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Old 02-09-2013, 09:32 PM
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11:1 on pump gas or add more gasket??

Ok guys. I recently picked up a SBC 383. It was to good of a deal to pass up. The motor was completly assemble when I bought it. The guy I bought it from wasn't 100% sure what kind of Compression it was making so I decided it would be a good idea to take it apart and take a few measurements. Know I'm faced with a decision. To get it around 10.5:1 ill have to run a .053 MLS gasket. With the pistons .005 in the hole that is going to put the quench at .058. I guess my questions is would you guys chance 11:1 on 91(I know there is higher octane pump gas out there but I want to be able to buy gas anywhere). Or throw a little more gaksket at it and suffer on the quech a bit??



SBC 350 40 over
Scat 3.75 crank
Eagle 5.7 rods
Speed pro pistons 2 valve reliefs 5cc (.005 thou in the hole)
Edelbrock performer RPM heads 64cc
Comp hydraulic roller 236@50
Performer rpm air gap
750 doble pumper.

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Old 02-09-2013, 10:01 PM
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Oh yeah. I meant to say that its going to be one or the other. I'm going to be using the parts I have. I'm not going to change pistons or buy new heads etc. I'm leaning towords running the thicker gasket. Even with aluminum heads 11:1 is rolling the dice a bit I think.
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Old 02-09-2013, 10:19 PM
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if it has aluminum heads on average you will lose one full number on your compress for heat loss. so even though its 11:1 you would be able to run pump gas for 10:1 but retain 11:1 compression performance. so now if you put more gasket in it, it will drop your compression more and the heat retention of the aluminum heads more as well
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Old 02-09-2013, 10:28 PM
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Quote:
Originally Posted by prostcelica View Post
Ok guys. I recently picked up a SBC 383. It was to good of a deal to pass up. The motor was completly assemble when I bought it. The guy I bought it from wasn't 100% sure what kind of Compression it was making so I decided it would be a good idea to take it apart and take a few measurements. Know I'm faced with a decision. To get it around 10.5:1 ill have to run a .053 MLS gasket. With the pistons .005 in the hole that is going to put the quench at .058. I guess my questions is would you guys chance 11:1 on 91(I know there is higher octane pump gas out there but I want to be able to buy gas anywhere). Or throw a little more gaksket at it and suffer on the quech a bit??



SBC 350 40 over
Scat 3.75 crank
Eagle 5.7 rods
Speed pro pistons 2 valve reliefs 5cc (.005 thou in the hole)
Edelbrock performer RPM heads 64cc
Comp hydraulic roller 236@50
Performer rpm air gap
750 doble pumper.
What cam part number or grind?
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Old 02-09-2013, 10:39 PM
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Comp XR288HR part# 08-433-8
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Old 02-09-2013, 10:59 PM
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Quote:
Originally Posted by prostcelica View Post
Comp XR288HR part# 08-433-8
Keep the quench tight, and run the best premium pump gas you have in your area and you'll be OK.

Listen and look at the plugs carefully for any signs of detonation- just like should be done w/any high performance engine.

This assumes things like the cooling system is adequate, the ignition timing not excessive (you should be able to run max power timing, but do not run anything more than what's absolutely needed), and the fuel/air ratio isn't too lean anywhere in the powerband.
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Old 02-10-2013, 01:18 AM
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There is no guarantee that even 10.5:1 cr will work on 91 octane.

91 is a mid grade fuel.

the best you re going to be able to do is to reduce the spark advance.
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Old 02-10-2013, 06:14 AM
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With 11:1 and a small cam its border line but a little bigger cam should work it out. Lots of ppl use 11:1 in there street cars but careful tuning will be needed.

Is the 11:1 measured or just calculated from what parts you have. The tight quench was probably done to hold back the detonation.

I use this calculator to figure compression. And dynamic compression.

United Engine & Machine Co. Incorporated

The larger quench is not ideal with alum heads and good cam it should be fine. Was the engine knocking before you took it apart?

Also what is the piston volume and type this can be very important.

Fyi: i got 10.8:1 when i ran the numbers. With 6cc pistons and 4.1/.041 gasket.

Last edited by hcompton; 02-10-2013 at 06:21 AM.
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Old 02-10-2013, 06:32 AM
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Dont change the gasket.

Polish the pistons and chambers to allow for a little better cr. It will also help with knocking. They do not need to look like mirrors just knock down all the sharp edges. Should be able to take a few cc out without much cutting. But in your case i think your good if you run good gas in it.

I only run exxon 93 or sunco 94 in my performance cars. You just cant stop at the elcheapo gas and fill up with What they claim is 91. That even goes for new mustangs or corvettes. They will not be happy on crappy gas and gas station owners are not to be trusted. Not even a little. Lol
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Old 02-10-2013, 09:34 AM
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Quote:
Originally Posted by hcompton View Post
With 11:1 and a small cam its border line but a little bigger cam should work it out. Lots of ppl use 11:1 in there street cars but careful tuning will be needed.

Is the 11:1 measured or just calculated from what parts you have. The tight quench was probably done to hold back the detonation.

I use this calculator to figure compression. And dynamic compression.

United Engine & Machine Co. Incorporated

The larger quench is not ideal with alum heads and good cam it should be fine. Was the engine knocking before you took it apart?

Also what is the piston volume and type this can be very important.

Fyi: i got 10.8:1 when i ran the numbers. With 6cc pistons and 4.1/.041 gasket.
The compression was calculated by the parts it has in it. I did not fisically measure the stroke or bore. I just pulled the part #'s of the parts. The only thing I measure was the piston to deck clearance.

The motor ran fine but was only ran on a run stand. It has never been installed into a car and been driven in real world conditions.

The pistons are flat top speed pro 2 valve reliefs 5cc
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Old 02-10-2013, 09:38 AM
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my engine is 10.75:1,I use 94 octane
I have topped up with 91,
I run 34 degrees total,I also have a bigger cam

no problems or indications of any problems
ten years running
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Old 02-10-2013, 09:39 AM
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Thanks for all the input guys. It always much appreciated.

I'm going to ask this question a different way. Out of the to setups below which one is more likley to suffer from detonation. It looks like there is quite a few stations around my area that have 94 so I will go that route.

11:1. With a quench of .045

10.5:1 with a quench of .058
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Old 02-10-2013, 10:12 AM
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prostcelica,
I know some will say I'm being irresponsible by suggesting something I haven't actually run, but I thought I'd suggest a couple of options you may not have considered, or at least something to think about.
Building a stroker myself, I ran into a problem with what I thought may be too much compression.
I tried to flycut the pistons I had, so that they resembled more of a trough type valve relief to gain some more volume. I checked to see that there was enough material under the location I intended to mill, and it was strictly just my opinion that there was. I first had them flycut straight accross from the back of the exhaust valve relief, but didn't get the volume I thought I wanted, so I had them cut even with the back of the intake valve relief by a different shop(much smoother). Went from 5cc initially to 7cc.
I later realized that I was removing material that would reduce the amount of the squish effect from the close proxcimity between piston and headsurface. Also the pistons had a little too much bore clearance for the 0.030" bores, so I bought some new ones for a .040" overbore.
The new pistons didn't have the same extra material between the 2 valve reliefs on the underside of the piston head that the previous pistons had, so I decided not to flycut them, and to try something else.
I had read about modifing the heads with grooves to suppress detonation. I don't know if they work, but some say they can make a difference between an engine that will run on pump gas, and one that won't. Honestly think that the grooves will fill with carbon to a certain extent, but that the velocity of the squish will keep them from clogging completely, but thats JMHO. They are supposed to creat a jet of mixture that promotes turbulence, thus quicker burn . I think that they also make for a path of flame to get to the far side of the chamber quicker than normal before the far side can detonate from exposure to the heat/pressure of normal combustion.
Like I said, I can't say from experience that they will help in any way whatsoever, but the logic behind them, and testamony of those that have tried them gives me enough courage to give it a shot. Mind you I didn't do it for any increase in power, but just to help the engine live on what I think is too much compression on todays fuel.
By the way I only gained around one cc polishing my chambers.
Also I've heard of more detonation problems by using a thicker gasket to lower CR than a thinner one originally used, but I haven't tried personally.
Like I said, just some things to think about FWIW.
ssmonty
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Old 02-10-2013, 10:28 AM
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Those heads are adavertised at 64CC chambers and I have seen most heads that the actual CC is higher then advertised,

That is a good size cam and with a 110 lobe sep it will bleed of a dynamic compression which will help.

Max timing should be right at 32 to 34 degrees for What I have seen on the dyno and should be all in ar 2500 RPM.

I think you should be fine.
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Old 02-10-2013, 01:06 PM
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I am running 11:1 with aluminium heads and a .043" quench on 91 Octane, 93 when I can get it. No issues. Cam is a little bigger at 253*@0.050" and 110LSA. I run 36*total timing in by 3000rpms. I use vac advance to add 8* more under cruise.

Read your plugs and add timing a little at a time.
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