Max compression for pump gas iron head

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qwk dust

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What is the maximum compression ratio that can be run with good 93 octane pump gas in an iron headed motor. I am building a 318 with 302 casting heads and want to get the highest compression ratio possible without getting too close to the ragged edge of detonation on good pump gas. The machine shop will be decking the block to get my ratio just where I want it with the 59cc heads so I could really use your advise. thx
 
Your lucky 302's are closed chambers so that helps. Shoot for zero decking the pistons and use a gasket that's .040-.045 thick. The cam also affects things. A long duration cam will bleed of cylinder pressure so you can run more compression. I'm running 9.9 to 1 compression on my 360 with open chambered X heads and a cam that's 234 degrees of duration at .050 and it never spark knocks on 93 gas with the total timing set at 34 degrees.

I'd say if you set it up like I described above you should be able to go 10.5 to 1 and be ok on 93. That's with a middle of the road performance cam.

If you polish the chambers real good and remove any sharp edges you might even get away with 11 to 1 comp.
 
it all depends on a number of factors. The main one being static compression which depends on cam timing. Gearing, converter, vehicle weight, altitude and fuel all play a part as well.
 
I am running 10.6:1 on my 360 with closed chamber magnum heads and a tight 0.040" quench, this is with a Comp XE268H cam which gives me a cranking presure of 180-190 psi. I have 15 degrees of intial timing with 33 total and a curve that has it all in at 2500 rpm.

I run this set up on 89 octane and 87 in a pinch if I don't get on it hard below 3000 rpm.
 
What cam are you putting in it? Like the part number? If your dynamic goes over 8.5:1, you will have an issue. Dynamic needs the intake valve closing event spec to be calculated.
 
Thanks for the replies. The cam is the Comp 268H .454 lift .218 @ .050. Rear gear is 3.90 and the converter will be what is prescribed by the converter manufacturer but I guess somewhere 2800 to maybe 3400. I don't mind always running 93 octane since the Vette, Mustang, Lexus and motorcycles all require that anyway and luckily Fl has pretty good gas. (that is if Ike doesn't sink us..) So am I reading maybe 10.3..10.5 max?
 
Which pistons? I'm looking at around a max of 10.1.:1, but the less quench you have, the lower you will want to keep it. A full dished piston will lose the quench effect totally.
 
If your useing the 302 heads with a zero deck piston and .039 gasket, you'll be preety much at the limit. If the heads are changed to an open chamber, you'll likely have a problem due to no quench and a still hi ratio and no quench.

Your reading right IMO, 10.5 is a pushing it issue on a quenched head and 10-1 on a non-quenched head. DGC has an interesting combo thats getting away with a high ratio and excellent quenched head.


OF course, many factors effect things that will allow the engine to run more ratio, or not. This is why I would rathger back it Down (and suggest so) a 1/2 point of compresion.
 
I was planning on using KB167 pistons but with the 59cc heads and after square decking the motor it looks like I would be over 10.5 to 1. Based on that I think I will use the 89 318 pistons which have a taller compresion height than stock and are cast not hyperuetectic. I wanted to get over 10 to 1 and less than 10.5 to one if you guys think that is reasonable. The 302 closed chambers are supposed to be a little safer with higher compression is what I was told. The cam will be the Comp 268H with springs lifters and a new timing set up if that matters. thx
 
I have a very similar setup to Dave. Mine is a 360 +.040, zero decked flat tops and Magnum heads. My cam is a Comp Cams Magnum 280H. My compression works out to 10.58 to 1. I can run 89 octane with no problems and 87 in a pinch. Remember though that I am in Canada where the octane is rated different. 93 in the US is the same as our 91.

Jack
 
I don't know the difference in the formula's, just that there is one. My guess would be your 94 equals about a 96 in the U.S.

Jack
 
The late 318 slugs are a flat top right? So are the KB167s. Zero deck is zero deck so use the taller pistons and save your poor machinist a few decking passes on the broach. You need to shoot for a quench height of .035 - .045 or so, and a typical composition head gasket will get you there really easy with zero deck.

Also, the KB167 is a little bit stronger than the OEM slug. It's still a cast piston, but it's a really nice cast piston.

Make sure there are no sharp edges in the chamber or on the piston, run a cold plug, pay attention to oil control and you shouldnt have any problem with pump premium.

I run almost 10:1 in my pickup on midgrade with shallow open chambers (Buick 350). Chamber prep helps.
 
Coyote, I think it's more to do with the size of the cam. The Magnum 280 is bigger and develops less pressure low down than the XE268 will.
 
The information I was given is that with the KB 167s height by the time I square deck the head (maybe .020 to get there) I will likely have zero deck or even out of the hole and that with 59cc 302 heads I would be well over 10.5 to 1. The flat top late model 318 pistons have less compression height than the KBs but more than stock so when I square deck the block I can still have enough room to keep the compression within reason. The late model pistons have a 1.745 compression height vs. the stock '74 pistons which are 1.720. I understand that with this set up appx .020 in the hole and a regular gasket I should be close to the 10+ to 1 goal.
Does anybody have the calculations with a 59 cc head 318 bored .030 with the '84 to '90 pistons at 1.745 compression height using either a regular or thin gasket what I would have? Thanks
 
You've been given a lot of "information", but what are the "facts" about your particular combo.

What is your current deck clearance? (OEM engines were all over the place)

What is the current compression height of the piston you are running?

What is the actual measured chamber volume of your heads after the valves have been set? (OEM engines were all over the place, and typically larger than the nominal value).

How "out of square" are your block and heads? .020 is a lot.
 
A friend of mine runs a true 11.2 w/915 closed chamber head and 87 octane
But it's all in the cam/tuning.
 
C130,
.020 is fairly average for a block, and or heads to be out as I see it every day. I've seen them out as much as .035 from side to side and end to end. So I would consider a block thats out .015 - .020 a good block and really square for factory specs.
 
Cheif, most of the info about what I am considering is in the posts. The current stock motor info is really not relevant since I am building a new one. Anyway, I am considering 59cc 302 heads, .030 bore, comp 268H cam (.454/218@50), assume a .039 gasket as Rumble referenced.
The stock pistons in the stock block sit .071 in the hole. Stock 318 pistions I understand have a 1.720 compression height. The 84 to 90 ones have 1.745 (.025 taller), KB167s look like 1.81 which is .090 taller. If I square deck the block and it takes .020 to do that I only have .051 to deal with before 0 and if I use the 1.745 pistons I am .026 in the hole. That looks like 10.28 to 1.. If as BJR says I have to go more off the block it obviously jumps from there.
I don't see how the Keith Blacks would really work.
Did I do this calculation right?
 
Looks about right to me. Is this with the thin gaskets, if so you can lower the compresion by using a thicker gasket but then you lose the quench that your trying to keep. As thin gaskets are very hard to come by right now, unless you want to spend $75-$100.00 a set. If you use a .038-.040 gasket and have the block decked to 0 then the quench will be there from now on. And cutting a block .040-.050 isn't that big a deal as this is still only 1 setup on each side.

It just depends on how you want to go about achieveing your goals.
 
Just so I feel better and want to be clear, I recomend the .039 thick gasket with a zero deck piston for good quench. The .039 spec is a typical Fel-Pro gasket that is on the shelf.

This is the way I would do it myself.

The taller 318 piston and the thin MP head gasket, if available, is a deccent way of doing tings as well.

I just like the first way better.
 
Rumble,
But what he is saying is that the KB167 pistons will be out of the block in his case and this isn't what he wanted, so what I said is to use the newer pistons and deck it to 0 and then use the .038-.040 gaskets. Or don't deck it so much and use the thin mopar gaskets. If he can get them.
 
Yeah, we're all saying the same thing here. The worst factory deck I had was .029" off end to end, and .017" accross the deck surface. It had to go below blueprint to correctly square it up. You know the math. I would square deck it, then order your pistons.
 
OOOOOOOOOOOOOOOOOOOOOOOO

OK! Thanks Bobby.
 
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