nastee383

I have a .003 deck, -4cc flat top pistons, 4.030 bore, 3.75 stroke and a .039 compressed head gasket which equates to @ 11.25:1 compression. The cam is a hydraulic roller with 242/248 duration @ .050 and the motor is carbed with alum heads.

Will this thing run on premium pump or am I gonna have to use race gas? Is anyone here on the forum successfully running this much compression on pump gas?

DJ Dep

You can always add this to up the octane:

http://www.maxlead2000.com/

Dep

Grinchia

Maybe...the guy a bought my car from claimed compression is 11.25:1, but he's not a completely reliable witness. I run 92 or 93 octane pump gas, add a little lead additive/octane booster to every tank, and it runs fine. I don't get pinging (since I adjusted the timing...12 degrees initial), but I DID get horrible dieseling when I had to put 5 gallons of 89 octane in one time when I ran out of gas. There are many factors involved in this though...I'd suggest you tank up and try it. If you get pinging, retard your timing a little until it stops, then consider a more permanent solution...I'm sure lots of people will chime in with suggestions. My guess is you'll be able to pull it off.

GrandSportC3

EddieJ82 runs 12:1 on pump gas with aluminum heads..

However, I doubt that you have that much compression because I believe that your bowls are blended.. This increases the size of the bowls and drops the compression..

My compression would be over 12:1 with the bowls untouched, but it is 11.71:1 with the bowls ported..

MotorHead

I am running 11:1 with the same duration cam, I usually run 94 octane but is OK with 92, had the timing up to 40 deg with no pinging, I don't run a vacuum advance which probably helps

Ironcross

I have several factory engines over 11-1, a 1970 LT1, 1966 L72 and a L78 in a 67 Camaro. All work very well with premium pump gas and without additives. My total advance is 36-38 degrees at 3000 rpms.

SkunkApe

Over at the Viper forum, some owners are running water or water/methanol(?) injection systems to prevent detonation. Not sure if this would help your situation.

lostpatrolman

I thought only boosted applications use water and/or meth injection. That would be alot of fluid if it was constantly being used!

turtlevette

what's the cc of the heads? You are probably calculating it on the high side. Its hard to believe you can get that high of a compression with flat tops and thick head gasket.

nastee383

That's what I DON'T want, to have to add additives every time I fill up.

nastee383

I'll find out what the bowls look like soon. I'm changing valve springs to beehives. Mark said it was about 10.7:1. I'd feel better about that.

By the way, can you run pump gas on a regular basis?

nastee383

That makes me feel much better.

nastee383

65cc heads. The comp. ht on the pistons is only 1.125. They are PROBE SRS lightweight forged. Their site estimates 11.58:1 with a .010 deck, .038 gasket, 4.030 bore, 3.75 stroke , 6" rods and 64cc heads. I have a .003 deck and am using felpro 1010 gaskets @ .039.

nastee383

Definitely NOT looking to go that route!

71coupe

Do you know what the quench distance is? A tight quench will allow you to run a higher c/r.

GrandSportC3

I should be ok if I'd run 30 degrees timing and would jet the carb on the rich side... but I probably won't try it... I'd have to re-jet again for the track etc... 11.71 is very borderline..
A lot also depends on the duration of your cam.. My cam is HUGE and bleeds some of the compression off......

BTW - your cam is also a pretty big cam (not huge but big) and should bleed some of the compression off. There are some sites where you can calcuaste dynamic compression ratio... I'm pretty sure that even with 11.25 : 1 and aluminum heads, you should be ok..

GrandSportC3

On my old iron head 383, I did the math and the CR that I calculated was 10.50 : 1 and the engine was advertised as 9.8 : 1. I called AR and they told me that blending the bowls chaned the compression ratio and that it was somewhere around 9.8 - 10.0. I always ran 93 octane in that engine without any issues... 10.0 is supposed to be the highest that you can go with iron heads...
If I remember correctly, they ported the chambers from 64 cc to 68 cc...

DJ Dep

Compression = power. The higher the compression the more power you make. Nothing's for nothing. If you didn't want to have the "hassle/expense" of high compression, you should have built or bought an engine with 8 or 9-1.

Dep

SteveG75

Here is a great quote:

"Because without reference to the camshaft specs, talking about (static) CR is next to meaningless!"

http://www.kennedysdynotune.com/Dyna...ion%20Tech.htm

What you really need to concerned with is your "Dynamic Compression Ratio". The above link is to a good tech article and a Google search will yield some free DCR calculators.

EDIT: Better link with a downloadable calculator: http://members.uia.net/pkelley2/DynamicCR.html

DJ Dep

At that same website....

Most custom grinds are $349.95.

Hmmmm....always follow the money trail.

Dep