i've been reading about engines & heads and compressions & all, and i'm getting worried about the engine i want to build.

ok... would i get detonation running 11.5-1 compression on closed 72cc iron heads on a 400 block bored out .30 over. flat top piston heads. of course.. on the best gas.. premium.. (i would do leaded, cuz then no combustion.. but. dumb gas ppl)

please say no, honestly.

thanks.

 

how are you going to get 11.5:1 with 72cc heads and flat top pisotns, unless you stick the piston out of the hole? i think you have more to work out now than if you'll have detonation problems later.

 

Yes, you will have detonation, running 11.5:1 compression with pump gas. From what I've read, about the highest you can go is around 10.5:1, and that is on a fuel injected engine with highly efficient swirl-port style aluminum heads. The rule of thumb is no more than 9.5:1 CR with iron heads and 10:1 CR with aluminum heads - any more than that and you are flirting with disaster.

 

k.

thanks.

 

Sorry, had to chime in. Your talking about static compression - SCR. SCR does not determine if you will have detination on pump gas by itself.

Your combustion chamber design, quench, head material (iron or aluminum), and mostly your dynamic compression are all factors in wheather you will encounter detination on 91 octane gas with a given SCR.

400sbc .030 over is a 4.155" bore with a 3.75 stroke - with

4.2" bore gasket
.039" thick gasket
72cc heads ( assumed they have been CC checked )
.020 deck height ( could be .025 if it hasn't been cleaned up )

Flat top piston with 2 valve relief @ +6cc

you get 10.13 static compression ratio and a .059 quench

A .059 quench is useless.

to get a 11.5:1 compression:

4.2" gasket bore
.030" gasket height
64cc heads ( true 64cc heads )
.012 deck height

Flat top piston 2 valve relief @ +6cc

You'll get 11.48:1 static compression and a great quench @ .042.

To run 11.48:1 SCR on pump gas, you need a cam with IVC of 79* measure from seat ( not .050 ). That will get you a DCR of 8.12 to 1. This is on the edge of detination. If you have aluminum heads then the general rule is about 8.3 DCR if properly setup and polished combustion chambers. Also - true heart shaped Vortec heads have a fast burn style combustion chamber that should be ok for 8.2:1 DCR - 8.3:1 DCR pushing it.

To give you idea of a cam that has a closing intake of 79* - take a look @ compcoms 306s-10 cam. I has a 260/260 duration @ .050 and .555/.555 lift. Pretty wild cam for a street car.

 

LT1's run about 10.5:1 stock, LT4's run 10.9:1 with no detonation.

 

quench?.... i'm reading up on my stuff... but havn't heard a thing about quench yet...

...what is it?

oh.. and i can't find the 306s-10 cam you said... i even looked on comp cam's site. they had 306-8 and 306-4, but no 10.

what about this crane cam. CRN-119841 at summit.
* Advertised duration: 296 intake/304 exhaust
* Duration at .050 in. cam lift: 232 intake/242 exhaust
* Gross valve lift: .539 in. intake/.558 in. exhaust
* Lobe separation: 112 degrees
* Basic rpm range: 3,000 to 6,500
* Rough idle, performance usage, good mid-range horsepower, minimum 3,000 rpm stall converter
* Cruise rpm: 3,800 to 4,600
* Recommended compression ratio: 10:1 to 11.5:1 naturally aspirated, 8:1 maximum with supercharger
* Aluminum-bronze distributor drive gear not required

i'm wanting power range from low to 6500 max. that's what all the other parts are rated for better power at. & the only things i care about is mileage and that it won't die as daily driver but weekend racer.

also, rods are 5.75.


thanks.

 

Quench ( or sometimes called squish <- old timers ) is the distance between the top of the piston and the bottom of the head ( flat part - not the chamber with the valves ). If the quench distance is .035 - .045, it pushs all the fuel mixture into the combustion chamber for a more even burn - which in turn leads to less detination.

If you have dome pistons though, you'll need a flame travel grove machined into the dome inorder to keep this effect. A good guench works best with flat top pistons or (special dish pistons if running really small closed chamber heads on a 400 sbc ).

The compcam CS 306s-10 part # is 12-225-4

David Vizard books have a good source of info on quench and blueprinting an engine.

 

Yeah, that cam has a 79* IVC. Thats a nice Hyd. roller cam for a 400sbc combo. With a good port/polish job on vortec heads and 2.05/1.6 valves - 750cfm 4bbl double pumper (holley or demon) - Victor jr. +1 tall intake (3500 - 8000rpm) - large tube headers and dual exhaust ( no cats ) - 11.5 :1 SCR and that cam would 500+ hp and 500+ TQ.

Why are you going with 5.75 rods? If I remember off the top of my head, 400 sbc rods are 5.565 because any longer would have pin/ring interference.

If you plan to make 450+hp then I would suggest ARP studs for the heads and main caps, ARP rod bolts, stronger aftermarket rods - eagle or scat ( but not longer rods), and most definitely a steel forged crank!

If you do run into detination problems and/or can't get much advance timming out because of it, then just run some 10% Toluene or 10% Xylene+lube to your gas.

 

ah, interesting... i PM'ed you...

and, i found the cam... i like the price ALOT more than the one i was thinking of origionally.... and it's better cept in the duration, of the cam they used in the stroker kit i'm looking at. erm, "strongarm".. maybe the same thing tho.

 

The compcom is a solid lift cam - that's why its cheaper than a roller cam. A roller cam will give a little better TQ and HP through out your power band though. You'll also have to use retro fit hyd. rollers for a 400sbc plus a cam button and possibly different timing chain set ( can't remember off the top of my head ).

Also, especially on a flat tappet cam, make sure you run the exact spring specs that the cam manufacture lists to go with the cam - best yet buy the springs from the cam manufacture. Too much pressure will wipe a flat tappet lobe and too little will cause valve float and/or valve leak when seated. On valve lift, make sure your maximum lift (total lift) is .050 to .080 before coil bind.

 

Remember there are two types of compression: Static and Dynamic. so you will need to find all the variable in your engine to find out the QUENCH and what type of gas you can use to fight off detonation. Here is an excellent information written by Pat Kelly. At the bottom of the article there is a calculator, download this calculator to your computer and run your engine setup. This is a great engine calculator for free:
http://members.uia.net/pkelley2/DynamicCR.html
Look for this hyperlink below in the article to download the calculator:
DCR Calculator with VB6 Runtime files
or
DCR Calculator without VB6 Runtime files 423 KB

 

 

Great answers !! There is a couple of other factors that will affect detonation. Altitude is a big one. The higher the altitude, the lesser your chances are for detonation problems...not that you won't have problems, just to a lesser degree. I see you live in Orlando. Altitude (or your lack of it) is not going to help you in this case.

Temperature is another factor. Not just ambient air but engine cooling temp. also. Streetable high comp engines love a cold air package...ram air...or what ever you want to call it. Avoid letting the engine breathe underhood air. Maintain water and oil temp. as low as you can. You can write a book on that topic.

I like the Vortec idea if you must run iron heads. Protopline or other newer style heads with the fast burn or heart shaped combustion chambers will work as well if you don't want to purchase the dedicated manifold. Every major aftermarket manufacturer has gone or is going to this design for good reason. I believe the measure for piston top to cylinder head deck clearance is .035" for steel rods (Someone correct me if I'm wrong). Use that as your "quench measurement" to shoot for. Definetly go flat top or even dished. Personally, if I were you, I would go no higher than 10:1 and use your cam to achieve the cylinder pressures you want. IE...shorter overlaps will achieve higher cylinder pressures and longer overlaps = less cylinder pressures. Give all your information to Comp Cams Help Line and they can give you recommendations or even have a custom cam built. Don't get stuck on that compression ratio. There are other/better ways to get the power your looking for without that high a ratio. The power increase of compression is not linear meaning what you get from 8:0 to 10:1 is not the same as what you would get from 9.5:1 to 11.5:1. It is dramatically less. The difference from 10:1 to 11:1,even if you could find the fuel, is really quite small.

 

don't push the limits with your compression on a street car.

run something you feel comfortabley safe with.

9.3:1 with iron heads is probably a good place to start.

9.8:1 - 10:1 with aluminum heads.

as stated above, heads with more effiecent chambers can take a few more tenths.

you don't want to build an engine, then when you go to fire it up realize that beacause you went with too high of compression for your gas, you have to cut timing, which cuts power.

usually every point in compression is worth about 1-2% more power.

so 9:1 compared to 10:1 on a 300hp engine, is about 3-6hp difference. nothing to write home about. and is 5hp worth risking an engine that pings and knocks all the way down the road without special gas?

 

The thing you must under stand is that SCR is not what determines detination - you can get detination on a static compression of 9.5 to 1. Dynamic compression is what determines detination the most - compression does not start until the intake valve has closed on a compression stroke - this gives you DCR (dynamic compression ratio).

Rule of thumb for DCR is 7.9 to 1 on premium pump gas - you can go with 8.0-8.3 to 1, but like what was stated already - you have to have proper quench, fast burn style combustion chamber, flat top piston, polishing the combustion chamber ( for carbon and pre-ignition problems ), aluminum heads also help, etc.

Although like Scottland stated, there is a general rule about what static compression to use if you don't understand all that required to blueprint an engine.

The way you should think about static and/or dynamic compression is this: Don't think of compression as power, but more of what the CAM requires to operate optimally. If you want to make a high rpm engine that puts out 500hp, then you'll need a cam with more wild profile which in turn needs more compression to operate.

You can make a 420+hp 350cid engine on 9.5 compression - without NOS or turbo changer, etc.

 

try some l98 corvette heads they have a 76cc combustion chamber and they are aluminum. They could be found pretty cheap on ebay or salvage yards. The larger combustion chamber will lower your compression to maybe 10.9:1 and because they are aluminum they can handle that.

 

All this about dynamic compression and intake closing point , big cams and all allowing 11.5:1 compression
on pump gas without detonation is all well and good for therory, Hot Rod magazine articles and dyno room testing but in the real world, it doesn't work.

An engine running in your car will be operating under much different conditions than it would in a 12sec drag test or on the dyno.

The air temperature entering the engine is a lot higher (in car) the water temp of the motor is higher and fluctuates.
11.5:1 is not going to work.
Octane booster is uneconomical and not practical.
Build for 10:1 (cast iron) and 10.5:1 (aluminum) max.
This is about the highest *practical* limit for the available fuel. You can make tons of reliable power at this compression ratio. 11.5:1 will only make 10hp+/- more. And only if it doesn't detonate.
Not worth it.
You do not have LT-1/ LT-4 reverse cooling and EFI computerized fuel and spark management.
Even a LT-1/ LT-4 will run better on 100 octane allowing full timing all the time.
Don't try to compare the two different engines.
Why build a motor with too high a compression ratio and then try to make it run with a way too big cam (for your purposes)? You end up with a slug that is neither as fast as it could be and bad on fuel.
We won't even mention driveability.
A comp 306s-10 is a big cam meant for a pro street/ bracket racer. The low end torque and cruiseability is not there.

Those that quote David Vizard on this subject are taking his info out of context.
There are real practical limits as to dynamic compression and its real world effect on detonation.

 

If you try the larger 76cc chamber and maybe use a LT1 cam it would probably work.