djtimodj

After reading cass's APS tuner kit thread i was thinking, I know by lowering the compression from 10.3:1 to 8.5:1 allows you to run more boost pressure.

If the stock block makes 400whp @ 9psi but a built motor with 8.5:1 comp will only make 330whp at the same boost why don't people use forged stock 10.3:1 internals?

If you make 500whp at 14psi on a built block then why not make 500whp @ 11psi on a higher comp ratio?

The engine would make more power off boost as well?

There my be a reason for this guys im just not sure??

Thanks for the info..

athenG

iTrader: (2)

In my Honda days, we ran low compression because we also boost 25+psi to get were we want but most build here only run 18-20psi. I'm also curious what is the point going 8:5:1 if you are only boosting 18psi.. Is there really any advantage going that low if you are only going to boost 16-20psi? You are going to boost less with high compression to hit 500whp which is a good power for street use..

350 Rookie

iTrader: (29)

In my opinion, stock compression is only good for a little bit. If you are reaching for 600+, stock compression might be a little dangerous without race gas, meth or any insurace mods.

thom000001

Its basically a safety mark for the boost. More compression means more heat no matter what. Go lower compression with boost, then you aren't "damaging" the air as much (thus less stress) during normal driving.

Basically if you are willing to put the octane in it all the time, run as much compression as you can weather you are boosted or not.

athenG

iTrader: (2)

yeah but to achieve the same power with lower compression you will need to boost more, with higher compression piston will need to run less boost. Like I said, most build here only run 18psi so with a higher compression piston, you will probably just need 16psi or less to hit less to hit 500+whp. I know one who has a GTM long block with 8:5:1 piston and the owner ask GTM what is a safe boost to run on them, the answer was 16psi - 18psi above is pushing it So 8:5:1 will be in a danger zone at 20psi?

I'm no expert in built motors coz I'm not an engine builder but will these built motors with all the bells and whistle running 10:3:1 piston cant handle 14-16psi of boost? How many built motor here really run 20+ with their twins anyways.. except the crazy ones like XKR with his crazy build but at 600+ most go with a stroker kit...

rcdash

iTrader: (18)

http://www.turbobygarrett.com/turbob...tech102.html#d

UpRev

iTrader: (5)

Like Thom said, it's about heat. You can't measure power ability in PSI. Air volume is how you need to measure it. Thats like saying a heavier car is safer, just not true.

Lower compression means lower cylinder heat, and you can run lower octane fuel per volume of air. The higher the compression ratio the higher the heat, and the higher the octane to reach the same "safe" power level. Depending on turbo or supercharger size you can reach a similar efficiency level with the lower compression set up, but you'll have to make sure your design is correct to get that efficiency.

In general terms, with a reasonable high compression motor you can run a small amount of boost. If you wanna play big dog though you're going to need to lower your compression ratio.

UpRev

iTrader: (5)

good call. I did a google search to explain it better and couldn't find anything but stupid SAE documents again. hahah.

athenG

iTrader: (2)

I'm aware about the heat but my question is does 14psi on high compression piston assuming turbo is the same so air flow will be the same will dramatically create more heat than a lower compression piston running 18psi.

I'm on my way home so I'll look at the formula and see how 16psi stand against 18psi (that is 2psi difference) on 2 different compression piston. Like I said, most build here only run 18psi so we are not talking about 16psi vs 30psi.. We have seen many stock block run 14+psi here but the problem is the Rods being weak...

QuadCam

iTrader: (8)

you should be able to make the same power in pump gas at 9-10 psi whether you have stock compression or lower. to run 10psi on stock compression, the timing has to be retarded a faur amount (losing power.) on a low compression motor, you can run much more timing. in general, timing makes more power than compression.

ttg35fort

iTrader: (2)

If you run too much boost with stock compression, you WILL have an issue with predetonation, which can destroy a motor. As other's have said, for a given fuel, you can reach a higher hp level when boosting as compared to boosting a stock motor.

You can run Meth/water injection or race gas to stave off predetonation. However, your hp still will be limited by your rods. A VQ35DE motor is supposed to be good to roughly 400 RWHP. A VQ35HR may be good to about 500 RWHP. I think you can get to these hp levels with 93 octane.

athenG

iTrader: (2)

What is to much boost then? Like I said we are not talking about crazy boost like our 4 banger cousin than run 25+psi, the VQ35HR is also high compression by the way and yet it is acceptable to run 500whp on that motor with boost, granted they are 2 different motor. The way I see it, mid 500whp and with stock compression or maybe a tad lower (dont have to be 8:0:1) with maybe increased redline with cams and headwork should be attainable with medium boost (13-15psi).. I'm not talking about 600+whp build here. We see stock motor run 10+psi with high 400whp and as far as I know the conscious here is that it is the rod that give first because it cant handle the TQ and not detonation. Like I said most build here are mid 500whp so is it really worth running 8:0:1 compression?

When I do my build I'm only going to shot for mid 500whp coz above that it is just bragging rights and I will definitely will not run 8:0:1 compression coz I want to attain 500whp with less boost as possible anyways, I'm thinking 9:5:1 is more than enough for 14psi-18psi, as a matter of fact I rather have a higher redline..

fstrnldr

iTrader: (4)

google dynamic vs static compression and boost, and you will find a lot of good articles on the subject. There are a lot of factors to take into consideration.

Heretic John

iTrader: (1)

nice thread

thom000001

<<<<<< Raises hand......

Run 25psi regularly on 110leaded....occaisonally turn it up to 30psi on the 110, but thats usually just to scare the absolute crap out of people.

Also 18psi is about the absolute max psi for pump gas no matter what (without water/meth). Not everyone has higher octane gas available in their area.....thankfully I do.

tom

fklentz

iTrader: (5)

Stock compression built motor boost will be limited at the point where preignition occurs, that will be based on ambient temps, fuel and timing. For a built low compression engine you will still be faced with a knock/preignition limit BUT more air and fuel volume will be inducted, Air + Fuel = Power.

fklentz

iTrader: (5)

If you're going to build the engine anyway and the 8.5 CR works for what you want why? If you later want to up the boost for more power? How about the tune, you could achieve what power you want with the higher CR but how safe will the tune be riding just below the knock limit or whould it be better driving around and not worrying knock.

athenG

iTrader: (2)

Coz I want to limit the lag, and we are not talking about real high boost here. I rather run 14-16psi than running 18-20psi. I think 9:5:1 is a good balance, for ex, here's a rear-mount running a T67 pushing 558whp@18psi running 9:5:1 compression piston and 623@20psi with race gas tuned by Jeremy.. Just because some run 8:0:1 mean we all should... I also have Water/Meth so I'm not to worried about knock since I'm not going to boost 30psi.. lol I dont care about pushing 700whp anyways

Edit:
I forgot the link https://my350z.com/forum/shop-builds...-torque-7.html





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thom000001

You need more info than just boost and compression.
Tune is more important....timing, A/F, etc....

Not apples to apples as mine was on a different dyno and not rear mount but
I made 546rwhp at 15.5psi, timing never got above 14 degrees max, and A/F was in low 11's......and I have 8.5:1 compression......

So there is more to it than just boost and compression numbers.

tom

athenG

iTrader: (2)

Like I said we are not talking about high boost here, most 4banger go with very low compression coz they all run 25+psi, I'm talking about 14-16psi so I think the 9:5:1 should be able to handle that. I also run W/M so that is an added protection. I mean is 14-16psi really that high that we should all go 8:0:1? I dont think Jeremy will put crazy timing either on that tune so how come he didnt complain about running 18psi on that motor? If it is to risky then he should've just run 14psi on pump gas and no higher..