"First off I’d like to say I made a typo… I did not mean a 4% loss in HP. The “rule of thumb” indicated 4-5% in power (aka torque) Not HP."

At any given RPM a 4% loss in torque=4% loss in HP. HP=TQxRPM/5252

 

5.3 with 317 heads is a dog out of boost. Speaking from personal experience here.

 

lol. I dont see an argument here. I just wanted to know if in your testing you changed cams?Im guessing that answer is no? because with higher compression you can cam it larger and have the same dynamic compression but a much better top end. i saw a NA 427 test where a 2 point change in compression was worth 100hp!!!

now what happens when you increase the mass with boost. now your "4-5 percent of 800-1000hp is now a big number no?

finally, ive a-b tested 862 heads vs 799(243) heads
the extra flow of the 799 heads DID NOT make up for the .4 loss in compression. it was dead even. so my testing tells me that id rather have the extra point in compression of the 862 than the extra head flow( and heavier intake valves AND more bore shrouding)of the 799/243/317

 

I think your turbos were maxed and thats why you didnt see the benefit of the better flowing heads.

 

I have to agree with ForceFed86 here. I've run dozens of combos with varying compression. The whole 'dog out of boost' thing for a V8 is total BS. The half point drop is marginal. You could make up for the loss in compression with a couple more degrees of timing. Compression doesn't increase power in a linear scale. You won't double your power going from 8:1 to 16:1. Diminishing returns kicks in pretty quick once you have enough compression to keep the engine running.

There are TONS of OEM 4-cylinders with 2.4 liters or less running 8:1 - 8.8:1 compression stock, and they drive just fine. My 2.4 liter SRT-4 came stock with 8.0:1 compression and was just fine to drive anywhere. To say an engine making more power N/A than these motors make with a turbo is a 'dog' is usually just someone whining too much. It will not be a dog, it will not require you to put more foot into it to get it up to speed or cruise through town.

If its a dog for those .3 seconds after you smash the pedal to the floor and you're waiting on boost to climb, then it will all be worth it with the added top-end power you are afforded with added boost from lower compression.

Saying a motor thats around 9:1 N/A is a dog compared to one at 9.5:1 is absurd. Most of the earlier small blocks and big blocks came with 8.5:1 compression for an N/A setup and drove just fine, which leads me to believe you're talking about wide open throttle before boost hits. If you're worrying about the performance of your turbo engine, why are you worrying about how it performs before full boost hits anyway? Doesn't that fall into the category of irrelevant?

 

Actually,I did this testing last year when it was n/a

Also, the fastest my car ever ran NA was with the stock 862 heads.

 

that makes no sense.
edit: after thinking about it for 2 seconds, in N/A form it would definitely be negatively influenced by .4 less compression, way moreso than a turbo combo would suffer, as cylinder pressure could be regained by a couple more psi of boost.
Out of boost additional timing can be used to regain throttle response.
I will be doing a little bit of experimenting myself as I built a friend a 5.3 turbo combo which will run stock 862's for the first season. it will be dyno tuned on an engine dyno, then ran at the track. after it is optimized, the following season we are swapping milled 317's on it(to get back close to stock compression). Ill be sure to share what I find.
from my experience the better flowing head will prevail unless another variable is holding the combo back.

 

I think it matters even more under boost. sure, more boost would make up for it, but at a cost of more intake heat-making it more prone to detonation, more exhaust pressure aka backpressure, the need to run less timing, etc etc.

I think in your test the better flowing head will prevail but only because you are bringing the compression back up to normal. me, I yanked the 799s off my car and sold them and Im going back to 862s. the higher compression, lighter valve, and less bore shrouding just make it a highly underrated performer.

 

I think that if the 2 different compression heads flowed the same, your above statement would hold true, but if the lower compression head flowed more air, it would take less boost psi to make the same amount of power..... meaning there would not be as much - " prone to detonation, more exhaust pressure aka backpressure, the need to run less timing, etc etc." - as you may think.

And I am certain that milling the 317's will make them head and shoulders above the 862's in this comparison, and I will share the results I find.

 

Why would you swap back to a head with less flow instead of milling the better flowing head? Do you think a .040 shaved 317 (64cc) would make less power than an 862 head? How does a shaved 317 differ from a 243/799 casting? If your theory on valve weight and bore shrouding was true, why do 243 and similar castings outperform the 862 casting on NA 5.3's/4.8s?

Not comparing apples to apples NA vs FI anyway. Assuming you could rule out charge temps and detonation all together, I could see what your saying working up until the 862’s became the bottle neck. (not claiming I have any idea what that point is) Then the 317 would still make more power after that point. Most don’t run race gas or alcohol, and detonation is a big issue.

IMO, Bottom line is… Higher the boost, higher the power. For your average pump gas build you’ll be able to run more boost at 8.6:1 than you could at 10:1. Take your snappy 10:1 862 headed motor at 14-15lbs VS a 8.6:1 317 headed motor at 21-22lbs… Line them up at the drag strip and I guarantee no one will think the lower compression motor is a “dog”.

Not that there aren’t many ways to make power, but here is an example of what a “doggy" 7.2:1 compression SBC is capable of… I’m sure it can barely get out of it’s own way below 4k…

355 cubes, std stroke
256/245@ 114 cam
.510 lift
Oem iron heads
162 cc intake runner
1.9 , 1.5 valves,
Twin 57mm turbonetics

(info on the top is just left over from another run pay no attention to it)

 

why would I waste money having the heads shaved when I could just run 862s? lol

why would I want to make the deck any thinner than it already is? why would I want to reduce my piston to valve clearance in any way?

you can give hundreds of why scenarios it doesnt change the fact that the lowly 862 outperforms and what it may or may not lack in flow it more than makes up for in the compression

I personally havent seen any stock 317s outperform a stock 862 in a back to back test so I dont know. there might be stock 243s out there that did better but I havent seen it either, and in my case there was no difference in performance.

Empirically, the 862 could NEVER be a bottle neck on a 5.3. the shortblock will scatter well before you reach the limits of the head. this is not just regurgitated info either. I have data logs of 862 heads on my 6.0 revving cleanly and strongly to 7500 rpms and making a ton of power for the boost.



Ill put my money on the higher compression motor /lower boost engine everytime - for performance, longevity of parts, and drivability
my 10.5:1 6.0 with stock 862 heads ran 6.0s in the eighth mile on the stock truck cam, at only 11 psi. !

my 10.5:1 ls1 346 recently ran a 5.99 at less than 9psi on a crap pass , so I know a little something about outperforming on higher compression and lower boost.

less back pressure
less intercooling requirement
less intake heat
can use a tighter converter
easier on valvesprings
better gas mileage
etc etc

 

Did you ever notice how every manufacturer runs lower compression when they put boost to something compared to their N/A counterparts? It's not an accident...

 

We’re buds, so this is all just for the sake of healthy debate.

If your all about the low boost NA performance, power production is all in the top end and RPM. The higher flowing head will come out on top power wise in or out of boost. With a similar size chamber on a 317 it will outperform a lower flowing head. Claiming anything else is ridiculous. A 243 headed 5.3 will make more power NA than the 862. The fact that you run methanol means you can get away with a lot of things most people can’t. The rules that apply to a methanol setup don’t apply to a street/strip engine.



Your claiming there is less longevity in a 9:1, small cam, low rpm, high boost motor than your 10.5:1 compression engine reving to 7500rpm that is cammed to take advantage of that higher compression/rpm? I disagree! Especially when we are talking about a street/strip setup spending most of it's time out of boost.

You are talking about a dedicated methanol fueled track car and MPG? What kind of MPG do you get out of that methanol fueled 347? What kind of cam do you run? I’m willing to bet it’s not geared toward valve train longevity, and mpg!

Personal examples are pretty useless when trying to sort out the facts. It’s like pointing out that I went 5.7 in the 1/8th on an 8.6:1 OE junk 5.3, OE cam, netting 24mpg and driving it to and from the track on pump gas (e85). There are a million reasons/loop holes that don’t apply to what we are talking about that allowed me to do that. Just like your setup.

Who's lower compression setup are you "out performing"? Assuming your engine could take it, your same setup at 9:1 and higher boost could easily double the power your making now. There is a reason the competitive turbo small block world doesn't run 16:1 compression and low boost. While your setup is cool it’s no street car and wouldn't perform well off methanol.

I don't get why running the numbers at lower boost seems to impress you. You are choosing to make cyl pressure with compression. When you could make it more efficiently with boost. Boost is more efficient at making power than compression. Otherwise there would be no need for forced induction.

I like this quote…

 

Right on I like healthy debate. Ive learned from it and it helps newbies make educated decisions.

lol. you are claiming something based on what head flow tells you it should be. Im telling you that experience(with data) trumps feelings. I HAVE tested 799 vs 862 and when you look at fuel flow data, neither one had an advantage over the other that was about a .4 difference in compression. the 317 will be about a 1.0 difference in compression and I will tell you a stock 862 will outperform a stock 317 hands down. compression wins over flow in these heads.
lets use our cars for comparison since they are both fairly light and fairly similar. btw, I love your car.


yours is 5.3 with a glide, 8.6:1 compression,s475 1.10 t4
mine is 5.7 with a glide, 10.5:1 compression,s475 1.32 t6

your car has run a best 5.703 at 122.72
my--car has run a best 5.995 at 116.92

your car 1.35 60ft launching at 24 psi boost
mine car 1.41 60ft launching at 1 psi boost

your car average psi through the run 23 psi
my car average psi through the run 7.4 psi

you would agree that this kind of makes it clear how big a disadvantage there is by running lower compression. your car has run faster, but it has taken 3x the boost and 24x the launch boost to do it. looking at the info, there is no other explanation I can give for the difference in boost numbers.

what does high boost mean?

1. your turbo is spinning much faster than mine at our boost levels
2 this puts more heat into the system
3. heat reduces density which causes less output per psi as boost increases
4. this heat makes your car more prone to detonate
5. this causes you to reduce timing
6. this causes you to run more boost, which causes you to further reduce timing and before you know it you are running 3x the boost to achieve the same result.

I wont repeat the other problems associated with running high boost since Ive listed them already.

I like these quotes:

"Knowledge is light"

"Give the people light and they will find their way

 

You can't compare your 2 combos accurately without ALL the details such as raceweight, and rear gearing, trans gearing, converter specs, and everything else about each combo.......also all the incrementals from those 2 quoted laps all the way out to the 1/4 mile

 

I agree.

Since both cars are running the same compressor, shouldn't each be limited on the compressor outputs? The higher pressure ratio setup moves more lb/min.

Quickly put both into Matchbot
5.7, 7.4lbs
5.3 24lbs

 

agree on the first point. the compressor ultimately limits the amount of power capable. I have however heard some say that when you are turbo limited, increase compression.

on the second point, you can have higher pressure ratio but a lower lb/min

agree. there are many factors that could come into play.

however,if we make an educated assumption that my engine is at the 550hp level NA, and that 2 pts compression equals 10%loss in power, then we know mathematically that what I ran on 7.4 psi would take his combo a bare minimum of 9.8 psi. this is not factoring in additional heat from extra boost. if we factor that in then its more(Im not at my chart so I cant see how much more right now)

then add the fact that I can run a much bigger cam even if we choose to keep the dynamic compression the same. this is going to allow my combo to breathe better upstairs with no loss downstairs.

so in all I would agree that not all the difference comes from compression, but I would say that much of it comes from the compression difference

 

I agree that compression increases efficiency. No doubt that more efficiency is a good thing. Question becomes, can we stay under the knock limit of a fuel with a less efficient setup burning more air and fuel? If so is this a better setup? I'm not sure, I guess it depends on your goals.

Great info in this thread.

 

71 chevy, the methanol fuel in your combo is a MAJOR factor that hasnt been mensioned. especially with respect to the compression.
As far as the head flow comparisson, I dont beleive forcefed86 turns his combo enough rpm to truly take advantage of his 317's either......

 

and that is the question- my own personal and untested opinion is that with the intercooling technology available today,even 9.5:1 is on the low side.


true talk. methanol allows one to run nice static compression.