Alright, I want your opinions on this.

I've spoken to my boy at Acosta Motorsports, and he said alot of people were making a mistake lowering the compression of the engine when rebuilding it.

He said that with forged pistons & rods, you should be able to run 15 or more pounds of boost without any problems. How does he know ? Well, they started to build the engine... they still have it in the back of their shop..

He basically told me that to run 15lbs on our compression would give insane performance and response, equal or better than running 20-25lbs on 8.5:1 compression. And he said the block can handle that without any problems.

What do you guys think ? Any unforeseen issues with this idea?

-slay

 

He is stupid. High compression and 15 lbs of boost equal boom. I don't think the rods or the pistons can take that. Yes there would be more performance but for a race only application. You would have to run really high octane to run 15 lbs of boost at 10:3:1 compression and excellent tuning. Oh and 20 lbs of boost on 8:5:1 would own 15 lbs of 10:3:1 especially with large turbos.

Who is running the most boost on stock compression on the boards?

 

I think each messageboard needs to hire a couple physicists, some professional mechanics, a couople mechanical engineers, and some professional machinists.

It seems to me that there's always a debate about something that should be very scientific and concrete.

 

I am not an engine expert however i f you look at all of theese 800 hp supras and all the insane 300 zx's running that kind of power they are all using 8-9 to 1 compression with 25lbs boost or more. Why does this guy think he can reinvent the wheel? I visited a shop in boulder and they are building two motors both with forged rods/pistons, both 8.5 to 1 comp, and both are going to run 25 psi.... this is a proven formula for huge power with turbos!

Slay2k this guy is correct in the fact that the block can handle that kind of pressure the shop i talked to said the block is good for 1000 hp as far as the cylinder walls are concerned and they are running .20 over pistons.

The problem with high compression and 15lbs of boost is the amount of heat this will produce is crazy and controlling detonation will be the biggest problem with this setup vs 25 psi on lower compression.

Well since they have already decided to build their engine like that for some uknown reason let us know how long it lasts!

 

Check out my disertion on Forged Internals. There are some great posts from very knowledgeable folks on this board...people that know waaaaat more than me. This will answer all of your questions...and then some. Personally, I will run 9 or 9.1:1 pistons when I switch over.

https://my350z.com/forum/showthread....threadid=68147

 

Thank you for the replies fellas

 

All I have to say is that they make 9.5 to 1 kits for the supra and eclipse guys. So i would say lower it to that. Funny on how N/A guys want lower and the turbo guys want higher compression. If the engine goes on mine I will run 9.5 compression and 15psi of boost. Not crazy just make sure you tune it right.

 

9:1, 1+ bar. no prob on this vq.
dont go too low compress.. else the car will be unresponsive when not on boost...

 

If I was building a forged motor, I would also recommend sleeving, and maybe even a semi-closed deck.

Yes...the VQ motor is sleeved from the factory...but only with a very thin layer of ductile iron. The rest of the bore is aluminum. When sleeving, you'd get that stock sleeve outta there....completely. Then insert a full 100% iron sleeve. NOW...you have a strong fricken block....just as strong as any full iron block, but many lbs lighter.

Without block reinforcements, the cylinders are going to start walking at some point. And since so little experimentation has been done on this motor, I'd sleeve the thing...just to be safe. The biggest drawback to block mods is they take FOREVER to complete. Consider youreself very lucky if you get your motor back in 1 month...and 2-3 months is not uncommon.

Junkyard VQ....here I come..to the rescue!

 

i heard that the maxima vq is a different mold, do you know for sure that the motor mounts are the same on both?

im thinking about doing the same thing with a 9:1 built block for turbo in the future, made to rev to 8000rpms

 

I don't care what type of pistons you run, if you detonate bad, you will vaporize them.

If you run 15psi with correct timing and fuel adjustments, the stock pistons will be fine. If you a tune that causes detonation at 15psi, you will melt your pistons, forged or not.

Don't forget, when a turbo motor detonates, not only does it lay waste to the pistons, but it will trash the combustion chambers too.

I suggest you look for a new engine building shop asap.

 

running 15 psi at 10.3:1 CR isn't inherently bad, but it requires very precise tuning. We all know pre-ignition and detonation are bad, but high-CR and boost aren't necessarily going to lead to detonation.. but they require better tuning -- there is less margin for error. Race teams can accept this challenge and can afford to replace a blown motor. For a street car, I wouldn't want the risk. I intend to go 9:1 on pump gas when I eventually build my car.

 

Sure, If you wan't to fill up with race gas every week rock that 10.5:1. Do it, it would be cool. But it's just not streetable or daily driveable.

I like Chebosto's choice in compression. That's exactly what I would; and eventually will be running.

 

So how about a poll on the best and most popular compression ratio for TT 350z?

a) stock 10:3:1
b) 9:5:1
c) 9:1:1
d) 8:5:1
e) none of the above
f) yes F. this $#!+ let's do a rb26dett or 2jz conversion

 

is 2jz conversion even possible?....

that would be awesome... you'd be at 2bar in no time! hehe.

 

im sure it could be done if an rb will fit. but it would be very $$$

 

g) 9.6:1 because that's what you get with the Nismo heads and the AEBS 8.5:1 setup. If the flow numbers on those heads are what they are rumored at, 14-17 psi + 7,500-8,000 rpm will get you ridiculous power

 

This is really stupid. Get him to read any beginners book on FI principles.

 

there are SO many consideratiosn to what the "ideal" compression ratio is guys...talk to an experienced engine builder to get it sorted. At a certain point with the heads (where the power is), you have no choice but to lower compression. the type of tubro, the amount of boost, the type of gas, how it will be tuned, all play into where the ideal CR is.

Also, realize that you would be very hard pressed to notice a difference with a .5 in compression.

Cheston - I always see people mention that low CR makes it "unresponsive" when off boost - who cares what the engine behaves like off boost....the idea is to be "on boost"..that's where the fun is at

lastly, what is this "safe to 15 lbs...15 lbs of boost with what turbo? 15 psi on a T25 is vastly different from 15 psi on a T100.
You cannot rate how much boost it's good to without stating what unit is making that boost

Adam

 

. well said.