I'm building a pontiac 68 400 and looking to put after market heads on.
With the goal of maintaining the ability to run 90 octane pump gas, I am
torn between some info
given to me.
The head manufacturer is recommending going with the 74cc heads. I am
looking at 270-290 cfm flow to put on top of stock flat tops with a RA IV
My gut is telling me to stay with 85cc heads, but that is all I have to go
Does anyone have any experience with a similiar set up?
Thanks and all the best.
Okay, I'm curious. I'm no Pontiac expert but build Chevy engines and am
up for the compression expression.
Are you aiming for a stock-type engine build or are you shooting for
performance with available components? I ask because you state the common
street performance desire to "run on pump gas" but I'm unfamiliar with the
"RA IV cam".
What's the cam data, vehicle weight, gearing and application (towing,
mileage, racing or mid-life crisis hot rod:) Really interested in what the
compression ratio is with your current configuration too.
There's a lot that goes into avoiding preignition & detonation, which
sounds like your basic concern. But, the simple static compression increase
that comes with reduced head volume might not be a problem at all if the
dynamic compression if affected appropriately by altered valve timing and
combustion chamber shape. But, there are definitely things that you can do
to help, like cylinder-to-head clearance.
First I'm not too familiar with that cam either but like James says cam
timing has a lot to do with it. You can use a cam to make compression or
take away some by the valve timing.
We are currently building a high torque engine and when I talked to the cam
place we use he asked what compression ratio we were using. He said he can
make the cam to match the pistons to get the compression to where we want it
to make high torque...
Also You are better to use smaller cc heads than using a domed piston to
make compression. I have read that a flat top can take higher compression
without detonation than a domed piston due to flame travel over the dome
causes more heat on one side and uneven flame travel.
Hopefully someone in here knows that cam and what you can get away with on
Hope some of this helps you a little...
He says the cam specs are as follows.
" The RA IV cam is .516 lift with 230/240 duration (GM #9794041). It was run
in the Ram Air IV motors during the '68 thru '71 heyday."
Yeah. All things being equal, I Think, raising the compression with
smaller heads is preferable to domed pistons. The domes inhibit the flame
front (= lost power to a certain extent). It may even help with detonation
through better quench (But, now I'm just thinking out loud). It does make
sense to not have a piston crown up there to pimp-slap the head, though:) A
race engine makes tradeoffs that we don't need to approach which is where
domes might be the right choice.
Now that we have the cam, need to do the math to find out the static
compression. LET's GET IT OONN!!!
P.S. I'm jealous. Wish I still had my '69...
Found 2 different specs for the cam... one says intake closes at 43 deg ABDC
other says 48 deg... I took 45 as a point to go by and my calculations are
as follows... if someone comes up with a different answer please correct
me... I did this by using 45 degrees and using trig to come up with position
of the piston as approx .375" ABDC therefore the 3.75" stroke now becomes
3.375" anyway here is what I came up with ...
P.S. cam spec was @ .050" which should be ok if anything using 0" it would
lower the ratio even more.
using a .030" thick head gasket
74 cc should give you approx. 10.12 to 1
85 cc should give you approx. 9.03 to 1
using a .060" thick
74 cc should give you approx. 9.441 to 1
85 cc should give you approx. 8.473 to 1
Anyone else come up with any ratios ???
P.S. this is considering the pistons being at TDC right at the top of the
cylinder not below or above the deck. Also not counting the distance from
top of piston to rings, and also I figured it out using 6" rod (couldn't
find a spec on rod length).
It's funny how you can buy a flattop that'll give you a .5 difference from
I'm more use to doing these calculations for motorcycle 2 stroke engines
which is easy to measure how far up in the bore the port timing is and the
dome is easily measured...
Looks reasonable and it agrees with our gut feeling, as expected. I'd
definitely agree with the recommendation for smaller heads. Pump gas
shouldn't be a problem in that neighborhood. Depending how close you want
to get to a magic compression ratio, you can do some extra math (there are
free downloadable calculators or I can volunteer via e-mail) and consider
some tweaks to your plan, i.e. shave the heads, various gasket thicknesses,
block decking, etc.
Do they have a calculator that figures all that out for you ? If so give me
a link I would like to try it. We always did it the old fashioned way using
calculus, algebra and normal math... the only part is the metric conversion.
(ci to cc) if you do have a calc to do that how close were we to what it
came up with ?
Sure. There's plenty of 'em out there. I don't have all the engine
specs, so, I didn't run the calculator. Google "compression calculator" and
it'll give you a slew of 'em. Here's a good link;
There are plenty other calculators and a search should be able to include
a link to a site that has many of them. Some are more thorough than others
(ex. this one accounts for gasket thickness but not gasket bore size like a
4.125 bore head gasket that sits in a 4in bore small block) but it's plenty
good enough. There is some outstanding software out there depending on your
wallet and need to push the limits of math for that razor's edge.
He says that the stock engine lists 8-something:1 (I forget). Using some
generic specs, I came up with 9.3 in stock form. I figured he'll come in at
around 10.4:1. That might seem like it's pushing the limits for "pump gas"
but, even if it's the true static compression, his cam should make that just
right IMHO. I think my numbers were conservative and they're either correct
(good) or actual compression will be just a little lower (still good).
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