F/I Blowing motors facts

[dj2020]

Originally Posted by SPCorBUST

if you're looking to retain stock 12.5 CR, is there any reason to upgrade from the OEM pistons?

Per this thread, apparently most aftermarket forged rods will not fit the stock pistons unless they're machined down. Since the motor is already opened, seems like most people just upgrade to forged pistons as well. I believe you can custom order forged pistons and retain the stock 12.5 compression ratio.

[GsxrMe]

Give JE a call, they have a 13.5:1 I'm sure they can grind down to 12.5:1 for you.

JE also has a 10.5:1 that looks tasty.

[Andew727]

Originally Posted by SPCorBUST

if you're looking to retain stock 12.5 CR, is there any reason to upgrade from the OEM pistons?

i just finished mating some bc rods to the stock pistons. light machine work needs to be done. top of the rod needs to be tapered down at a slight angle on each side and an oil shoot on the bottom end of the rod also on each side. cost me about $100... saved me $700 so i added a more efficient intercooler and tranny/shifter parts with the budgeted money. i wont be over 450 hp... maybe 400 to the wheels. i also lowered compression a bit with a thicker head gasket. money well spent? o should i have made new pistons with a 12.5:1 cr?

[Xero-Limit]

Originally Posted by SPCorBUST

Looking like the San Andreas fault line on the top of my motor...



I'd like the next motor to be 12.5:1 CR, 7.5k redline (maybe slightly higher if it can be managed safely), C38, HBP, E85, I'm exploring what the motor needs to be safe under those conditions.

Ouch, that sucks. I wouldn't get too caught up about the 12.5:1 CR. After building several 10.5:1 and 9.5:1 and driving/tuning them after the fact, you'd be hard pressed to ever know that it was lower compression aside from the fact that there's much less knock at high boost. With a turbo/SC it is such a minor difference that unless it will be a strictly e85/meth car, then the low CR is not a deficiency. Just my opinion...oh wait, no, lets add some data





Hint, one of these is on pump gas, the other on e70. Both with the same exact car, same turbo, same everything other than built motor dropping 3 points of compression. This allows not only 5 psi more boost, but also allows more aggressive cam and ignition timings. AFR is the same. Those are turbo journal bearing results.

A roots/twin screw can benefit from the higher CR on 93 or alcohol. But on pump most blowers out there right now can be spun down enough to generate more hp/heat then the high compression knock ceiling. So still a good idea to drop a couple points.

EDIT for clarity, red lines are LOW compression. Blue lines are stock pistons.

[CSG Mike]

Originally Posted by GsxrMe

Give JE a call, they have a 13.5:1 I'm sure they can grind down to 12.5:1 for you.

JE also has a 10.5:1 that looks tasty.

CSG can have any compression piston you want made, as long as it'll physically fit inside the engine.

[SPCorBUST]

Originally Posted by moto-mike

Ouch, that sucks. I wouldn't get too caught up about the 12.5:1 CR. After building several 10.5:1 and 9.5:1 and driving/tuning them after the fact, you'd be hard pressed to ever know that it was lower compression aside from the fact that there's much less knock at high boost. With a turbo/SC it is such a minor difference that unless it will be a strictly e85/meth car, then the low CR is not a deficiency. Just my opinion...oh wait, no, lets add some data





Hint, one of these is on pump gas, the other on e70. Both with the same exact car, same turbo, same everything other than built motor dropping 3 points of compression. This allows not only 5 psi more boost, but also allows more aggressive cam and ignition timings. AFR is the same. Those are turbo journal bearing results.

A roots/twin screw can benefit from the higher CR on 93 or alcohol. But on pump most blowers out there right now can be spun down enough to generate more hp/heat then the high compression knock ceiling. So still a good idea to drop a couple points.

EDIT for clarity, red lines are LOW compression. Blue lines are stock pistons.

How does changing CR affect emissions? Being in CA, CARB is a major concern and is one of the main factors for my build. It's the main reason I may not go for any head work like porting or oversized valves. I could get away with appearing CARB but I think that sniffer is another story.

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[CSG Mike]

Originally Posted by Luftwaffel

Yeah, I have a hard time grasping how the actual fuck the individual components and a DIY rebuild are almost the same price as just ordering a short block and then getting the core charge back. It really makes no sense and even seems a little shady to me, actually. Some price fuckery going on IMO.

Perhaps you're getting lower quality components in that pre-built short block than if you order the components yourself. Most people ordering a pre-built engine aren't going to be taking it apart to check everything. I've seen many shops go down over the years, after eventually getting caught on switcharoos.

[Jaden]

Originally Posted by SPCorBUST

How does changing CR affect emissions? Being in CA, CARB is a major concern and is one of the main factors for my build. It's the main reason I may not go for any head work like porting or oversized valves. I could get away with appearing CARB but I think that sniffer is another story.

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It was my understanding that we won't even have to do a sniffer test. That they will only care about not throwing any codes... I could have heard wrong though..

Jaden

[SPCorBUST]

Originally Posted by Jaden

It was my understanding that we won't even have to do a sniffer test. That they will only care about not throwing any codes... I could have heard wrong though..



Jaden

I think it's different for me since I've been sent to the ref previously. I think it's more stringent from here on out, lol.

[Jaden]

Originally Posted by SPCorBUST

I think it's different for me since I've been sent to the ref previously. I think it's more stringent from here on out, lol.

I went because stupid cop said I had an "inadequate" exhaust... I passed even without my baffles in, which the ref required since they were removable.

So now I carry around the the ref cert with me so if any cops question it I just show them the cert.

Jaden

[SPCorBUST]

Originally Posted by Jaden

I went because stupid cop said I had an "inadequate" exhaust... I passed even without my baffles in, which the ref required since they were removable.

So now I carry around the the ref cert with me so if any cops question it I just show them the cert.

Jaden

Yeah I had to do a complete inspection, all evap lines etc. Think I have a slightly lighter version of that each year now.

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[dj2020]

@moto-mike or anyone else: I was always under the impression to keep the stock 12.5:1 compression when it came to the Rotrex kits, c30/38. I think I remember reading it some where in the JRSC thread. But if lowering the compression a couple of points for safety has no big detrimental effects on off boost drivability, then it's the way to go it seems like. I'm trying to completely understand the pros and cons.

[CSG Mike]

Originally Posted by dj2020

@moto-mike or anyone else: I was always under the impression to keep the stock 12.5:1 compression when it came to the Rotrex kits, c30/38. I think I remember reading it some where in the JRSC thread. But if lowering the compression a couple of points for safety has no big detrimental effects on off boost drivability, then it's the way to go it seems like. I'm trying to completely understand the pros and cons.

Turbo =/= SC.

Compression is just one piece of a highly complex system, which results in the horsepower you want. I would recommend you chat with your engine builder and tuner to determine your goal, and then choose the path you want to take to reach that goal.

[DarkPira7e]

INSANE necro thread revival. I wonder how much our understanding of this has improved? My ostentatious logic says that a large contributor to this is from load on the rods. Turbo tends to pile on low end load quickly, as torque comes in much harder and faster than on most supercharger types people are running on this platform. Also, people who put load on at low RPM (lugging the engine WOT at 2,000RPM) are going to stress the rods a lot more than those who are softer with their input.

Just wanted to open this up for discussion again, as after reading this thread, I feel more success and more failure has been had. Any good experiment's data has a large test pool, right? Too bad we have no control..