What is the standard on the most compression

-

boosted

Life Needs Air
Joined
Oct 26, 2008
Messages
1,410
Reaction score
383
Location
Muscogee/Harris County GA
For street use and pump 93 octane fuel? I know it will be a tuning issue to some level but there has to be that one step that you cannot tune your way to it running good. Some of the old school guys are like 11 to 1 but I thought there are some on up in the 12.5 to 1 area.,
 
Unless you on E85, safe bet as the max imho is 11.5 and that will require the right heads and cam as well as tuning to make it happen.
 
Scr has almost no relationship to the gasoline.
It 's all about the heat in the chamber and the ability of the fuel to survive the heat, without spontaneously combusting before the proper time. To that end, cylinder pressure is the best guide.

That is to say, you could take a given engine at a given Scr, and vary the pressure with ICA , and create a monster detonator, or a pussycat, all on the same Scr.
I don't build engines for resale;
Just for myself, and
I collect data.

Let's take the 360, my favorite engine cuz you can easily dial the Scr in to just about anything, pretty cheap. It falls together, with 68cc heads at about 84cc total chamber volume with the .039 FelPros. The swept at 4.03 bore is 748cc.. Making an Scr of ~9.9.. So far, it's just a number.
Lets put a stock Magnum spec cam into it, retimed to it's proper install, IMO, of 106*. The Ica will be 51* and
Static compression ratio of 9.9:1.
Ica of 51*, 500ft elevation
Effective stroke is 3.08 inches.
Your dynamic compression ratio is 8.66:1 .
Your dynamic cranking pressure is 178.62
PSI................... 178
V/P (Volume to Pressure Index) is 168 ............................. 168

Fantastic engine with aluminum heads, but you cannot run 178psi with iron heads.
EDIT; Well YR says you can, and I wouldn't want to argue with him,I don't doubt that HE can, but can you?
The 168 VP says it would make an awesome tow-truck engine, or even a sick streeter.

Ok, lets put the monster 292/108 cam in it, at an Ica of 70*, otherwise it's the same engine.
Static compression ratio of 9.9:1.
Ica of 70*, 500ft
Effective stroke is 2.64 inches.
Your dynamic compression ratio is 7.56:1 .
Your dynamic cranking pressure is 149.21
PSI. .................. 149
V/P (Volume to Pressure Index) is 121 ............................... 121

There ya go a pussycat that burns the crappiest gas you can discover in the back of your garden shed, but the VP says the bottom end is real doggie.

So from 51* to 70*, in the same 9.9Scr engine You might have a full-time detonator even at PT (Part Throttle) to a race engine using a hi-stall to get rid of the soft zone, that should run just fine on 87 never mind 93.
So what would be the ideal Ica for hi 9.9 engine? you might ask.

Well it could be this one, burning 91plus with crappy often detonation-prone open-chambers
Static compression ratio of 9.9:1.
Effective stroke is 2.79 inches.
Your dynamic compression ratio is 7.94:1 .
Your dynamic cranking pressure is 159.28
PSI. .................. 159
V/P (Volume to Pressure Index) is 136 .............................. 136

Or this one, with tight-Quench and smoothed chambers on 91
Static compression ratio of 9.9:1.
Effective stroke is 2.88 inches.
Your dynamic compression ratio is 8.16:1 .
Your dynamic cranking pressure is 165.15
PSI. ................. 165
V/P (Volume to Pressure Index) is 146 ............................. 146

Or this one, if you have an 87 gas ceiling
Static compression ratio of 9.9:1.
Effective stroke is 2.72 inches.
Your dynamic compression ratio is 7.76:1 .
Your dynamic cranking pressure is 154.50
PSI. .................. 155
V/P (Volume to Pressure Index) is 129 .............................. 129

Notice the PSI/VP swings; all on the same 9.9Scr engine

Lets go back to the 292/108 cam,Swap out the .039s for .028s, install some aluminum 63cc heads and go racing.
Ok so the new total chamber volume will be 63+8.8+5 less 2.1= 74.7 and the Scr goes to ~11/1

Static compression ratio of 11:1.
Ica of 70*,500 ft
Effective stroke is 2.64 inches.
Your dynamic compression ratio is 8.37:1 .
Your dynamic cranking pressure is 170.79
PSI. ................. 171
V/P (Volume to Pressure Index) is 138 .............................. 138

Hey, looking pretty good!,and this is still a fall-together engine.

But some guys here on FABO, have figured out how to run 170psi with cast iron.
And some guys have figured out how to run 200 with aluminum.
So if you want to be competitive with that, well open up your wallet.
<<<<<<<<<<<<<<<<<<< >>>>>>>>>>>>>>>>>>>
I don't build engines for resale;
Just for myself, and
I collect data.
From the internet, mostly from FABO, I have gleaned the following seemingly safe data, if guys are reporting accurately.
Ask your MOPAR builder; not some chevy-guy.

for tight-quench iron;
91 gas might support 165 psi
89 might support 160
87 might support 155
For an iron open chamber quenchless design, deduct at least 5psi
For an aluminum tight-quench design add 30psi,

As to street-VP

100 is so poor I wasn't even gonna include it, think slanty, not dissing slantys boys, don't get all riled up, it's just a comparison
120 or less sucks big time, think smogger-teen, stay away from here
130 requires a hi-stall, then it will work
135 is a typical iron headed result, ho-hum
140 is where the fun starts, hard to get to, with anything smaller than a 360
145 is dynomite on the street
150 too much more than this is just tirespin bragging rights
160 IDK; that's stroker territory, I haven't run it, but I suspect it's extreme overkill for the street.Ok wait, I did run it, 164VP actually, my favorite combo; just killer! Totally overkill, and my next combo sacrificed sumadat for top end. Good trade.
5VP ratio increments, are like 5psi pressure increments; the increments don't look like much but make surprisingly big differences.

As to power, here's the punchline

going to 11/1 from 8/1 with no other changes represents an absolute power increase of perhaps 5.2%; this from David Vizard. Do the math; this is perhaps a lil more than what you get from the next bigger cam.
So chasing compression, for absolute power, is best left to the drag-racers.

Whilst for streeters, the VP tells the tale; cylinder pressure is everything, and will make or break your comb, in the rpms we do most of our driving at,namely below 3500rpm. But yeah, you can get around that doggy soft bottom-end with a hi-stall.
>>BUT if you are a manual trans streeter, your engine is married to the tires and crappy VP will be a crappy combo, of that I can guarantee you.
The softer the bottom-end,(low VP) the more timing you will want to run, and all that does is make the off-idle jumpy, to the point of being undriveable down there.
The stronger the bottom end (large VP), the less timing you will need to run, and the slower the engine will be able to run and still pull itself without the rocking-horse,bucking-bronco routine. And, the less shifting,you will need to be doing.

A big cam on the street is not needed, it only raises the operating range unnecessarily. Cams in the 210 to 225, IMO are just right with a 360. A 225 will power peak around 5200, and want to be shifted at about 5600, but can pull to 6000 pretty good in a pinch.And a 225 can still have a strong bottom end, if the clearance ramps are not too stinking long.
By 230* the fuel economy is taking a dump. By 240* you will no longer want to know, and start planning roadtrips around where the gas-stations are, AND carrying extra gas.
 
Last edited:
Most engines I build are relatively stock. I like to keep a street engine at no more than 10.5
Im sure you can get away with more with aluminum heads and a well tuned decent timing management system.
 
Most engines I build are relatively stock. I like to keep a street engine at no more than 10.5
Im sure you can get away with more with aluminum heads and a well tuned decent timing management system.

We know that this is true based on factory stuff that exists/existed.
 
I'm going to do a 380ish inch 12:1 pump gas engine next year (I hope). Carb'd. It shouldn't be an issue.

Compression ratio and pump gas is combo dependent. I'm already running 11.08:1 on pump gas and I really should pull the heads and slip the thin head gaskets in there and make it 11.25:1 because...well, because there is no reason not to really.

You ain't going to do something like this buy off the shelf cams, and not understanding what the proper combustion chamber shape should look like.

If you get those two things ironed out, 12:1 isn't unreasonable and 11:1 should be the standard.

I forget to mention you need to be able to read a spark plug and making tuning decisions from that.

If you do that, you can do more CR than most. Most guys are scared and believe what they read in magazines and on line.

An aluminum head will NOT take more CR than iron. It's just not true. Whatever you do with aluminum you can do with iron.
 
Is this going to be a street car? Driven in Georgia? During the summer? When it's hot and humid as all HADES? If so, keep it to no more than 9.5:1. Sure you can cheat the numbers and make it run with a higher static compression ratio, but that'll have to be the same as a controlled environment with perfect temperatures and humidity. With little to no load. Give it a 95 plus degree Georgia day with 70% and above humidity in stop and go traffic under a load and yeah, I wanna be there to see how happy your 12:1 is on pump premium.
 
Well I am not one for the heat an the old barracuda will be without a/c. Aluminum headed aluminum block 426 hemi with two F1 throttle bodies and the motor will be in the 500 inch size. I have just decided to use the 12.5 to 1 pistons I have and run some 116 with my 93.....

And Kim you are wrong.. It was my Aunt. :)
 
Just throw water meth on and cool those temps.... :rofl:

Is this going to be a street car? Driven in Georgia? During the summer? When it's hot and humid as all HADES? If so, keep it to no more than 9.5:1. Sure you can cheat the numbers and make it run with a higher static compression ratio, but that'll have to be the same as a controlled environment with perfect temperatures and humidity. With little to no load. Give it a 95 plus degree Georgia day with 70% and above humidity in stop and go traffic under a load and yeah, I wanna be there to see how happy your 12:1 is on pump premium.
Agreed
 
For street use and pump 93 octane fuel? I know it will be a tuning issue to some level but there has to be that one step that you cannot tune your way to it running good. Some of the old school guys are like 11 to 1 but I thought there are some on up in the 12.5 to 1 area.,
For the sake of ease and just giving a generic guide line..
87 - up to 8.5-1
89 - up to 9.5-1
93 - up to 10.5-1

This is a very flexable answer & assumes a reasonable size cam shaft is used along with the rise in compression for the performance factor and reasons. If the cam is to small for a high ratio, you’ll have detonation issues.

I run a Hyd. roller cam @ 224@.050 w/.538 lift in a (just ever so shy) 11-1 - 360.
The cam isn’t a super stout ramp cam. It is a Comp Cam on a 110 C line. 93 octane only as it doesn’t like the next grade down, 89. I wasn’t sure about this but another member local time came over and said I could do it and I should go for it, just watch the tune. The distributor timing is key along with lean or rich conditions.

This engine also has a good custom Pro Torque converter and 3.55’s out back on 26 inch tires. The car is on the light side.

It’s really a package deal.
Everybody working together.

I was just looking at a Snow kit for it...
Boosted, are you intercooled?
 
Being a Hemi (needed information that was left out of the first post, duhhhhh), you might can get by. There's a guy over on The Hamb with a first gen 392 Hemi with 13.5:1 and runs 93 octane. The Hemi chamber (especially the early Hemi) has superior anti detonation proporties. You can "try". I would massage all of the sharp edges off the pistons and smooth the chambers as much as possible. Anything such as sharp edges that could provide a hot spot for detonation to begin should eliminated. You'll have better success with a Hemi than anything else......but I still think for best results you'll need to spike it with race gas.
 
Last edited:
the chebby ls1 runs 10.2 on 87 octane.Its all about the quench.

That's right and every single small block Chevy since 1955 has had quench ready heads. Every. Damn. One.
 
That's right and every single small block Chevy since 1955 has had quench ready heads. Every. Damn. One.
and you never hear any body in the chevy camp talk bout quench! and when thay do its bout exotic piston crowns and odd shaped dish's! as cheap as thay are to play with guess thay can afford the high octane fuel than can burn for it gose pop! lol
 
and you never hear any body in the chevy camp talk bout quench! and when thay do its bout exotic piston crowns and odd shaped dish's! as cheap as thay are to play with guess thay can afford the high octane fuel than can burn for it gose pop! lol

Those in the know do. I know a few guys around here that mess with the old gen1 small blocks. Whatever you're driving, if you pull up next to any of them, you better pack a dang lunch.
 
Those in the know do. I know a few guys around here that mess with the old gen1 small blocks. Whatever you're driving, if you pull up next to any of them, you better pack a dang lunch.
first i ever herd of it was late 80s hotrod artical trying to build power on 87 octaine and pass smog test in komie-fornia... attention to detail can make power with anything!
 
first i ever herd of it was late 80s hotrod artical trying to build power on 87 octaine and pass smog test in komie-fornia... attention to detail can make power with anything!

Yup. Just like this thread around here where everybody is talkin about the 340 Duster whoopin up on a big block early Vette. In their dreams. ONE car pulls off the seemingly impossible for probably some tunung or technical issue on the other and suddenly Mopar is king. I got news. Mopar lost their fat share of races, too. I don't think a lot of these guys have ever stood next to or ridden in a 396/425 Vette. I have. NO stock 340 Duster would make it around one in good tune. Ain't happenin. Dream on. lol
 
-
Back
Top