Can you relief cut the pistons?

 

Detonation is going to depend on dynamic compression not static compression. What cam you have is going to affect things greatly since you are really on the edge. I have to watch what I do with timing running 10.2:1 with 93 octane, but that is in Texas where the air temp causes you issues too.

I have to call BS on 11.5:1 on the street unless you are filling up with race gas or running alcohol, E85, or something.

 

double post

 

Urban--didnt you warn me about a problem you had with AFR heads?

the cam was custom ground by ed curtis for the application. something like 560 lift with 255 duration at .050. Thats off memory though dont have spec sheet. He also told Curtis that he would be running 91 oct and 10.8-1 comp. he runs the rpms up to 6800. motor only has about 8 passes on it. You can still see the machining in the block.

Im in California so air temps should be similar to texas.

Can we get spell check...LOL

 

why don't you just get the block and then make up your mind how that's gonna fit ... :-) I would

 

I love the performance on my AFR 205 heads, but the valve quality is crap. I look forward to pulling them and putting in good SS Ferrea valves and good valve springs. I would recommend AFR heads, but I would get bare heads and build them out myself if I did it again.

 

Aluminum has something like three times the thermal conductivity of iron, so the Al heads will run a bit cooler. I've heard it's worth about a point increase in compression.

That said, at 10.8 I'd be more comfortable with an engine that was running a carefully tuned EFI rather than a carb.


Norm

 

Well you have a 4.030 bore, or maybe a little more. Bore diameter has an effect on these things., as does chamber shape, head material, altitude, humidity, temperature, and a host of other things. Some guys in Seattle WA can run more compression than guys in Charleston SC, cooler air with milder humidity means less detonation. So it depends on where in So cal you are and what your weather is like, Texas can be brutally hot and humid, spent enough time in San Antonio/Wichita Falls to know.

So what does that have to do with anything? I have an engine with a 4.18" bore, with aluminum heads that is running 10.8 static and it didnt get any rattles on 91-93 in Charleston SC. A larger bore is more prone to detonation than a smaller bore, so the 4" bores of the small Ford help with controlling preignition and other badness.

The cam will only help with the lower rpm if it is bleeding off compression with overlap, once you spin that sucker up the cam will end up packing more air in the cylinders and if you are marginal on CR then you will rattle and possibly break things. When you break things like that it is at a higher rpm with more of a load on the engine, things go boom real quick. If you are going to idle around and never do a 1/4 mile pass at WOT then you can run a bunch of static with a 250 @.050 cam, just dont stomp it or you will do some damage.

All that being said, you could run E85 if it was close to you, but California is behind the times with that stuff. You would only have to mix a bit in, not run it completely if you didnt want to. A mix of about 30%-40% ethanol would enable your engine to run even on 87 octane that was mixed with it, and 93 would be a no brainer. A slight jet change is all it takes, nothing radical, no drilling on the carb, just jets under 50% ethanol. Your problem is finding the stuff in commie land.

I would surmise you can run those heads, but you should run either race gas or some ethanol with it if you plan to open it up, just cruising around it should do ok. Mine did WOT passes on 92 octane camel whiz and showed no signs of detonation, and its a much larger engine at 467ci.

 

well E85 and race gas are out of the question. I know a couple places that run 100 octane here in so cal but I dont want to have to rely on that. This guy was running this motor in AZ with no detonation issues at 10.8-1 on 91 octane. He claims thats what the cam is disigned for.

Regardless do you think I would be better off swapping out my heads for a 10-1 CR? because I do a lot of cruising and of course Im going to make 1/4 passes who builds a motor to idle around?

dodgestang runs 10.8 cr on his 408. where does he live?

 

you'll be fine with premium pump gas..... most motors are a little lower compression than people seem to think they are anyways.

you'd think a zero decked block with flat tops and a 60cc head would give my 289 like 10.0:1 or something, but in reality its more like 9.0:1.