compression for 87 octane gas

[aj's tonka truck]

I am re-building my tbi 350 in my 91 blazer. I have purchased a set of cast iron e-force heads with 76 cc chambers. I am trying to pick a flat top piston for it but I keep getting differant info. on what compression I need to shoot for to run regular 87 octane gas. I have been told anywhere from 8.5 :1 all the way to 9.5:1. I have talked to the summit techs and comp cam techs, and various techs with the various parts houses on the web and no one gives me the same info. I am about to give up. I am going to run a comp # 12-388-4 cam with 206/212 dur.@ .050 and valve lift of .425/.440 and lobe sep of 112. I believe I need between 8.7:1 and 9.0:1 and I know that your quinch plays a factor but with iron heads what is the best compression to shoot for? This will be a hunting blazer that needs good mpg but good low end torque. I also know that in the hole depth and deck heith all plays a factor but my machine shop said I need to figure what piston I want and bring it to him so he can start the block work. Can anyone help me?

 

[Wyomingboy]

From what you describe as having now, your should be in the neighborhood of 8.75 to 1.

Are you going to have them deck your block any? Or mill your heads?
If not, your ok with what you have. Even if you do deck the block and mill a little off the heads, your still going to be under 9 to 1, with 76cc heads.

Just use the stock pistons, as your setup is very mild and stock is all you need.

 

[nsxxtreme]

I am re-building my tbi 350 in my 91 blazer. I have purchased a set of cast iron e-force heads with 76 cc chambers. I am trying to pick a flat top piston for it but I keep getting differant info. on what compression I need to shoot for to run regular 87 octane gas. I have been told anywhere from 8.5 :1 all the way to 9.5:1. I have talked to the summit techs and comp cam techs, and various techs with the various parts houses on the web and no one gives me the same info. I am about to give up. I am going to run a comp # 12-388-4 cam with 206/212 dur.@ .050 and valve lift of .425/.440 and lobe sep of 112. I believe I need between 8.7:1 and 9.0:1 and I know that your quinch plays a factor but with iron heads what is the best compression to shoot for? This will be a hunting blazer that needs good mpg but good low end torque. I also know that in the hole depth and deck heith all plays a factor but my machine shop said I need to figure what piston I want and bring it to him so he can start the block work. Can anyone help me?

Flat top pistons with 76cc heads you should be fine. But are you sure you have 76cc heads? A lot of the people make the assumtion that the stock heads are 76cc and they are wrong. In my case my stock heads were 64cc.

 

[aj's tonka truck]

Flat top pistons with 76cc heads you should be fine. But are you sure you have 76cc heads? A lot of the people make the assumtion that the stock heads are 76cc and they are wrong. In my case my stock heads were 64cc.

Yes, I am sure they are 76 cc heads. They are the cast iron E Force heads. I have been told that they are made by Dart but not hold me to that. They are 170 intake runners, 1.940 intake valve and 1.500 Ex. valve, with 76cc combustion chamber and I was told that the heads are +.010 over factory to compensate for a milled down deck height. I have been told that several of the after market head companys do this because they know that usually the heads will be put on a block with the deck shortened. I am going to have to bore the block .030 over to clean it up as well as take .010 off the crank. And yes, my factory heads were 64 cc chambers. My friend that is helping me said that no way were they less than 72 cc so I had to fill them with fluid from a tester to prove to him and boy was he shocked. They are the 193 swirl port heads. I told him that looks don't tell you everything. Anyway, there are so many variables to consider and I need to pull the trigger and order the heads cause the machine shop said they would need the pistons so they could start boreing the block. I guess I will try for a piston that will put me at about 8.75:1 compression with a compressed gasket thickness of .040 and a Deck clearance of .010 which is in the middle of .025 and .005 deck clearance and the .040 gasket compressed being the middle of .039 and .041. Then I guess I will have to hope for the best on 87 octane gas. Also, I hope the 76 cc combustion chamber wont give me problems since the factory 193 heads were 64 cc. What do you guys think? Any and all help is welcomed.

 

[Wyomingboy]

With 76cc heads your not going to have a problem with 87 octane. Most people run 64cc on a 350 for any kind of performance apps. The 76cc heads are for 383's & other strokers.

Just my opinion, but since you have these gargantuan head chambers, I would deck the block 0.020" at least then use a .040 or 041" head gasket. That will give you the maximum (conservative) squish. You can go less, but let's don't get into that.
It won't matter what amount you cut the heads(if you do) just make sure they are flat !
You'll still be under 9 to 1 after everything. Then get stock flat top pistons and you should be good to go. just my .02c

 

[aj's tonka truck]

With 76cc heads your not going to have a problem with 87 octane. Most people run 64cc on a 350 for any kind of performance apps. The 76cc heads are for 383's & other strokers.

Just my opinion, but since you have these gargantuan head chambers, I would deck the block 0.020" at least then use a .040 or 041" head gasket. That will give you the maximum (conservative) squish. You can go less, but let's don't get into that.
It won't matter what amount you cut the heads(if you do) just make sure they are flat !
You'll still be under 9 to 1 after everything. Then get stock flat top pistons and you should be good to go. just my .02c

Do you mean take 0.020" off of the deck or cut the block to a 0.020 deck clearance? And are you saying that the 76 cc heads are going to kill my low end performance because this is a hunting blazer in river bottom country and I am looking for low end power & torque.

 

[K85 Octane]

I would have guessed 9.3:1 but it seems I'm over. Lots of things change that though, like cam timing. My cousin ran a 383 @ 9.5:1 on 87 octane, but pulling a trailer through the desert would cause a little pinging. So he'd run 91 when towing. I don't know anything about his setup though.

 

[Wyomingboy]

AJ,

The factory block has a 9.025" deck height. That means the pistons, at TDC are still .025 down below the deck surface. Many times the deck will warp over time. So you can have the deck cut down .020, which will give you a better compression, and squish, plus a flat sealing surface. For sure you want to have the heads checked for flatness. I bought new GMPP heads and after only 20k, one of them was warped and had to be cut flat. I didn't check them when new, which I should have!
Your 76cc heads will be just fine in this application. You shouldn't have any problems with stock timing and a 9 to 1compression ratio. Your mild cam will not adversely affect our CR.

So have your deck checked for flatness and cut down if needed. Have your heads checked also, whether they are new or not.

 

[Stomis]

Playing on the safe side Id say. 9:1 with iron heads. 9.5:1 with aluminum or vortec heads.

 

[folkenheath]

Do you mean take 0.020" off of the deck or cut the block to a 0.020 deck clearance? And are you saying that the 76 cc heads are going to kill my low end performance because this is a hunting blazer in river bottom country and I am looking for low end power & torque.

He means cut the block to zero deck clearance (flush with TDC) and then use a .040" thick gasket for .040" quench.

 

[4X4HIGH]

You will be fine with anything up to 9:1 CR. In my professional experience building engines most of the time the factory deck height is somewhere between .020"-.025" (piston below the deck surface at TDC). The ideal quench is .040" which means you would need to zero deck the block and use either a .039" or .041" compressed thickness head gasket to achieve the ideal quench. Also a dish top piston promotes better flame travel for a better burn.

 

[aj's tonka truck]

You will be fine with anything up to 9:1 CR. In my professional experience building engines most of the time the factory deck height is somewhere between .020"-.025" (piston below the deck surface at TDC). The ideal quench is .040" which means you would need to zero deck the block and use either a .039" or .041" compressed thickness head gasket to achieve the ideal quench. Also a dish top piston promotes better flame travel for a better burn.

I tried the calculations using several dish top pistons but can't get the compression above 8.6:1 with a .005 deck clearence and using a .040 compressed gasket thickness and a dish top with 76 cc heads. Looks like I will have to use a flat top piston. Is it better to use a flat top with 4 valve reliefs or 2 valve reliefs? With 4 valve reliefs I can get a compression of 9.02:1 using the 76 cc heads, .005 deck clearance, .041 compressed gasket thickness. 9.04:1 with .040 gasket, 9.05:1 with .039 gasket thickness, or get as low as 8.66:1 with same piston using a .025 deck clearance, .041 compressed gasket thickness. That is without any work on the new heads. What do you guys think?

 

[Wyomingboy]

Just use the stock, 2 valve relieve, flat top pistons.

 

[4X4HIGH]

Some quick checking and you won't even get 9:1 with a 76cc head and a zero decked block with a flat top piston with 7cc's and a .039" gasket. This combo would net you 8.934:1

 

[aj's tonka truck]

Some quick checking and you won't even get 9:1 with a 76cc head and a zero decked block with a flat top piston with 7cc's and a .039" gasket. This combo would net you 8.934:1

The piston I am going to use is a seal pro # SLPWH345ACP30 with a coated skirt and the new digital diamond profile. The Summit web site shows the piston head volume to be a +5.00 for the flat top with 4 valve reliefs. I called the Seal Pro tech line and the tech told me that the volume is a - 6.90 cc, I questioned him about the - and he said that any dish piston or a piston with valve reliefs, you should use a - instead of the +. This is differant from everything I have read or been told. It should be a + for dish and - for a dome but the Seal Pro tech said that everything I have been told is wrong??????? This is how all my research has been when it comes to picking the right piston and also on how much compression you can have and still use 87 octane gas. I have been told everything from 10:1, to can't go over 8.7:1 to run 87 gas. I am so confused....

 

[aj's tonka truck]

Has anyone rebuilt their tbi 350 and kept it fairly mild? What compression did you end up with and were you able to run a low octane fuel like our 87 octane? My cam is 206/212 @.050 and lift of .425/.440 and lobe sep. of 112, power is from 1000-5000 RPM.

 

[folkenheath]

The piston I am going to use is a seal pro # SLPWH345ACP30 with a coated skirt and the new digital diamond profile. The Summit web site shows the piston head volume to be a +5.00 for the flat top with 4 valve reliefs. I called the Seal Pro tech line and the tech told me that the volume is a - 6.90 cc, I questioned him about the - and he said that any dish piston or a piston with valve reliefs, you should use a - instead of the +. This is differant from everything I have read or been told. It should be a + for dish and - for a dome but the Seal Pro tech said that everything I have been told is wrong??????? This is how all my research has been when it comes to picking the right piston and also on how much compression you can have and still use 87 octane gas. I have been told everything from 10:1, to can't go over 8.7:1 to run 87 gas. I am so confused....

The + vs - thing depends completely on the calculator you are using. It is not a rule or a fact, it's just whatever point of view you look at it from when you write the calculator. I have a dynamic compression ratio calculator in the COG forum available for download, and I use a negative number for a dished piston. That makes more sense to me, because the dished piston takes up less volume, results in less compression, the dish sits lower in the combustion chamber, etc.

However, a dished piston will give you a larger swept volume, so some people look at it that way.

Just make sure you know what calculator and what piston manufacturer you are using(they write the specs for their pistons), and use a positive or negative accordingly. Many flat top pistons still have a slight dish effect because of the valve reliefs.

And you are getting different numbers from different people because it's not a number set in stone. There are a lot of variables for maximum compression, such as camshaft profile, operating temperature, cylinder head material, combustion chamber shape, connecting rod length, spark plug location, RPM, load, etc. So you can't just say, "9..0015:1 is the limit for 87 octane".

These guys are steering you on the right track, 9:1 is probably fine. The dynamic compression ratio is a better indicator of maximum compression for your engine. But setting a limit for maximum dyn compression for 87 octane isn't really an exact figure either.