C/R and Turbo

[Hybird]

Well I got some cash now and soon to have income tax payed back. So I think I will be buying my set of pistons for my proposed turbo'd 3.1/3400. I dont know if I should just save up and learn more, but then I'll likely spend the cash.

Thing is I want to get a high compression turbo charged engine. I want to run 9:1 comp or close as possible. I know curtis runs 8:1, and what I propose to do is likely not the smartest for a starter. The stock C/R is 8.9/1, and if I cant do 9:1 I will most definitely do 8.5:1. Numbers are still in the air, I think I will go to a couple performance shops and see what they have to say before I get this decided.

So questions are, can I acheive this with just piston pin relocation? I assume I can, but could be wrong.

Second, reason for the high comp idea is I plan to do some pretty good air/water intercooling, with a standalone comp like colin bulocks holley comander, so I'm hoping to stray from pre-ignition. Keep in mind this is a long term project, and I do plan to go through with it over the course of several years.

Hoping to start around 10psi and make it up to 15 then work from there. Turbo is yet to be choosen

Opinions please?

[oil pan 4]

RE: C/R and Turbo

I have a nice turbo calc.
Lets see if you put a nice cam and port the heads and up the V.E. to 80% and keep a 9:1 c/r at 10psi you will need 100octaine gas.
That sucks.
Down the c/r a point to 8:1 and keep evey thing else the same you will need 97 still sucks. It can be done, you will need a lot of ICing and water and tricks like that.
I'm not sure what all the turbo calc uses maybe hot air for this but I don't know.

[Guest]

RE: C/R and Turbo

umm 9:1 compression is totally streetable with boost.

The honda guys have been known to run 12:1 with boost. Well one guy at least but he is pushing 12 psi on his engine with 12:1 compression.

9:1 and boost is pretty low by todays standards. You will be fine with a good tune. 8:1 is way too low in my opinion. its not 1978 anymore people. Cylinder head technology has come a long ways to keep detonation down with higher compression and boost as well as the eff of the turbos is better then they were even just 5 years ago. I plan to run 9:1 if I boost. I would never go lower then 8.5:1. Its pointless. 9:1 and you can still run up to 15 psi on pump gas with a good tune.

Go 9:1. You will be able to use pump gas no prob.

[sharkey]

RE: C/R and Turbo

you dont need real low compression anymore for boost. 9.5:1 would be fine fopr mild to moderate boost, as long as you have a way to tune it. im building an ironhead motor right now with 9.5:1 compression, and next winter i plan on boosting it with a blower, running up to 10lb boost.

[Hybird]

RE: C/R and Turbo

Well thats pretty much what I was thinking. I'm just wondering if I will have head sealing problems. I should probably go studs instead of bolts, but I dont think there is a company out there that makes them..

[bszopi]

RE: C/R and Turbo

ARP does...

[Doubt_Incarnate]

i know the aluminum heads are good, i dont know if iron heads can do as well. i guess so if gm once-upon-a-time made 12.5:1 pistons for them. but hondas have a pent roof design and all that jazz witch controls the squish better, i think i read somewhere one guy had 13:1 with boost on a civic.

for durability reasons i'd play it safe and stay under 9:1 for up to 10psi, and over that drop it to 8.5. just my opinion, faster car for years versus fastest car for months. the more "high strung" the motor is, the less room for error.

[bszopi]

Iron heads will handle higher boost than aluminum heads will.

[Doubt_Incarnate]

by strength or design?

[TurboZ24]

Well, this subject is up in the air. I went with 8.0:1 because I intended too run 25 psi of boost on 92 octane.

There are a lot more factors that go into the engine than just static compression. The cam profile, flow rates of the heads, intake, etc all impact on the dynamic compression of the motor. My engine has a pretty high dynamic compression given the static compression.

The real question is how much compression can you run with X boost before you have too pull out so much timing with a given octane before you start loosing more power than you might have minimally gained from the compression ratio.

Also, remember that no amount of tuning is going too prevent pre-ignition (not detonation). The possible gains from jacking the compression really high really arn't all that significant. The real problem is if you do a certain compression ratio and you want too run X boost, but when you try, you get pre-ignition, there really isn't an easy way too go back down on compression, you will have too simply settle with the lowered boost, add water/alcohol/methanol injection, or tear apart the motor. It's easier too err on the side of caution.

I would say

9.5:1, 10 psi seems like it would be ok (you will not really know until you try)
stock 8.9:1, probably 12-16 would be ok
20+ psi, I'd probably go for the low 8 range.

[TurboZ24]

We are talking about durability also, yes. I know a lot of guys with high compression motors experiencing pre-ignition and several have had engine failures on just 4-6 psi (Honda's, LS1's, LT1's). You want too build a setup that will be everybit as reliable as possible.

[2M6]

AFAIK most factory turbo cars are in the 8.5:1 range. This allows good off-boost drivability while remaining safe at moderate/high (15-20psi) boost range. If I build a turbo 3400 it will be 8.5:1 for sure.

[Doubt_Incarnate]

Hey sharkey? how are you gonna boost? centrifigal?

[sharkey]

roots. im thinking of the same supercharger as that supercharged s10 pickup that was floating around here before. id have to raise it off the manifold and figure out fuel rails (i still want multiport). it will be a lot of work, but once i have the motor im building in the car, im gonna be bored again.

[Doubt_Incarnate]

i see, thats 614streets truck i think. the faegol unit. i thought of adapting a roots, but i think the centrifugal would be easier. more compact, and you can intercool it easy. not to mention the fuel is added after the compressing, helping to keep the temp slightly cooler. i think when i move and get settled, i'll boost the 2.8 with a centrifigal in place where the AC used to be, low in a cooler spot, and right where i can put a FMIC.