what is the max streetable compression for a stock cast iron head setup?

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-=1987 Formula 350=-

Member Greater Cincinnati F-Body Association

 

Usually somewhere in the 10 to 10½ range. It depends on what heads, and alot of other factors.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Too many factors involved. Old school thinking allowed 9:1 but now it's usually around 10:1. A properly designed engine could theoretically go to 12:1 on premium pump gas.

Flat top pistons can tolerate a bit more compression becasue of the better flame travel.
Electronic fuel management can get away with a bit more by adjusting fuel requirements to engine load and retard the timing if knock is detected.
Higher altitude will allow more compression.
Cam profiles can allow more compression depending on the grind.
Retarding timing allows more compression.

It all depends on how the engine is built and not all engines are built the same. My new race engine should survive easily on 92 octane here in Calgary but would need at least 100 if I went to race at sea level.

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Follow my racing progress on Stephen's racing page
and check out the race car

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
461 Big Block installed and ready for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

depends on heads chamber size i have 601's
thats 58cc and flat top pistons. 12:1 and ive changed head gaskets to prove it its streetable with high octane gas and additives
i wouldent recoment this set unless you were dead set on runing a "almost diesle" engine.
64cc which are stock on most 305s should boost a 350 to 10:1 with flat tops which would run fine on mid/pre gas.
Also Stephen 87, im in colorado and they claim that a carbed car will run fine on 85 how does 100oct come into play?


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85 Camaro RS T tops 350/350, stealth intake. holley street avenger 670. ported 58cc heads. comp magnum 270H cam,Crane 1.6 roller rockers,edelbrock accu-drive, MSD 6 series ignition on HEI, K&N extream air cleaner, true dual exhaust with 18 inch glasspacks.
82 Firebird 305/250 action plus intake, accel distributer with ccs 8mm wires foam air filter.
The only "cat" i have uses the litter box

 

If your car runs fine on 85 in Colorado then try it in LA. That much lower in elevation will probably cause a lot of pinging requiring a higher octane. What they "claim" depends on your setup. No two cars are the same. Just because one person said his car runs fine on 85 doesn't mean yours will.

601 heads on a 350? They came on 267 engines (with very small valves) from 80-82 and 305 engines from 80 to 86. Although they have 58cc chambers I doubt they flow enough for a good 350 especially with the stock smaller valves. Even a good port job and 2.02/1.6 valves if they would even fit would probably be only as good as a set of stock 350 heads.

My engine is 10.3:1. It could probably survive here on 87 octane but I'll be using 92 just to be safe.

[This message has been edited by Stephen 87 IROC (edited March 24, 2001).]

 

i have stock heads on my 350 now and it has 9:1 compression if i put a set of ported 601's with 1.94 valves and 1.5 what would my compression be and would it still be able to drive on pump gas. By the way the 350 is out of a 70's camaro if that helps any. Also would they flow better then the heads that i have now?

 

If the 350 is out of later 70s Camaro, like mine, its probably got dished pistons. I would think with 58cc heads, it would be about 9.5-10:1

 

does anyone know what size the combustion chamber is in the late 70's stock heads. I would think it would have to be 64cc if the 58cc only raise the compression up to 9.5 or 10?

 

Depends on what fuel you'll be useing.

Althou different engines under different operating conditions
( temp, elevation etc) will vary a little in how much compression they will tolerate with a said octane fuel,
they don't vary that much. Not near as much as some people
try to claim. Forget octane boosters. Not economical or effective enough.
For 92 octane gas:
Build for a true 9.6/9.8:1 compression with conventional
cast iron cylinder heads. Vortecs will allow slightly more. (10:1)
I have used 10.3:1 with stock 882 heads and tight quench but the motor was real sensitive on ignition timing and spark plug heat range. ( would not run right with stock heat range projected tip plugs.
(94 octane gas)

Not worth it for every day best performance.

Do not get caught up in the big cam big overlap story that
is said to allow more compression. Very misleading.
Sure You will be able to cruise @ low speed/ part throttle {because of high E.G.R. and low cylinder pressure ( cam overlap)} but any full throttle will need retarded timing and/ or over rich mixture (less power) to avoid detonation. Unless you're at real high altitude
or real cold weather even a race motor with a big race cam will not operate at full throttle and full optimine timing without detonation.
A street motor with a over sized big race cam will be real hard on gas too.
Not practical.

Build for 9.6/9.8:1 and concentrate on induction/ exhaust improvements to get the power you want.
You can get over 500hp at 9.8:1 in a well built 350 street motor.
( read that "radical street")

Leaves ya a lot of room.

 

late 70's 350's had 76 cc heads

even the high perf motors did from 1971 on up.

 

The 601's are not really the best 305 heads to start with.
Unless you already own them and want to do up a set of 305 heads look for the 416's.
Once you've sweep out the chamber walls to fit the larger valves
you'll be at about 60/61cc's.
If your 70's 350 has dished pistons you'll be well within the tolerance of street gas (92 octane)
If your 70's 350 has flat tops and is not exessivly decked
then you'll still be well within the tolerance on 92 octane gas.

: After changing over to these small er chamber head.

Note the stock 305 type heads have smaller valves and do not flow as much air as a stock 350 head.
But once fully ported and upgraded with larger 1.94x 1.60 valves
they will out flow and out perform a stock 70's 350 head.
A good 70's 350 head like 441's or 920's can be improved with porting too remember. The ports are about the same.
They can be shaved to reduce the chamber size and raise compression too.
Both heads flow about the same with full porting and equally sized big valves.
But the modded 305 head is easier to bolt on higher compression and improved breathing.
What is the head casting # on the 350 heads you have now?

 

i will go out tomarrow and check to see what the casting number is

 

i couldn't find the number on the head because they are in the car on the motor. Is there a way to see the number without taking anything off?

 

My '50 pickup ran just a tad over 11:1 on 93 octane, but at reduced timing to prevent detonation. that was on iron factory heads, 76cc chambers, TRW domes, and Crane cam with enough overlap to kill some of the dynamic compression pressure.

 

What 4 bolts to remove the valvecovers are too much?

Can't be that important to ya.

 

the larger cam does help bleed off pressure allowing you to run a larger cam. with the higher compression it is going to help offset the ffects of the larger cam a little bit.
another thing I have heard through honda builders and some high compression domestic motor tuners is at low rpms the cam will bleed off the pressure allowing you to run a lot higher compression ratio then normal. but when you get to the higher rpms where the cam is going to start filling your cyl a little better is b/c of the rpms detonation doesn't quite have enough time to happen b/c things are going on so much quicker. and these are guys running 11-12:1 compression on a street car.



I have seen a mag where they ran 12.3:1 compression on a 502 using ONLY 87 OCTANE and it ran fine without detonation problems. and made over 650lbs/foot of torque


but things that affect your comression max are heads alum resist detonation better being they are able to cool the combustion proccess down. downside cooler combustion chamber leads to less power.
cumbustion chamber shape. if you can create turbulance insidie the combustion chamber it is going to wrinkle the flame front leading to a quicker burn. hence less time for detonation to form


swirl inside the combustion chamber has the same affect. and if you put swirl inside the intake manifold where the fuel is it should atomize the fuel better promoting a quicker burn which again leads to less of a chance of detonation. problem though is if you have too much swirl you can create a pumping loss that would overcome the gains.

flat top/dished pistons with a small CC head gives better flame front travel which again leads to more detonation resistance. if you put flat top pistons in there the flame front is goign to have some isues going up and over the hum in the piston creating a chance of detonation.

puttting the spark plug in the center of the cyl head (not something that can really be done to heads... just have to buy them that way. gives better burn inside the cyl as well.

polishing your head off is kinda up for debate right now. some say with the head polished it will reflect more heat into the combustion chamber which can build pressure and make power but more pressure means more of a chance of detonation. others say with a slightly rought finish it will creat turb inside the chamber with will atomize the fuel better creating more power... problem there is you have a chance for hot spots which can cause detonation


running evans coolant can help also being it prevents hot spots from forming inside your combustion chamber. downside it doesn't have as good of heat transfer as water creating more pressure which can lead to detonation.. so that one is also up for grabs.


retarding the cam timing can give you some more detonation resistance being it allows you to bleed of some pressure when the rpms are low.

bore size will affect it as well. the larger the bore the more prone to detonation.

and I think rod/stroke ratio has a part to do with it but I can't recall it right off the top of my head.... oh well


there are prolly many things I am forgeting but hey I'm really sick right now so I'm just giving what I can


hopefully vader will come in here since he is the tech ***
:hail: :hail: :hail: vader

 

My heads are ported and polished 601s and I can tell you that they vastly outperform my old stock 416s.

They are funny looking in their bowls but man did they ever add some get up and go once worked over according to F-Bird'88's specs :hail:

If 601s are what you have, then don't worry--you can definitely make them FAST

 

I have the 993 casting heads are these worth taking the time to port out and give them a good work over or are they junk?

 

They are not the worst of the smog era heads.

They can be improved with full porting and larger valves.

The issue on you're motor is compression thou.
I perfer the smaller chamber 305 heads to use once ported
and up graded, especially if your workinfg with a dished piston
motor.
Its easier to get the cr up to where it needs to be with
ported 305 heads.

It's up to you.

I would shave them .060" and use a thin .015" gasket.

 

Old old one here... I am running a 350 .030 over with 187 58cc heads with 202 intake and 1.6 exhaust valves, ported also. I will be taking it to the dyno after this deployment is over and post numbers. Compression shold be in the area of 10.2-10.6 depending on quench. I need to do some checking to get the magic .040.
With 92 oct fuel and not over kill on the timing all should be fine.

 

SCR at 9.5 and DCR at 8.5.Quench @ .040.Go anywhere and do anything you want.

Anyone who has a friend that tells you not to listen to Alky,has a fool for a friend.

Now going on two yrs I have been trying to "teach" a certain member.So here is how

that lesson is going to play play out.Sooner or later we will meet at Cayuga in time

trials.We will actually get off it to let him catch up before we dust him.Then I will

introduce myself and once again remind him of his usually high risk advice and well out

of date home porting.He'll get the nickel tour of a sportsman pro race operation where

knowledge and skill has been the standard for a very long time.Just maybe,just maybe,

that will turn him into enough envy that he will start to learn..........


It is certainly such a waste of a Firebird with he has done to it.

 

I ran a dero-decked 60-over 305 with flattops, and milled, big-valved, and ported '76 heads, and a Comp 12-388-4 cam, but it was a high-elevation build, driven at high elevation with no less than 3.42:1 gearing and 25" tires. That's like a 255/40R17. Also had a loose converter. There are tricks to make compression work, other than the late intake closing, or too much overlap, or both. Why hurt performance when the combo can't breathe to the high RPM necessary to take advantage of the compromise cam?

 

Ya Gary1 Be sure to bring your money Eh!....

Idiot

 

Not really enough grip in a .015 shim head gasket to the heat up and numerous cool down cycles of a street driven vehicle.

Here again we are trying to incorporate race only parts on street cars.

 

For a flat-enough block and re-surfaced heads that weren't done on a big belt sander, they're fine for 400-HP-350 daily drivers with iron heads. Before trying to run them you must verify your quench, and you must get a 4" machinist's straightedge and a good set of feeler gauges. Clean the block decks with a wire wheel or a razor blade, but NOT a "ziz" wheel, or any other abrasive pad. If you can't get a 0.002" gauge between the deck and the straightedge, you can run the FelPro 1094s. Likewise the heads must not allow a 0.002" gauge. Otherwise the GM 0.028" composite gaskets are really good. The 1094s don't brinnell aluminum heads, but don't combine them for daily commuting.