Compression Ratio?

Discussion of the 331-354-392 HEMIs.

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NE57
Posts: 192
Joined: Fri Mar 27, 2009 12:54 pm

Compression Ratio?

Post by NE57 »

For a street, carbed, 392 bored and stroked to about 456CI what would be a good CR to specify to the piston maker?
I'm leaning to an aggresssive cam which I think higher CR works well with. Something around 230 degrees@.050 or so. But what lobe seperation?
Am I on the right track with this?
What kinds of CR/Cam combos are some you using and are you happy with the results?

Thanks much
polyspheric
Posts: 57
Joined: Wed Apr 09, 2008 1:03 pm

Re: Compression Ratio?

Post by polyspheric »

The cam is the last thing you choose - after the head work, manifold etc. are done.
For a 450" motor, 230° is far to mild for serious power, think more like 250.
mart
Posts: 536
Joined: Tue Mar 22, 2005 2:06 pm

Re: Compression Ratio?

Post by mart »

polyspheric wrote:The cam is the last thing you
choose - after the head work, manifold etc. are done.
For a 450" motor, 230° is far to mild for serious
power, think more like 250.
-------------------------------------
Also, a longer duration cam will allow you to run a bit more
compression and/or more ignition lead for a given fuel octane
by lowering cylinder pressures at low to medium speeds.
- mart
=================================================
NE57
Posts: 192
Joined: Fri Mar 27, 2009 12:54 pm

Re: Compression Ratio?

Post by NE57 »

I was hoping someone could chime in with a specific experience. As in..."with X ratio and Y cam I had or didn't have detonation" .

Dare I go for 12:1 if went with the 250 degree cam? Pump gas.
George
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Joined: Wed Aug 25, 2004 4:12 pm
Location: Fl

Re: Compression Ratio?

Post by George »

NE57 wrote:I was hoping someone could chime in with a specific experience. As in..."with X ratio and Y cam I had or didn't have detonation" .

Dare I go for 12:1 if went with the 250 degree cam? Pump gas.
As far as octane is concerned you can run 1 point higher comp. on a hemi with the same oct as a wedge engine.If you run 93 in a 383 @ 10:1, you could run 11:1 on a Hemi
392heminut
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Re: Compression Ratio?

Post by 392heminut »

I have 10.2 to 1 actual measured compression in my 392 (stock cast iron heads). The cam is a Racer Brown hydraulic with .510 lift and 230 degrees @ .050 duration and 109 LSA. I run 10 degrees initial timing with 30 degrees total, all in by 2500 RPM. I run 91 octane gas and have never experienced any detonation with this engine.
Owner of the Poor Man's Hemi Cuda
NE57
Posts: 192
Joined: Fri Mar 27, 2009 12:54 pm

Re: Compression Ratio?

Post by NE57 »

>.510 lift and 230 degrees @ .050 duration and 109 LSA

What kind of vacuum does that give you at idle? Enough for power brakes etc?
392heminut
Posts: 488
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Re: Compression Ratio?

Post by 392heminut »

It was pulling about 12-14 inches of vacuum at idle. I don't know if that was enough to run power brakes as I had standard brakes on the car.
Owner of the Poor Man's Hemi Cuda
polyspheric
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Re: Compression Ratio?

Post by polyspheric »

you can run 1 point higher comp. on a hemi with the same oct as a wedge engine

Why?
mart
Posts: 536
Joined: Tue Mar 22, 2005 2:06 pm

Re: Compression Ratio?

Post by mart »

polyspheric wrote:you can run 1 point higher comp. on a hemi with the same oct as a wedge engine
Why?
========================================
Why? Because of better and smoother flame travel due to the nearly
centraly located spark plug and the smooth, rounded contours of the
funnel-like shape of the himisperical combustion chamber. It makes
for very rapid, but also very smooth, even and consistant combustion.
While a hemi chamber will tolerate slightly more compression for a
given fuel octane than a wedge chamber, it will also usually require
less ignition advance for a given fuel octane and compression ratio.
This is were some guys get in trouble. They assume that because the
octane requirement is reduced, that they can, or need to, also run
more ignition advance too. Not so with a hemi. A few other
considerations as well. Gasoline quality and consistancy can vary
quite a bit regardless of the actual octane number shown on the
pump. One brand or even one batch of "93 cctane" might perform
like another company's "91 octane' etc. Some (most) companies even
blend their fuel for a specific season (ie - summer and winter blends)
or for a specifiic geographic region - ie - high or low altiitude regions
and for certain states that require varying amounts of ethonal or other
oxygenates in their fuel. All of these things affect the way the fuel
behaves in an enigine, regardless of the theoretical octane number
on the pump. Likewise with fuel mixture too. A late model engine
with port-EFI can generaly run a higher compression ratio than a
simllar engine with a carburetor, simply because with port-EFI, the
fuel mixture is much exact and more consistant and even in every
cylinder.With a carburetor and standard 'wet' intake manifold even
if you get the air-fuel ratio from the carb, theoreticly at least, 'dead
on' for the engine, you're still going to have some cylinders running
richer or leaner than others. If you lean the system enough so the
"rich" cylinders can make maximum power, the" lean" cylinders will
then be too lean and prone to pre-ignition and detonation. It's all
a compromise. For the real world, for a street-driven car with a
carburetor, running 'premium' pump gas and cast iron hemi heads
- regardless of the theoretical octane benefits the hemi head design,
to be safe, I'd go a bit conservative and set the compression ratio
at no more than 10 or 10.5 to one. You *might* get away and stil
be safe with at 11 or 11.5 to one with a hemi, but it'd be on the
ragged edge and a dice toss. With aluminum head (like Hot Heads)
though, 11 to 11.5 to would safe on 93 octane pump gas, with maybe
a point or point and half higher if your willing to gamble. That's my
opinion, anyway.

mart
==============================================================
George
Posts: 694
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Location: Fl

Re: Compression Ratio?

Post by George »

Aluminum conducts heat so much better than iron that the comp ratio must be increased by 1 point to compensate for the heat loss, al vs iron heads. Heat is energy, & energy is horsepower.
NE57
Posts: 192
Joined: Fri Mar 27, 2009 12:54 pm

Re: Compression Ratio?

Post by NE57 »

Boy, lots to chew on there, thanks all. One thing that I've been wondering about...the theory that a bigger cam would allow more CR. At some point wouldn't the increased cylinder filling that comes from a big cam override the "calming" effects(vis a vis increased CR, IF I understood that right) of the longer duration at low RPM? Like when the cam comes full on, wouldn't that be when detonation is most likely and most dangerous?
mart
Posts: 536
Joined: Tue Mar 22, 2005 2:06 pm

Re: Compression Ratio?

Post by mart »

George wrote:Aluminum conducts heat so much
better than iron that the comp ratio must be
increased by 1 point to compensate for the heat loss,
al vs iron heads. Heat is energy, & energy is
horsepower.
========================================
Yep. No disagreement here. All things being equal, an
aluminum head engine will make *less power* than an
otherwise identical cast iron head engine, if the
compression ratio is not increased by *at least* one full
point to compensate for the increased heat dissipation
of the aluminum heads. I still go with my basic, albeit ,
somewhat conservative recomendation of about 10 to
10.5 compression for a cast iron head, premium pump
gas, 392 -stroker street engine and 11 to 11.5
compression for the same engine with aluminum heads.
Some guys have safely run than this more on the
street with pump gas but there are too many variables
to say that more, or more more correctly, exactly how
much more, will safely work in all cases. As a *general*
rule of thumb though - a lighter car with a lower rear gear,
a longer duration, high overlap cam and richer and cooler
overal fuel/air mixture will tolerate more compression
than the opposite, for a given fuel octane. A compression
ratio that might work fine on the street in a radiclly
cammed, 4.56 geared, 1800 lb.T-bucket might rattle it's
brains out in an RV cammed, 3.00 geared, 4800 lb "lead
sled" street cruiser.

mart

P.S. About a long duration cam bleeding off cylinder pressure
at low and medium speeds, but increasing it when the cam
"comes on" higher up. Yeah, it does that and that *could*
cause problems. But the there's a few factors that hopefuly
help compensate. First, with a long duration cam, on the
street at least, the engine is not usually run way up in the
rev range where the cam really starts working for more than
a few seconds at a time. Most of the time, it's running at a
point way below where the cam becomes "efficient".
The key words here are "on the street" though! (if you're
talking about something like an oval track car or a marine
engine, where they run wide open and "on the cam" most of the
time, all bets are off!) Secondly, when the engine is at high revs
and the cam is "working" and increasing cylinder pressure, the
carbs(s) are usually wide open and hopefully providing a much
richer fuel mixture then when cruising. Also, by the time the engine
is "on the cam", the car is usually already rolling at a pretty good
clip. It has momentum now and has overcome the initial inertia
of getting the car rolling from a dead stop, so the load (actually
the pressure working against the pistons and cylinder pressure)
is somewhat reduced.

=======================================
NE57
Posts: 192
Joined: Fri Mar 27, 2009 12:54 pm

Re: Compression Ratio?

Post by NE57 »

Would it then be a fair assumption that a vacuum advance(actually vacuum retard if I understand it right) distributor would be helpful in avoiding detonation in high load/ low speed situations in a high CR motor?

I'll be needing to pick out and buy a distributor and this aspect would be a definite consideration. A strictly mechanical advance dist. would not be a good idea for the street in any case. Do I have that right?

Its interesting and fun to see how all these things inter-relate. Making an uninformed choice in one part of the motor may have unforseen consequences later on. I'm really glad I stumbled on to the collective wisdom on this board. Might save me from making an expensive mistake. Thanks again to all. Yup, I'll have more questions.
mart
Posts: 536
Joined: Tue Mar 22, 2005 2:06 pm

Re: Compression Ratio?

Post by mart »

NE57 wrote:Would it then be a fair assumption that a vacuum advance(actually vacuum retard if I understand it right) distributor would be helpful in avoiding detonation in high load/ low speed situations in a high CR motor?

I'll be needing to pick out and buy a distributor and this aspect would be a definite consideration. A strictly mechanical advance dist. would not be a good idea for the street in any case. Do I have that right?

Its interesting and fun to see how all these things inter-relate. Making an uninformed choice in one part of the motor may have unforseen consequences later on. I'm really glad I stumbled on to the collective wisdom on this board. Might save me from making an expensive mistake. Thanks again to all. Yup, I'll have more questions.
=================================================
You want a vacuum advance on the street because it will improve fuel economy and overall drivability ast low and medium speeds. How much compression your engine can tolerate on a given fuel octane is more or less set by the load on the engine, the rpm and the amount of total ignition advance. What the vacuum advance does is dial in more ingition advance when the engine is under light load and thus not in a detononation prone sitution anyway. When you nail the the throttle, the vacuum essentialy goes to zero, so the vacuum advance is not adding any timing to the engine. That's when the mechanical advance takes over and contols your ignition timing. The vacuum advance will allow you to run less initial timing for easier starting and crank in enough extra 'light load' advance for decent economy, power and throttle respomse at low and cruising speeds . It can also allow you to ward of detonation at higher speeds and loads, because since the vacuum advance will supply the ignition advance needed at low speeds and light loads, you can then use a slower, less aggreesive mechanical advance curve .

mart
polyspheric
Posts: 57
Joined: Wed Apr 09, 2008 1:03 pm

Re: Compression Ratio?

Post by polyspheric »

......if you say so.
dan miller
Posts: 86
Joined: Wed Dec 27, 2006 5:05 am

Re: Compression Ratio?

Post by dan miller »

On our Engine Masters engines, with aluminum Hot Heads, 10.5:1 compression, and a cams in the mid 240's, we observed no detonation with the supplied 91 octane fuel. It made best power at 26 degrees lead (fixed).

Danny
krooser
Posts: 58
Joined: Sun Sep 01, 2002 8:21 pm

Re: Compression Ratio?

Post by krooser »

NE57 wrote:Would it then be a fair assumption that a vacuum advance(actually vacuum retard if I understand it right) distributor would be helpful in avoiding detonation in high load/ low speed situations in a high CR motor?

I'll be needing to pick out and buy a distributor and this aspect would be a definite consideration. A strictly mechanical advance dist. would not be a good idea for the street in any case. Do I have that right?

Its interesting and fun to see how all these things inter-relate. Making an uninformed choice in one part of the motor may have unforseen consequences later on. I'm really glad I stumbled on to the collective wisdom on this board. Might save me from making an expensive mistake. Thanks again to all. Yup, I'll have more questions.
Sounds like you're trying to build a monster motor for the street when a fairly mild Hemi will make you squirt in your pants...

Never seen a 'vacuum retard' dist except in an 80's smog motor... a CD box with retard can be used but I usually only see 'em on blown engines... BIG blown engines. you don't want all that race stuff for a street motor... IMHO. Blowers are fine but keep the set-up mild.

Build an 10-1 hemi with a mild cam, stock heads, good ignition and a proper carb and you'll be a happy camper. If you like to wrench on your stuff every week and want to put up with overheating in traffic, poor fuel economy and such by all means build that Rat Roaster.

I've seen strong Hemi's do 15 mpg on the hwy without an OD trans... not bad.
Last edited by krooser on Sat Aug 01, 2009 5:52 am, edited 1 time in total.
China...Great Food...Crappy Tools
54 fool
Posts: 2
Joined: Fri Jul 31, 2009 1:14 pm

Re: Compression Ratio?

Post by 54 fool »

Newbe to forum-1st time posting.I would read the post written by Gene Adams(Racing the early Hemi) or something. He covers just about everything you will need to know when you are actually screwing a combo together. If the post doesnt answer your specific question,call him up(his #is in the Tex Smith Hemi book ). He is a very polite and knowledgeable guy-just keep things quick as he is very busy since he is THE MAN when it comes to these ancient old motors that we all love.

Good luck!
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dan miller
Posts: 86
Joined: Wed Dec 27, 2006 5:05 am

Re: Compression Ratio?

Post by dan miller »

Gene has a new number. (530) 357-5570

Best to call between 0900 - 1500 California time

Danny
oldngood
Posts: 121
Joined: Thu Mar 13, 2008 5:14 pm

Re: Compression Ratio?

Post by oldngood »

NE57 wrote:I was hoping someone could chime in with a specific experience. As in..."with X ratio and Y cam I had or didn't have detonation" .
Dare I go for 12:1 if went with the 250 degree cam? Pump gas.

Your cam choice is a good one and about the maximum you can run with a street engine and still be sanely driveable- I run a 470 CID "brand X" motor with a solid lifter cam 250 @ .050" and really like it. You have the cubic inches with 456 to run that much duration.

the 12:1 is too high for pump gas. At WOT it would ping like hell.

The best way to make power with pump gas is, maximize cubic inches, port the heads to maximize intake port volume and airflow, improve manifolding, run a solid flat tappet cam. Keep the CR at 9.25 to live on the 93 octane.

12:1 would require 110-116 octane racing fuel to live at WOT for any length of time.
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