Static/Dynamic compression, cam choice, SBC - am I doing something wrong?

So....

I started out building an old-school style low-buck small block Chevy in the school Auto shop (I teach high school Mechanics).

350 Chevy, .020" four-eyebrow flat tops. 305 heads, 60cc, heavily ported, back-cut valves. Howard's Cams - 296/296 adv., 230/230@050, .470", 108LSA, 104ICL (old school I know)

I'm pretty sure similar combinations have been put together over the past 40 years, but as I learn more about Dynamic Compression Ratio, I'm not so sure it's wise.

According to http://www.rbracing-rsr.com/compstaticcalc.html I end up with a theoretical STATIC compression ratio of:

10:1 with composite head gasket (.025 down the hole, assuming .050 head gasket) 10.9:1 with steel shim (.015")

The cam intake closes at 39°ABDC@050. Punching all this into http://www.rbracing-rsr.com/comprAdvHD.htm (which asks for closing angle at 050) tells me I get a theoretical DYNAMIC compression ratio of:

9.3:1 w/composite 10.1:1 w/steel shim.

If I extrapolate when the valve is fully closed (based on adv. vs 050 duration - I have not measured), I get a dynamic compression ratio of:

7:1 and 8.2:1 respectively

I have been reading that 8.5:1 should be a better reality for pump gas.

If I punch my numbers in here (whether I use advertized duration, or intake closing angle) http://www.projectpontiac.com/ppsite15/compression-ratio-calculator I get:

9.97:1 static 7.5:1 dynamic with composite 10.86:1 static 8.15:1 dynamic with steel shim

Am I fretting too much? Is one of the calculators wrong? Am I wrong? Should I just buy an LSx and a Miata?

10:1 with a 230 degree cam is a very street friendly, pump gas friendly engine. Don't worry too much about the calculators.

SkinnyG wrote: So.... Am I fretting too much? Is one of the calculators wrong? Am I wrong? Should I just buy an LSx and a Miata?

Yes.. An LSx and a Miata and an E36 M3

the only thing i'd change would be the heads for a set of vortecs...

Car Craft is full of stuff like this. Get the school to drop $10 for a year's subscription for the library.

Keith Tanner wrote: Car Craft is full of stuff like this.

While this is true, I'd kind of like an article on 305 heads on a 350 with this cam and these pistons in the next issue. Not likely going to happen.

I have read through a multitude of other forums, articles, and technical reference websites, and I'm posting here because I would like to be closer to the horse's mouth. I value the information given here, on THIS website, far more than many, many, others.

What I'm after is someone looking at what I'm doing, and telling me if my number are out to lunch, or "it will work fine, stop fretting" (Thanks Knurled).

Knurled - if I run 10.9:1 with the steel shim gaskets (.040 quench), will I run into bigger problems (higher compression) than if I ran 10:1 (.070 quench)??

I'd probably toss the heads in the dumpster. If you have E85 around, compression isn't a problem, and high rpm they won't flow enough air to create any significant cylinder pressure anyway. Drop a kilobuck on a set of real aluminum heads, or find a set of Vortecs. You will make 50-100 more horsepower with the rest of the combination the same.

I think some of you guys missed the first line about how this is an "old school", "low buck", school(i.e ultra low buck) project.

You're massively over thinking this, which is, given, better than underthinking it like most American V8 builds go. Build it with thin gaskets, if it won run on 93(and if my limited understanding of the subject at hand is correct, it will be fine) with typical timing settings, the students get to learn even more. I'd be far more paranoid about proper cam break in and lubrication, more importantly what happens when that doesn't. Stock up on Rotella T6.

I'm assuming since you didn't mention it, this is a 2 bolt main, cast crank, stock rods job that shouldn't be revved very high anyhow. So high RPM (what, maybe 6000rpm redline?) breathing with heavily ported heads that are known to flow like crap as is might not be as bad as everybody is making it out to be.

*I'm not an engine builder(other than the occasional leaf blower ring job), nor do I play one on TV while yelling at my family

I'm waiting for a 50% off sale at Pick-N-Pull with a set of Vortec's that haven't been picked and pulled. In the mean time....

register at www.chevelles.com and ask questions in the engine section... that site has by far the deepest knowledge base of pretty much any car related subject you can think of, and a few very reputable engine builders that will tell you exactly how much power it will make and where it will make that power...

Thanks novaderrik, I indeed found my answer there. Plus found a number of other sites when I searched "squish" instead of "quench."

Apparently ensuring you have the proper .040" squish (quench) is vital for this head swap to work.

When the block comes back, we'll see what shakes up. I'll have to post a video, you know!

Thanks again.

SkinnyG wrote:

Keith Tanner wrote: Car Craft is full of stuff like this.

While this is true, I'd kind of like an article on 305 heads on a 350 with this cam and these pistons in the next issue. Not likely going to happen.

Well, they do post pretty much all of their articles online. Like this one. Anyhow, it was a general suggestion, not a recommendation on how to solve this particular problem.

SkinnyG wrote: Thanks novaderrik, I indeed found my answer there. Plus found a number of other sites when I searched "squish" instead of "quench." Apparently ensuring you have the proper .040" squish (quench) is vital for this head swap to work. When the block comes back, we'll see what shakes up. I'll have to post a video, you know! Thanks again.

what did the experts have to say?

Keith Tanner wrote: Car Craft is full of nonsense

FTFY.

That's an ok cam, and although the lobe centers are tight, (but fine) the intake centerline is way too early. Try moving it around in your calculation to 108 and 110 and see what happens. The combination looks fine, but I wouldn't install that cam at 104 degrees.

novaderrik: It took digging, but found a specific mention of "squish" needing to be .040 to .045 in order to reduce detonation with specifically a 305 head / 350 block combination.

Since the combustion chamber is smaller than the 350 bore, you end up with a ring of "squish" around the perimeter, which (when thin enough) encourages turbulence into the combustion chamber at TDC which significantly improves combustion. As I understand it. Too large (as I would have if I were to run a composite head gasket) and you lose that turbulence, and you become much more prone to detonation.

Jeff Swisher is a main proponent of 305 heads on Chevelles.com, as is F-Bird88 on nastyz28.com There are others, but few mention details about "squish"/"quench"/"head-gasket". Many nay-sayers though. They'll build torque, which is what I want in this low-buck square body truck project.

Zomby Woof: Thanks. One of the reasons I chose it was specifically the tight lobe centers. The cam is ground on a 104ICL by the company (Howard's Cams).

Is your reasoning in changing the ICL to 108 or 110 to reduce cylinder pressure?

On this calculator: http://www.projectpontiac.com/ppsite15/compression-ratio-calculator

With the cam installed as ground (104°ICL), the dynamic compression is 8.15:1

With the cam installed at 106° I get 8.00:1

With the cam installed at 108° I get 7.85:1

With the cam installed at 110° I get 7.69:1

Howard's doesn't grind any of their flat-tappet cams with 110° centerline, but they do have what appears to be the same grind at 106° intake centerline. According to their catalog, this cam comes on 800rpm later - not what I want at all.

They have other cams with a 108° centerline, but those are ground with 112° lobe separation - not what I want.

(sigh) All this thinking for a truck that we're going to be painting with Tremclad and a roller...... Most of my ideas are prefaced with "Because I'm stupid....."

Is your reasoning in changing the ICL to 108 or 110 to reduce cylinder pressure?

No, it's to show you the effect that the early intake centerline is having on your calculations.

Almost everybody would either grind or install that cam between 106 and 112. I'm not sure why it's so short but I wonder if it's one os those "thumper" type cams for an 8-1 smogger motor with auto trans and lousy gearing. It will shift your torque peak down, and boost your low RPM power, something you probably want, but at the expense of possibly having to reduce ignition timing, something you don't want to do.

Like Kenny says, you're way over thinking this. Stuff like those calculators, and discussions about quench and squish are all valid, but secondary considerations (and mostly entertainment) in a build like this.

Disclaimer: I am a cam grinder, and used to build motors like this for money.

Update so far:

Degreed the cam.

Intake opens at 6°@050, closes at 45°

Cam card says this should be happening at 11° and 47° respectively.

Contacting the manufacturer now....

Bringing this thread back from the dead, one year later.

Howard's Cams said to just set the cam the way it should be, using ICL. No concern that the numbers were off. Luckily I had an indexable timing set.

I ended up using the 0.015" Felpro steel shim head gasket. Proper "squish" seemed logical.

The cam survived the break-in. I used plenty of cam lube, and Comp Cams break-in oil, pre-oiled the engine, ran at a varying 2500 rpm, and now currently run the engine on Lucas "Hot Rod" high-zinc oil.

I have the timing set at around 34 to 35° max, with 20° base timing. Vacuum advance is getting manifold vacuum and is limited to 12°. Engine was idling at 14" in park, and 8" in gear with the stock converter. I ended up putting in a 2500 stall converter since the stocker was too tight, and haven't checked vacuum since.

The Rochester was rebuilt, the throttle shaft re-bushed and all the jets and air bleeds and whatnot tweaked in accordance with Cliff Ruggles' book.

Also added an MSD box since it allegedly burns everything better. No noticeable improvement whatsoever, though the tach is now berserk. MSD unit was free - came in a car I parted out.

Runs and drives perfectly fine. Runs just fine on 91 octane (might have up to 10% ethanol in it). Haven't noticed any detonation at all. It has a lot more low-end torque than other "cammed" engines I've driven, probably because of the compression and the advance of the cam? Probably still less torque than stock, but I never drove it before the rebuild.

The choppy idle is like therapy. Pulling into the school parking lot every morning and listening to the engine lope away puts a smile on my face every morning. People pay large sums of money to counselors and therapists to try and find peace in their world. I'm essentially doing the same, just paying at the pump instead. It's also good marketing, since I am the Mechanics Teacher at the school.

I'm averaging about 12mpg (US) so far. Considering what it is, that's not that bad.

For a low-buck build, this worked and does exactly what I want.

Now that you have a nice converter in it, I'd see how it runs with the vacuum advance on ported vacuum. Running it to manifold vacuum is kind of a crutch sometimes, and can result in an idle that hangs at different RPM or is slow to return to a steady idle. Plus, it's not really hard to do, just move the hose from one place to the other.

SkinnyG wrote: The choppy idle is like therapy. Pulling into the school parking lot every morning and listening to the engine lope away puts a smile on my face every morning. People pay large sums of money to counselors and therapists to try and find peace in their world. I'm essentially doing the same, just paying at the pump instead. It's also good marketing, since I am the Mechanics Teacher at the school.

This is exactly why I drive a bridge ported RX-7.

In reply to Knurled:

Yeah ported vacuum, at least in my case, greatly increases advance at tip in from idle, making it come out of the hole a lot better.

At least in the mgb world (not very much related to American v8s admittedly), there are distributors setup from the factory for manifold vacuum and others setup for ported vacuum.

All I know about it is if you hook up the wrong combination, your car will run like crap and you will blame the carb and then buy a weber. About half by pure luck hook up the vacuum advance correctly with the new carb, and believe that su is crap from then on. The other half, of course, believe the Lucas electrics are to blame and they sell their mgb.

In reply to rcutclif:

Yeah, there are vacuum advance distributors, and then there are vacuum retard distributors. I also think there's a lot of variation in what "ported" vacuum is. In my case it behaved exactly like the manifold, except it would jump, not fall, when you blipped the throttle.

For the kids who missed one of the classes - where's the vacuum source on the ported setup?

Isn't ported vacuum from outside the throttle plates? I.e. Low vacuum at idle and higher with open throttle, whereas manifold is inside the throttle plates so vacuum is high at idle and lower with open throttle?

I think that is what it means.

Take your big diagnostic vacuum/fuel pressure gauge you keep with the timing light and dwell meter, there will be 2-3 small vacuum ports on/near the base of the carb, one of them them vacuum may or may not read 0, and will rise, not fall when you blip the throttle, this is ported vaccum. With anything not stock you really just have to experiment to see which works best. A lot of emission controlled engines ran a switching circuit, and would switch to manifold vacuum if the engine was overheating or cold, to up the idle speed and spin the fan faster.

I pulled this from google images, claims to be part of the autozone repair guide for a 67 Corvair, I think this is a generic info sheet though, I remember seeing the same one unsmogging the Yugo, which was full of 80s Ford smog bits. "EGR vacuum"=manifold vacuum, "spark port"=ported.