the stock rev limiter helps alot also. when it comes to daly driving I almost like the fact that I can shift at 3500 rpm's and stay out of the boost, works great in the rain etc. I dont think I would go mutch bigger though IM thinking I'll go with a true GT35r when the 61 bites the dust.

Well said.:D

I like my powerband mid to high end too and boost lag really helps for a DD since it's not boosting so that's fuel saved. When power is needed it's just a downshift away. Isn't turbo lag aka traction control to some guys too.

[1ViciousGSX]

My 2.4L would hit boost very quickly and hard on my old GT3076/7cm. I could hear it spinning up while driving at anything above idle. If I remember correctly I hit 29psi around 3300-3500rpm with it. Fun car, but,........