I've been going some research and have found answers all over the board on this. I'm trying to determine if my car will be pump gas friendly (93 octane) with the changes I'm making. Currently my car has iron heads and a pretty stout cam. The previous owner told me it was rebuilt to 10.3:1 compression ratio. I'm changing the cam in it to something more street friendly (along the lines of 220/230 duration and 110-112 lsa, around .475-.500lift) My understanding is that the large cam that is currently in the car allowed for some overlap which helps reduce compression. I want to make sure with a cam change that I won't get pings or knocks on pump gas and that I won't have to lose a lot of perfomance by changing timing/etc. So..

1) Is 10.3:1 going to be okay on 93 octane? This will be with iron heads/1.6 ratio rockers and a cam in the 220/230 @ .050 probably 110-112 lsa
2) If it's too high is there a way to lower it without changing pistons/heads? The heads are coming off to fix a bent valve so is their something the shop could do to bring compression down a couple of points that's not a budget buster?

The car is going to an engine builder in 2 weeks. I'm going to run these questions by the builder and see what they have to say. Just wondering what the forum thinks.

 

Bigger Lobe Sep angles mean the intake closes later (good to lower dynamic compression and give good idle) pic 112-116degrees.

What heads are they? Later designs are better than earlier.

Unshroud head chamber around intake valves to lower compression. Pick valves with a bit less margin and even a recess in the valve head to lower compression. Dont stack up head gasgets, instead use a gasket tha keeps the piston top to head clearance at 35-40thou.

Other methods to lower risk of detonation include get your motor to run cooler (radiator/pump, thermostat etc), use cold air pickup for carb, block off exhaust heat riser in manafold and insulate bottom of manifold from the hot oil.

Run your setup through one of the many dynamic compression calculators.

 

Still, you should be fine running 10.3:1 on 93 octane. Altering the heads can hurt performance if not done correctly, as it will undoubtedly change the flow characteristics (besides the fact that lower compression hurts performance). If it were me, I'd leave it alone and trust the engineering that went into it. Many automakers run 11-11.5:1 in production today (granted, often with knock sensors). Good luck

 

Until you find what pistons you have (domed, flat or recessed piston) you won't know what your compression is or will be. IMHO, a 10:1 ratio would be a flat top piston w/ a 64cc chamber head. I'm no expert here (just learning/studying the forums) just layman terms, but I'd love to hear if I'm right! The vets will chime in and further help you...

 

I think it you will be borderline. I have a 388 with a tad over 11:1 with modern vortec heads and 222* cam. It is a no go with 93 octane.

 

best bet is to change to aluminum heads,or lower comprission 10:1 is still cutting it close,what do you come up with on compression test?
I had a 10:5 compression motor with a comp cam 305 magnum with iron heads ,ran 91 octane in my Amx and broke ring lands on forged pistons due to detonation,Im sure if you retard timing and don't run it hard you might not have a problem ,listen real close on exceleration for pinging .

 

Why doncha wait 'til it's at builder where/when you can accurately determine what piston volume is/will be and how far down in hole they are/will be? From that & w/head & gasket knowledge ... you'll KNOW CR. Until you know that ... this here is nothing but speculation.

*edit* a coupla points'll drop CR from 10.3 to 8.3 ... probably not what you want. If you've got flattops & it's truly 10.3 ... a set of dished pistons'll drop it to about 9.4-9.7

 

I am running sportsman II iron heads with flat top KB pistons and a 224 236 cam on a 112 LSA. I have had zero problems with 93 octane. I have sprayed a 100 shot several times and a 150 shot on occasion and still no problems. Just watch the temps. As long as they are between 180 - 200 deg you will be fine.

Sorry the cam is a Comp XE268H 224 230 on 110 LSA it has been a while since i looked at the specs.

 

This is the answer more or less.

You need real measurements and intake valve closing points to get the right match between compression and cam timing. Dynamic compression ratio calculators will tell you how close you are to being able to run pump gas.

-Mark.

 

Just do a simple compression test. If cranking compression is over 180, you're a no go for pump gas. Just be mindful of your tuneup if the cranking compression is in the 170 to 180 range on pump gas.