High compression engine and pinging

[cppursell]

Originally Posted by Ticker

See this post; long-winded but pretty good and although it looks a little intimidating it's not all that hard to do - and well worth a couple hours investment.

http://67-72chevytrucks.com/vboard/s...d.php?t=290498

It sounds like a weekend project. I'll see if I can get to it. Hopefully it'll make a positive difference.

[cppursell]

Originally Posted by nd72c10

Do you know anything about the engine that you just stuck these random heads onto? cam part #?

What is the casting number from the cylinder heads? How was it determined that they are small chambered 283 heads?

Don't know about the cam (I didn't put it together) but the head # is 3795896 or
3884520 determined by the single triangle symbol, per this guide:
http://www.chevellestuff.com/qd/head...ng_by_year.htm

Only word I got on the cam right now is that it isn't stock.

[cppursell]

Originally Posted by SSedan64

I think you have the wrong Heads for the engine. 283 Heads have tiny 1.72" Intake valves & chambers. Larger chamber & valve needed.

Yeah, well I guess the experiment with this engine is how efficient and lean it can run, so I guess my cousin's idea with putting these heads on was to see if the higher compression (and small intake) would equal +MPG.
I think the heads are great so far, 'cause I got this thing on the freeway and it accelerates more than my old (stock) 350 with powerpack heads.

[cppursell]

Originally Posted by LEEVON

Just a thought...have you checked total timing? Could it be that you have too much mechanical and vacuum advance? Adjustable vacuum canisters are cheap, you can dial it back and get the total timing correct. I agree with the others, bring your initial back to about 12 degrees and see what happens. I run about 10.5:1 in my 350 and don't have detonation but then I make sure the total timing is in range too.

I agree, I definitely need to time my engine more accurately. I just haven't had time yet (school). I'll try to get to it this weekend.

[cppursell]

Originally Posted by Ticker

Cool - but my only suggestion is stop experimenting until you get the timing set right as I noted previously.

It's not just about the initial timing but the total timing including advance, which from your description is where the issue is.

Thank you ticker for your previous link. Yes, I'll try to get to that this weekend.

[cppursell]

Originally Posted by bradh

If you get the total timing set and it still knocks try a cooler plug. If I'm not mistaken that's a .375 reach gasket type plug on 283 heads. A Champion RJ12YC should be in the ballpark.

I have new AC Delco plugs in the engine. My cousin set the gap but he kept it modest.

[Bad70sbchevy]

Originally Posted by Ticker

As previously noted, a cam with a lot of overlap on an engine making less than about 1.4 HP/CID just wastes power, gas and driveability. Your engine would make significantly more power (on the order of 10-15% or more) with a lower static CR and the RIGHT cam. And it would make far more power where it spends most of it's time - below 4500 RPM.

This isn't intended to be harsh, but using DCR to run a high static CR is just bad engine design on a low-power, low-RPM engine. It's a crutch to make a bad combo work - but it doesn't work well. 1 point of CR is worth maybe 1.5 - 3% MAX on an engine making less than 1.4 HP/CID or so...so shooting for high CR for a street engine *does not* make power. It's a design technique for high-powered racing engines that has made it's way into street design.

I agree, a big cam is NOT good for engines seeing low rpms, but a big cam will make more power, it just happens at a higher rpm due to overlap helping cylinder scavenging thus increasing volumetric efficiency. Also on engines, shouldnt you always shoot for the highest compression ratio possible (without detonation) since it results in higher efficiency thus leading to more power? Any time you raise compression you should always see a power gain (in theory), even if its a little bit. I wouldnt call it a bad combo, just probably not for a PURE street engine.

[pockets]

i agree that running a larger cam to reduce DCR isn't a great idea, just that it can work. a real street engine that never/rarely sees the track will be far better off with the right cam and compression.

I also agree that a higher compression ratio will generally mean more power, assuming the cam is up to the task as well as the fuel. Race engines use high compression to make more power, but they also have the fuel to compensate.

[Ticker]

Originally Posted by Bad70sbchevy

I agree, a big cam is NOT good for engines seeing low rpms, but a big cam will make more power, it just happens at a higher rpm due to overlap helping cylinder scavenging thus increasing volumetric efficiency. Also on engines, shouldnt you always shoot for the highest compression ratio possible (without detonation) since it results in higher efficiency thus leading to more power? Any time you raise compression you should always see a power gain (in theory), even if its a little bit. I wouldnt call it a bad combo, just probably not for a PURE street engine.

A big cam isn't going to make more power unless the heads can flow to the level of that lift. As an example, a big cam with 882 heads even if they're ported to a crazy extent isn't going to make power at high RPM...it just can't flow enough to use what the cam offers. And overall - a big cam making power at a crazy RPM level doesn't really do any good unless the car is regularly at that RPM level.

Compression ratio does improve volumetric and combustion efficiency...but again, only if the engine can make use of it. The more flow the engine can consume, then the more important CR becomes. Premium pump gas limits CR to around 10.2 with full ignition advance and an .040 quench....that's just as far as we can go on the toilet water we have for gas today. Going higher than that means giving up power - backing off on ignition advance or the previously mentioned DCR. For an engine running on pump gas, that 10.2 or so is the Golden Rule for a GEN I. We can push that a fair bit on a GEN II or LS due to the dramatically better combustion chamber shape, and even a bit with top-notch heads on a GEN I...but the gains are minimal unless it's a very hot engine. Vizard has an interesting chart that shows the benefits based on HP/CID - I'll see if I can scan and post it.

I would call the OP's engine a really bad combo. It can be made to work, but it's never going to work particularly well. The only thing we can do is figure out the crutches it's going to take to make it work without coming apart, and there have been a lot of good suggestions around that...but if budget is not an object, swapping the heads to get the CR into a reasonable place is the best choice.

Again, not trying to be but this is the way it is

[cppursell]

Thanks for everyone's replies and advice. I simply don't have time to work on my truck 'cause of school, so I have my timing set to as advanced it will get without pinging, 15. It's a quick and dirty solution for now, but I still plan to take everyone's advice when I get the chance.
Thanks again and good trucking!