When I rebuilt this 87 350 4 bolt main, I had 11:1 compression pistons put on it. After getting it running, it works on my mind that the car isnt acting like it should to full potential. I have all the right parts for good amounts of horsepower, but something doesnt add up. I have a stock ignition system on it, and the engine still turns over and starts very easily, like stock. Believe me, it's a pretty good distance away from stock. I thought I would feel some resistance or have it trying harder to turn over and start with such high compression.

So I began to wonder. I lost my records for the heads I bought. I know they are Dart Iron Eagle's. Does anyone know what sizes the cc's come in for that type of head? What I'm thinking is that I had maybe ordered too big of cc's on the head, which would be lowering my compression? Any suggestions?

 

I have heads that are stock on another 350. I also have heads that are stock for a 305. Would those with lower cc's work to give me my compression ratio back? Can I do anything with the heads I have on it now? Milling?

The heads are Dart Iron Eagles 64cc 2.02 intake, 1.6 exhuast 1.25 springs

 

Are you sure you want an actual 11:1 compression ratio for
motor that runs on street pump gas?

a 11:1 compression gen 1 carbed motor will need about 98-102 minimum octane to run without ignition retard.

Yes the smaller the cc the cylinder head the higher the compression ratio will be.

 

no not actually 11.1 but bring my compression back up. For some reason it seems way too low. 64cc isnt that big though. Im not sure what to do.

 

I have 13:1 compression ratio and have no problem cranking my engine over. It all depends on the cam grind. If the valve overlap is wide, it allows cylinder compression to bleed off and makes cranking the engine over easier. Very high cylinder pressures means you need a beefier starter to crank the engine over.

Do a compression test to see how much cylinder pressure is being built up. A "good" engine should be in the 150 - 180 psi range.

Even a 13:1 ratio engine will run on pump gas as long as there's no high load put on the engine. Once the engine starts to work, low octane fuel will start to self ignite under the high compression. A higher octane fuel burns slower and won't detonate as quickly.

64cc was the original small chamber performance heads back in the early 70's. The very small 58cc heads were on 305's but don't leave a lot of room for bigger valves. Typical smog heads are 76cc. The size of the combustion chamber doesn't really mean much for compression ratio because piston dome can change the ratio. You can have 12:1 with a 76cc head or 8:1 with a 58cc head just by changing the pistons.

Your "11:1" pistons should have a combustion size given for that rating.

 

I know the combination of the two makes the compression. I was looking for the cheapest/best way to get more compression out of it. I dont want to spend the money on new heads, or take the engine all the way down to put new pistons in. I was asking for any suggestions. I'll will have to check the psi for sure, but I'm pretty sure its lower on compression than I want it. It isnt so low it wont run good, I just thought I should be getting more out of it than that.

 

Depending on what pistons you used, using a smaller cc head to raise the compression ratio may or may not work. A flat top piston won't care but if you have a dome piston, it might not fit into a tiny combustion chamber. The piston manufacturer should be able to tell you.

Another way to slightly increase the compression ratio is to use a thinner head gasket. Typical composite head gaskets are around .035". You should use a factory style metal shim head gasket that's as thin as .018".

Your heads can be milled down to decrease the size of the combustion chamber which will increase the compression ratio. This usually involves an angle cut so that the intake will bolt back up.

Other than that the only other way to raise the compression ratio involves a tear down. Having the block zero decked, using pistons with a bigger dome.

 

Do you know how much it would cost to have mine milled? How much should it be taken down to lower one cc, and so on.

 

Does anyone know what psi that comes out of the plug would equal high compression? Like 170 psi=10:1 compression? Or is that not a good indicator?

 

It's not a good indicator.

The pistons that you used in your motor to get "11:1" do they have a dome top or are they "flat tops"?

 

flat top TRW's

head gaskets are standard size

 

if you also have 64cc heads your compression ratio is not too low. You're barking up the wrong tree.

 

mean83z...

No way you got 11:1 with flat tops and 64 cc, be more like 9.72 if your pistons are 4 valve reliefs flat tops (-6.1 cc)

Are you sure you need and want more compression?

Unless you have a VERY BIG cam you are gonna run into detonation like crazy on pumped 93 octane.

I run a true 10.18:1 ratio with a 272/284* cam and I have to keep timing pulled back to 34*, jetted carb fat, cold air set up, and keep water temps down low, to keep from pinging on 93 octane. My cranking compression readings are 215-217 psi on all 8 cyl. This is really way to high for a street driven engine. 190-200 psi is best for high performance street engine

The being able to run higher compression, cyl psi readings, etc is made up of the compresstion ratio and cam size. Bigger cams will "bleed" off more cyl pressure. This is why if you stick a big cam in a stock-ish type engine, it becomes a *** at lower rpm ranges

Compression is great, but it's not that big of a power gain after you get over about 9.3:1.

If your useing the norm blue fel pro head gaskets (4.166" x .041") that comes in most rebuild kits, and most common sold in parts stores, you may be a twitch lower compression, and your quench height will be wide. More a nice mild gain you could go to a 4.100" x .015" steel shim gasket. It will bump compression up some, that thats not the main place the power gain will come from it, will be from the tighter quench height

DO NOT REPLACE YOUR DART HEADS WITH STOCK HEADS!!!

The darts flow alot more air than the GM heads.

Give me a run down on all your specs and info and i'll try to give you good info on what to do to get the power out of it. Most of the times it's as simple as tuning. Other times people may have picked 1 or 2 wrong parts thats a big mis match to rest of combo.

I need to know....
bore
pistons
stroke (3.48" for a 350)
did you have block decked/milled?
cam size. Give me part number and all specs like seat duration, duration at .050" lift, valve lift, LSA
Head info. I know what you said so far, but what intake CC runner size did you get?
intake
carb
timing
timing curve
carb jetting
fuel pump

 

alright well it sounds like i might get those metal shims for the heads and just leave it. I'm friends with a guy that builds race cars, and Ive had him tune it before. But I've changed a few things since then, so I guess I'll take it back up to him to tune it the way it really should be. Right now I'm doing a manual to auto tranny swap, so I'll deal with the engine after that.

As far as specs, I'm limited to what I can give you for now. I'm not at home right now, where cam info card is. Hopefully I can find it. I know thats a big part in determining things, sorry bout that. I'll get it this weekend tho.

11.1 flat top TRW's
Dart Iron Eagles 64cc
Cam- all i know right now is Crane 510* lift
350 bored 30 over, milled and decked
stock crank
1.5 roller rockers
polished alum. dual plane intake
Holley 750 dp with jets two sizes bigger than stock(I think)
New distrib. so timing isnt set right now-Mallory
stock fuel pump

I realize this cant help much, I will be able to give you the full specs after this weeked