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Old 03-17-2008, 12:19 AM   #1
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Default Max comp. ratio?

What would you deam the safest maximum compression ratio on a 3.0 liter v6 on normal gasoline? 13:1 too ambitious? I'm looking into getting a z32 300zx after my miata so that i can have at least one un-boosted car because my 240 is getting a t3 next. I think they come with 220 bhp with like 10:1-10.5:1 from the factory don't they? Increasing the compression and the rev's seems to be the bluntest way for more power to me, other than a stroker . So is 13 or 12.5:1 too high?

oh yeah forgot we have 93 here, i don't know if that will make much of a diference compared to 91 that i've seen at other places
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Old 03-17-2008, 12:24 AM   #2
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Just ask Mr. Decisive, Husler. I dunno, depends what your trying to do you'll have to run premium if you don't have to all ready. Perhaps cams and headwork that would help a lot I would imagine after the basic bolt ons.
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Old 03-17-2008, 09:58 AM   #3
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Originally Posted by mazda/nissan View Post
What would you deam the safest maximum compression ratio on a 3.0 liter v6 on normal gasoline? 13:1 too ambitious? I'm looking into getting a z32 300zx after my miata so that i can have at least one un-boosted car because my 240 is getting a t3 next. I think they come with 220 bhp with like 10:1-10.5:1 from the factory don't they? Increasing the compression and the rev's seems to be the bluntest way for more power to me, other than a stroker . So is 13 or 12.5:1 too high?

oh yeah forgot we have 93 here, i don't know if that will make much of a diference compared to 91 that i've seen at other places
It's gonna be limited by bore size more than any other factor, which I forget the size on. I would think that you are going to be pushing your luck at 12:1 to be honest, and that 11.5:1 is a more "honest" goal. Remember in some cases you can have VERY high static compression, if your dynamic compression varies enough at low RPM. (In other words it is possible to build a 93 octane 13:1 motor with the right cams.)

While compression ratio is useful in raising power, it is not the single most important determinant, flow is. You want to make that sucker flow some air.

Your logic should lead you to looking at it something like this:

Intake/Exhaust
Cams/RPM mods
Port work
Cubic inches
Compression

Good luck,

Mark

PS - http://www.popularhotrodding.com/eng...tio/index.html

David Vizard has forgotten more about high compression street motors than I will ever know.
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Old 03-17-2008, 03:29 PM   #4
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thx for the info mark, that link is great i was planning on doing the stuff in that order just was unsure what to do when i got there. I was planning on high flow exhaust for high end power even though i'll lose some torque. I'm also hoping to make it rev higher to take advantage of the exhaust minimum flow restrictionz. I understand that adding some stroke will increase displacement and therefore power, but won't a longer stroke inhibit my rev-happyness or does a heavier rotating assembly hurt the revs more?
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Old 03-17-2008, 03:33 PM   #5
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i wish i still had the video of me drag racing my nissan altima against said 300zx with breathing mods and i win
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Old 03-17-2008, 05:04 PM   #6
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i wish i still had the video of me drag racing my nissan altima against said 300zx with breathing mods and i win
And what were the specs on your Altima?
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Old 03-17-2008, 09:08 PM   #7
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Originally Posted by mazda/nissan View Post
thx for the info mark, that link is great i was planning on doing the stuff in that order just was unsure what to do when i got there. I was planning on high flow exhaust for high end power even though i'll lose some torque. I'm also hoping to make it rev higher to take advantage of the exhaust minimum flow restrictionz. I understand that adding some stroke will increase displacement and therefore power, but won't a longer stroke inhibit my rev-happyness or does a heavier rotating assembly hurt the revs more?
Longer stroke motors are less "rev happy" to a point. However we're not talking a huge difference here, since you're not going to add huge amounts of displacement either way.

Downsides of a large bore, increased likelihood of detonation
Downsides of a large stroke, higher piston speeds

This is why exotics run 12 cylinder engines.

It's not that the motor is less "rev happy" but that the piston speeds become unacceptably high. A longer rod and custom piston can help in these cases. Lightening the rotating assembly makes a HUGE difference in how "rev happy" motor is, taking 2 or 3 lbs off a BP crankshaft and using an 8 Lb flywheel makes for a very rev happy motor, but also one that is easy to stall off the line too.

A sensible exhaust system won't give up much low end (or high end) power. However it's more economical to give up low end torque in favor of high end HP. I would give up alot below 3000 RPM in order to have serious gains from 4000-8000 RPM. After all, this isn't a tractor, its a sports car.

Mark

PS - Remember, this is mechanical advice coming from a psychology student!
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Old 03-18-2008, 01:23 AM   #8
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well do you have any idea how many rpm's would be too many with a bore of 87mm and a stroke of 83mm? i think they redline at 7000 but like the miata i'm sure stiffer springs would help it get to 8, but how much is too much? (before it goes kaboom)
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Old 03-18-2008, 09:26 AM   #9
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well do you have any idea how many rpm's would be too many with a bore of 87mm and a stroke of 83mm? i think they redline at 7000 but like the miata i'm sure stiffer springs would help it get to 8, but how much is too much? (before it goes kaboom)
Well it's gonna be a function of piston weight, rod strength, and remember that it has the stroke of a 1.6 motor and less stroke than a 1.8 motor!

So with good rods and light pistons, 9000 RPM on the bottom end should be doable. After all the Honda F20C has an 84 mm stroke and an 87 mm bore. This assumes that the bottom end is stout, which on the VG30 series motors is the case. It's the valvetrain that gets tricky, it's important to use a good dual spring setup and lightweight components on the valvetrain. I know that the VG30 series motors are capable of in excess of 8500 RPM when properly built.

Mark
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Old 03-18-2008, 05:02 PM   #10
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well that is reassuring news mark, thx. Any good webisites for z32's? I keep coming across links for z32r.com and that they had some nice N/A vg30's but the site as since gone away. Nicoclub and whatnot is just a bunch of "what spoiler looks best on my bang me in the *** blue car?" it seems to me
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Old 03-18-2008, 05:09 PM   #11
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And what were the specs on your Altima?
2 bump in timing. 15.7 @ 89mph. best the 300zx was doing was 15.8-16.1
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Old 03-18-2008, 07:24 PM   #12
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sounds like an expensive and possibly fruitless project. maybe look at an ls1 swap into that z32 chassis, problem solved.
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Old 03-18-2008, 07:58 PM   #13
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sounds like an expensive and possibly fruitless project. maybe look at an ls1 swap into that z32 chassis, problem solved.
why? i know it will be expensive but it is not going to be a straight line car i'd like for it to tear up some twisties and maybe a few laps around the carolina motorsports track.
http://www.carolinamotorsportspark.com/trackabout.html
i think the vg30de is a good candidate for an N/A buildup, it is just often overlooked because of the vg30dett. If i can get it into the 13's then that will be plenty fast enough for me that and i don't ever want to deal with the hastle of a motor swap
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