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Discussion Starter #1
Hey desktop dyno folks, can you shed light on an ongoing debate? How much of the 75 hp gain the Ford crate 392 realized came from compression versus the heads? The Ford 392 crate motor delivered 537 hp after switching from 9.7:1 cr gt40X's to the 10.8:1 AFR185s. The 185's had 54 cc chambers vs the stock 64cc chamber gt40x's. So the hp increase wasn't just the diff in the flow of the heads.
So, can you break down the 75hp diff into its components? How much does DD say a stock 392 should make at 9.7:1? How much at 10.7? How much with AFR 185s?





<font size=-1>[ This Message was edited by: R Code on 3/29/02 3:41am ]</font>
 

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I was reading that article the other night and I noticed the samething with the compression. I have been curious about the comparison between the two heads also. I own a set of 64 cc gt-40x heads currently on a 302. I know that the 185's would have been too much for my 302 but I'm hoping that once I build a 347 the X heads can be ported comparably to the 185's. Any thoughts?
 

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I won't be on my computer 'till later tonight, but until then, I'd guess that the compression accounted for around 20hp, and the rest of the gain was from the heads. I'll check out what DD has to say later tonight.
 

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Discussion Starter #4
Thanks Motorhead. I look forward to the comparison "dyno runs". Even AFR has added fuel to this fire by now running ads touting the 75hp diff. (Edit job - OK they aren't THAT far off. I ASSumed the split was going to be bigger. Learn something every day on this forum. I did spark a bit of conversation, though
)

<font size=-1>[ This Message was edited by: R Code on 3/29/02 3:45am ]</font>
 

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For what it's worth, Engine Analyzer says that going from 9.7 to 10.7 compression is worth 12 HP on the GT40-X heads and 11 HP on the AFR 185's.
 

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My two cents ...

First off ... I don't have a computer Dyno Program ... I just have friends that do


OK ... THIS IS MY TWO CENTS and it doesn't really answer much of the question, just my thoughts after reading the question (and a couple of responses) ...

It seems to me that the idea of "High Compression" creating goobs of power is way over rated.

Everything I know tells me that while raising compression does indeed increase efficiencies ... continuing to raise compression does not raise efficiencies equal to the incrumental increases in compression. (if that made sense)

An increase from 8.0 to 9.0 have a greater affect on engine efficiencies than will another step from 9.0 to 10.0 ... or, 10.0 to 11.0

It is my understanding that once you pass the 10.0 to 11.0 range the advantages of higher compression are about meaningless to the average "Gear Head" - "Street Hot Rodder" ... which probably describes a bunch of us guys in here.

To add to that ... once you get past that compression range the problems created by the increases are more than most guys are wanting to deal with.

All you Thouroughbred race guys are urged to disregard everything I just said. Your engines most likely need every bit of power you can squeeze out of'em and longevity & fuel selection are most likely not the most important issues.

With that said ... Anyone that wishes to kick me in the shins ... I'm ready


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<font size=-1>[ This Message was edited by: monstermach on 3/28/02 10:47am ]</font>

<font size=-1>[ This Message was edited by: monstermach on 3/29/02 2:54am ]</font>
 

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I didn't want to be the one to burst everybodys bubble but since you did Larry I guess I'll add to the burst. Totally true, the higher the compression gets raised. the less of a gain you'll get and the gains are mostly in the low end/midrange while the top end higher RPMs don't gain as much. At higher RPMs, the engine is working much more efficiently especially if over cammed for the lower compression because of the column velocity is helping to somewhat ram the air/fuel in the cylinders but at lower RPMs there isn't enough but the added compression makes the engine more responsive and efficient at the lower than ideal RPMs for the particular hypothetical engine being used. Low compression engines are usually down on low/mid RPM torque and HP but still come alive at the "magic" RPM band where it all works. Add compression and the RPM band widens moreso to the lower end of the RPM band but a little up top too. I would never consider a general HP number for each point increase because it depends on if compression is optimum before raising it, if it's higher than the cam really needs, if it's lower than the cam needs(before the added compression), if you go from 8:1-9:1 compression, go from 13:1-14:1 compression, and the cubic inch of the engine. Too many variables to come up with a generalized HP number for each raised point in compression. A big block with 8:1 compression and aluminum CJ heads with solid roller may pick up 50-75 ft/lbs torque and 40HP by going to 10:1 compression but a 289 with 10:1 compression with AFR 165s and hydraulic roller may only pick up 5-10HP(or less) and 10-15ft/lbs(or less) torque by going to 11:1 compression. It all depends (no, not the old people diapers) on the circumstance and engine combination.
 

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Also most of the overall gain will be below the torque peak and some at the torque peak but more so below peak all the way to off idle where the engine becomes more responsive and runs cleaner. High compression especially helps out the torque of engines using heads that have intake ports that are too large for the RPMs the engine is running so added compression restores a lot of the lost low end the same as compression helps out a cam too big.
 

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Just want to cover my a$$ here a little and say that I don't have ANY experience building small blocks, and very little experience building engines period. I'm just a 22 year old college student that's done lots of reading and lots of playing with Desktop Dyno, and the 20hp was purely a guess. I know you guys weren't directing anything bad toward me, far from it, but I just wanted to get that out in the open so everyone realises where I'm coming from and how much I really KNOW about this stuff (not much at all!). Anyways, enough with the disclaimers
, here's the numbers I got:

GT40X & 9.7:1 418hp @ 5000, 480tq @ 3500

GT40X & 10.8:1 438hp @ 5000, 498tq @ 3500

AFR185 & 10.8:1 506hp @ 5500, 520tq @ 4000

So according to Dyno2K, the compression was worth 20 horse (dang, I guess good!
) and the heads were worth 68(!) hp negating the compression gain, or a total gain of 88(!!) hp by bolting on the heads. I don't know, does that seem a little inflated? This is where the experience part comes in, which I have none of!


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<font size=-1>[ This Message was edited by: Motorhead on 3/28/02 3:10pm ]</font>
 

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89 coupe ...

As you explain that ... it shows me a little about why my "new" build (351C 4V) seems to have more power in the lower and mid range than my "old" build.

I had a machine shop do my first rebuild, The guy built at 9.0:1 compression (open chamber heads). The engine was truely a pig below about 3,200 RPM ... not bad once you got above that though.

The second rebuild, I did myself using Quench heads which we worked to achieve 11.1:1 compression along with a more agressive mechanical roller cam. Being a 4V the power band is from about 3,500 to 7,000, but with the higher compression and bigger cam, the added power below 3,500 is very noticeable compared to the "old/first" rebuild at 9.0:1.

Anyway ... seems this thread will make clear to anyone out there that has been lead to believe that compression is the "pot of gold at the end of the rainbow" for power production ... You always have to build with "TOTAL PACKAGE" in mind, A good friend taught me that early on and I'm here to tell anyone that will listen ... He was correct


OK ... I'll go away now




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<font size=-1>[ This Message was edited by: monstermach on 3/28/02 10:27pm ]</font>
 

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Sorry, but I want to tangent this thread for a second. Can I port a set of GT-40x heads to be comparable with 185's? And will this be too much head for a 331 or 347? Thanx
 

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I took a picture of a dyno sheet I saw at the AMC nationals in Houston, TX last year because I thought I was to good to be true. It was a 401 stroked to 422 w/ a .650 solid roller and stock ported dogleg heads with larger unshrouded valves w/ 9-1 comp that made 650 hp. DAMN
 

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Discussion Starter #13
On 2002-03-28 03:06, Motorhead wrote:
So according to Dyno2K, the compression was worth 20 horse (dang, I guess good!
) and the heads were worth 68(!) hp negating the compression gain, or a total gain of 88(!!) hp by bolting on the heads. I don't know, does that seem a little inflated?
Well if you mark it down to match the reported 75hp gain you get 18 and 57 split. So 75% of the gain was due to the heads. Alrighty then. I still think the author should have made this exact point. Good journalism is hard to find. Anyway, now when someone mentions that article and the response is "yeah, but they kicked up the cr" we have a factoid. Many thanks Motorhead, 89Coupe, et al.
 

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sticking to the exact combo of the ford 392W crate engine. I think it is pretty obvious that the stock GT40X heads are not the optimum head for that combo. The only real way to find out how much the compression increase is worth would be to run the GT40X heads with both 64 and 54 cc chambers AND run the AFR 185 heads with 54 and 64 cc chambers. Im still betting you wont see 75 hp diff between these heads given the same compression...either at 9.7 or 10.7 to 1.
 

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On 2002-03-28 12:01, Jimmys66 wrote:
Sorry, but I want to tangent this thread for a second. Can I port a set of GT-40x heads to be comparable with 185's? And will this be too much head for a 331 or 347? Thanx
IMO yes. AFR's are great heads but it has come to pass that they are the end all to the small Ford power producing spectrum. Most people don't realize the attention that is given to the AFR head out of box. They basicly have a stage 1 or 2 port job on them from the factory. Combine this with good cross sections and you have an awesome head. Does that mean the Gt-40x's, edelbrock, twisted wedge, or any other out of box aftermarket Ford head is junk now? Absolutely not. Give them the same level of refinement and they will produce close to the same way, it just may cost you more if you farm the work out. IMO the Twisted Wedge is still the best bang for the buck. I went and looked at a set, a friend is selling, last night, that have been ported by a shop out in LA. They are reported to flow 290/[email protected] .600. I could easily replicate the port work, with average porting knowledge, and TW's are less than a $1000 new. With the same level of refinement the gt-40's will be a sound investment.

<font size=-1>[ This Message was edited by: jdsgallops on 3/29/02 9:21am ]</font>
 

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Thanks for the reply, I needed a set of heads that could "grow" so to speek as I needed it. I knew the 185's were to much head for a 302. The X-heads I bought used from a police officer in Virginia. He was selling them with 13,000 miles and had them ported and freshened after 10,000 miles on them. He used the money to buy 185's. I found it curious that he didn't spend a few hundred bucks or a few hours to finish porting them. I may end up spending the same as a new set of heads but I can use them now and for a future (bigger) build.
 

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Last year I had 13.4 to 1 compression ratio, using the same cam, but different heads. (old worn out windsor sr/s.). Those old sr/s were ported to hell, port matched, 2.06 /1.66 valves, all the goodies, besides being made of iron. That combo made 572 hp and 517 ft lbs tq.
This year all I changed was the heads, (Ed Curtis prepped 205's), and bigger headers, (plus had Biggs go thru the 950 HP).
It made 610 hp and 544 fts tq. I am currently running around 11.4 to 1 compression. I lost 2 full points with the aluminum heads, via porting/cutting, and just the fact they're aluminum.
My dyno operator was shocked at the HP/TQ numbers we were able to produce this year.
All the HP and TQ we gained was due to just plain old AIR movement. Velocity.
It totally offset the compression losses.
My .002.

(next year will be a totally different story tho. HUGE roller cam and 15 to 1 compression.)
 

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jdsgallops:

I agree

Only unless your going to step it up big time to say a race combo...I cant see why the tw heads(and others) with proper porting wont work in almost every hot street motor combo under 400 cubes. If you look at the results of the work that TEA got from a set of Winsor jr heads(an article on the website)...Im convinced that I dont have to dump my tw heads just yet....and really that is the point...if you already have a set a decent heads it seems to me it would take a real change in the combo to justify having to buy another new pair of cyl heads.
 
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