# What compression will I need to be at to run 87 octane



## my67goat (Jun 9, 2011)

I finally was able to get my dream car. A 1967 GTO with 400 HO, with the 670 heads and an automatic. Numbers matching engine. Now, the real news, the engine has to be rebuilt. So I want to run 87 octane. So far from the info I've gathered on this website( just a wealth of info from experienced folks) Mr. P body recommendes a xe 262 h cam for lowered compression. What should be my CR range? Also will the heads need to be reworked, they already had the hardened exhaust valves seals done by the previous owner. I know the pistons need to be dished but what would be the ultimate CR for 87 octane unleaded gas. I'm ok with midgrade if I have to use it. Will more work need to be done on the heads. I plan on 30 over,new pistons,cam, balanced and heads redone if needed. Please advise the newbie, as I would like to have an intelligent conversation with the machinist I don't want to spend more than I absolutely need to but I want to do a quality build done right the first time.


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## Pontiacpurebrred (Jun 22, 2011)

Anyone jump in that knows more than I, but it's my understanding you'll want to be in the 8-8.5:1 range if you want to run pump gas readily available today. There was also someone on here talking about running them on E85 with only minor modifications to the carburetor needed. You may want to look into that.


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## Instg8ter (Sep 28, 2010)

to run those heads you will need to get dished pistons to get your compression down, 8 -8.5:1 for 87-89 octane. Thumpin455 does the carb conversions for E85 trying to get the choke setup on my holley so i can send t to him. I am sure Jim (MrPbody) would be happy to lay out a plan of attack if you call or mail him with your plans and desired usage.


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## Thumpin455 (Feb 17, 2010)

The E85 depends on where he lives, but it is the easiest route since you just build it and dont worry about compression. Some places dont have it available, like Nevada anywhere outside Vegas. Some people just dont like it even if it does make more power and run cooler. If he wants to know about that I will give him the info.

You need to get it under 9:1, which would probably be easier with a different set of heads. Any head after 1971 will work to lower compression, some more than others. 

You can open up the chambers on the 670 heads, that will help unshroud the valves and drop the compression. Even with the chamber mods you will probably need dished pistons, depending on where you live. Higher altitudes are less detonation sensitive as is cooler dry weather. I know you will probably knock the upper rod bearings out of a 400 with 670 heads on pump gas, been there done that. Gas is also formulated differently for different areas, so what works in one state might not in another.

The only ways around lowering the compression is changing the fuel or running aluminum heads that dissipate heat faster.


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## dimitri (Feb 13, 2009)

8.5 - 8.7 max for 87 octane and you will be able to keep you timing at 36 degrees. I built an engine with 8.5 and ran regular cali gas, 85 octane with my timing at 36 all day no problems.


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## BearGFR (Aug 25, 2008)

Howdy and welcome. 

As others have said the vicinity of 8.5 -8.7 :1 with your combo will "generally" be safe, but unfortunately there's just not a "one size fits all" answer. 

A lot's going to depend on some variables, like rear end ratio, how you're going to use the car, how comfortable and proficient you are at "staying on top of" the tune up (especially fuel mixture), the condition of your cooling system, etc. First thing, you really must cc those heads (measure the volume of all 8 combustion chambers) so you know what you're starting with. Pontiac heads are known to vary quite a bit from the published factory specs sometimes, plus you don't know for sure if they've already been milled at some point in the past, so you can't know for sure what your real compression ratio is until you measure those chambers.

Two main things work against you as far as detonation: heat and cylinder pressure. Heat is why getting the fuel mixture exactly right, and the condition of your cooling system, are so important. Iron holds more heat that does aluminum. Cylinder pressure is determined mostly by two things: static compression ratio and cam shaft profile. If you have a cam with more "attitude" (duration) than stock that means the intake valve will be closing later in the cycle - so it will build less cylinder pressure than a milder cam will at the same static compression ratio. Cam selection is where where things like intended use, gear ratio, etc. come in. If you want to keep good street manners, vacuum assisted power brakes, "more or less" street converter, etc. then that's going to impose a limit on how rowdy you can go with the cam shaft. If you want or can tolerate a little nastier cam, than you can get away with a little more static compression ratio due to the later intake closing event. Those 670's are closed chamber heads and have lots of quench area --- so if you go too extreme on the piston dish it can kill the quench, which will reduce chamber turbulence, which can make the motor more likely to get into detonation. If at all possible you want to use D-shaped dishes in the pistons in order to preserve that quench pad area. That's also why you don't want to solve the compression problem by using thicker head gaskets, unless you don't have any other choice --- they tend to kill the quench area.

So, the place to start is to answer yourself some questions, like... how are you going to be using the car most of the time -- street toy? highway distance cruiser? race car? some mixture? Do you want to be able to build it and just drive it without ever having to raise the hood or are you comfortable/happy with the extra time and effort involved in making sure the tune up is "on" all the time?
What matters to you most.. max performance, ease of maintenance, fuel economy...?
And of course... what's your budget for parts and labor looking like?

Bear


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## geeteeohguy (Feb 2, 2008)

I agree with the above posts. A wise Pontiac guy once posted somewhere that the general rule of thumb for Pontiacs and octane was to put a decimal between the octane number and you'd get the compression ration you needed: 87 octane: 8.7 compression, 89 octane 8.9 compression, 91 octane: 9.1 compression. I've found this to be pretty much dead on. I retrofitted my '67 GTO with 87cc iron heads, and have a CR of about 8.8 or 9 to 1. It runs on 89 octane. Good choice on the Comp Cam....I've had experience with a couple of them now, and with a lowered compression Pontiac, they sure scream!


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## pontiac (Mar 6, 2011)

don't mess up a good set of 670 heads and hope for lower compression. swap to later heads and be done with it. You can not get enough dish with the 670 and still be practical for 87 octane. It will be a really dog. PM me about a swap for your 670s. You can not get enough dish for 8.7 compression with the 670 heads and be practical. You will have to run higher octane even with the max dish. Also, not just any machine shop can dish pistons since the jig to hold a 400 piston is not common for most lathes and there is only so much thickness available on the dome of the forged 400 piston. Cast won't work. And if you cut the 670 head for more CCs, you will get into an internal passage and mess up the valve seats.


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## my67goat (Jun 9, 2011)

Thanks for the info guys. I really want to keep the original 670 heads (original engine). Dishing the pistons is probably the way we will go, the machinist said he will measure "cc" the chamber, or at least I think he said something to that effect. I am not familiar with quench, but I will bring it up to the engine builder. The car does have a 3.55rear and 400TH. I plan to cruise at highway speed and I am concerned about mileage(price of gas you know). Want to be easy to maintain. I may someday bring it to a strip, just once, to say I did it. I want it to have a little zip off the line, just for my own fun and pleasure. I'm not rebuilding a strip race car engine but hate to say it, I will sacrifice a little performance. Now that I'm older I want more of a highway cruiser but at the same time maintaining the nostalgia of this car. I'm not going the way of E85, I want to maintain a little of the original integrity of the carburator. I really appreciate all the info, it will help me discussing the rebuild options and alternatives when I speak to the engine builder. Hoping for a little more input and food for thought. Thanks.


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