1G 9.1:1 vs 8.5:1 Compression

[AntmanAndLittleDi]

I'm preparing to rebuild a 6 bolt and use it inside my 2g. So far the setups on Jackson Auto Machine are what I'm aiming for, the Stage 2 motor and Stage 3 head setups in particular. My only question left (so far after loads of research) is in regards to the differences of 8.5:1 and 9.1:1 compression ratios. My plan is to basically hold the horsepower at 550-600 until we get used to driving it, then open it up to 700-750hp. I'd like to only change the boost setup and retune the ecu accordingly to make the jump in hp. Can I build this motor at 9.1:1 and still throw enough boost at it to accomplish this? Or should I keep it at 8.5:1? I just like the idea of a little more giddy-up off the line with the 9.1:1, but with needing to receive more boost to make the final hp mark I'm also understanding in the need to lower the compression. Any comments are appreciated.

 

[Biddy420]

What fuel are u planning to run? My build will see high boost and running Mahle 10.1 comp pistons, on e85 u can get away with much higher compression.

 

[1990TSIAWDTALON]

10:1, E85, 2g head and alot of boost here. I haven't run into problems with static compression and boost up to a little over 40#'s. 60 lbs/min. Link (bench racing) says 614hp/515tq.
Just for comparing.
Every motor is different.
Bigger cams coming.

 

[AntmanAndLittleDi]

What fuel are u planning to run? My build will see high boost and running Mahle 10.1 comp pistons, on e85 u can get away with much higher compression.

I'd prefer to stay on 91 octane since it's readily available here. We have one pump in town with E85, but they jack the price accordingly. However, I did get permission from the city to purchase aviation fuel from our airport (choice of 146 or 162 octane). Plus I'm still waiting for a call back from the ethanol plant to see if I can buy pure ethanol from them. I have choices, but still want to stick with the 91 premium that we've been using. And no, I didn't plan on running straight aviation fuel I'll blend it if I need to go that route.

 

[jakk220]

If you are going to run 91 octane I would personally go with 8.5:1. With 91 octane and 9:1 compression pistons you are going to have a smaller tuning window. I would go with the lower compression if in doubt.

 

[AntmanAndLittleDi]

As an added note I looked up what exactly the airport had on hand, it's 100LL AvGas. (There was a chart there probably from a past date I saw with the two other choices.) The 100LL is rated as having a lean automotive octane of 100 and a rich aviation octane of 130 and is currently $4.80/gal.

 

[1990TSIAWDTALON]

I was buying Av gas from the local Fuel Distributor but I still didn't get the performance I wanted that I did get from the E85. (I couldn't get the timing up as high).
It's a shame they jack with your price, my local E dealer (20 miles away) is always cheaper than 87 octane gas.

 

[brian9397gsx]

I had jam build my motor and with speaking with him he made my mind up just to do 9:1. Jerry is a very good guy but is plagued by forgetfulness . Oh and dont be in a hurry took almost a full year to get mine done which wasn't a big deal since I sent it in at the beginning of the car build and it showed up right when I needed it. The work was outstanding in quality.

 

[AntmanAndLittleDi]

My only problem is that I want to do the build myself. I understand I can't do Everything, but I'm not about to mail off my motor. I'd rather buy it piece by piece and go from there, call a machine shop for something special once in a while but not for the whole thing. Coming from a family of grease monkeys, then marrying into another one, it's hard to find a reason to call a shop other than for specialty items.

 

[SasaniFab]

I had jam build my motor and with speaking with him he made my mind up just to do 9:1. Jerry is a very good guy but is plagued by forgetfulness . Oh and dont be in a hurry took almost a full year to get mine done which wasn't a big deal since I sent it in at the beginning of the car build and it showed up right when I needed it. The work was outstanding in quality.

Did you say 1 year? Thats insane and totally unprofessional

 

[brian9397gsx]

Did you say 1 year? Thats insane and totally unprofessional

Well I did tell him I was in no hurry or rush. We touched base every month or so. He is an incredibly nice guy and honestly it wasn't an issue for me because I sent it out well in advance of needing it. Like I said when it showed up I had just finished fab and mock up with an old junk motor and was able to yank it out and drop it in. The timing honestly couldn't of worked out better in my favor not having to worry about getting junk in a fresh motor or having it on a stand in the way. He also took extremely good care and compensated me for being so patient.

 

[SasaniFab]

Yah i feel yah..... 1 year would drive me nuts though. The two weeks my blocks are away are already enough for me.

 

[Biddy420]

My block took about 2 months, my head about a month.

 

[motomattx]

I have had a block sit in a machine shop for 4-5 years or more, eventually I got ready to use it and picked it up and took it elsewhere. Shops now days seem to be able to pick and chose the work that they do, they arent interested in small money 4 cyl stuff especially if they arent doing every piece of it, it IS very unprofessional though, but the alternative is to have someone unkown that you dont trust screw with it.

 

[AntmanAndLittleDi]

That phrase, "someone unknown that you don't trust" scares the ever-living daylights out of me. Glad I have gear heads and grease monkeys for family. So far my eclipse has been a learning experience for everyone since they're mostly into American muscle.

 

[ThunderChild]

That phrase, "someone unknown that you don't trust" scares the ever-living daylights out of me. Glad I have gear heads and grease monkeys for family. So far my eclipse has been a learning experience for everyone since they're mostly into American muscle.

Technically the eclipse is American muscle

Having anyone else do work for your car can be nerve racking and annoying, especially when you're capable, but time or lack of equipment necessitates farming it out.

Just had new tires and an alignment done after new suspension pieces. The shop I took I to (I used to work there actually) first claimed they couldn't do my tires because they were too low profile (215 45 17 ) and then said one if my front tire rods "fell off" while doing the alignment...mind you, this was a moog unit with 15k on it and no prior issues, my alignment was supposed to be more focused on the rear toe/camber and front camber.

Ended up having to have them replace the tie rod as I didn't have time to go down and sort it out, and once I picked it up and looked at the old tie rod, I was confused. There was NOTHING wrong with it, it looked brand new. Asked them what happened and eve. the techs explanation just didn't add up. Told them I'd pay for the labor, but wasn't covering the part cost as they could have re-used the moog piece. Turns out the place had changed management after I stopped working there and hasn't been for the better.

Took the car to another place down the road from my house for the tires, and am now looking into making an alignment setup for my shop.

Still have that tie rod in case of a rainy day

 

[1990TSIAWDTALON]

Lots of folks "claim" to be mechanics.............Still, I wouldn't trust a single one to know a dam thing about my DSM, because they don't.

 

[AntmanAndLittleDi]

Lots of folks "claim" to be mechanics.............Still, I wouldn't trust a single one to know a dam thing about my DSM, because they don't.

Tell me about it. I've been sold industrial spark plugs because they said Mitsubishi on them, promptly switched my terminology to include NGK. At another place I had to spell Mitsubishi for them. Hurray rural farming communities.

 

[1990TSIAWDTALON]

Hey..., you're not alone, this corner of the state is hurting for any knowledgeable mechanics and just plain counter help sometimes. I just go behind the counter to find what I need.

 

[brian9397gsx]

Lol I'm a lead tech at ford but I'm well known in my community and currently I'm working on a local dudes well built evo8 at work. Getting ready to align it should suck full coil overs and adjustable everything on the suspension ittl probably take a couple hours to align it right .

 

[AntmanAndLittleDi]

Back to my original post though. Anyone have anything else to add on 9.1:1 vs 8.5:1 compression ratios? I still haven't received any email replies from the handful I sent out, unless you count the delivery failure to Boostin.

 

[iceguru1114]

Back to my original post though. Anyone have anything else to add on 9.1:1 vs 8.5:1 compression ratios? I still haven't received any email replies from the handful I sent out, unless you count the delivery failure to Boostin.

Given your stated desire to stick with 91 octane fuel, combined with your power targets, it seems like 8.5:1 is a no-brainer if you want to have more knock margin. That being said, if you're going be revving the engine up high to make your power and you're ok with knock limited ignition timing and the poor low-end torque that goes with that, 9.0:1 could work.

The limits are going to be 1) go too high on the compression ratio and you have to run knock limited advance which limits you at low engine speed and high load 2) go low on the compression ratio and have lower cylinder pressures at low engine speed and high load. Either scenario is going to limit you in the danger zone of low engine speed and high load because that's just a quirk of flame propagation in IC engines. Every IC engine ever is challenged in that zone of operation.

If it were me, I'd go higher on the compression ratio and limit boost and spark down low to avoid knock, but, I'm comfortable tuning near the margin, so, IMO it comes down to how comfortable you are in your ability to tune against knock.

 

[AntmanAndLittleDi]

Nobody I know has ever tuned a turbo motor, let alone a dsm. Tuning is going to be a learning curve, but the more I research about it, the less scary it seems. Will probably play with the 2g a bit first while the 6 bolt is being built, get comfortable, then do the swap. Thinking of getting EvoScan for the 2g and probably Haltech or AEM for the swap afterwards.

 

[1990TSIAWDTALON]

You don't think Link will get you to your goals? Lots of help here and on ECM's site for that platform.
No matter what system you use, don't let tuning scare you.

 

[AntmanAndLittleDi]

I thought about Link, but being I already have a '99 gst auto, it means that ecu isn't a candidate to even be chipped. I also have a '95 gsx ecu, but it's for a manual trans, I want to keep the auto. I haven't even looked to see if the 95 ecu is eprom or not. Using Link would require me to find an eprom ecu for an auto trans, which from what I've seen is hard to find, everyone wants manual so that's all that's being sold. Even then, most applications require my auto to get turned into a forced auto, so to speak. It would still be an auto, but running full pressure all the time and I'd have to physically shift it through the gears. I'm not really set on Haltech or AEM, but would really like to find something that keeps the auto trans acting like an auto.