TechInspector: was reading your post in another thread about compression ratios..

his particular 406 was set for zero deck height... I didn't measure the deck height when I assembled my 350 (one of the many lessons I learned after my first build)

do you have a general figure for deck height/volume I can use for calculating my SCR?

I suspect my SCR is higher then I expected to be because my engine has turned out to want premium only if I want to run enough timing to make power..

it's a stock 350 block, bored .030. I don't believe it had ever been apart before I disassembled it. (got rainwater in a cylinder, thats why I had to go .030) i'm running a basic felpro composition head gasket.
with SpeedPro PowerForged flat-top pistons with 4-valve reliefs.
L2256F appears to be their part #.

Let's just go through it for the benefit of others reading here.
A 350 bored +0.030" will have a bore of 4.030" and a stroke of 3.480". Using our cubic inch formula of .7854 times bore times bore times stroke times 16.387 = 727.4 cc's.

The stock 350 crank will have a radius of 1.740" (half the stroke). The stock rod length will be 5.703". Your pistons have a compression height of 1.563". If we add up these values, we find a stack of 9.006". Now, given that the nominal block deck height of a 350 is 9.025", that would theoretically put the piston crown at 0.019" below the deck at TDC. We would use the same formula to figure the piston deck height volume. .7854 times 4.03 times 4.03 times .019 times 16.387 = 3.97 cc's.

I don't know the particulars of your motor, but for the sake of finishing up this post for the benefit of others reading it, I will plug in some values to reach a conclusion.

Let's say we have 64 cc chambers in the heads. And we know that the pistons you are using have 6.1 cc's in the crown.

727.4 in the cylinder
64 in the heads
3.97 in the piston deck height
6.1 in the piston crown
9 in the gasket (generic 0.040" gasket)
total- 810.47
total less cylinder- 83.07
divide larger by smaller = 9.75:1 static compression ratio.

The squish (piston deck height added to gasket compressed thickness) in this example is 0.019" plus 0.040" for a total of 0.059". This is not ideal according to many builders, but it is also not "awful". It may or may not be contributing to detonation depending on fuel quality.

 

TechInspector: was reading your post in another thread about compression ratios..

his particular 406 was set for zero deck height... I didn't measure the deck height when I assembled my 350 (one of the many lessons I learned after my first build)

do you have a general figure for deck height/volume I can use for calculating my SCR?

I suspect my SCR is higher then I expected to be because my engine has turned out to want premium only if I want to run enough timing to make power..

it's a stock 350 block, bored .030. I don't believe it had ever been apart before I disassembled it. (got rainwater in a cylinder, thats why I had to go .030) i'm running a basic felpro composition head gasket.
with SpeedPro PowerForged flat-top pistons with 4-valve reliefs.
L2256F appears to be their part #.

Taking into account all the calculations that Techinspector did for you, one has to conclude that having a 9.75:1 SCR is asking a lot from today’s fuels.

To a fair extent this is cam timing, combustion chamber shape, port size, and cylinder head material sensitive. It is also sensitive to engine operating temperature, intake manifold temp, mixture ratio, ignition timing in rate and total amount of the advances, as well as brand of gasoline, vehicle weight and overall gearing.

Picking some of this apart:
- Less cam timing against a high compression ratio will make the engine more detonation prone. This gets into the Dynamic Compression Ratio (DCR). The DCR should fall from the high 7 sevens to about 9 to 1. It is an adjustment for the apparent reduction in stroke and how that affects cylinder volume when that is adjusted for the point where the intake valve actually closes. One really shouldn’t build an engine without doing this calculation.

- Heads greatly affect detonation resistance. Modern combustion chambers of the so called heart or kidney shape are much more tolerant of high compression ratios than the older open chambers. The port size to engine size plays a role in mixture velocity. Ports that are too small or large increase detonation tendencies by affecting cylinder fill thus mixture density and mixing. The material the heads are made from has an effect, aluminum moves heat faster between the combustion chamber and the coolant than cast iron which makes these heads less detonation prone and more acceptable to higher CRs

- Temperatures of the coolant and the inlet air/mixture greatly affect detonation resistance. Cooler temps of either or both increase detonation resistance.

- Mixture ratio plays into detonation resistance, richer mixtures help suppress detonation. Fuel bases and blends also play a part, especially with the effect of temperature. Many brands using the R+M/2 calculation of octane actually loose a lot of rating as their temperatures increase.

- Vehicle design plays a role a heavy vehicle more prone than a light one. High gearing netting low engine speeds, especially against a heavy or resistive design where the engine has to work hard for its RPMs will have a greater tendency to detonate.

Not discussed very often these days is water-alcohol injection to suppress detonation. This is an old and effective technique that is simpler and cheaper than tearing the engine apart to reduce compression. Like nitrous injection, it was used extensively in the piston aircraft engines of WW2 to extend their power range at critical times of take off and in combat where the engine has to contribute at its maximum capability.

Bogie

 

Bogie, not to hijack a thread, but can methanol injection work on a naturally aspirated engine? I thought it was mostly a forced induction thing?

 

Bogie, not to hijack a thread, but can methanol injection work on a naturally aspirated engine? I thought it was mostly a forced induction thing?

Works very well.

Bogie

 

Hmmmmmm......To the garage!!! :mwink: As you were gentlemen!! LOL.....Muuahhhhaaaa.....Excelent!

 

Hmmmmmm......To the garage!!! :mwink: As you were gentlemen!! LOL.....Muuahhhhaaaa.....Excelent!

Easiest to procure and work with is isopropyl alcohol (rubbing alcohol) from the drug store. You'll want to end up with about 50/50 alcohol/water. Isopropyl is normally 70% alcohol as found in the stores, so for instance 3 pints of off-the-shelf isopropyl and 1 pint of pure water mixed together would produce a half-gallon of 50/50 roughly. I used to use a Ford windshield washer bag with the built-in pump for spraying the mix into the carb throat. Just pour the 50/50 mix into the bag. I don't know if you can still find them, because back in the 70's is the last time I did it. But anyway, use a blank Holley jet and drill it 0.030" and rig it up so that the jet will blow the alky down into the carb throat. For initiating the flow, mount a 12v microswitch (Radio Shack) at the carb linkage so that when you reach 1/2 throttle, the microswitch activates the 12v pump in the windshield washer bag.

All I'm sayin' here is that you don't have to spend hundreds of dollars to rig up an injection system. Air out your brain a little and you may come up with something even better than I have outlined here.

 

wow thanks for the info Tech Inspector..

Here's some more particulars to continue the discussion

cylinder heads I _BELIEVE_ are 67cc. I can find virtually no information on them on line. . however, one of my chevy books shows a picture of all the various size combustion chambers, and from comparing my heads with the pics, I'm pretty sure they are 67. Casting # is 3998920

that being said.. since you did the calculations with 64cc, i don't feel bad about stepping up to Vortec heads with the 64cc chambers.. 9.75 isn't horrible.

This is a scan of the page in the book where I cut and pasted a picture of one of my chambers for comparison sake..

the cam I'm running right now is a Summit Racing brand cam. 214°/224° @ .050 with .442/465 lift, 110LSA Link to the cam specs

the rest of my combo is long tube headers, twin 2.5" pipes, no crossover, dynomax ultraflow muffers and 2.25" tailpipes out under the back bumper

inake is a Holley 650 vac. secondary, sitting on a 1" spacer, on top of an eddy. perf. rpm (not airgap) intake.

ignition is MSD ProBillet ready-to-run, and an MSD 6AL with Blaster 2 coil, Taylor wires, and AC Delco plugs (two plugs are Acell shorties)

i can't remember where ihave the timing set, but i seem to recall it's around 14° initial.

engine temp is a steady 165° normally. climbs a bit when I'm sitting.

air intake temp is ambient air temp, I have a 4" cowl induction scoop, and the top of the air cleaner sits proud of the top of the hood (have a 1" spacer between the air cleaner and carb too)

i'm planning to swap in a lunati VooDoo cam in the spring, and then eventually upgrade to real vortec heads and intake :>

 

ok I ran the numbers with my 67 cc heads.. this is what I came up with

727.4 in the cylinder
67 in the heads
3.97 in the piston deck height
6.1 in the piston crown
9 in the gasket (generic 0.040" gasket)
total- 813.47
total less cylinder- 86.07
divide larger by smaller = 9.45:1 static compression ratio.

that's actually right where I believed it to be.. (mid to high 9's)

i can understand it needing premium with a static compression that high and a small cam.. so If I'm understanding this right.. a larger cam will probably ease my pinging situation a bit by reducing cylinder pressures?

 

That is correct^^^ about the larger cam. It would also be beneficial to swap to a thin(018-.020" thick) steel shim type head gasket to reduce the quench distance(although only do this if you are sure everything is flat, head surface and block). Even though this will raise the compression ratio a bit, the engine will be less sensitive to octane with tighter quench.

It will also be less sensitive to octane with the Vortec heads better combustion chamber shape even though the compression will go up with the use of their 64cc chamber also, your current chamber shape is of the open truck/smogger style and will not like being pushed on compression ratio on pump gas. Chamber shape makes a big difference here.

Even though the heads you have are listed at 67cc, don't take that as the real # without checking them, most Chevy heads are 1-2cc bigger than published #'s when you actually cc them.

To me this sounds like a case of all the wrong little things adding up to octane sensitivity at the near 9.4-1 compression ratio. 10.5-1(and more) can be done on pump gas with iron heads(I've done 10.85-1 with no problem, carbureted), but it takes attention to quench, chamber shape, timing curve, and cam duration to do it,....and all your dominoes seem to be stacked in the wrong direction unfortunately . You're learning though, and thats what counts

Just out of curiousity, you said you are using an initial spark timing of 14°...but what is your total?? and at what rpm is it fully advanced??

 

Short answer on deck SBC deck height.

Usually around .025", that figure works well for estimating the compression ratio of an undecked block. I have seen some older 400's that run into the low .030's, but for guesstimating, .025" works well without going into writing a novel......................

tom

 

TechInspector: was reading your post in another thread about compression ratios..

his particular 406 was set for zero deck height... I didn't measure the deck height when I assembled my 350 (one of the many lessons I learned after my first build)

do you have a general figure for deck height/volume I can use for calculating my SCR?

I suspect my SCR is higher then I expected to be because my engine has turned out to want premium only if I want to run enough timing to make power..

it's a stock 350 block, bored .030. I don't believe it had ever been apart before I disassembled it. (got rainwater in a cylinder, thats why I had to go .030) i'm running a basic felpro composition head gasket.
with SpeedPro PowerForged flat-top pistons with 4-valve reliefs.
L2256F appears to be their part #.

You may be having some issue with not enough quench. With your undecked block and composition gaskets, your quench height with the L2256F is around .060" (.038"+ .025"). Regardless of your dynamic compression ratio, that much deck clearance will excacerbate any possible need for higher octane gas.

 

hrrm... don't see a delete post button

 

Even though the heads you have are listed at 67cc, don't take that as the real # without checking them, most Chevy heads are 1-2cc bigger than published #'s when you actually cc them.

I don't know why i keep getting stuck on that 67cc.. the page in the book says 69cc and I tend to believe that.. I had these heads sitting next to the 76cc heads I had from another engine and it was pretty obvious that the 76's were larger.

When i do my cam swap I might pull the heads and check them, since I need to change the springs anyway.

To me this sounds like a case of all the wrong little things adding up to octane sensitivity at the near 9.4-1 compression ratio.
...
and all your dominoes seem to be stacked in the wrong direction unfortunately . You're learning though, and thats what counts

I agree.. originally this cam was meant only as a stop-gap. my previous engine had rounded two lobes off and I did a quicky cam swap to fix it. however, my buddy and I swapped all 16 valve springs heads still on the engine.. we got all but the very last spring right.. a couple months down the road, it dropped a valve.. the engine was a total loss.
The head
The Piston
The pieces of piston I found in the oil pan

the cam and lifters were the only things that wern't hurt! so I took each lifter out and bagged and tagged it, took the cam out, and they all went back into the new block
I was surprised at how much power I had on EIGHT cylinders that I decided to run with it for a while
I built this whole engine for about $1500. machine costs included. and I even spent $300 on forged pistons. so that should tell you a little bit about what kind of a build this was LOL. I reused the entire stock bottom end except pistons..

Just out of curiousity, you said you are using an initial spark timing of 14°...but what is your total?? and at what rpm is it fully advanced??

[/quote]
I didn't post this last night becuase I had to look it up LOL. I'm going by the specs on paper, i've never actully put a light on it to verify it.

I'm using the blue stop bushing which is 21° total advance for a grand total of around 35° it's "all in" by 3200 rpm (2nd fastest curve available)

 

"I didn't post this last night becuase I had to look it up LOL. I'm going by the specs on paper, i've never actully put a light on it to verify it."

Now THAT'S a great way to wreck an engine in a hurry.................

And P/O your machinist.


tom

 

huh? wreck an engine in a hurry how? it's not pinging.. and even if it does, I have forged pistons

i've just never tried to check my timing at 3000+ rpm.

i know my base is set at 14 by an advance-light.
and it's a brand new balancer so I'm fairly certain the mark is in the right place

 

Even a forged piston will melt, detonation creates heat even before it creates an noticable knock. Very easy to have detonation that is inaudible, especially with loud mufflers or open headers, but that doesn't mean damage isn't occuring.

This is why every time you see posts on timing, someone who knows what they are doing askes what the total timing is, it is more important than the initial ever will be.

Lots of different possible timing curves, and different amounts added, decided by the weights, springs, and travel stops in the distributor. Without knowing how much it adds, and at what rpm it is fully added, you are in the dark about on of the most important tuning parameters of your engine.

If you have an advance adjustable type light I can't understand why you haven't checked this, it is usually the guys with just a plain light that haven't done it.

 

mainly it's not having 3 hands that's stopped me from doing it.. it's difficult to hold the throttle open and be sure I'm staying above 3000 rpm while trying to operate my advance light..

 

mainly it's not having 3 hands that's stopped me from doing it.. it's difficult to hold the throttle open and be sure I'm staying above 3000 rpm while trying to operate my advance light..

Don't worry that much about the rpm right now, just throttle the engine up until the advance quits moving on the balancer and record that total. You can figure out what rpm it happens at later.

You'll see guys at the track(drag, circle, mud, whatever) just winding the motor up while reading a timing light, quickly spot checking before a race, all they are concerned with is making sure the total is what it should be... hasn't moved higher, which could result in engine death, and hasn't moved lower, resulting in missing power.

The rpm it is all in by they figured out at home, when they had the time, just to improve part throttle drivability.

 

most of the circle track guys i know ran locked out timing .. but i get your point.


back to the original subject

I ran my #'s thru the wallace racing DCR calc.. this is what it spit out...

Static compression ratio of 9.45:1.
Effective stroke is 3.27 inches.
Your dynamic compression ratio is 8.94:1 .
Your dynamic cranking pressure is 183.47 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.94 :1.
V/P (Volume to Pressure Index) is 184

DCR of 8.94:1 seems to be exactly where I want it to be?

my cam closes 34°ABDC

probably at this point, a few details on the car might help

it's a 1972 Monte Carlo (4,020 lbs on the scales, with me in it)
TH700r4 trans, 2400 stall converter, 3.55:1 gears, 255/60-15 street tires (27.01") and 26x10 Drag Slicks

i know it could stand to use more stall.. but I'm balancing performance with driveablity.. if I can find a nice 'tight' converter rated at 3000 rpm I'd go there. but I don't want a loose 'light switch' type converter that sucks to drive around town!
(nice thing about a converter with the lock up clutch.. I can cruise below my stall speed on the freeway) I run 2250 rpm @ 70mph with my setup.

 

Looks well thought out to me, should be a fun driver :thumbup:

 

hrrrm that's interesting.. I never ran gear calcs with 4.10's... guess I just assumed they'd be too high... but 4.10's put me cruising at 2500 rpm and I could definitely live with that. i think that's pretty close to what I ran with the TH350 and 2.73's.

and it'd sure wake the car up even more but I'm a little worried about how short those 4.10's would make that 3.06:1 1st gear...

(slight pause)

ran 3.06 thru the gearing calc.. I could possibly drive 38 mph @ 6000 rpm.. I guess that isn't TOO short.

the only reason I even have the 3.55's now is they were already in the axle when I bought it (my old axle was open, this one is posi)

i think I see some 4.10's and a detroit locker on the horizon :> :> :>
one thing tho, I'd have to SERIOUSLY improve my high rpm power to run 4.10's. I'm already running out of cam before the 1/4 mile with 3.55's!
(as in.. moving my power band up at least another 2000 rpm)
I trap around 4500 with the 3.55's.. it'd move up to 5500 with 4.10's)

back to the original question
running the Lunati VooDoo 60103 with 64cc vortec's puts my DCR up in the 9.0 range.. this is really pushing it for pump gas correct?

 

Looks well thought out to me, should be a fun driver :thumbup:

thank you i did my best to match the components up as best I could with what information I had on hand. my only regret now is that I went with the full flat top pistons.. I think I would have been slightly better off with a smidge lower compression then 9.45:1 (more like 9.0:1 I think)

 

What you are doing is really pushing my patience.

Then stop reading this thread if you don't like it, I wasn't aiming this discussion at you to begin with.

On top of which, I don't know where you got the assumption that I'm selecting a cam based on my DCR.. I've already selected the new cam, and I'm comparing DCR and other specs to make sure I don't wind up with a motor with excessive compression that requires more than 92 octane.

 

C'mon guys, let's keep it civil and simple.


tom

 

I edited your post.

To emphasize parts of a word, circumventing the profanity rule, is specifically not allowed, the word is "assumption", no caps anywhere.

Like Tom said, play nice.... :thumbup: