Performance Difference between 8.3:1 & 9.6:1 compression in a 454 [Archive] - Chevelle Tech

: Performance Difference between 8.3:1 & 9.6:1 compression in a 454


RAMBO
Jan 5th, 10, 5:26 PM
Doing some contemplation with my Low compression 454 build...

The motor came with stock 781 heads on it... Haven't CC'd them yet so for now we'll assume ~119cc

Contemplating trading for a set of 390 98cc heads.

With this engine setup:

454, 4.370 bore, 4" stroke
Flattop pistons (.080 over) eyebrow valve reliefs)
~.022 DITH
.02 shim head gasket
Lunati 60203 cam 4degrees advanced

Kettbo estimated the 781's would put it ~8.3:1 comp & with the 390's it would be around 9.6:1

------------

The question is- What would the real world performance difference between them be?
How much HP & Torque would i be giving up staying lower Comp. Can anyone crank out a desktop dyno of the two combos so i can see what the differences look like?

idea is that down the road a year or two if i rebuild this motor and change the pistons i might be glad i kept the 781's. A full go through is not in the budget now, and see no reason the motor can't go back together with shim headgaskets & performance cam and be enough to make me happy for a while. Plus it would be nice to run cheap gas in it... However as i said, if the performance difference is significant, then to hell with the cost of gas :)

Thoughts?

EDIT: Additional

Couple ???
Carb & intake?
Exhaust ?
Cam's 4's over the ground in 4's ?

Holley 800 DP
Edelbrock Performer 2.0 gasket match ported
Hedman 65002 Full length 2" primary 3" collector
2 1/2" H-pipe exhaust w/ full tailpiples
Dynomax Super Turbo Mufflers 19" long case


For cam advance this is what was recommended:
X2 on the 60203. Run it on a 103-105 ICL and it'll rip

Mike
Jan 5th, 10, 5:32 PM
Couple ???
Carb & intake?
Exhaust ?
Cam's 4's over the ground in 4's ?

Rough guestimate is 404hp vs 412 hp @ 4500 & 540tq vs 566 tq @ 2500
Used 750 carb ,dual plane and headers w/muffler's.

RAMBO
Jan 5th, 10, 5:42 PM
added some more detail to the bottom of my first post...
I'm not really up on the whole cam advance/ICL thing- just posted what Darryl recommended for me..

Mike
Jan 5th, 10, 5:50 PM
OK 3's over the 4 already there.
Changed carb ,now get 401/405hp and 541/566tq

Bowtie91
Jan 5th, 10, 5:58 PM
It appears you may be better off with the lower compression engine for now, and as you said later on you can go through the bottom end and up the compression a bit to add onto what you already have. Sounds like an interesting build though, keep us up to date on it!

RAMBO
Jan 5th, 10, 6:12 PM
I'm surprised the HP peak is only at 4500... sure looks to be a torque monster though...

Just for kicks... what would the HP & toruqe be if it had a domed piston in it- putting it closer to 10.1:1 comp

Also for kicks, 60204 Lunati cam

Thanks for punching this stuff in for me!

Brettd85
Jan 5th, 10, 7:15 PM
Rambo, I had the 60204 cam with 10.5:1 compression and my power peak was at 5600 if I recall correctly. I'll check the dyno sheet when I get home.

Mike
Jan 5th, 10, 7:22 PM
I'm surprised the HP peak is only at 4500... sure looks to be a torque monster though...

Just for kicks... what would the HP & toruqe be if it had a domed piston in it- putting it closer to 10.1:1 comp

Also for kicks, 60204 Lunati cam

Thanks for punching this stuff in for me!

W/781's it's 467hp @5000 & 565tq @2500
That's w/cam at 4's.
W/your additional 3's it's 450hp @5000 & 568tq @2500.

RAMBO
Jan 5th, 10, 7:42 PM
W/781's it's 467hp @5000 & 565tq @2500
That's w/cam at 4's.
W/your additional 3's it's 450hp @5000 & 568tq @2500.

is that the 60204 cam or with the domed pistons?

Mike
Jan 5th, 10, 7:43 PM
different cam & c/r.

RAMBO
Jan 5th, 10, 7:44 PM
different cam & c/r.

can you show me the cam with the lower comps?

Twins Fan
Jan 5th, 10, 9:14 PM
With your numbers and the 781's, DynoSim came up with 8.47 to 1 compression. 60203 cam made 408 HP @ 5000 and 504 TQ @ 3500. 60204 cam made 420 HP @ 5000 and 486 TQ @ 4000.

With the 390's it came up with 9.92 to 1 compression, which seems high but with the big bore it could be right. 60203 cam made 441 HP @ 5000 and 537 TQ @ 3500. 60204 cam made 449 HP @ 5000 and 516 TQ @ 4000.

Just for fun either heads (don't have different flow numbers) with 2.19/1.88 valves and 10.5 to 1 compression came in at 495 HP @ 5500 and 540 TQ @ 4000 with the 60204.

Brettd85
Jan 5th, 10, 11:52 PM
Just for fun either heads (don't have different flow numbers) with 2.19/1.88 valves and 10.5 to 1 compression came in at 495 HP @ 5500 and 540 TQ @ 4000 with the 60204.

Considering thats basically my combo but my heads are ported and that was just about right on the money. :thumbsup:

EDIT: Dyno sheet calculates out 483 hp and 570 ft -lbs assuming 15% loss.

I had 484 RW ft-lbs at 3800RPM and 411RWHP at 5200RPM. Remember you have a couple more cubes on me...

I wonder how my current pistons and cam compare... :confused:

RAMBO
Jan 6th, 10, 1:21 AM
With your numbers and the 781's, DynoSim came up with 8.47 to 1 compression. 60203 cam made 408 HP @ 5000 and 504 TQ @ 3500. 60204 cam made 420 HP @ 5000 and 486 TQ @ 4000.

With the 390's it came up with 9.92 to 1 compression, which seems high but with the big bore it could be right. 60203 cam made 441 HP @ 5000 and 537 TQ @ 3500. 60204 cam made 449 HP @ 5000 and 516 TQ @ 4000.

Just for fun either heads (don't have different flow numbers) with 2.19/1.88 valves and 10.5 to 1 compression came in at 495 HP @ 5500 and 540 TQ @ 4000 with the 60204.


Gotta be an error with the 390's... 9.9:1 seems too high of a jump between 119cc heads and 98cc heads.

thanks for running them though!

Brettd85
Jan 6th, 10, 1:41 AM
Ben, I know you havent CC'd your heads but using stock head specs, haven't you run the compression calcs? That will tell you what it really is... I probably wouldnt run the 60204 if under 9.5:1 compression. I would think a better flowing head is more important than higher compression, I'm sure theres a happy balance there though.

kettbo
Jan 6th, 10, 2:43 AM
Interesting thread.
Just how much better flow with the 781 (non-tricked) vs the #390 castings and a sizeable jump in CR?
I am having 049s shaved so I can get 9.5. /9.6:1 to run the "402A3" cam. Solid version of the 60204.

wotr
Jan 6th, 10, 1:13 PM
Doing some contemplation with my Low compression 454 build...

The motor came with stock 781 heads on it... Haven't CC'd them yet so for now we'll assume ~119cc

Contemplating trading for a set of 390 98cc heads.

With this engine setup:

454, 4.370 bore, 4" stroke
Flattop pistons (.080 over) eyebrow valve reliefs)
~.022 DITH
.02 shim head gasket
Lunati 60203 cam 4degrees advanced

Kettbo estimated the 781's would put it ~8.3:1 comp & with the 390's it would be around 9.6:1

------------

The question is- What would the real world performance difference between them be?
How much HP & Torque would i be giving up staying lower Comp. Can anyone crank out a desktop dyno of the two combos so i can see what the differences look like?

idea is that down the road a year or two if i rebuild this motor and change the pistons i might be glad i kept the 781's. A full go through is not in the budget now, and see no reason the motor can't go back together with shim headgaskets & performance cam and be enough to make me happy for a while. Plus it would be nice to run cheap gas in it... However as i said, if the performance difference is significant, then to hell with the cost of gas :)

Thoughts?

EDIT: Additional



Holley 800 DP
Edelbrock Performer 2.0 gasket match ported
Hedman 65002 Full length 2" primary 3" collector
2 1/2" H-pipe exhaust w/ full tailpiples
Dynomax Super Turbo Mufflers 19" long case


For cam advance this is what was recommended:

You say your bore is 4.370. ?

You say your pistons are .080 over. ? Correct? LJ

RAMBO
Jan 6th, 10, 3:29 PM
You say your bore is 4.370. ?

You say your pistons are .080 over. ? Correct? LJ

pistons are .080, i don't have a caliper to measure, but someone else mentioned that 4.370 was the bore w/ .080 pistons.

Brettd85
Jan 6th, 10, 3:37 PM
pistons are .080, i don't have a caliper to measure, but someone else mentioned that 4.370 was the bore w/ .080 pistons.

I believe it's 4.25+.08=4.33

Rambo, did you get on those compression calculators yet? I'm curious what the real difference is on those heads rather than Kettbos estimates. If not I'll do it for you when I get home.

RAMBO
Jan 6th, 10, 4:42 PM
I believe it's 4.25+.08=4.33

Rambo, did you get on those compression calculators yet? I'm curious what the real difference is on those heads rather than Kettbos estimates. If not I'll do it for you when I get home.

You are correct, 4.33- i think he was looking at the head gasket bore size is where i got that number.

Just ran a couple of the calculators and his estimates are pretty close ..
119cc 781's= 8.23:1
98cc 390's=9.58

Still looking for anyone to confirm what the first guy Mike calculated for the performance difference between the two?

Is it really only worth ~5hp ~20ft/lbs for a full point+ compression difference?

Mike
Jan 6th, 10, 5:06 PM
Here's a Dyno program if you care to DIY -it's free
http://www.compcams.com/camquest/default.asp
You'll need to enter the cam data as it's from Comp.

Tom Mobley
Jan 6th, 10, 8:49 PM
the traditional number is 4% increase in power for each full point of CR. This can vary a lot depending on whether or not other factors are optimized.

in general, you'll get better response and drivebility with lower compression and more aggressive timing and carb setups.

Busted Knuckles
Jan 6th, 10, 11:50 PM
The biggest gain in compression is the first - 8:1 to 9:1 will yield 4% - 5%, but you won't get that much going from 9:1 to 10:1, and the same applies as you get up toward 14:1.

kettbo
Jan 7th, 10, 12:55 AM
Gotta be an error with the 390's... 9.9:1 seems too high of a jump between 119cc heads and 98cc heads.

thanks for running them though!

I re-did the CR, best recollection of the numbers...figured .019 for the head gasket. So Brad the Twins Fan is in the ballpark.:yes:

8.2:1 w/big chamber 781 with 119cc heads. Remember, you may have to shave the 781s to get down to 119cc.

Here are some ratios, I just changing chamber volume we calculate
98cc is 9.53:1, 97cc and 9.6:1 with then 96cc is 9.68:1, 95cc is 9.76:1.
Remember, this is 58 cubic inches crammed into something the size of a Dixie Cup (bathroom size) of volume.

Best guess on Brian's old 461 engine now is 9.5:1 give or take.

badrad
Jan 7th, 10, 1:18 AM
One comparison is the 70/71 LS-5, HP went up slightly but TQ took a huge hit. On a street engine I'll take TQ any day over HP, I don't tend to drive around at 5000 RPM.

novadude
Jan 7th, 10, 10:44 AM
in general, you'll get better response and drivebility with lower compression and more aggressive timing and carb setups.

Help me understand this.

Take 2 engines w/ the same cam, heads, etc - one 9:1, the other 10:1. I would think the 10:1 engine would have more torque and throttle response throughout the entire rpm range - just what you want for drivability. :confused:

shonuff
Jan 7th, 10, 1:19 PM
Help me understand this.

Take 2 engines w/ the same cam, heads, etc - one 9:1, the other 10:1. I would think the 10:1 engine would have more torque and throttle response throughout the entire rpm range - just what you want for drivability. :confused:

10:1 with iron heads, and too agessive timing, could run into possible detonation...... So its not as simple as two engines, same cam , same heads becuase both will have differet timing holdig all things equal....... But in theory yes the 10:1 should make more power and and more torque......

Now the possiblity of being able to get away with more compression and timing is increased if a larger cam is used do to the intake valves closing later.


So we go back to your statement :

Take 2 engines w/ the same cam, heads, etc - one 9:1, the other 10:1. I would think the 10:1 engine would have more torque and throttle response throughout the entire rpm range


Yes but then you run into possible detonation problems with the smaller cam(with intake valve closing earlier as the piston is further down the cylinder) at 10:1 vs the 9:1 (the piston would be further up) At 9:1 with a cam that intake valves closes earlier still can be wicked( torque monster).... ie more torque along lower points on the torque curve......

Tom Mobley
Jan 7th, 10, 1:53 PM
if the high compression engine pings and you have to retard the timing and fatten up the carb the drivebility goes to pot in a hurry. It uses more gas and wants the best ($$) gas you can find. Cars like that end up sitting in the garage a lot.

RAMBO
Jan 7th, 10, 1:57 PM
thanks for all the input & responses guys!

Decided i'll be sticking with the 781's and be able to run 87 octane :)