Compression 8.3:1 , 8.5:1 or 9:1

[thilaksharma]

Hi guys , I would like to clear my doubt on the piston compression choice
my current specs

FP Green
FP 2 cams
Stock 7.8:1 1G vr4 block
MMC MMLS HG

it got 363WHP on dyno dynamics , 30psi of boost , 14* degree max ,
93octane fuel, should be about 400 on dynojets

I am currently deciding on my pistons for my new built
what I have in my mind would be
ARP Head & Main Studs
Scat Rods
King Bearings
Cometic 1.3mm HG

should I run on 8.3:1 , 8.5:1 or should i run on 9:1 compression for boost levels of 30-32psi on 93 octane.

I know you would ask me to search, but i am having mixed reviews, dsmtalk guy say it wouldnt even pass 16psi on 8.5:1 , some evn say 9:1 pump gas and 30psi dint have a knock. What would u say ?

Thanks a million

 

[Rockbmx1]

I have 8.3:1 cr right now, but when I build a new motor I will be going with atleast 11:1 cr. I would say to go with 9:1. I know a lot of local guys that run 9:1 and make 450+whp and one guy that makes like 527whp on pump gas and 8.8:1.

 

[95blackGsTurbo]

Since you have 93 readily available 9:1 is no problem at all. Most people who build a motor choose that option. Couple the 93 octane with meth. injection and you can reach your goal. On straight pump no. Probably 22-23 max.

 

[thilaksharma]

Since you have 93 readily available 9:1 is no problem at all. Most people who build a motor choose that option. Couple the 93 octane with meth. injection and you can reach your goal. On straight pump no. Probably 22-23 max.

so i guess with 8.5:1 and 93octane i should be able to hit 30psi with straight pump and some ignition trimming on my megasquirt ecu

 

[Rockbmx1]

No, that is too much boost for straight pump if you want any sort of timming added. The difference between 8.5:1 and 9:1 tuneiblity is hardly different. If you want 30psi you are going to need e85, race fuel, or a meth injection. I would still suggest the 9:1, but only about 25psi max if you want a safe and reliable tune. You can do what you want, but I will laugh when I see a new thread started "I didn't know 30psi would blow my motor "

 

[thilaksharma]

No, that is too much boost for straight pump if you want any sort of timming added. The difference between 8.5:1 and 9:1 tuneiblity is hardly different. If you want 30psi you are going to need e85, race fuel, or a meth injection. I would still suggest the 9:1, but only about 25psi max if you want a safe and reliable tune. You can do what you want, but I will laugh when I see a new thread started "I didn't know 30psi would blow my motor "

i am currently running 32psi on stock 1g block , bt i guess 1g block has very low compression and thts the reason it was ok ? correct me if im wrong

 

[Jst4Cyl]

No, that is too much boost for straight pump if you want any sort of timming added. The difference between 8.5:1 and 9:1 tuneiblity is hardly different. If you want 30psi you are going to need e85, race fuel, or a meth injection. I would still suggest the 9:1, but only about 25psi max if you want a safe and reliable tune. You can do what you want, but I will laugh when I see a new thread started "I didn't know 30psi would blow my motor "

Ok, so you're saying that 30 psi and pump gas isn't possible? Not to nit pick but E85 is technically pump gas, as it is pumped straight from the pump.

YouTube - Steve93Talon's 1g dsm S258 turbo, bone stock motor on pump gas dyno 457whp/421tq

YouTube - 600 AWHP PUMP GAS

As for the OP, you will find that less boost typically is needed on higher CP motors to make good power. As seen on this thread below. But the tuning is on a closer edge also. There are two ways to make power, more timing less boost or less timing more boost (lower comp. motor,30 psi, realistic timing, can be safe). I feel more boost is the "better" route but that's me http://www.dsmtuners.com/forums/dyno-talk/324626-our-sleeper-1g-makes-619whp-out-fp3065.html

IMO, I would go pump plus meth.

 

[nytescion]

I run 30+psi on gt30r w/ stock 6bolt block without my meth on with no knock. I'm only running 10 degrees timing at redline though so maybe that's the reason why. I don't know what kinda power I'm making (haven't been dyno'd). I also run 93 octane.

 

[95blackGsTurbo]

haha don't even begin to compare E85 (~105 octane) with alcohol in it to 93 octane unleaded. For a daily driver I wouldn't do this. Unless you have another car.

 

[1gDSM4g63]

...but when I build a new motor I will be going with atleast 11:1 cr.

11:1 is a bit to far, I'd say 10:1 is alright if you're running on e85 unless you're going to be running methanol as a gas.

 

[jrohner]

It all depends on your tune. You can run almost any compression with any boost (within reason), you just have to tune for it. I'd go with 9:1, unless you do have E85 available, then 10:1+

 

[95blackGsTurbo]

Problem with running high compression motors is, if it's your daily driver and you are low on gas or want to road trip and have to run 93 pump, your going to run into some serious issues unless you have a 93 map as well.

 

[mitch86]

I have a similiar setup to you and net 360ish WHP per DSMlink at 22psi. I have Ross 8.5:1's and Eagle Rods. If I turn the boost up to 28-30 I don't think 400 would be a problem....I'm running 93 with only 11 deg timing max.

 

[95blackGsTurbo]

I don't think E85 is even available in Asia.

 

[dsm-onster]

E-85 is from a pump, but still not gas (well 85% not gas). That arguement gets old to those of us that really have no access to it. Call it pump ethanol; not pump gasoline. Pump gasoline is a completely different animal. A weak and finicky animal.

Anyway. I think you should be looking at 8.5-9.0:1. It's simple. MBT goes down with higher compression because of the increase in flame speed. And, [playful sarcasm] what do you know [/playful sarcasm], you need to lower timing anyway to prevent detonation. If you have full timing control you can run 9:1 with pumpgas and see 400whp with a SPOT ON tune.

But I would really consider just throwing some water/meth in the setup.

As well it comes down to piston brand too. Wiseco and Ross have lower silicone content in their pistons. More silicone means lighter piston and technically able to stand more cylinder pressure. But Wisecos and Ross's have been tollerating 800+whp all over the place. So it's enough for your goal . The advantage of having less silicone is that the pistons is SOFTER, being able to tolerate sudden spikes from detonation more frequently and at higher levels of average peak cylinder pressure. A higher silicone piston is more brittle.

I say wisecos and 8.5-9.0:1CR. And water/meth injection is ALWAYS a good investment for pumpgas numbers.

 

[thilaksharma]

looks like i will be going with the 8.5:1 1400HD since maperformance offered me a superb price on it.

i have a water injection tht has not been fitted yet, it has been sleeping for long, its an AEM , so i guess, i will start spraying the water injection once it starts boosting at about 22psi , hopefully i have nice numbers at the end of the day

i be keeping the fp2 cams aside n testing them with kelford 272's, hopefully i will be able to give a good review on the cams

one more diffrence between octane level tht effects our tuning i guess would be the sea level

 

[dsm-onster]

Great choice. MAP did me good with my forged internals as well.

The matchup between the fp2s and kelford 272s with your setup will be very nice too see. Kelfords are really just 288s with a quicker "pre-ramp".

Yes, lower elevation means more power with less aircharge heating boost.

 

[TunaTalon]

And at higher altitudes any given tune will tolerate lower octane without detonation. At high altitude the absolute pressure in the combustion chamber is limited by the lower ambient air pressure. That's why there is no 93 octane in Denver CO. At higher altitudes the PSIG boost from the turbo may be the same as for flatlanders but the PSIA inside the engine is lower, making 91 octane just fine.

I found this out the hard way with Talon Approaching Cody WY and trying to power into an opening that was plenty long enough to pass at WOT. Opps; the boost came up but the power was just not there. Fortunately, my wife raised the pressure in the car enough to get around with the fenders intact.