compression produces torque at all rpm ranges. going from 9:1 to 10:1 will give a 3%-4% increase in power over the entire rpm range. every full point will do the same. several engine builders told me that 9.5:1 is the highest you can go with iron heads before you start to get into detonation problems. you can run higher than that but you will need to back the timing down or run a cam with enough duration to keep the cylinder pressure low at lower rpm. i would not recomend doing either and i know of people that ended up with broken piston ring lands trying to do it. you can run 10.5:1 with aluminum heads because they dissipate heat faster than iron heads do.

 

Since your cam is pretty aggressive, I'm thinking you can get away with 10:1 static compression. Although, you would have to run premium fuel all the time. I also think it would be worthwhile. As mentioned, higher compression makes a big difference all through the RPM range.


I think you'd be disappointed if you lowered the compression. Likely, your cam would not work as well either. I'm not a cam expert, but yours may be designed to operate with high static compression. Lowering the compression could really take the wind out of Jane's sails. Remember, the mid 1970s 302 engines were pretty anemic and low compression was a big factor.

 

I think you'd be disappointed if you lowered the compression. Likely, your cam would not work as well either. I'm not a cam expert, but yours may be designed to operate with high static compression. Lowering the compression could really take the wind out of Jane's sails. Remember, the mid 1970s 302 engines were pretty anemic and low compression was a big factor.

To clarify, my old compression was 8.9:1, so any change I make will be improving compression by at least half a point! :smile2:

Knowing what I do about the engine now, I'm actually kind of surprised how zippy it was. Next time I'm back in Colorado I'll have to take you for a spin so you can see what it's like when it's not suffering from a failing oil pump! :lol:

 

Best would be to have the heads air flow tested and CCed great info to help with mating parts along with desired usage

 

That zip you like is torque, which is simply force (compression ratio) times distance (stroke, or displacement). Go for as high a CR as you can handle without having to really retard timing. The cam and ported heads will like it more (better breathing makes better use of the higher CR), and you won't lose the punch in the back the torque gives you with the higher CR. And stay with the premium gas.
Again, I'm certainly no expert either but all the above is just sort of common sense engine-wise as I see it... hope it helps a bit.

 

The only way you're really going to know is to check the opening and closing events on the cam to get an accurate idea of dynamic compression. That will give you the most realistic idea of what octane gasoline you can run. Until you measure the cam, everyone here can only guess.

 

What year heads? When ported did you put big valves in? The '66 289 heads I'm preparing for the next build together with mildly dished pistons (don't remember how dished) with a good felpro head gasket will yield 9.3:1 in a +.030 '66 289 short block. That's a good CR to work with, although I might need to run premium gas. I do in my current ('68 302) build that has flat tops and milled/ported/big valve '68 302 heads.

 

The difference in the compression ratio between the 2 pistons is negligible. For detonation resistance it will be more important to set the proper quench height.

 

Kelly, they called it "advertised" compression. You have to know exactly how far below the deck the pistons are in order to get an accurate calculation. A lot of oversized pistons are usually below the the deck surface to offset any increase in compression from being a larger displacement. You also have to know how much by me the head gasket adds. I'll bet your motor is closer to the low 8's. The A code piston has 10 CC less volume then the Center code's 13 CC and that will make about a true low to mid 9:1 CR IMO. I had both of these motors and measured them.

How compression you can run depends on a lot of factors. Look at today's cars. A lot run over 11:1 on 87 octane. Combustion chamber design and quench which is how far down below the deck the piston is will be big determining factors. I don't buy into the aluminum having an effect to any significant amount. The stock heads have 50+ years old combustion chamber technology. Today's aluminum heads have modern combustion chamber technology. That's what I think.

Years ago David Freiburger of Hot Rod wondered this also. He took a small block Chevy and tested it with 2 sets of heads. I think they were Dart. Both heads were the same except one set was the cast iron version and one was the aluminum version. Both sets made exactly the same power, used the amount of ignition timing. Both heads made the same compression. He said at the of the article he didn't find that aluminum head argument allowing more compression over caster iron to be true. Speak to your builder for advice.

Don't rule out a good used Explorer GT40 or GT40P motor. These can be very cost effective alternatives. That's what I'm doing.

 

Kelly,


I must agree with Tom. You have to closely measure everything on your engine to know the exact CR. The bore size, deck height (is it really 0-decked), how thick is the head gaskets you are using and what CC's does you cylinder heads have.


When you know these measurement, you can do the math and find your actual CR. There are different calculators on the net that can help. All the engine books I have, have the "math formula" for calculating it the old fashioned way.


I did that with the stroker engine I am building and I ended up with a CR of 9.3 - 9.5 :1 (depending on little things like how far down on the pistons are the rings seated and stuff like that). I was told by several smarter and more experienced guys than me, that between 9.2 and 9.5 would be a good aim for a conservative build that would not destroy it self.


Cheers
Bebob

 

the higher the compression you run, the more torque the engine produces, but there is a point of diminishing returns.

the higher the compression you run, the greater the risk of detonation, but there are ways to mitigate that to an extent.

for the street i would keep my compression ratio at about 9.5:1. if you feel you have the need to bump it up more, you can mitigate detonation somewhat by going with a cam that has narrower lobe separation angles, going from a 110 to a 108 cam, will allow you to run a higher compression ratio.

how does this work? it does so by changing the dynamic compression ratio, tricking the engine into thinking it has a lower compression ratio than it really does. and it does this right at the point where detonation tends to happen, at moderate speeds under load.

 

Maybe a little off topic but an interesting read about combustion chamber design it this site. Endyn look into the archive section on "the soft head" and Bob Glidden articles. Good read and interesting whether you believe it or not. The Soft Head articles were from about 1987 on Cleveland heads and compare what is said to when the auto makers started to redesign their heads later on in the 90's.

 

I say go 8.0cc pistons on one side and 6.5cc on the other side, That way you have the best of BOTH worlds!!

That is probably what the PO was trying to do with the mixed cylinders

 

The only "con" I can think of is if you plan to use a supercharger. Paxton superchargers don't work as well with compression over 9.5.

 

On another forum I hang out on one of the members built an altered wheel base straight axle 62 Chevy II gasser. It has a 6-71 supercharged BBC running high 9's low 10's. In street trim. When asked what gas he uses he said the finest 87 octane money can buy. He also uses a stock single point distributor and coil.

Personally I'd use only A code spec pistons with 3 CC dish. With stock heads you come to a out 9.5:1. I wouldn't worry or think twice about it. Most 289's have problems making too much more. Most aftermarket heads are meant for 5.0 engines to end up with around 9:1. They usually have around 65 CC which works out for about a half point loss in compression on a 289.

 

The compression ratios I calculated were with the following stats:
Bore size: 4.030"
Stroke length: 2.87"
Head gasket thickness (compressed): 0.035" (taken from specs on head gaskets I'm using)
Head gasket bore diameter: 4.010" (taken from specs on head gaskets I'm using)
Deck height: 0.016" (stock), though this will actually be less as the engine is getting decked this time and may have been decked last time it was rebuilt. Unsure and will find that out this week
Combustion chamber volume: 53cc (C5AE-B heads)

So that's how I'm getting the 9.38:1 or the 9.55:1 ratios based on piston volume.

So far I'm hearing a mixed bag of opinions. Bummer, as I was kind of hoping there was one definitive response! But hey, it's the vintage Mustang world... there's never one answer

 

So that's how I'm getting the 9.38:1 or the 9.55:1 ratios based on piston volume.

So far I'm hearing a mixed bag of opinions. Bummer, as I was kind of hoping there was one definitive response! But hey, it's the vintage Mustang world... there's never one answer

I had the same answers - so apparently nothing in the Mustang world is Black & White.

Good to see that you got your math done right. Did mean to insult you or anything the like.... not all double check what the suppliers write on their products.

Cheers
Bebob

 

I think your calculated CR will get you about where you want to be Kelly. You like your cam (maybe adjust you lsa as suggested a little), and you general setup as a whole...this will give you a little more, without having to pull up to the premium pump every time.

Don't expect it to feel like you dropped in a stroked out 351, and you'll be happy with it.

 

There are two points that everyone seems to agree with here, 9.5:1 is the max recommended CR and your going to need premium pump gas. To be clear 9.5:1 max meaning trouble free, noting to worry about! You have plenty of cam to offset a little more compression anyway. As someone mentioned also, the difference between your two choices is really minimal, nothing worth sweating over. My vote, go for it.

 

piston numbers

Kelly do you have part numbers you are considering for Jane ? I ask because some flattops are 1.605 tall, while others are only 1.585 tall. The shorter pistons will lower the compression. If Jane were mine, Id use Silvolite 3101HC.030 pistons. They are 1.605 tall, and also have standard and budget friendly 5/64th, 5/64ths, and 3/16ths ring grooves. The rings you'll want are 2M139.030 from Hastings. You'll be fine. LSG

 

Kelly do you have part numbers you are considering for Jane ? I ask because some flattops are 1.605 tall, while others are only 1.585 tall. The shorter pistons will lower the compression. If Jane were mine, Id use Silvolite 3101HC.030 pistons. They are 1.605 tall, and also have standard and budget friendly 5/64th, 5/64ths, and 3/16ths ring grooves. The rings you'll want are 2M139.030 from Hastings. You'll be fine. LSG

Yep!

+8cc piston is the Speed-Pro Hypereutectic H273CP30. Listed 1.605" compression distance. Also uses 5/64th, 5/64th, and 3/16th ring grooves.

+6.5cc piston is the Keith Black KB Performance KB115-030. Listed 1.608" compression distance. Also uses 5/64th, 5/64th, and 3/16th ring grooves.

 

With all due respect to the experts that have already weighed in...

You can run an actual 10.0:1 CR with stock iron heads (HiPo heads) and have 40 degrees of total advance WITHOUT having ANY detonation issues. And that's running today's gas with no extra octane booster added. If I can do that over many years (1991 thru 2015), and tens of thousands of miles, anybody can.

Z

 

With all due respect to the experts that have already weighed in...

You can run an actual 10.0:1 CR with stock iron heads (HiPo heads) and have 40 degrees of total advance WITHOUT having ANY detonation issues. And that running today's gas with no extra octane booster added. If I can do that over many years and tens of thousands of miles, anybody can.

Z

I'm glad you chimed in because I was thinking the same thing. But without currently owning the exact setup I didn't want to get into a fight where I didn't have a dog. However, my uncles '66 coupe ran that and probably a little higher with no problems.

 

What about building it to run on e85? I'm about to have my engine built and I was wondering what it might be like to up the compression to around 11+ and setup my carb, pump and lines for e85 to take advantage of the 103+ Octane level. I know compression has to be fairly high to take advantage of it. I was thinking it might be a matter of $1000 or so to get it setup for E85 and the HP to dollar ratio might be worth it, I've heard some pretty high HP claims from doing it.. Just thinking out loud.

 

Tom is right about the necessity of knowing the finished deck height. With that info you can then choose a piston that will give you optimum quench, which is going to be a major factor in controlling detonation.

If, for instance, your block will end up at 8.200" (taking six thousandths off), then you want to select a piston and head gasket that will give you optimal quench of .040" for a SBF. A KB SKB192-030 with 1.608" compression height combined with a 8.200" finished deck height will put the piston at zero deck and a head gasket with .040" compressed thickness will provide the desired .040" quench. The 12cc volume of the dish and valve reliefs on said piston will yield a 9.13:1 compression ratio which should work quite well. If your finished deck height is less than the 8.200" then look at a piston with a bit longer compression height.

Sure, you could go with something like a KB311 with a 6.50cc relief volume, which would up your compression to around 9.78:1 but I think you'd have no choice but to run 93+ octane premium fuel where the lower compression combined with .040" quench should let you get away with 91 on a regular basis.

Another "hint" while the block is being machined.... If you're going to plan on spinning it past 5,500 rpm on a regular basis, you might want to consider having the main caps and block machined to accept a 1/2" diameter 351W main cap bolt. The cost isn't horrible...around $150...and increases the clamping force on the cap by about 40% and vastly reducing the chances of cap walk from stretching of the 7/16" bolts.

 

If, for instance, your block will end up at 8.200" (taking six thousandths off), then you want to select a piston and head gasket that will give you optimal quench of .040" for a SBF. A KB SKB192-030 with 1.608" compression height combined with a 8.200" finished deck height will put the piston at zero deck and a head gasket with .040" compressed thickness will provide the desired .040" quench. The 12cc volume of the dish and valve reliefs on said piston will yield a 9.13:1 compression ratio which should work quite well. If your finished deck height is less than the 8.200" then look at a piston with a bit longer compression height.

Whoa whoa whoa, can you pump the brakes a bit there and go back and explain what it was that you just calculated? Where does the 1.608" compression height go with respect to deck height? What's the equation?

 

I agree with zray. Even using the crap we have for gas I'm running 10:1 in the '66. No-longer-made TRW replacement pistons, cast iron lightly
ported factory '66 heads, recurved distributor and about 13-14 initial timing with 23 in the distributor. Hipo mechanical cam. Gas I use in that
car is 87 octane; used the same octane for open track events too.
The '68 has almost as much cam, wickedly ported aluminum heads, Edelbrock EFI and 9.3:1 CR. It also uses the 87 octane option at the pump.
My "feeling" is that I went too low on compression and could have gone nearly a point higher on the same grade of fuel. Mainly the precision of
the EFI and the better chambers & aluminum construction of the heads. I say this without empirical evidence, just having 45 years experience
with numerous 289s.
I think Mr. Freiburger is all wet on his comments about aluminum heads..... if he said what was quoted earlier. Chevy heads are a different
animal in the combustion chamber and aluminum heads and iron heads DO NOT run the same ignition timing. Ever.

 

i would keep the compression at 9.5 max. if you have an engine that makes 200 HP raising the compression from 9.5 to 10 is only going to make an additional 3 to 4 hp max. in the world of professional racing every tiny bit of hp helps win a race but for street driving you wont feel that. you can get lots more power with a street performance cam, performance valve job, performer or stealth intake , headers , 600 cfm carb and dist curve job.

 

If you laid a flat piece of glass on bottom of head its the volume of liquid you could pour into the chamber with valves installed. All '65 non-HiPo should be standard at 54.5cc.

Can you reference that in his book to put in context with information?

 

I'll see if I can scan in that part of Bob Mannel's book next week when I'm back at it! Left it in NC and I am in TX for this week.

Bart - thanks for the explanation, had no idea where those numbers were coming from! Will see what the engine builder says deck height is and then go from there, I guess. Having read up on quench, it seems I should keep a real close eye on it and order pistons accordingly since I have a couple different options as far as compression height goes.

 

Comparing Aluminum And Iron Cylinder Heads - Car Craft Magazine

Ok, so I was a little off but still interesting.