OK guys, I blew a head gasket on my 383 and the gasket showed signs of detonation. We did some calculations on my setup and found out the compression ration was at 11.17 to 1. Also, the heads are aluminium!
Can I use 93-94 octane (reg pump) fuel without causing any problems?????

I have heard that my setup will be fine with using reg pump gas. I also was told that it's not fine and I should stay under 10.5 to 1 for reg pump gas.
Note---the cam is rather large (#'s) and the heads flow a ton of air for a SBC.

What's your answers?

Thanks....I am probably going to swap out the heads to 78cc heads and lower the comp. ratio to 10.2 ish!

thanks....Heavy

 

They SHOULD be able to handle that no problem, as long as you have your cooling under control, plugs are not too hot, timing isn't turned up too high. Techinially speaking, 93 octane can handle 12.5:1 CR.


There are people here running pump gas on that compression.

 

Is that 12.5 with a gen-II and III reverse flow cooling? Ive been told that 10.5:1 with my to be aluminum headed gen-I SBC will be too much for 93 octane.

 

Mikael--I don't see how that's possible...12.5 to 1 on pump gas. Maybe with severe timing retard. And was it on an LT-1 (reverse cooling)?

Striker- if you really want to use that low of an octane fuel, my suggestion is to run your gas tank as close to dry as possible and then put in 5-6 gallons and then drive around with the radio off and the windows down. LISTEN for engine knock! If you hear it then you got your answer.

Personally I would never use anything less than 93 octane in any performance vehicle.

thanks for the replies.

 

Actually, the big thing you need to worry about is not compression ratio, but cylinder pressure. I'm going to ramble a bit, as I have not yet had my second cup of coffee, but....

A higher c/r can cause a high cylinder pressure, but thats not the only factor involved. Short duration camshafts will have the 'virtual effect' of rasing cylinder pressures by opening/closing early, and allowing maximum pressure to develop insire the chamber.
Valve overlap, ie: late (intake)valve closing, can reduce cylinder pressure, by allowing some of the pressure generated in the compression stroke to effectively 'bleed off', allowing the use of a higher static compression ratio, but with the result of a overall LOWER cylinder pressure. This is determined by the cams lobe separation angle....and how late in the compression stroke it stays open. If you think about the way the compression stroke works in slow motion, you will see this. At some point, the intake charge of air comming in from the open intake valve, is 'overcome' by the growing pressure inside the cylinder( as the piston moves upward in the stroke), and some of that air is forced back out into the intake manifold. The intake valve then closes as the piston moves upward, and compression builds normally, as fuel is injected into the combustion chamber. Since the intake valve was open for part of the compression stroke, and some of that air was 'bled off', the overall cylinder pressure is not as high as it would have been if the intake valve had closed when the piston started it's compression stroke. Follow me? Again, lack of the 2nd cup of coffee is hindering my ability to explain it clearer.
The combination of high compression pistons (ie: 12:1), and a wide lobe separation angle, can allow you to run on the street, with pump gas. In fact, this is just what GM did in the LS1/6 and LT1 engines. Also, as you all well know, ignition timing also can have a dramatic effect on reducing detonation, but thats another can of worms.....
A good article to read that explains just this, is in the September issue of Chevy High Performance. I believe it explains it a bit better than I have.
Gotta get some more coffee!

Good luck.
Jeff

 

Jeff, go drink your second cup of coffee cause your thoughts are all over the place!
Just joking......Your explanation was exactly what I needed. This explains why my engine builder said that I would be fine at 11 to 1 with using my solid cam.
The cam is a Comp Cam, solid, 248 258 @ 050, lift is somewhere in the high 500's (I don't know exact numbers)

I understand know but I am still going to reduce my comp. ratio to around 10.2 to 1.

thanks

Heavy

 

Glad you were able to follow me. Now that I have more coffee in me, I am able to think a little clearer!

One of the easiest things you can do to reduce your c/r is to run a thicker head gasket (but you already know this). This slightly increases your cylinder volume, and lowers your static compression ratio.

Changing to a hyd roller cam can also slightly lower your cylinder pressure..the valve opening/closing tends to be a little smoother (and SLIGHTLY slower) and this will help in your endeavor.

I see you live in Philly....I amin Voorhees, NJ, about 15 miles from Philly. Maybe we can start up a group meet in our area.

Best regards,

Jeff

 

Changing the head gasket is indeed an easy way to decrease the compression, but you also increase the squish height in the process. This might make the engine more susceptable to detonation than if you had just left things alone. This is more true with the newer small-chamber heads than with older chamber designs, so your results may vary.

 

Our M.I.A. mentor Chuck had a great thing he wrote up one day on quench, detonation, and CR & his experiences varying it on a blown 5.0.

 

The data I have indicates that octane requirement drops 1.0-1.5 octane units per 1,000ft of elevation increase. Sounds like you will be OK (do they even sell 91 octane where you live?). Why guess..... try the lower octane fuel and monitor for knock retard.

 

I'm running 11.8:1 with 91 octane at 500ft elevation. My total timing is 30ish, depending on how hot it is. If it gets hotter than 210ish I will pull as much as 7 deg. My chambers are reworked, and I have a quench distance of .038. When I run VP fuel, I can run 38-40 deg advance (MAJOR power difference), any more (45ish) it looses HP even though I don’t have any retard.


Sean
94 Z M6
353 Heads/Cam/Headers

 

Great replies people! Thanks alot.

I appreciate all the inputs, facts, and opinions.

Sean94Z- I would think your engine would gain a significant amount of power if you started at 30 total degrees for normal driving and then bumped up to high octane fuel and 10 more degrees of timing. Sounds like your comfortable with your setup. thanks.

HotZ4me- I live in NE philly and I am an avid "race enthusiast" I read your sig and I would like to check out your ride. My work schedule is extremely random but I try to make it to "the NE Philly spot" and the "Ltown spot" when I can. Email me at markusleo@mindspring.com

thanks again for the posts and I will update this thread when we get the engine put back to gether and running.

Late....Heavy

 

Hey Fred, I am at ~1000-1500ft. If I have my heads milled .030" to raise the compression ratio 1/2 a point, it would then be 10.5:1, up from 10.0:1. They will be ported LS6 heads on an MTI 382 Stroker and B1 Cam (221/221 .558/.558 114 LSA). All we have here is 91 and 92 octane fuel. Is this sufficient?

If anything I will AutoTap it and watch for KR, just wanted to get your opinion...thanks.

 

depends were your at
here in socal we have low elivation in general but crappy 91 oxagenated gas.

currently I run a calculated and verifyed 11.2:1 CR on 91 w no detonation at the track at Fontana in 100 weater NA. But I do my own programing and I cleand the pistons myself :-)

TAD

 

Is there a calulator or a formula to use to figure out how much cylinder pressure you have at certain RPMs? Doesn't DeskTop Dyno2000 have something like this built in?