Thread: CR vs weight

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A question for the engine guru's...

How much of a factor is the weight of a vehicle when determining how much compression you can run in an engine on the street? An example, would a 2400 pound roadster running 10.5:1 compression be less prone to high compression related problems then the same engine in a 3600 pound sedan??? Is there any rules or formulas that take weight into consideration when determining the ultimate compression ratio that will work on pump premium, or is the overall weight even a factor????

Hey Dave,don't do this to my mind so early in the morning. I would think that it is possible that the lighter load would be more forgiving,but when you really think about it you start confusing youself. I've seen articles from back in the early seventies where they played with valve overlap and duration to make a high compression engine live on pump gas.The big boys have been showing us the tighter you can control fuel and spark the more squeeze you can put on it. Hank

Early??? Heck, I've been up for 3 hours!! Allready took the dog for a walk, and went through the Sunday paper!!!

I'm kind of old school on this, more compression equals more horsepower... maybe a retard module on the ignition to pull some timing out at high RPM??? Mostly just thinking on paper and looking for some input....

I know I've tried high compression on 4x4's and it doesn't work great. You have to downshift and keep the revs up, they're doggy at low end, pinging, heating, all that fun stuff.

Light cars accelerate quicker so the detonation won't last as long.

Dave,

I'm not one of those "engine gurus," but here's my logic:

Detonation occurs mostly under heavier loads, i.e. when the engine is lugging a bit. If we increase the weight of the load (car,) we increase the liklihood that the engine will lug down sooner and longer. To me this indicates that higher CRs work better on lighter cars.

That's just an observation, and I have no way to relate weight to CR numerically. Of course, I could be totally out to lunch on this one.

Well, then that would be two of us that are completely out to lunch, my thinking is the same. Curious if there is some sort of equation that adds vehicle weight into the equation??????

I used to run a hi-revving roller cammed 289 in a '70 Maverick. Car had 'glass fenders, hood, doors, deck lid, lexan windows, weighed 2380. Had 11.5 CR and ran it on pump premium!!! Was I just lucky, or is weight that big a factor???

huh?
Sorry- still early here too. I think your talking torque Dave? Maybe? I think that your sbf motor had a 106 maybe duration that helped with the low end torque?
I don't know- hell I'm guessing, I should get some coffee.....

Naww.. The thing had no low end torque, didn't make any power till about 4 grand, pulled good all the way to 8500. Could barely get itself moving below 4. Question is, was it the light weight that let me get away with the 11.5 CR????

My 73 Monte Carlo (3800 lbs) with 11.5:1 350 would detonate slightly. i indexed the spark plugs and solved that one.

My 68 427 Vette (3200 lbs) at 11.75:1, oval port heads required 50/50 mix of premium unleaded and 108 leaded race gas (what a pain that was).

I have not ever seen a formulation with CR as a function of weight, although it seems plausible to me. 10.5:1 is approaching the gray area perhaps, but a retard function ignition module, either open or closed loop, would probably fix you up. If was real bad, but before race gas, you could inject water also.

I think in the end you build it and see though .........

Regards, Kitz

heres m 5 cents i had the opportunity to drive and tune the caddy eldorado my 500 came out of before i put it in my t bucket. the timing was very sensitive in the caddy. one could not just bump it ahead a little bit and expect for it not to ping cause as soon as i did that it pinged under acceleration.
put the same motor in a 2500 lb car with 3.55 gears and 32 inch tires no ping i have about 10 deg timing in it and it runs good i also have installed a dist spring kit to open the weights up quicker and it seems to be fine with it, i can advance the dist to the point the car wont turn over after being shut off and it still wont ping . part of this may be the engine heat factor but i still think the removal of 2000 lbs of steel definitly helps

what abut elevation and comp ratio,,how do those 2 interact?
anyone out there ?

[QUOTE]Originally posted by Dave Severson
Naww.. The thing had no low end torque, didn't make any power till about 4 grand, pulled good all the way to 8500. Could barely get itself moving below 4. Question is, was it the light weight that let me get away with the 11.5 CR???? [/QUOTE i had 5 big blocks12to1 run it on pump and one 548 10to1 with 10 psi of boost run that on pump a 548 14-8 to1 i did run pump in it but did not crank that one past 5000 rpm with it in it all the times i ran pump in them i was thinking of holes in my pistons but did not happen all of them were big soild roller cams and much dura and were on 110or 108 lsa and were O.deck or damn close and piston tops and head chambers were polished and smooth on the corners like a old bar of soap and i am sure the hi v oil pump help keep the pistons back side cool to they all were in 3500 car so i would say it can be done in a car with no load on it .

You can kill some of, if not all of the detonation problems with a slightly too high of a CR by selecting a cam with a late intake close.

This bleeds off some of the compression so you can run the engine without detonation problems.

If you can't find a cam with a late intake close, a cam that's the next step more radical will do the same thing.

We did the reverse in my bother-in-law's 413 cross-ram Dodge quite a few years back.
The car ran great, but we got into the gotta have more HP bit, stuck in a more radical cam with symettrical timing and ended up with detonation both at light, low rpm throttle settings and full-on blasts.
Then we found out ChryCo's factory cams had the late close so as to avoid detonation problems.

Even with the better and higher octane gasoline from times past.

With today's low octane gasoline and low comp engines - far as carbureted, non-computerized engines go anyway, a mild cam will make better HP than will a radical one.

I do miss the rumpity-rumpity sounds though....

Well Dave- seems that the less load the less ping and I got a big old 'Burb that pulls at 8:5/1 without ping - so go figgure.

Now that i'm thinking a little clearer, I'm not called half tanked for nothing, you might have gotten away with your cr on the little ford because of the duration and overlap. On my old street stock, I had angle milled,fully ported aftermarket heads with flattop pistons, and I couldn't even test run it on pump gas. Yes , a retarded ignition would help, but to me it seems like pouring dollars in only to drain them back out. Most of my engine building has been for stock cars, but just think of what kind of power it takes to run a 1/4 mile with 4 turns in 15 seconds,with a 2 bbl carb! I know aluminium heads will allow more cr as will smoothed and coated pistons, and un-shrouding valves to eliminate hot spots will give you a little more. Then you need electronic ignition and fuel injection o2 sensors and a computer,which will allow some more. You could also play with rod length, which if you lengthen gives a little more time at tdc for burn to occur, works good on sbc, haven't played with too many fords. Hank