I was looking in the archives and am troubled about the comp ratio on my rebuild. Is 9.2 : 1 the best I will be able to achieve running 93 oct? I remember a long time ago a huge discussion about this topic, but it was purged when Geoff revamped the site a while back.
Not neccesarily. I've heard it said that you could possibly go as high as 10:1 with iron heads and higher yet with AL. It's second hand info. I can't verify by personal experience. The general rule of thumb is 9.5 is the breakpoint for comfort. Evidently it all hinges on how well it's tuned.
Going to 10 with iron heads and pump gas requires eliminating all the possible detonation sources and having the right curve on your distributor and a very, VERY well-tuned carb.
With aluminum heads it's quite easy since you can go a full point over iron with an equivalent short block.
Many of the steps you'd have to take can be expensive and are probably not necessary for what you need. Most people can't tell the difference between 9.2:1 and 9.7:1 on the street, even with a calibrated SOTP meter. On top of that, as Bob mentions, keeping it tuned becomes a bigger part of your hobby than just enjoying the ride.
IMHO, build the biggest 9:1 street motor you can and spend the $5k you save on the coolest paint job and the best interior.
My car has a .030 over 350 with 69 cc #16 heads, cast pistons. I run it on 93 octane pump gas with an occasional (~every third tank of gas) bottle of cheap octane booster. It rarely pings under any circumstances. It runs at 160 to 180 degrees with a 160 degree thermostat. The only thing I've deliberately done to deal with detonation is using the coolest plugs that are appropriate for my engine.
Pontiac said the 350 HO had 10:1 compression. Classical Pontiac's compression calculator says my setup is 9.4:1. I think Pontiac overstated compression ratios.
If you want to run high compression there are two things that help the most. First is to have the block 0-decked. and second use a cam with at least 112 deg and early intake valve opening.
I just don't get the mind set that people have a 400 car or put a 400 in a car and then de tune it so every small block chevy or Subaru goes blowing by. If you want less power get a 350 or 326. I just say don't let the good rep of high preformance Pontiac get laughed at .
My own car has 400 0-deck 68 #16 heads crower 60916 cam 1.65 rockers and I have put 5,000 miles on it running 93 octane with no sign of ping or detionation.
Alsow,I have demonstrated to the locals enough power that they don't try to beat me on the street.
If running higher compression zero decking for a tighter quench area and smoothing all sharp corners is a good idea. However, you want a cam with a later intake valve closing as that's when the cylinder begins to seal. I would ultimatley pick a cam that gives me the dynamic compression I want for the intended use. Measure everything and use a dynamic compression calculator otherwise stay in the mid to low nines for your static compression. I'd recommend degreeing the cam installation regardless.
Dave was co-partner with ace mechanic Demetri Kokkoris, the team known as SUPERJET SPECIALTIES of Queens, NY. Superjet rebuilt and blueprinted musclecar engines, specializing in modification and custom tuning of Rochester Quadrajet carburetors.
Below is an article written by Dave regarding camshaft and cylinder head selection for the Pontiac V-8, one of several articles that appeared in Pontiac musclecar related publications.
Following is some additional info by highly experienced Pontiac ace, Jim Hand.
Due to time constraints, Dave is no longer able to answer automotive questions.
Valve timing and compression pressure are the two most important factors that effect an engine's performance. All other modifications will be futile unless the valve events and compression pressure have been carefully dialed in per desired state of tune. Through experimentation, we have developed the following "formulas" which can help when attempting to create an optimum parts combo.
The optimum (maximum) cranking cylinder compression pressure for a performance/street application using 93 octane fuel = 160 -170 psi
Changing the position of the camshaft (and thus intake valve closing event) by 4 degrees changes cranking cylinder pressure by about 5 psi. (Advancing increases pressure, retarding decreases pressure.)
Changing the compression ratio by 1 point changes cranking cylinder pressure by about 20 - 25 psi.
CAMSHAFT (all cam and valve event figures measured at .050" lift)
The first thing is to choose a cam grind that will yield the desired manner of operation. We know from experience that a cam such as Pontiac's 068 - "H.O." (211/225 - 116 lobe sep.) or similar will produce a broad torque band in a 400, and thus excellent all around power, while yielding a pleasant idle and relatively good fuel economy. We also know that a cam such as Pontiac's 041 - "RA IV" (230/240 - 114) or similar will produce max torque at a higher rpm, thus greater horsepower, but at the expense of lesser low end torque, poor idle, poor economy, and excessive emissions.
Any Pontiac enthusiast who seeks improved performance but intends to use his/her car as a "daily driver" should aim for a maximum camshaft intake duration of 210 - 220 degrees, depending on the engine/situation. Exhaust duration can be a bit greater without an adverse effect. For those who are ONLY concerned with obtaining the lowest possible quarter mile e.t. ("race" use), cam durations upwards of 225 degrees will be more effective. Such larger duration cams are NOT appropriate for use in "daily drivers" because of their adverse effect on economy and street behavior. A car that is used nearly everyday, city and highway, heat wave or snow storm, must be dependable, efficient, and convenient to use. DO NOT make the age old mistake of creating a "race" type vehicle which is actually intended for "daily driver" usage.
Remember that engine displacement is an important factor when choosing a cam grind. Bigger engines can tolerate larger durations. A Pontiac 068 grind will act a bit "wild" in a 350, but act "mild" in a 455. Also, heavier vehicles, and vehicles with low numerical rear-end gearing will be better off with a lesser cam duration, where lighter vehicles, and vehicles with higher numerical rear-end gearing can get away with a greater duration.
COMPRESSION
The "compression ratio" (also referred to as "static" compression) is determined by the cylinder head combustion chamber, head gasket, deck height, and piston face volumes (dead volume), with respect to the total cylinder volume swept by the piston (swept volume).
The intake valve closing event is determined by the intake duration and intake lobe centerline of the camshaft, as well as the actual position of the cam as installed with respect to the crankshaft.
Actual compression pressure, also referred to as "dynamic compression", is determined by both the "static" measured compression ratio, and the intake valve closing event. The dynamic compression is the compression pressure the engine actually sees, regardless of the measured ratio, and is all that really matters. Consideration must be given to the matching of the cam and heads to yield the desired compression pressure. It is too common an error that a new cam grind is swapped in without paying attention to compression, resulting in too low a compression pressure.
Optimum cranking compression pressure when using 93 octane fuel is 160 - 170 psi, at least with old Pontiacs. Too high a compression pressure will yield too high a peak combustion pressure and thus cause detonation. Too low a compression pressure will yield a low combustion pressure and thus poor performance and poor economy.
Some well-blueprinted Pontiacs have been known to handle pressures upwards of 170 psi when using 93 octane fuel without detonation. But considering the inconsistent and often poor quality of today's fuel, it is much wiser to aim on the low side of what might technically be the maximum useable pressure. Detonation must be avoided at all costs. We have found that pressures of 190+ psi will cause detonation, overheating, "run-on", etc., unless 98+ octane fuel is used, or ignition timing advance is reduced way below optimum. Remember that you should expect to often receive fuel with an actual octane rating of less than what you're paying for. Also, at any given time, a few cylinders may be running a tad hot/lean due to one of several common problems (uneven coolant flow through the block, uneven fuel mixture distribution, etc). For street use, we vote to dial in a "safety zone" and stay detonation-free.
EXAMPLE
Begin your project by measuring the existing cam and compression specs as a starting point, and then use the previously mentioned "formulas" to get an idea of what parts are needed to create an optimum combo. It is very important to measure all the variables of your own project engine. Do not rely on existing texts/manuals for measurements. Texts are often inaccurate, and original parts of your engine may have been modified or swapped prior to your ownership.
We recently disassembled an excellent bone stock 1975 Pontiac 400 from a Trans-Am. Actual compression ratio was measured at 7.8:1. Cam was a Pontiac 066 and measured in at 197/206 - 112, as specs call for. Even with the original stretched timing chain, cam was found positioned at a 107 degree intake lobe centerline, also as specs call for. This places the intake closing of this cam at 26 degrees abdc, as measured. Before we had disassembled the engine, cranking cylinder pressure was measured at 135 psi across the board.
In order to increase performance to the desired level, we decided to replace the original 066 cam with a Pontiac 068 cam. If we install the 068 cam "straight up", the intake closing event will be at 39 degrees abdc. This is 13 degrees LATER than with the original cam (13 degrees retarded from original). Refer back to our formulas, and note that the engine would now produce roughly 15 psi LESS compression pressure with the 068 cam due to the later intake closing - from 135 psi down to 120 psi. A compression pressure of 120 psi is too low to support optimum efficiency. Desired pressure is 160-170 psi, so we need to increase pressure by 40-50 psi. Refer back to our formulas, and note that it is possible to gain 40-50 psi by increasing the compression ratio by 1.5 to 2 points. Our target compression ratio is thus 9.5:1. In order to reach this compression ratio with a .030" overbore and typical flat top replacement pistons, we'd need heads with 82cc combustion chambers. Since we cannot readily get our hands on heads with such a chamber size, we've chosen a pair of cheap and plentiful 93cc "6X" heads (off a `77 350). These heads will be milled .040" to achieve a chamber size of 86cc. The compression ratio will be about 9.2:1. Estimated compression pressure is 150-155 psi - close enough, considering the ease and low cost of this plan.
To gain a bit more psi, we can advance the cam. Or, we can try a different cam grind with similar duration to the Pontiac 068 cam, but with a tighter lobe separation which will automatically advance the intake closing event with relation to the other events. The amount of overlap gained by reducing the lobe separation from say 116 to 112 degrees on cam with 068 size duration will not cause any measurable adverse effect on economy.
A pair of older "16", "62", or similar "high compression" heads could have been chosen, but with their small 72cc chambers, estimated pressure would be up in the 185+ psi range with our combo - too high for our purposes.
Keep in mind that if the wild Pontiac 041 cam was installed "straight up" in the above engine, the intake closing event would be 47 degrees abdc, 8 degrees later than with the 068 cam. This alone would drop pressure by about another 10 psi, and thus the 72cc heads (10.4:1 c.r.) might just slip by as detonation-free. Estimated pressure is 175 psi - right on the line.
OVERLAP
A common misconception is that camshaft overlap reduces cranking cylinder compression pressure. Not so. Overlap indeed reduces peak combustion pressure by diluting the incoming intake charge with exhaust, and by sending some of the combustible mixture out past the exhaust valve before ignition, but this does not affect compression pressure. The intake valve closing event is the only cam related factor that noticeably affects compression pressure.
An experiment was conducted: a given cam, as installed in a custom 9.9:1 c.r. 455, yielded about 185 psi. Detonation was evident. Cam had only 1 degree of overlap, and the intake closing was at 39 degrees abdc. A new "larger" aftermarket cam with a full 14 degrees of overlap, but SAME intake closing as the previous cam (39 degrees), was installed. (New cam had greater duration and a tighter lobe separation.) Cranking cylinder pressure was exactly the same - 185 psi. Other engine characteristics changed, but detonation was still present. (Fuel metering and ignition timing were correctly tuned for each application.)
The first cam was then refitted, but was retarded 10 degrees from the original position so that the intake closing was now at 49 degrees abdc. Pressure dropped from 185 to about 170 - the tendency of detonation was noticeably reduced. (The optimum set-up for this particular engine was ultimately achieved by reducing the compression ratio and running the smaller cam slightly advanced.)
Cylinder compression pressure effects the amount of peak combustion pressure to a greater extent than does the amount of overlap, per degree of change. So, for those trying to reduce detonation problems in original "high compression" engines by merely swapping the cam, be sure to get a cam with a wide lobe separation and thus a relatively late intake closing. Note that TOO late an intake closing event will adversely effect street performance. Such allows an excessive amount of fuel mixture to be pushed back into the intake, creating significant reverse pulses which will hurt low end performance and can even upset carburetor operation.
The above information is general, and is intended as a guide. Figures are approximate, and will vary with different situations. Also, remember that "budget" measuring devices often have large error margins. (Inexpensive compression gauges will vary from each other by as much as 15+ psi, etc.)
It's always a good idea to consider tech articles and recommendations, but you can never avoid experimenting with your own project when optimum results are desired. If you want it done right, you must make your own educated decisions based on your own specific data, and do some trial & error testing, because every situation is unique.
Concept and article by Dave Miranda Mechanical tests conducted by Demetri Kokkoris and Dave Miranda
ABOVE: The Official "Elf-Mobile" - 1968 G.T.O., owned and raced by Demetri Kokkoris.
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Due to time constraints, Dave is no longer able to answer automotive questions.
Additional info directly related to the above subject, emailed to me in 1998 by highly experienced Pontiac ace, Jim Hand. Thanks for the input, Jim! -Dave
When looking at cams and compression, the real relationship is when the intake valve closes in relation to BDC. Actual compression of the fuel/ air mixture does not begin until the intake valve closes, regardless of the static compression ration (CR) rating. My Performance Trends Engine Analyzer will calculate the dynamic CR based on the two factors - static CR and intake closure point. Here are several examples, all with an assumed static CR of 10:1 with a 455:
Factory Grind 066 Intake is 197 with LC at 107, dynamic CR = 8.21
Factory Grind 068 Intake is 212 with LC at 113, dynamic CR = 7.25
Factory Grind 744 Intake is 224 with LC at 113, dynamic CR = 6.67
Factory Grind 041 Intake is 230 with LC at 112, dynamic CR = 6.65
Comp Cams 268 Intake is 218 with LC at 106, dynamic CR = 7.62
My Wolverine 234/244 Intake is at 107 with LC at 107, dynamic CR = 6.84.
What is the significance of these numbers? The 066 and the CC 268 would be real pingers in an engine with higher rated static CR. The 041, 744, and my Wolverine would be less likely to detonate. The earlier lobes (intake closing points) of the 066 and CC 268 make the engine think that it has higher static CR. Longer durations tends to minimize this tendency, but does not preclude it.
For example, the same Ultradyne cam grind - 239-247 at a different lobe separations (LS) show the same characteristics. The normal 239/247 has the intake at 104 (exhaust at 116) and an LS of 110, and has a dynamic CR of 6.90. However, using the same lobe profiles but placing the intake at 100 and the exhaust at 112 for a 106 LS (which is essentially advancing the intake closing event by 4 degrees compared to the previous), the dynamic CR jumps to 7.19. If the engine was marginally OK with the 110 LS cam, it could be ruinous to install the 106 LS cam.
Be very careful of any aftermarket versions of Pontiac grinds. Invariably, they have advanced the intake lobes in order to improve low rpm throttle feel, but give the engine more dynamic CR!
Dave, I have enjoyed reading your sensible post concerning compression and valve timing. Your testing seems very comprehensive, but I don’t believe that it can be considered a benchmark for dictating the maximum CR of all other combinations of blocks, deck clearances, heads, cams, timing, gas quality, etc. Like you, I try to help others get a little more out of their engines, while suggesting the cheapest and easiest way. How do we help the guys that know the technical aspects to live with optimum CR without leading others astray?
I know someone who is extremely well informed about a wide range of auto subjects. He has been working for several years on his dream 400 setup. But he was scared so bad about the dangers of high CR that he set his 400 at just 9.25. Yet, he has spent a lot of money on ignition, head work, special pistons, etc., but the thing that will make the most difference in actual engine performance is not set to optimum! In addition, he is going to try to run a "specially selected" cam with the intake lobe at 112. Not only does he not have adequate static CR, but the 112 lobe cam will make it even worse.
From my view, anyone could safely run 9.7 to 10 CR on a daily driven street/strip car, providing the engine was setup for that with correct assembly, proper ignition and carb work, correct engine cooling, and dependable pump gas. My brother, friend, and I have all done it for years with not a trace of problems.
Here are the steps I take to minimize high-CR detonation related problems:
When preparing the heads, as part of the porting we lay back the shrouding on both sides of the intake valve. We start cutting at the base of the overhang (chamber floor), and straighten and tilt back the overhang such that it does not get closer to the valve as the valve lifts. All chamber edges are rounded/smoothed. The entire chamber, including both valves, are polished to a bright luster. The piston tops are prepared by rounding the edges of the piston and the valve relief's, and then they also are polished. This practice is based on my desire to eliminate any potential hot spots, as well as suggested by KB Piston Co. KB states that the polished surfaces will tend to reflect heat back into the chamber thus generating more combustion pressure and power. We also carefully prepare each spark plug by filing all edges to a rounded surface. So do any of these things help? They certainly can't hurt, and by minimizing every possible hot spot, I feel even safer.
Now for the most important step of all regarding CR. Set the engine deck to 0. This will vastly improve quench/squish, which in turn improves fuel air mixing. Better fuel mix burns faster and more predictable, thus requiring less ignition advance. Presto! We have won twice! More CR due to the 0 deck, and less timing required so we can run even more CR. When I made this simple change (about .020 off to get to 0 deck), my engine ran quickest and fastest with 30 degrees total timing as opposed to 34 total with the .020 deck. This should be the first step in setting the final CR in every serious engine build-up! On the reverse side of this effect, cutting the pistons tops (and thus increasing effective “deck volume”) may aggravate a detonation problem because of the poorer fuel mixing! Even though the static CR is lower, the timing has to be advanced to get any power, and we are back into the detonation problems.
And don’t forget temperature control! I was at the drag strip last night in 86 degree weather. While in line, the engine got up to 190, so I sped the rpm up to about 1500 for 20 seconds. It dropped to 180, and by the time we hit the finish line at 111.5 mph, it had dropped down to 175.
Jim’s above approach using “dynamic CR” figures (via an analyzer or calculator) as a guide for dialing in the optimum cam/head combo (optimum peak pressure) is essentially the same premise as using measured cranking compression pressure as a guide for the same. In both approaches, we are studying the relationship between the actual physical ("static") compression ratio and the timing of the intake valve closing event... which together make up the "dynamic compression", or compression pressure the engine actually sees.
The method of calculating dynamic CR on paper (or on a computer) is nice since it can be done without actually assembling and testing a physical engine. Of course, assembling a proposed combo and then measuring the results will deliver an actual physical indicator of what is really taking place. As well, it is good for comparing the measured stats of your current set-up to a new proposed set-up in order to observe the true degree of change and establish benchmarks for your own engine.
As Jim has pointed out in the past, the “optimum” measured cranking compression pressure figure will vary from engine to engine… while 170 psi may be “optimum” in one set-up, 200 psi may be optimum in another set-up, and so on… and then of course we must remember that there will be a good deal of error with typical compression gauges, and differences in the measurement procedure itself (condition of battery, rpm of starter motor, ambient temperature, ambient pressure, temperature of engine, etc… all may effect a cranking compression pressure reading by easily 10% or 15%.)
So it appears that the only way to arrive at the optimum dynamic compression ratio figure for one’s own situation is to experiment… which may mean more than one swapping of cam/heads. Many weekend hobbyists may not have the time or budget to reconfigure their engine more than once, so this is why, when in doubt, I personally vote to aim on the low side when plotting “optimum” dynamic CR. Just my personal opinion.
For STREET performance use where no serious competition is involved, running a slightly lower than “optimum” dynamic CR will still net you excellent performance and economy, even if you might be down on power by a tiny amount compared to true "optimum" dynamic CR. HOWEVER, running a slightly higher than optimum dynamic CR may yield realistic headaches such as detonation, run-on, hot operation, etc. If you can only rebuild your engine one time, which scenario would you rather have? I personally vote to stay detonation-free at all costs, even at the possible expense of a tiny amount of performance.
However, needless to say, if one is serious about squeezing out every last drop of performance for competition use, then experimenting is absolutely necessary to arrive at the true optimum state... be prepared to swap parts more than once. But after all, isn't that what hot-rodding is all about?
Due to time constraints, Dave is no longer able to answer automotive questions.
That post belongs in the Hall of Fame Forum. That shed a lot of light on things for me and was extremely educational. Thank you for the time to post all of that info.
I've heard good things about Paul Kelley's calculator. I downloaded it with the VB6 Runtime files but couldn't get it to work for me.
What kind of trouble did you have?
The download completed. The unzip worked to show three files: Project, setup, and setup text (I believe it was a readme file). I click setup to install. The installation said completed. The only file I saw to do next was Project (nothing else showed up on my desktop) so I clicked that. If I remember right, nothing happened and I think the window actually closed on me. I tried it a few times. I just decided to delete the files. If you know what went wrong, let me know.
Your welcome Hoody on the calculators.
"An ignorant man thinks he knows everything, a wise man knows he doesn't."
No, sorry Sheri, you could always give it another try. I have used other "online" calculators as well but I like this one the best. There can be some confusion about the value to enter for the closing angle (.050 vs actual closing). Previously I was using a handwritten formula to find the actual closing angle but the P.Kelley calculator has a provision for this which I have found to be very close to the formula I was using. It just makes it easier and I'm confident it's accurate. I can toggle between static and dynamic figures while making adjustments which is helpful too. Also, I don't have to be online to use it.
I will give it another try sometime. If it doesn't work for me, I will send Paul Kelley a message to see if he can help. He did say contact him for questions and help. I have heard from others too that his calculator is accurate.
"An ignorant man thinks he knows everything, a wise man knows he doesn't."
Some people already have Visual Basic 6 Runtime files on their computer and some do not. It's like some people may have Adobe player to run pdf files and others may not. I understand it as Runtime is needed to run this program. Visual Basic is a computer program designed to make computer programs. I take Paul Kelley used this program to make a program for compression calculations.
Last edited by Sheri; 01/29/0905:20 AM.
"An ignorant man thinks he knows everything, a wise man knows he doesn't."