Guys,

I've got a 1969 427 that I've got it almost completely built on the stand with everything done but valve adjustment (waiting on rockers). I've been doing some reading and I'm concerned about my cam/static compression combination causing detonation and I'm running out of time to do any major changes with the motor out of the car. Here in good ol Alabama we've got 94 octane at BP and I'm hoping this is enough with what I've got.

Comments, successes/failures, current setups etc. appreciated.

This is what I've got:

-427 (.060 over = 439cid) or 4.310 bore and 3.76 stroke
-101cc closed chamber oval port iron heads w/2:19 - 1:90's mild bowl work
-16.8cc domes on forged pistons
-.045 quench (.030 Multi-layer steel gasket, .015-.020 deck clearance... I think... forgot to check )
-Comp 274H Hydraulic Cam
Lift - Int:552 Exh:.555
Dur - Int:274 Exh: 286
LSA 110

With my calcs I'm running a 10.35:1 static CR and 8.38:1 Dynamic CR and I'm starting to get concerned about detonation from some recent google searches on my cam. This is mainly because of the high ramp rates and narrow LSA leading to a higher cylinder pressures (ie. DCR). Most people agree that 8.5:1 DCR is max for pump gas especially with iron heads.

Some thoughts:
I knew I was on the edge running a high static CR trying to match what came in my car from the factory (10.25:1) but apparently this cam that I've got isn't high static CR friendly. This is why I shot for a tight squish with the .030 MLS gaskets to reduce detonation as mentioned in this article.

http://www.chevyhiperformance.com/te...ide/index.html

Installing a thicker head gasket (.045) would get me down to 9.97:1 but based upon what the article says I might be doing more damage than good with a quench of .060-.070 actually leading to more detonation at lower static CR.

As a side note the tech at Comp Cams said that with 32 degrees of timing I might be alright, but I'd hate to sacrifice power from taking timing out if I can compensate in another way.

I also considered just rolling with my setup and possibly switching to 1.65 rockers to reduce the duration on the cam if I ran into trouble because a rocker swap is easier in the car than a cam swap. Besides 1.65 rockers would put my lift at the next size smaller version of my cam anyway at around .530".

Or... Worst case scenario...I swap out the cam for one that makes a lower DCR.

What do you guys think???
RICK

 

Have you thought about switching to E85? That would be the easiest solution. You can run alot higher compression on E85. It like cheap race gas.

 

What's the advance on the cam? You might be able to get what your after by moving the advance of the cam...

 

10.25 isn't that much compression; just run premium. I think you will be fine and I would keep the 1.7 rockers. 1.65 will slightly decrease the intake valve closing point after BDC. This will increase your DCR. You can decrease DCR by retarding the cam if you're going to lose sleep over it. Four degrees will lower it by about 1/4 point.

Keep your quench at 0.050" or less. More than that will increase detonation.

 

I think he is more concerned about the DCR being too close to the maximum for pump gas, which is a valid concern.

 

I can't give you a definitive answer to your question but I do have some other info here that could be really important to you. I'm running the closed chamber head design like you are and you should also be concerned with valve to piston clearance. Rather than restate a bunch of info I'll post the link to a prior thread that covered this topic. Look at post #4 for my personal experience and then hit the link there to an article that discusses this too. Good luck on your build. I'm sure you'll get it all sorted out. Here you go- http://forums.corvetteforum.com/c3-t...alum-head.html

 

No "expert", but I'm with Ben on this one too, and would most likely install that cam at zero advance (retarded 4* from as ground) with 10.25:1 and those heads. And, yes, good quench is important here.

 

I would definitely like to use a little more cam to help absorb some of that static compression...BUT you didnt mention what intake/carb setup you are running? You dont want to cam it too much if you are running a stock intake... I'm guessing you likely running the aluminum 69' 427/390hp q jet intake? If the heads and intake are going to be the bottle neck, you don't want too much cam...It will be a dog down low then run out of breath up top...


Personally, I would take a little less quench and run the thicker gaskets and get the static below 10.0 but....There are different trains of thought on this and I am no pro engine builder....

I do think you can get away with this combo, even with the smallish cam, you will have to be conservative on the tune up... I agree with 32-33* of timing total and no vacuum advance.

Not exactly the same combo but... I am running real close to 10.0 compression, smallish 238/248* hyd roller, 110lsa and degreed to 107 icl. Iron GM oval ports. I also run no more then 32-33* and it makes power fine at that range... Although I have ran it at 38* and blown a head gasket and started to melt pistons....this was within one weekend of running it hard...but I never heard it ping so dont believe you can listen for ping.

My tips:
*timing at 31-32* total
*Have the cruise/light throttle afr in the mid 12s, WOT afr in the 11s.
*R43T spark plugs
*Keep the engine as cool as possible all the time, if it is over 180* water temp, DON'T POUND ON IT....especially during the summer. I normally don't run mine hard if it is over 175*.....and at the track when really pounding it, I never run it over 150*.

Yes you will be giving up some power having the timing at 32* but....it will likely stay together and not push gaskets....Otherwise, you need to go to a more radical cam, with MORE duration and MORE overlap...Its going to want to rev more with that cam so you need the heads/intake to feed it..

Just my thoughts....take it for what it is... Lots of people will tell you 10-1 with iron heads and 36* is fine....you'll just have to see for yourself what works... After you change head gaskets once with the engine in the car, you'll keep the timing low...LOL

 

Yea.. you called it... stock 390hp aluminum intake with a jetted Q-jet. So I don't want a "peaky" cam because I'm intake and head limited on flow anyway. I've got my cam installed on the dots... neither advanced nor retarded.

As far as keeping the car below 180 F I don't know what I'll do about that. Before I pulled the motor I sat in line at the track on a warm Alabama night last summer and the damn thing was running 230+. I plugged the crossover passage for the carb in the intake and I'll probably get a reproduction Harrison aluminum version of my radiator which will all help. But I'll probably end-up using additive like a blend of Xylene and 94 octane or Torcco Accelerator if it's hot outside and I feel like beating it.

Opinions on E-85? There's a station that carrys it not too far way from my house. Not sure my GM reproduction fuel hoses or rubber carb internals will handle it.

 

Les...

I pulled all but the "baby" valve spring on my exh valve on #1 cylinder with the head gaskets off and pushed down hard in a few places in the crank rotation as I watched the cam and lifter. Didn't have any contact anywhere before or at TDC.... I was really pushing hard too each time that I checked. I was actually trying to get the valve to hit the piston so I knew what my total clearance was.

I may be wrong - COMMENTS - But from what I hear the most likely place for valve contact is with the exhaust valve when the piston is coming up on the exhaust stroke. Didn't check the intake valve yet but I've got to remove my inner springs for cam break-in before installing rockers, so I'll make a point to check it then.

 

You can't run e85 without completely recalibrating your carburetor. I'm not sure if can even be done with a qjet. An engine uses something like 30% more fuel when running that stuff so your mileage will go to you know what.

 

Yeah, but it also cost alot less than 93 octane, like $2 a gallon. It's a wash cost wise. It's very cheap race fuel. I've heard it's equivalent to 100 octane rating gas (although it does not have an octane rating). There are other advantages too, like your engine runs alot cooler, similar to alcohol engines. I imagine passing the sniffer test will be a breeze on E85, although I doubt the OP will have to worry about that anyway considering the age of his car.

Some racers have been running it for years. The only disadvantage is availability, and there has been talk of it completely going away.

Also, I thought Qjets could be rejetted just like any carb. Just the computer controlled ones can't.

 

427 engines run best with the cam straight up, no advance or retard....the piston chases the exhaust valve home so a retard will place the valve closing later. No much, but closer than straight up....Compression ratios of 11-1 are still safe with pump premium at 93....If you want better performance from a 427 run or build to a L-71 or L-72 specifications....and hang on....its all you need

 

This totally depends on the cam your running, specifically the intake closing point. Also depends on head material, chamber design, timing, advance curve, A/F ratio, Quench, Temperature the engine runs at and many other factors. To throw a blanket statement out there that you can run 11 to 1 compression on 93 octane without taking all the factors above into account I would have to disagree with.

 

I just had my L71 rebuilt....030 over, Comp Cams solid lifter duplicate of the factory grind, Comp Cams spring set, speed pro pistons for an 11.1 to 1 comp ratio, the original tripower and had it balanced at Boucher Racing in Rowley, MA
Try to run a mixture of 10 gallons of Sunoco 93 and 7 gallons of Cam2 (110) and that averages out to around 100 octane. She runs pretty darn good. Every once in awhile last summer while on long cruises I'd put just 93 in and didn't seem to be a problem at all. And I always beat on it/ drive it hard playin' around. I'm going to forget about the E85 but it sounds interesting...my two cents on octane.