gnfast87

Building an LQ4 twin turbo in a Buick GN. I plan on running 2 TE-60 turbos with Methanol injection.

Is it better to run a lower compression (8.5 ish) and crank up the boost or run compression around 9.5 with lower boost??? On the Buick V6s, we usually ran compression pretty low so we could crank up the boost and push methanol injection.

Also, I keep reading that people tear apart these motors and they look great inside. Does this mean that a motor with say 150,000 doesn't have a lot of cylinder wear??? Maybe only needs to be bored .005 instead of going .030??

engineermike

A few years ago, the trend was to go with more compression and less boost. However, if you think about it, you will always make more power with less compression and more boost.

12/1 and 0 psi won't ever be as fast as 8/1 and 20 psi. Every full point of compression is about 5% power, so going from 12 to 8 will drop you 20%. However, adding just 15 psi boost will get you 100% more power. You see where I'm going.

Some will say that higher compression will get you more response and torque off-boost, and they are right. But, are you building an engine to be fast off-boost?

The Alchemist

iTrader: (19)

I agree with what you're saying, and the next question has to be how you intend to drive the car. If it's just a strip car, then you have little concern about part throttle or low rpm drivability, but for a 100% street car, having that part throttle, off-boost response is a nice feature.

Personally, I kept my setup at 10.3:1 CR and am running 15psi of boost and the car is just amazing to drive on the street. Part throttle it drives like a stoudt heads and cam car, but roll into the throttle enough to close the bypass valve, and you get instant boost and it's like a progressive shot of nitrous.

engineermike

Counterpoint. . . many years ago, I had a 8.4/1 360 SBC. I changed direction on the project and converted it to a 11/1 383 with everything else being the same. It was a very large change in compression ratio, but part-throttle response and torque differences were hardly noticeable to me. I like to remind myself from time to time that these are big v8's in mid-weight cars, so the part-throttle response with even low compression will be far better than heavy trucks or 4-6 cylinder "typical" vehicles. I drive an Ecoboost F150 crew cab now, which weighs 6000 lb and is only 3.5 liters and 8.5/1 compression and, to my surprise, even it has decent part-throttle off boost response.

As a side note, I figure it gained about 50 hp from the compression and another 20 or some from the cid. It went from 12.0's at 113 to 11.3's at 121.

As another side note, high compression will always make more power than low compression, as long as you keep it from detonation. So, adding a full point of compression to an 800 hp turbo v8 will get you around 40 hp(!). With a good Methanol system, you can get away with a lot more compression and boost than before, netting you the best of all worlds.

spoolin turbo

If you plan to run the same psi (15psi) on low or high compression then it just comes down to the tune.

Phil99vette

iTrader: (11)

With the parts available today, 9.5:1 would be the lowest I'd go on a pump gas street motor. If you get into E85 or 98 than the sky is the limit. Personally I'd do 10-10.5:1 on 93 with methanol injection.

MUSTANGBRKR02

iTrader: (91)

I say over 10-1 SCR and boost till you feel content. For a street car I really like the throttle response.

With the tuning abilities and safety nets as in E85, methanol etc I really think anything in the 8-1 SCR is a waste of horsepower and potential in a streetcar/drag car.

MUSTANGBRKR02

iTrader: (91)

Damn Phil beat me to it

gnfast87

thanks for the advice guys. We don't have E85 or 98 up here in Vermont.
But I do have a dual alkycontrol system from my Turbo V6. Man, I used to love to crank up the boost and spray methanol.

I think I'm going to shoot for 9.5:1 with my ported 317 heads with 72cc chambers. Then if I upgrade to better heads with smaller chambers I might be able to get close to 10:1

71 chevy

iTrader: (18)

i am not for or against boost vs compression. but I think there is more to it than just a 5% loss per point.


first, everything else being equal, i believe that 1 point of compression is more than 5%. case in point, the lq9 is only .5 more compression than the lq4, but it makes 6% more hp. so i would say that 1 point at the range we are talking about, would be closer to 12%

now, take the 12% increase in power, and double it for 15 psi, and the higher compression motor is making a lot more hp.

lets do an example

stock engine 325 hp, at 15 psi boost = 650 hp
add 1 point of compression at +12 percent
is
364 hp, att 15psi boost = 728 hp

so, 1 point of compression at 15psi is worth about 78hp


now, go back to the stock engine:

it gained 325hp with 15psi or 21.66 hp per psi

to make up the additional 78 hp, it would require an additional 3.6 psi, so now you are looking at running 19psi to equal the higher compression engine.


WHICH increases inlet temps approximately 30 degrees, which means a loss of power so even running 19 vs 15 psi, the lower compression engine is not making as much as the higher compression engine.

so far weve compared same cam scenario.


8.5:1 compression is limited on how much cam you can put it it before it becomes a dog at low boost and low rpm. you could kill the dynamic compression ratio by putting a big cam in it. so it might make good peak power, but be dead slow on the transbrake, or off idle.

with ,say,10.5:1 compression, the cam that you can run will be much bigger to keep the same dynamic compression, which means more power.

put a 234/234 cam in an 8.5:1 engine and you just killed all low end and low rpm response. the engine probably wont recover even at high rpm.

put that same cam in at 10.5:1 and it still has good dcr and would make more power everywhere. would be fun to drive, it wont kill the spool time, and it would just be overall more efficient.

this is assuming no detonation.

NicD

iTrader: (7)

Less compression / more boost, especially on pump gas.

I was also pretty sure it was well understood that every point in compression was about 4% in power production at 1 atmosphere, generally of course.

I don't understand where people come up with the idea that a motor at 8.5:1 compression is a dog compared to one at 9.5:1 and wouldn't even recover at high RPM or spool on a transbrake. I personally would shoot for 9:1 or 9.5:1 compression with a turbo setup now a days for a pump gas setup w/methanol.

This example is ignoring the timing aspect of the whole thing. Being octane limited changes things and you are almost always going to be octane limited in some aspect. A lower compression motor is going to be less so.

allout06

iTrader: (11)

AES is building my 390, they didnt want me going over 9.5:1 compression. I told them i want 1000rwhp, on a twin turbo setup, 100% street car running a methanol kit. he said hed like to get me in the 9.4:1 range. We have specified a cam yet because he has to flow some heads for me and match me up a custom turbo cam.

71 chevy

iTrader: (18)

hearsay is good, but real numbers are better. the lq9 has a 6% power advantage over the lq4 and only .5 difference in compression.


it is easy to get a motor to be a dog. not rocket surgery. go out and put your total timing to 5 degrees btdc and report back on how it ran. same thing with dcr. you put too big of a cam on an engine and it wont build enough compression and will be a turd.

also, it is easy to see how less power means the car is slower on the transbrake. EVEN if they make the same max power, the low compression engine is going to make less power and be at a lower stall rpm on the brake. so, it starts from a lower level and builds boost slower, making it take significantly longer to get to launch boost

71 chevy

iTrader: (18)

you also should remember that these cars make a whole lot of power under boost, so the converter you have to use needs to be very tight. a converter thats too tight, and with too low a stall rpm, can make ANY engine feel slow

NemeSS

iTrader: (127)

I have always wondered what my old 11:1 6.0 trex cam motor would pump out with 15psi.
Seriously thinkin of recreating it as such. That old combo use to be strong as hell and ripped thru 7000rpm like nothing. I can only imagine pumpin 15psi with meth or e85

Phil99vette

iTrader: (11)

I'd be ok with 9.5-9.75:1, truck manifolds, 2" pipes between the manifolds and turbos, twin 3.5" exhaust or so, depending on what the heads flow a straight or reverse split on a 116*118 lsa. What manifold ya thinking?

If u do headers 1.75 stepped to 1 5/8 would work.nice.

NicD

iTrader: (7)

Ahh yes, a real comparison. My LQ9 in my truck dyno'd 15 horsepower higher than the LQ4 in my buddies both having headers. Blah blah blah yeah it's two different vehicles but nobody is going to swap out the pistons to do a back to back. Let's not forget, the LQ9 needs 91 octane or it will ping and the LQ4 does not and it's actually 6/10ths of a point in compression difference and not half a point. I guess I should also add that every engineering chart I can find for theoretical increases from a single point of compression vary between 2% and 4% depending on what you are going from/to. At any rate it's probably better to not quote GM for actual power figures seeing as how the f-bodies were rated at 305 horsepower and we all know what they really made.


And setting your timing to 5 degrees has nothing to do with the difference in static compression ratios. It's easy to make generalizations about too big of a cam, etc but the fact remains a point of compression does not make it a turd all things the same. Anybody who says otherwise is making **** up unless your definition of a turd is a 20 rwhp drop.

Ahh yes, that 15-20 horsepower difference from a point of compression really makes a significant difference on the transbrake.

71 chevy

iTrader: (18)

being that a typical torque converter has a 2.5:1 multiplication ratio during initial acceleration , yes, you will feel the difference in 40ftlbs of torque difference from 1 point of compression, assuming the low comp engine can even get on the converter lol

NicD

iTrader: (7)

I can see at this point you are just arguing for the sake of arguing because that's all irrelevant. It's as easy as turning up your 2 step another hundred RPM or so to build another pound of boost to compensate nor does that 20 horsepower really have any affect on being able to get on the converter.

djfury05

iTrader: (30)

Interesting thread.. be nice to see some other real world info.