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Old 11-21-2011, 10:52 AM   #1
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Default 5.3L build 862 vs 317 heads (compression)??

In the process of my build now. I just picked up a set of 317 heads. Now my question is: Do I surface them to clean em up only or do I mill them to up the compression as I have heard going to a 6.0 head (317) you loose compression. I am planning on running a 76mm turbo.

Does anyone have actual compression numbers from experience?
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Old 11-21-2011, 12:24 PM   #2
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I figured it to be 8.6:1 compression on my 5.3L... and i wish I hadnt. If you are limited to like 91 octane maybe run ls1 or ls6 heads.... but not the 317's... drops compression way too low and it will be a dog out of boost and slow as a cool summer breeze. Your stock heads are not limiting you so run them....thats my opinion based on experience.
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Old 11-21-2011, 01:04 PM   #3
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I figured it to be 8.6:1 compression on my 5.3L... and i wish I hadnt. If you are limited to like 91 octane maybe run ls1 or ls6 heads.... but not the 317's... drops compression way too low and it will be a dog out of boost and slow as a cool summer breeze. Your stock heads are not limiting you so run them....thats my opinion based on experience.
Not really limited with octane. This car will see more track time than anything else. Not against race gas or hell even E85. I just want a good compression out of boost, but not too much during max boost.....
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Old 11-21-2011, 01:46 PM   #4
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I have 317's on my 4.8L....dogggy dogggy dogggggy.
I am actually going to slap on stock heads with some better springs this winter.
The slow acceleration followed by the boost hitting is pretty annoying after a while.
The only fun part is how people think the car is slow at first because they see your foot hit the floor and the car isn't moving out....then the turbo screams...followed by the tires....followed by the passenger!
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Old 11-21-2011, 01:50 PM   #5
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....followed by the passenger!
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Old 11-21-2011, 02:08 PM   #6
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I have 317's on my 4.8L....dogggy dogggy dogggggy.
I am actually going to slap on stock heads with some better springs this winter.
The slow acceleration followed by the boost hitting is pretty annoying after a while.
The only fun part is how people think the car is slow at first because they see your foot hit the floor and the car isn't moving out....then the turbo screams...followed by the tires....followed by the passenger!
lol.... thats exactly what I thought would happen, turd out of the hole then it violently comes alive. I want something much more manageable considering i'll be at the track a lot with this set up.
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Old 11-21-2011, 02:18 PM   #7
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I have 317s on my 5.3 with a 78mm turbo. It is an absolute dog out of boost. There is no reason to drop the compression.
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Old 11-21-2011, 02:27 PM   #8
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So a vote here: Screw trying to mill the heads and just leave the 862's on there?
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Old 11-21-2011, 02:53 PM   #9
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On a stock 5.3 with arp rod bolts, Head studs, LS9 gaskest, about how much boost is the 862 heads good to? 20lbs?
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Old 11-21-2011, 03:36 PM   #10
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On a stock 5.3 with arp rod bolts, Head studs, LS9 gaskest, about how much boost is the 862 heads good to? 20lbs?
I dont think there is a really cut and dry answer to that,it all depends on tune,octane,running meth/alki, timing...more factors come into play then saying these heads are good for 20 psi...

i do like the opinions of 317 heads,i pics up a set(317) for my 5.3 which i think has 243 heads now,,,should i run the 243 or 317 ,turbo will be a mp70 or mp76...what convertor is everyone runnning
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Old 11-21-2011, 04:57 PM   #11
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If the tune is good and you keep the AFR #'s consistent and see no knock, 12 lbs of boost should be fine.

I think for a street tune, it would not be pushing.
I am hitting 18 lbs on the street and drive my car almost every day but my head stud kit was more then the cost of the tire motor so I really don't mind if it push the engine too far. It will just give me an excuse to put something bigger between the fenders.

Maybe set it up for 15 lbs to run at the track for some numbers.
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Old 11-21-2011, 06:12 PM   #12
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I have 317's on my 4.8L....dogggy dogggy dogggggy.
I am actually going to slap on stock heads with some better springs this winter.
The slow acceleration followed by the boost hitting is pretty annoying after a while.
The only fun part is how people think the car is slow at first because they see your foot hit the floor and the car isn't moving out....then the turbo screams...followed by the tires....followed by the passenger!
This is exactly how my car runs!!! LOL I have the 317 heads off right now switching back to 862. I did put a looser converter in it and with the trans brake it isn't too bad. But I still wouldn't recommend the 317, definitely a turrd before boost.
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Old 11-21-2011, 06:24 PM   #13
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It's amusing how a naturally aspirated 362 lb-ft of torque is a 'dog', but 374 lb-ft is not?

This is obviously just an example of one specific combination, but enough to show the thermodynamic cycle trends with CR variation. Results from a simple gas dynamics/engine sim model from work:
Click the image to open in full size.

A single point of CR is not going to drastically change the behavior of your car. You will notice as much and more difference in torque due to atmospheric condition variation.

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Old 11-21-2011, 07:21 PM   #14
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It's amusing how a naturally aspirated 362 lb-ft of torque is a 'dog', but 374 lb-ft is not?

This is obviously just an example of one specific combination, but enough to show the thermodynamic cycle trends with CR variation. Results from a simple gas dynamics/engine sim model from work:


A single point of CR is not going to drastically change the behavior of your car. You will notice as much and more difference in torque due to atmospheric condition variation.
agreed, i think having a mismatch part (intake,cam etc) in your motor combo will make loose much more power/torque then low compression, you also have to consider other things as well as far as drive-gears, weight and trans, you need to have everything matched to work with each other and i guarantee you would not notice you had a low-comp motor

i myself have p&p 317 on my 4.8 and out of boost the car drives pretty much like stock
also if you guys are waiting around forever to get into boost, then you should be looking to making changes in your combo anyways.

but on the flipside of thing i also would not even waste my money on headgaskets to change out the heads if your not planing on making 14# or more boost or if you dont mind running race gas

my car will be seeing at least 18psi with 91 and meth
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Old 11-21-2011, 07:45 PM   #15
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agreed, i think having a mismatch part (intake,cam etc) in your motor combo will make loose much more power/torque then low compression, you also have to consider other things as well as far as drive-gears, weight and trans, you need to have everything matched to work with each other and i guarantee you would not notice you had a low-comp motor
Yeah, exactly my point. Some people seem to worry way too hard about the CR, but then may ignore other aspects of their combo that would make much more of a difference.

I think some people are just expecting too much out of their car off of boost due to being de-sensitized by the large amount of on-boost torque. It's kinda like what happens when people put a cam in. They get this large increase in top end torque, and consequently think they lost a lot of their low end. However, 'usually' upon comparing the dyno graph to stock, there wasn't much change in the ~1500 RPM torque. It's all because the increase of torque at higher RPM's (whether due to cam or boost), makes the low end seem lethargic.


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but on the flipside of thing i also would not even waste my money on headgaskets to change out the heads if your not planing on making 14# or more boost or if you dont mind running race gas

my car will be seeing at least 18psi with 91 and meth
I'll be going with a 5.3 build in the near future. The car is on E85, and I will be shooting for 20 psi, so 317's it is. I will gladly give up ~10-15 lb-ft NA (25-30 lb-ft under boost) for the chance of getting it to last several months.
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Old 11-21-2011, 09:27 PM   #16
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Who builds a turbo car to take advantage of the low compression out of boost power by running a tiny cam and small intake, low gears???? No one.

Drivability of my car I couldnt have cared less because of low compression when it was a 5.3 w/ 317s, its a turbo car and I want it to work in its entirety. low compression = slower spool time.

You want to run 20psi through a low compression setup w/ E85? With more compression, a larger turbo, you'd spool it up, make the same power with less boost.
And E85 you shouldnt need the low compression....thats the wonder of it.

N/A Dyno graphs dont mean ****... go drive a car with a 8.6:1 compression turbo 5.3L in it and see.
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Old 11-21-2011, 10:19 PM   #17
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What if u have a lq4 370 w/ flat top pistons 317's 10 1/2 compression
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Old 11-21-2011, 10:38 PM   #18
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Who builds a turbo car to take advantage of the low compression out of boost power by running a tiny cam and small intake, low gears???? No one.

Drivability of my car I couldnt have cared less because of low compression when it was a 5.3 w/ 317s, its a turbo car and I want it to work in its entirety. low compression = slower spool time.

You want to run 20psi through a low compression setup w/ E85? With more compression, a larger turbo, you'd spool it up, make the same power with less boost.
And E85 you shouldnt need the low compression....thats the wonder of it.

N/A Dyno graphs dont mean ****... go drive a car with a 8.6:1 compression turbo 5.3L in it and see.
I think you've mis-interpretted the intent of my post.

Why would an NA graph not mean anything when the topic it was addressing was off-boost torque? As you'll see, most comments in this thread are just saying that they felt the 8.6:1 CR had poor off-boost (NA) performance. My point was that it would still be terrible even if the CR was 9.5:1. A 5.3L out of boost is just not going to knock your socks off.

As for 'in boost' performance, I'm still doing some analysis to see which route I will go. I'm mostly looking at simulated piston surface temps at a given power level (900 crank hp)- with varying combinations of compression ratio, boost, and the 3 cams I have in my garage. What I'm hoping to see is that more boost, lower CR yields lower combustion temperatures than less boost, higher CR. I could be completely wrong though - or it may be a wash.

I have not finished my analysis, but I have a feeling I will likely run the 317's on my 5.3. Partly because the car has an F1A Procharger and thus I have to make up the 90-110 hp required to drive the blower. i.e. making 700 rwhp will stress the motor like it's making near 800 rwhp. I would like to do everything I can to keep the motor together for a little while at the 700 rwhp level and I'm thinking the low CR will aid this.
Secondly, the car has a 4400 stall and a TH400 so, any pedal is going to negate any concerns of off-boost performance anyways.

As for spool time vs. CR. I have always been a bit suspect that it's as much of a driver as people believe. What drives a turbine is mass flow and temperature extraction accross the turbine. Mass flow is not going to be affected at all by changes to CR. A higher CR will result in a higher inlet temp to the turbine however. So, I could see this having some impact - I had just assumed this would be pretty negligible. When i finish my procharger analysis, I will have to build a turbocharged engine model and do some transient analysis of turbine acceleration as a function of CR.
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Old 11-22-2011, 12:08 AM   #19
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You got think though, those graphs were probably on a engine with good flowing exhaust, not pushing all that air though a small turbine like a turbo motor does. Run that same motor with a blocked off small frame exhaust housing and see what it makes.
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Old 11-22-2011, 12:23 AM   #20
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Quote:
Originally Posted by FSAE_Junkie View Post
I think you've mis-interpretted the intent of my post.

Why would an NA graph not mean anything when the topic it was addressing was off-boost torque? As you'll see, most comments in this thread are just saying that they felt the 8.6:1 CR had poor off-boost (NA) performance. My point was that it would still be terrible even if the CR was 9.5:1. A 5.3L out of boost is just not going to knock your socks off.

As for 'in boost' performance, I'm still doing some analysis to see which route I will go. I'm mostly looking at simulated piston surface temps at a given power level (900 crank hp)- with varying combinations of compression ratio, boost, and the 3 cams I have in my garage. What I'm hoping to see is that more boost, lower CR yields lower combustion temperatures than less boost, higher CR. I could be completely wrong though - or it may be a wash.

I have not finished my analysis, but I have a feeling I will likely run the 317's on my 5.3. Partly because the car has an F1A Procharger and thus I have to make up the 90-110 hp required to drive the blower. i.e. making 700 rwhp will stress the motor like it's making near 800 rwhp. I would like to do everything I can to keep the motor together for a little while at the 700 rwhp level and I'm thinking the low CR will aid this.
Secondly, the car has a 4400 stall and a TH400 so, any pedal is going to negate any concerns of off-boost performance anyways.

As for spool time vs. CR. I have always been a bit suspect that it's as much of a driver as people believe. What drives a turbine is mass flow and temperature extraction accross the turbine. Mass flow is not going to be affected at all by changes to CR. A higher CR will result in a higher inlet temp to the turbine however. So, I could see this having some impact - I had just assumed this would be pretty negligible. When i finish my procharger analysis, I will have to build a turbocharged engine model and do some transient analysis of turbine acceleration as a function of CR.
In theory more compression may not decrease spool time. However if the engine can accelerate faster due to a higher compression ratio will this not
decrease spool time? In theory?
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Old 11-22-2011, 12:23 AM
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