[Will]

Good thing I'm going to be in charge of my next build.

All these builders go conservative on the compression for reasons like "in case someone puts 87 in it." I can guarantee that will never happen to me, I don't smoke weed any more. (Famous story among high school friends of me putting diesel in my Formula back in the day after toking up...)

In my search for a new machine shop, one got ruled out because the builder insisted I couldn't run more than 10:1 with aluminum heads. I wasn't about to argue with him, I'm just not going to give him my money.

I'll let my choice of cam dictate the CR. Whatever it needs to be to meet my goals is what it will be, but I figure with some KRE heads I ought to be able to at *least* run 10.5:1 and probably more depending on the final cam selection. We have 92 E10 at the pumps around here. There are a few stations around that carry "pure/no ethanol" 92 also, but they're far and few in between.

["QUICK-SILVER"]

Originally Posted by gtofreek

How much you lose highly depends on the cam being used. 50 ft. lbs., and 50 hp per 1 point reduction? I don't believe that for a minute. Maybe 15 HP, and Ft. lbs., but not 50. I wonder if Taylor was mis-quoted, as someone misunderstood him.

I remember that Taylor article from years ago. It went more like how much you gained on a 455 when going from 8:1 to 9:1 and seems like it was 50hp. Then seems like the hp was cut in half with every point after that.
From fading memory
8 to 9 = 50
9 to 10 = 25
10 to 11 = 12.5

Clay

[Will]

Again, it's just not that simple. Really depends on the cam and the rest of the combo. A tiny little cam isn't going to gain 50 HP from a compression bump.

I guarantee that if I left the stock cam in my '73 455 and installed the 6X-4 heads I have which would boost the compression to around 10:1 from the stock 8:1 that I'm not getting 50 more HP. I'd probably get an engine that would detonate itself to death instead.

[Steve C.]

SD Performance has developed our own CNC machined 87cc combustion chamber which we machine into Edelbrock 72cc heads (old style castings), on a back to back dyno comparison our 87cc chamber required about 2 degrees less timing when compared to the std. 87cc Edelbrock combustion chamber (less timing indicates better combustion efficiency which equates to more power with less fuel required and less chance of pre-detonation).


.

[Formulajones]

Since we are on the subject, here's a pic of the combustion chamber after Tony reworked it. It CC's at 89.

To keep compression pump gas friendly at 10.84:1 he had these custom dished pistons made.

[Half-Inch Stud]

AAAaaaaaahhhhhh i studied the Post 11 Chart in the 2000's and dropped compression ratio to 9.0:1. After several re-cams downward, i can say WOT has been nice and ping-free on all 3 cams, with idle improving each time.

Butt hey there aint't a full HP result. There is a penalty in running 12.2 ETs with 3850 Lb Low-Comp, instead of 12.0 with 3650 Lb with High- comp. Hey looks like no HP was lost! MPG was certainly lost; feels like in Half but not so, like 15 instead of 18 mpg.

[Formulabruce]

Originally Posted by Formulajones

Since we are on the subject, here's a pic of the combustion chamber after Tony reworked it. It CC's at 89.

To keep compression pump gas friendly at 10.84:1 he had these custom dished pistons made.

Those pistons look like the 30cc dish ones that KB makes. What is the CC of those sitting at zero deck?

[GTO JONES]

Originally Posted by Formulabruce

Those pistons look like the 30cc dish ones that KB makes. What is the CC of those sitting at zero deck?

There a -18 cc dish on Ross pistons and the piston deck of .0064 clearance

[Cliff R]

If one is simply looking at this sort of thing on paper or with a static mind-set the power differences from static compression aren't really that great.

However, compression is your friend with these things and unless you haven't got a clue when selecting engine components and parameters one would never build a low compression engine with a huge cam in it expecting both good power and user friendly for "normal" driving.

The gains by raising compression are HUGE with street engines. IF I were building a nice pump gas friendly street 400 with 6X-8 heads around 8 to 1 compression I can make good power with a relative small cam, good idle quality and user friendly. It would take about 204-210 @ .050" intake duration to accomplish that task.

So for conversation sake let's say I chose the Summit 2800 cam for it, 204/214/112 LSA. It will idle relatively smooth, great street manners and around 330-350hp and at least 400 ft lbs torque.

Take the very same engine and raise the compression to 9 to 1 with milled 6X-4 heads and increase the cam to 214/224/112 and it will idle equally as good, same basic power and efficiency in the "normal" driving range, but it will not only make more HP from the added duration, it will make more torque and push the torque up higher in the rpm range, so best guess around 350-370hp and 420 or so torque.

Take the same 400 and top it with early big valve heads and raise the compression ratio to 10 to 1 or a tad higher, install a cam with around 10 degrees more duration @ .050". The Crower 60916 (221/229/112) will make 419hp/453tq and pull hard to near 5500rpm's and the slightly larger 60243 cam (228/234/112) will make 424hp/465tq and pull closer to 6000rpm's.

Our 400 will still idle well with either of those cams although the 60243 starts to produce some "lope" and lower vacuum but still very acceptable for a 400cid street engine on pump gas.

Best part is, all of the engines described above are fine on pump gas with great street manners, but for sure the last examples will make considerably more power over a much broader rpm range than the first ones. So basically if one is looking to build one of these engines and contemplating the compression ratio to use for pump gas, they also need to look at the cam choice, use of the vehicle, and goals for the project.

I've built samples of all mentioned above, and a 400 with a cam around 204-210 @ .050" is a high 13 to low 14 second car at 3600-3800lbs. A 400 with the 60243 cam and some decent head work will push similar weight caers deep into the 12's and we have a few into the 11's in full street trim with that combo. Quite a difference if one starts looking at this topic from that angle........Cliff

[glhs#116]

I love this thread. I'm so glad that this stuff is finally starting to make some sense to me. This cam duration business was such a mystery to me for so long. Great to learn from you guys. One day the HO will get a real build. But I'm hoping the current build holds together as long as I need it to.

Sam

[Cliff R]

The Pontiac 455 HO and Super Duty engines have great potential, and they don't need a super high compression ratio to make fantastic power.

Folks screw that deal up by putting tiny cams in them and LSA too tight. We can EASILY get 1hp/cid from any of them with nothing more than tightening up the squish and custom ground camshaft. Not the first grinder or sanding roll need touch anything in the heads, intake, or exhaust to get those numbers.

I attached a dyno sheet from the last 1973 Super Duty 455 that we did, and it used one of the smaller cams we offer.........Cliff

[pastry_chef]

Originally Posted by glhs#116

This cam duration business was such a mystery to me

If you are really curious one of the best purchases would be this:
https://www.amazon.com/Chevrolet-Sma...dp/0879385952/

In progress (not released) is a specification program:
http://www.davidvizardperformancesem...r-Software.php
A member here is directly involved.

Many here would suggest this nearly 800 horsepower engine with a 107 LSA would detonate on premium, let alone 87 octane. It will idle at 450 RPM.
https://www.youtube.com/watch?v=diaKgpqzoA8

Here is track run with a similar 509 engine with a Chris Straub cam (765 horsepower).
https://www.youtube.com/watch?v=65zFVO6su-g

Hyd roller
244/248 @ .050
650/620 lift with 1.7 Ratio
107 LSA , 101 intake centerline.

[Formulajones]

Pastry Chef,

Dave and Dal have been racing this pair of chevelles around here for a long time, one shown in your last link.

https://youtu.be/AwhUQFSIGIc

Actually related to this very subject both of these cars are an excellent example. They both run nearly identical combos and they are dead nuts simple. They started out with GM crate 502's with low compression (about 9.6:1) with stock untouched bottom ends, 305 AFR heads installed out of the box, and both running custom hydraulic roller cams from Straub. Both cars easily ran mid 10's.

Fast forward a bit and they bored the 502's .030 to 509's and put larger dome pistons in them for more compression. Both cars run in the 9's.

I was in contact with Dal for quite a while since I want to build a similar combo out of my crate 502 with AFR 305's that I have sitting here. I'm told they now run Clay Smith cams.

[pastry_chef]

Originally Posted by Formulajones

Both cars run in the 9's.

Small world. That run I linked to was 9.68 @ 137, using the cam I posted.
Straub and Clay Smith formed a partnership a few years back. I don't know if that remains.
http://www.performanceboats.com/gn7-...hnologies.html
Unfortunately Dave passed away.

[Formulajones]

Originally Posted by pastry_chef

Small world. That run I linked to was 9.68 @ 137, using the cam I posted.
Straub and Clay Smith formed a partnership a few years back. I don't know if that remains.
http://www.performanceboats.com/gn7-...hnologies.html
Unfortunately Dave passed away.

Yeah I heard the news, sad to hear for sure. Dave and I talked quite a bit. I lost his emails when I switched computers but he sent me a dyno sheet on the first low compression 502 crate/AFR head/Straub cam setup along with the cam specs, and it was just over 700 hp and tq at that time and running 10.60's at 128 mph, he told me at that time he was street driving it quite a bit and ran pump gas. With just a compression bump and a cam change it now goes 9's.
I was more impressed with the earliest combo since that's what I'm building, seeing what can be done with low 9:1 compression, using a good set of heads and the right cam. I was reluctant for a while not liking the 9.6:1 that a 110CC head will give me on the crate 502, but these guys show how it's a very strong and simple combination that works. In my head though I still want to raise the compression more, lol.

[pastry_chef]

Dave's heads and intake were worked a lot by Foxwell, probably worth 50 HP.

Another.
10.0 flat @ 130 (not the best air) Chevelle.
496 with 4.25 stroke.
Out of box AFR 290 - 18 cc Mahle dome pistons
93 pump gas
Straub Hyd Roller
248/256 @ .050
.665/.638
107 separation, 101 intake centerline
Actually dyno'd 790 - owner does not believe it.

[Formulajones]

Yes they are now. Originally the AFR 305's he used on the first 502 were out of the box and milled to 110cc for a 9.6:1 compression.

Same heads I have that flow 370 cfm so more than capable of hitting the power level he showed me.

[ponchorob]

had an chrysler 440 compression article...500 hp engine

500hp at 11.1c.r

486 hp at 10.1

460hp at 9.1

440 hp.8.0

but ill think with stock pontiac heads raising compression give even more gains...cause of the smaller efficient runners.... one example take a 70 400engine with stock 330hp gross with 067 cam with unrestricted carb it has 350-360 gross hp this engine should put out at least 310 net Hp vers a 71-72 400 with 300 grossHP with 8.2cr, 067 cam we know has a offical rating of 250 Net HP so the jump from 8.2 to 10.2 nets 60HP

[ponchorob]

or take a stock 73 superduty with the 8.4cr and put in a 067 cam i dont think it puts out more then 270 net HP thats my opinion so ill think compression plays a big role in certain combos more than someone thinks...

[chiphead]

So a lower compression engine can still make good peak power. But the tradeoff is loss of idle quality and ragged low speed behavior. You take a combination running well at 10.5 and drop it to 9.5, it's going to get lazy at lower RPMs. If you use a smaller cam to gain back the street manners, peak power is reduced further than just the compression ratio change alone indicates.

This is why everybody (including me) is trying to push that edge of 9.5, to use larger cams and mitigate the low RPM penalty. This is also the big upside of roller cams, you can get large valve lifts without the huge duration penalty of FT cams. This gives good power in streetable RPM range, below 5500. This is the street section, where port velocity is kang.