I have decided to go carburated so I need some advice on my new motor. It will be a 355 with edelbrock E-tec 200 heads, Super Victor for the vortec heads, Demon carb, .533/.555 cam with 254/264 duration at .050, 1.6 rockers, 3500 stall, 4.10 gears, hedman headers. Now here are my questions. What is the safest compresion ratio to run 93 octane with an additive, what size carb should I run, and finally what kind of power do you think it will make. I appreciate any input and advice. If anyone feels like they have a better combo in mind feel free to let me know. Thanks

 

you should get some decent power with that setup. what is the most lift you can get out of the e-tecs? i ran a similar setup and was using the 750 speed demon with no problems. with that combo you might even be able to run the carb right where it is set with little fiddling. i heard horror stories about them, but i have no problem.

 

Forget the "additive" its just not practical or reliable.

I would build the motor so it can operate at optimium ignition timing on the available pump gas in your area, everday.

You'll see some people quote a motor making x amount of power on a dyno with quite high compression ratios using pump gas.

Keep in mind that the actual running conditions "in car" and on the dyno are quite different.
One example is the temperture of the air entering the carb.

70*F air is a lot cooler than typical under hood temps of 120 to 150*F. Cooler air entering the engine will help supress detonation. Allowing a little more compression ratio.

So when you see a quote of someone running 11:1+ compression on pump gas, take it with a grain of salt.
A 7sec dyno test is a lot different than the conditions your street driven car will see.

I would build you combo with no more than 10.3:1 compression. The actual differnce in power between 10.3:1 and 11:1 is near nill.
You'll make great *** reliable*** power at 10.3:1
Even on the hottest days.

I would strongly consider a Performer RPM air gap intake over the single plane manifold for you combo.
Much stronger mid range power. Only a few HP less at the peak rpm. pairing a performer RPm with a larger carb like a tweeked 750 (820cfm) will make equal top end and much more mid range power. Not to mention it will fit under a stock hood without a scoop.

Put a remote electronic thermometer under your hood or in the aircleaner of you car and go for a ride and watch the actual airinlet temp on a in car motor. Big difference than what you see on a dyno even on the hottest day of testing.
You can find these in any auto parts store.
usually they are good to 155*F

If you just got to have sky- high compression look into water injection. Little cans of octane booster are not going to cut it and will just emply your wallet. Mix ing pump gas with high octane unleaded race gas is much more effective and costs less.

Take note of the compression ratios used by professional engine builders that sell "pump gas" "crate race motors". You'll see that the cr's are quite modest even thou the motors are making great power.

 

Well I know that with aluminum heads I can run at least 11:1 with no problem because of the heat transfer. I was looking for about 11.5:1 or 12:1. I am definetely running the victor. The etec heads handle .550 lift but I will be replacing the springs. Thanks

 

If you know you can run 11:1 why are you asking others opinion?

12:1 will require very high octane gas regardless of what you're hoping for, or have read in some magazine.

The laws of physics are very un- forgiving.
rebuilding an engine cause the rings butted from an overheated pinged/knocked to death motor is real $$$pricey$$$ many times it takes the cylinder block out with it.

Yes aluminum heads trasfer heat faster. I already took that into account at 10.3 to one.

You may see a EFI engine running that much cr and getting away with it. (under ideal conditions and electronic engine management)
But a carbed motor is less than idea and so are the conditions you'll usually experience. You won;t have a EFI computer knocking back the timing at the slightest hint of detonation way before you hear it. You won't have a computer that watches individual cylinders AFR and ignition timing and adjusts it on the run.
Your engine will not be breathing 70*F air.

You asked for a **reliable*** maximum compression, not some theroretical ratio based on ideal conditions, split second dyno tests, and or experience found on a totaly different engines design than yours. You'll never see a pro at the track running "octane booster"

11:1 is exessive for carbed street motor that has to perform every day even with aluminum heads.

If you can find someone that will unconditionally garrantee in writing that their 11:1 carbed sbc will not fall apart under real street conditions (IN YOUR CAR) or your $$$ back on 93 octane pump gas.

Put it on a dyno at WOT max rpm and go to lunch. if its still alive when ya get back,
BUY IT!

In the real world an properly tuned 11:1 motor will need 98 to 102 octane to live on the street.

If you build a motor with exessive cam overlap and tune it with exessive rich afrs to try and mask detonation, you will not make as much power or go as fast as a properly tuned motor with less compression.

 

Thanks for the reply. I asked the question even though I knew it because I wanted another opinion. This will not be a daily driver so I know that under normal conditions 93 octane will be insufficient. With the cam i have it will allow enough cylinder bleed off to drive on the street ocassionally. I plan on running race fuel thru it when it went to the track but I wanted to know if it was possible to run 93 octane thru it if i pulled timing out and drove it on the street on nice days.

 

I wanted to know if it was possible to run 93 octane thru it if i pulled timing out and drove it on the street on nice days.



Yes Iwould not go full WOT thou.

The long over lap cam will only "bleedoff" cylinder pressure at low rpm when the induction system and cam timing are out of sync. When the motor comes "on the cam" the cylinder pressure is high.

Dynamic compression therory forgets that even thou late intake valve closing and valve overlap reduce cranking pressure and low rpm cylinder pressure caused from some of the air being back pumped or passing right thru the motor.
The heat of high compression does not change.
Compress something x amount and it heats up y amount over ambient. Compress it higher=more heat. This heat of compression stays even thou some of the pressure is bleed off at low rpm.
It's the exessive heat of the highly compressed fuel/air that contributes more to detonation (auto ignition)

According to strickly dynamic compression therory if you had enough overlap and late enough intake valve closing on the compression stroke you should be able to run any old static compression ratio you want. as long as the cam timing is big enough or too big.
Because of the constant "heat of compression" gas law, this simple therory has a flaw. It doesn't work.

Yes you can cruise around at part throttle fine and do fairly ok with overrich mixtures and retarded timing, but you'll make less power ( than you could) , run hotter and waste fuel.
You can cheat the usable compression ratio up a bit with a big cam but not near what simple dynamic therory suggests. The cam you chose will allow about a 1/2 more ratio than a "small cam". There are other factors involved but their effect is reletively small.

Build an engine that breaths better. (good heads detail etc) and use more modest practical usable compression and you'll go faster , farther, longer and for less fuel.
On a dual purpose street strip car street cruising on race gas gets old in a hurry. ( draining the tank, switching fuels) Rich mixes and retarded timing means the power is not there for ya when ya want it.
The real difference between a pump gas friendly 10.3:1 and higher octane requireing 11:1 is going to be less than 3% power. (with the right fuel) Is it worth it?

One way around it is to have two completly separate fuel systems on board. 93 in the tank and a small under hood fuel cell and plumbing for 110octane race gas.

This the setup I had/have on my car when the motor was 12.65:1.
Take a que from my experience. This motor came unglued
( detonation, broken ring land)
while passing a car on the hwy while driving to the track.
Forgot all about the fact that the timing was max set for race gas and was running it on 94. Didn't hear a thing either. Even if you just scuff the rings from detonation this scuffing will destroy the ring/cylinder seal (power). Now you have a prematurely "old tired motor"
How further are you ahead now?

Luckly it was not an expensive motor. Needed a full rebuild and a cylinder sleeve. One ported 305 head was destroyed.
Was fun but didn't last long. (this engine ran fine for 10 years in more than 1 car at up to 10.4:1 compression Was touchy at the 10.4:1. 1/2degree too much timing would ping even on 94. Running it at 12.65:1 compression on 94 killed it. Cam in the high cr combo was near identical to yours.
I know what you have in mind cause I've been there.
You'll be very happy with it at 10.3:1.
This is what I recommend.

Dynamic compression therory is all fine but you have to look at the big picture.

 

I really appreciate the time you put into the post. I believe I will stick with a compression of about 10:1 to 10.5:1, somewhere in that range. Thanks