Lots of variables....actual pistons and the distance from piston to head. Thick gaskets and aftermarket pistons that drop height to allow for block decking can really kill compression. A factory 454 with flat tops can actually measure 7.8-8.5 at best.

All that said....you can still make low 400's on HP (flywheel) with a very, very mild 454. Any more info on the cam?

JIM

 

My engine is 60 over as well. Im running a nice cam and have pistons with a slight, mild raise in them ( I dont remember specifics) I want to say when I did the guesstimations I was about 8 to 8.5 compression. I guess Im making about 400 HP and 500 FT lbs of torque. Totally a guess though, based on my components and seat of the pants feel.

 

 

Carl- before I started messing with the 454 NOM in my 68 convert, I did a dyno run on it to see my starting point. The engine was .030 over with a cam similar to yours. cast iron peanut port heads. Chinese no-name intake. flat top pistons. roller rockers, long tube headers. stock point and condenser distributor. 750 cfm carb. It showed 306 rwhp and 340 lb/ft torque

 

 

Peanut heads run good to about 5k. Then they run out of breath. OP. What is the casting number on the heads. They can vary from 100 to 122cc's. Most common are 119, but you could get lucky...

 

 

Carl, I have an almost identical engine with 28 cc domes. Mine is estimated (Desktop Dyno) at 475 hp and 510 lb ft torque. It runs mid 12 second quarter miles at 117mph (if memory serves me) in my 3600lb Chevelle...

 

781 are 119cc heads. Also considered the best flowing of the oval heads. The 100cc heads have extra meat shrouding the valves that the true open 119cc heads do not. Now, compression is not as big a thing as people think. From 8 to 10 it is worth about 4% a point. So the diff between 8 and 9 to 1 is 12 to 16 hp tops.

 

The difference in throttle response, engine vacuum etc as well as acceleration will be a good increase with another 2 points of compression. It especially helps at lower levels...diminishes as you go higher...but compression is always a good thing.

JIM

 

Carl- the car drove and felt good and was faster as I added stuff. I got caught up in the "more power" dyno racing bug. I added stuff in stages, but here is a list. Aluminum CNC'ed square port, 119 cc, 2.25/1.88 undercut ss valves, 320 cc heads from Shaw City Engines, Weiand Stealth intake, polished and port-matched heads to intake, HEI ignition, 25cc Wiseco domed pistons, Scat H beam rods, Howard's roller cam .618/.618 lift, Comp cams rocker arm stud girdle, delete belt driven radiator fan, electric fuel pump, dbl pumper mechanical secondary QFT 750 carb. I just got the pistons and rods installed and have not taken it for dyno yet. The last run before those additions was 395 rwhp 365 lb/ft. I'm hoping for 420-430 rwhp. Keep in mind this is for a car I have never driven faster than 75 mph. (but it gets there fast)

 

This......every "low compression" (10:1 or so/less) never ran or sounded right....compression absolutely makes on overall difference you can hear and feel.
Run as much as you can without having to pull timing to drive it you wont regret it. Wish Id gone 12:5 not like its driven often

 

I keep reading that lower compression makes the engine sluggish, but I can't find the physics or thermodynamics behind it. Why would higher compression respond faster? I googled this for months and can't find an explanation, only the stament Can somebody send me in the right direction?.

 

 

Compression creates a more complete burn of the fuel in the cylinder. This increased burn increases cylinder pressure after spark ignition which pushes the piston harder. You can recognize if by increased NOx production in engines with higher compression ratios. The reduction in compression ratios were part of trying to reduce pollution. These days...we've got other ways to clean it up AFTER the burn so even OEM's are pushing 12.0+ compression ratios.

You will also notice an increased vacuum at idle due to the rapid change of cylinder volume of a cylinder with a high compression vs one with a larger area at TDC. It's always interesting to take a carburetor that's run fine when the engine was low compression...and do nothing but increase the compression by milling heads or something and all of a sudden you have to turn the idle speed down. Why? More complete burn and increased power production.

Think what happens when you advance the ignition timing. Spark occurs earlier which increases cylinder pressure. Idle speed goes up and it makes more power as RPM rises with the increased timing also. Same concept achieved another way....increased cylinder pressure.

At the track...the higher compression engine will create MPH and acceleration that is more impressive than the HP increase would lead you to expect.

The diesel world has been dealing with this also. Even though by design they have very high compression ratios....they can increase power and performance by raising them even more. The problem is increased NOx from the higher cylinder temps. Two ways to deal with diesel emissions have evolved. Massive EGR where exhaust gases are used to control the cylinder temps by introducing exhaust that takes up space in the cylinder but doesn't burn again. This is a very difficult system to control so that you can maintain performance as well as emissions. The other way is the use of DEF (Diesel Exhaust Fluid or UREA) systems. In this version they've increased compression ratios and tuned the engine to create high cylinder pressures and make power as well as get better fuel economy...and then use DEF along with catalytic converters and Diesel Oxidation Catalysts as aftertreatment devices to clean it up afterward. That's pretty much how all the engine MFG's have done it. Navistar tried the Massive EGR idea and are still dealing with the lawsuits.

To the extent the available fuel will handle it..compression is always good.

JIM

 

To add to what Jim said here, more compression adds more power throughout the entire powerband from idle through redline. It is because of this it shows greater gains at the track then what the peak torque and horsepower numbers would lead you to believe. That is why when people say you only gain 3 to 4 percent increase in horsepower per point of compression they are really missing out on the wide range of effects of higher compression.

With higher compression you can run a larger cam without giving all the bottom end power away, which will give more peak torque and horsepower than the internet 3 to 4 percent wisdom. Increased fuel mileage, better throttle response, better bottom end power with larger cam, more power throughout the entire powerband with larger peak torque and horsepower numbers are more than enough reasons for me to run the almost 12 to 1 compression I have. Of course gas becomes an issue, but most of these cars are not daily drivers, so not a big deal. I wish I had gone to 13.5 to 1, I would run E85 which is everywhere in my state and all the surrounding states.

Mike

 

 

It's a 454 with decent heads, it should still make decent power and run on regular gas, not all bad.

Mike