compression ratio

[88bluestar]

How much of an increase of torque/hp by changing ratio from 9.4:1 to 9.9:1?
93 octane gas
355 cid

[MadBill]

According to a chart by David Vizard, the basic increase would be ~ 1%. However a big cam engine would gain more.

[88bluestar]

780cfm smi carb
victor jr intake
large port vortec bowties
1 3/4 headers.
cam is a 4/7 swap 252int 255exh at 108(4 degrees advanced) .540/.550

[MadBill]

That's probably enough duration for the CR increase to pick up the bottom of the curve by maybe 2% but the top end only a couple of tenths more than the chart's 1%.

[88bluestar]

It lacks torque, so will this increase be a sizable difference as far as torque?

[Scotthatch]

780cfm smi carb
victor jr intake
large port vortec bowties
1 3/4 headers.
cam is a 4/7 swap 252int 255exh at 108(4 degrees advanced) .540/.550

???

Should be a standard sort of motor 500 HP at 7000 and 400 ft lbs at 5500 rpm

Does the car have no gear?

[88bluestar]

Dirt oval, tight under a 1/4 mile. 700 final.rpm range is 3500-7000, rev limiter is 7000 by rules
3200 lbs car

[MadBill]

It lacks torque, so will this increase be a sizable difference as far as torque?

Likely ~ 6-8 lb-ft. max between 3500 & 4500; but a worthwhile move . Is 10:1 CR max a rule? If not more would be effective and possible with good quality 93 octane.

[David Redszus]

Based on an engine with a 4.0" bore and a 3.5" stroke, at 8000rpm a static compression ratio of 9.4 (dcr=7.18) would produce a BMEP of 10.52 and 542.1 hp.

The same engine with a static compression ratio of 9.9 (dcr = 7.56) would produce a BMEP of 10.62 and 547.3 hp.
A gain of .0095% or just under the 1% estimated by DV.

The performance gain is not determined by the SCR of 0.5, but by the change in DCR.

If an earlier closing camshaft were fitted, (115 btc vs 10 btc) the SCR would remain unchanged but the DCR would increase and produce a power increase of 1.7% instead of 0.9%.

[F-BIRD'88]

The SBC victor JR is not a vortec head style intake. Thats why it lacks torque.
Get a "vortec" intake that matches the vortec heads raised intake port.
Change to a RPM air gap dual plane vortec intake. It will make a lot more torque and match the power of the MIS matched vic JR because the port alignment is better. Play with the valve lash a bit.
Looser valve lash shortens the duration. enhances low end torque
Move the cam to a 99deg intake C/L and adjust move >>>retard as needed.
Combine these changes with the compression ratio bump.

[Geoff2]

Improvement [ if you could feel it ] probably not worth the effort/$$$.

[mag2555]

Please remember that that running any more octane then what you need to be safe only serves to cost you power and even slows the engines rev's in the extream!

[groberts101]

It lacks torque, so will this increase be a sizable difference as far as torque?

Not much in the lower rpm range you're going to really feel at lower throttle angles. However, the added compression will help liven up the induction tract and make the booster slightly more responsive which serves to make it feel slightly more touchy to throttle transitions under peak torque. Probably slightly better tuning resolution as well.

The other thing many get wrong.. even seasoned engine vet's.. is failing to tune ignition curves for maximum intake manifold vacuum at idle and part throttle. Bigger cams can easily accept upwards of 40 degrees or more at idle/light cruise rpms. That added transitional throttle response will easily overshadow the 4-6 ft/lbs you'll likely gain from that increase in compression. Gains could be cumulative and very noticeable if combined.

[zums]

780cfm smi carb
victor jr intake
large port vortec bowties
1 3/4 headers.
cam is a 4/7 swap 252int 255exh at 108(4 degrees advanced) .540/.550

Not trying to be a downer on your combo but in my opinion, the heads are too big for that cube and rpm and you could use more compression, more than you are asking about
Tom

[88bluestar]

780cfm smi carb
victor jr intake
large port vortec bowties
1 3/4 headers.
cam is a 4/7 swap 252int 255exh at 108(4 degrees advanced) .540/.550

Not trying to be a downer on your combo but in my opinion, the heads are too big for that cube and rpm and you could use more compression, more than you are asking about
Tom

I'm stuck with the head for the time being, but have plans in the offseason for smaller vortec bowtie heads(or promaxx), and a jones race cam/lifters, and likely increase the comp at that time. We have a limit on comp ratio so i have to be careful. I had considered a rpm intake or tall dual plane intake however whatever I choose we have a 7" rule from top of block to base of carb.