crazeinc

This is only for curiosities sake, but if you bumped up the CR from 8.5:1 to 10:1 with a new pair of heads without changing anything else, how much HP/TQ do you expect to gain?

------------------
1982 Camaro Z28

should be running 11's this spring...we'll see
Check the website for progress http://members.fbody.com/Pro82Z/

AlkyIROC

I don't have the formula handy but by increasing the compression ratio you only increases the torque. It doesn't change where it peaks, just increases it.

If your peak torque with a low compression engine was at 300# at 3000 rpm and you increased the compression ratio, the peak torque could now be 350# at 3000 rpm. That's assuming all you did was increase the compression ratio. By changing the cam, you change where the peak power is.

------------------
Follow my racing progress on Stephen's racing page
and check out the race car

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
461 Big Block installed and ready for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

crazeinc

well...torque is good too

The ODB

Sorry but there is no direct corellation in reality. The end power result depends on a multitude of factors like piston speed, valve events, chamber shape, etc.. There is really no way to slap a given quantity to a compression change, although I have seen a % efficiency increase in print before.
Bottom line is you have to actually test every case individually.
Yeah usually you could assume a small increase (percentage wise) in power output and efficiency from bumping compression up, but like I said there are way to many factors involved to claim any true numbers. Serious racers will generally target a certain compression ratio to match the fuel blend they plan to use.
So you see the fuel and the comp ratio have to work together to get the increase in efficiency.
For pump-gas, racers tend to shoot for 10:1 with iron-heads, and 11:1 with aluminum heads, unless they take certain anti-detonation measures.

well that's what I know. Sorry I can't give a simpler answer.



------------------
White 1986 Irocz, 305/383 with Edlebrock Performer-RPM intake and Performer #1407 carburetor, +110hp shot of crack, 700R-4 tranny, 3.25:1 rear, Mcreary Road-Stars, single 3" Borla exhaust, Linginfelter-TPI camshaft pulls 17" vacuum solid. N/A runs 10.9 @124, Crack-runs 10.3 @135... haven't run at track since Oct-99

RB83L69

That depends on the heads and on the rest of the motor. Your topic and your description are 2 different sets of numbers.

If the heads in scenario #1 are identical (like 2 models of Edelbrock Performers or something like that) you'll probably gain about 6-8% or whatever HP you were making before. So your 350 HP motor would probably end up at around 375-380.

The scenario in your body text cannot happen with a 350, as the numbers don't work out.

What are you really asking?

------------------
"So many Mustangs, so little time..."
ICON Motorsports

crazeinc

Alright, I was trying to keep it as simple as possible, but here goes:

I've got a rebuilt 350 w/ the Performer package except for an RPM intake and stock 74cc heads..The compression ratio is about 8.5:1 or at least that's what the previous owner told me so I just won a set of double humps on ebay that have been ported, bigger valves installed, magnafluxed, decked, the whole nine yards and they were milled to 63cc. This, I was hoping would bump up the CR to around 10:1. Rather than further complicate the situation by saying I'm getting a new cam and converter to match the heads (which I am), I was just curious how much HP/TQ would be had by just swapping the heads and keeping everything else the same?

I know it doesn't make any difference since I'm changing other parts, but I was just curious...

RB83L69

OK...

I don't know what heads you've got, but that doesn't matter very much. A flat-top 350 with 64cc heads is about 10.3:1. However, 76cc heads on top of 12.5cc dish pistons gives 8.6:1. So it's anybody's guess what you've really got.

Without knowing what heads you have, there's no way of guessing what the difference between them and your new ones is.

When you get it torn down, tell us what kind of pistons are in there, and maybe we can make a less uneducated guess.

------------------
"So many Mustangs, so little time..."
ICON Motorsports

crazeinc

fair enough, thanks RB.

burnoutrpm

are you putting the heads only on or are you puttine the cam and stall in tooo?? if you put in all 3 the you could get about 100hp at least! more compression, better air flow allround, bigger better cam and stall! yeah should give you minimum about 70-100 hp and lots of tq! enjoy let me know about the cam and stall (if you put them in!) just email me !

------------------
1989 firebird formula

Mods: converted from T.B.I. to a carburator 305 to a 350. Flowmater exhaust,hedman shortie hedders,202 160 (882) heads,.447/.447 lift 222@.050 duration speed pro 327/350hp cam 350 .40 over (356) Edlebrock torker2 and a 600 edlebrock manual choke (1405)

Future mods performer rpm air gap intake (polished) comp roller cam, and way better headsa 400 defintely in the works!

Box of Rocks

Crazy,

(My favorite Patsy Cline song, except it was written by Willie Nelson)

Eh, like RB83L69 said, we cannot really determine what the change would do with accuracy, since we don't know what you actually have, BUT......

In very general terms, with no other changes, the latest numbers I've seen on a 350 street motor like yours is about 4% per point of compression. So, 1.5 points is about 6%.

On a 300 HP car, it becomes 318 HP.

BOR

crazeinc

Alright...I'm still clueless about my 350 shortblock because I haven't torn it down yet except for the 76cc heads, but here's what I'm putting on it:

-quoted from person I bought them from "Small block chevrolet iron fuelie (or double hump) heads. They are fresh from the shop where they were magnafluxed, surfaced, 3 angle valve job. valves ground and back cut, and blueprinted again. The heads are 63 cc's and it has 1.943 intake valves and 1.64 exhaust. It has the bigger exhaust because I intended on using them on a 327 with a lot of nitrous but as it ended up I only used them with 2 bottles and only 125 horsepower before I went to aluminum heads. I had the shop do them because I was going to use them again but I sold the car I was going to put them on. They have been cut for .500 POSITIVE seals and have the seals installed. They are ready for bolt on performance increases. The bowls have been blended along with port work. They have 115 # closed on the seat, with 340 # at .575 lift, and will coil bind at .615. Seal to retainer interference occurs at .640. The valves are one piece swirl polished valves. The springs, retainers, and locks are all brand new."

-Continental 10" 3000 stall converter, custom nitrous/boost style

-Comp Cams CS XE284H-10 .507/.510 lift (I plan I using 1.6 rockers, though) 240/246@.050


[This message has been edited by crazeinc (edited February 04, 2001).]

Basett Racing

What is the casting number on the heads??? The older heads, 62-67, had no accessory bolt holes in the front. These were usually( I'm using the last 3 digits of the casting number) #461, #462, #281 and there are some others. The later heads '68-'70, #186, #492, and others had the accessory holes. If none of these numbers are on your heads, go to http://www.mortec.com and you should be able to find the heads you have.
It would be hard to compare the HP numbers using only the compression ratio, because the "double humps" probably flow better than the open-chamber heads you had on the engine, Especially since they've had some bowl and port work. The 1.94int/ 1.62 exh set-up works well with nitrous, I know this because I have a friend who runs that set-up on his Nova, He's running 11.40's on a 150Hp shot with a 350 w/ported double humps.

------------------
82 Z28 350, Ported #882 Heads, Performer RPM cam and intake, hedman headers,650 Demon carb,
Trans: Turbo 350 w/ 4000 stall -- Rearend 7.5 w/ Richmond 4.10's, Auburn Minispool
Best ET: 12.52@107.2
Future plans: Dart headed, Roller cammed 383 in early '01


89 RS, L03 305, Hypertech Chip,cat delete, Dynomax exhaust,K&N open element Filter,160 stat, MSD coil --Trans:700R4 Corvette Servo -- Rearend: 7.5 GM 3.42 w/ posi-lock
New Best ET: 14.91 @92.9
Bassett Racing
Mid Atlantic F Body -82 Z28 Page

crazeinc

accessory holes aren't a worry since I run a low-mount alt and an electric water pump, but I plan on running a little bit of juice on the motor (nothing over 125)