Well I need some help. I recently got my car on the road. The engine is all new. I did the assembly myself. Problem is detonation. I am worried my parts recipe has resulted in a compression ratio that is too high for pump gas. The particulars are thus: new stock 2000 ford shortblock- flat top pistons.1970 351 windsor heads. I think these have 60cc chambers. Stock Maverick ignition system. This is the '76 electronic ignition. Pre duraspark type. I am using a Comp cams roller camshaft with .486 lift .I am running premium gas and that has helped somewhat, but detonation occurrs when I mush the accellerator pedal down with my size 12 right foot. I have tried playing ignition timing games, even jumping one tooth on the distributor gears(To and Fro). I have the stock rear gears (2.79), kickdown linkage fully functional. The carb is a rebuilt Autolite 4100.That is another whole issue there, cuz I need some help tuning it. I have not been able to set timing with my timing light because the marks on the damper are about two inches from the timing pointer. I have no idea what that is all about. Maybe the mix of new block and old ignition,distributor and balancer/ flywheel. How do I calculate compression ratio? I have a compression tester, but I don't know how to convert that data into a ratio. Thanks.
Compression tester won't give you the compression ratio. I would guess you're running too lean and probably have the timing way too advanced. Get that situation with your damper sorted out and I bet you'll be okay.
that combo should be ok. even if using flattop pistons instead of dished, 60cc heads, .015 deck, .042 gasket would give a 9.6 to 1 ratio. should run fine on regular if timing is set conservatively. timing is probably whacked. mismatch of pointer and balancer marks? try retarding timing a little at a time to see if things improve. the compression readings cannot be converted to the ratio because the readings are affected by the camshaft (bigger cam, lower cranking compression). do you know how to find top dead center with engine in the car to check the timing marks? that might be a good starting point
also, that rear ratio will hurt slightly.. the numerically low ratio will load (lug) the engine a little and could cause slightly higher cylinder pressure which would just compound the problem. if you cure the primary problem, the rear should not be a problem, though.
Hmmm... if your carb is running lean, wouldn't that cause the detonation too? Maybe? Maybe not? It certainly doesn't sound like your engine's compression is too high. A quicky calculator can be found here --> http://www.prestage.com/Car+Math/Engine+Building+Calculators/Calculate+Compression+Ratio/default.aspx more at http://www.google.com/search?as_q=calculate+compression&num=10&hl=en&ie=UTF-8&btnG=Google+Search&as_epq=&as_oq=&as_eq=&lr=&as_ft=i&as_filetype=&as_qdr=all&as_nlo=&as_nhi=&as_occt=any&as_dt=i&as_sitesearch=&safe=off
I did not gegree the cam. Guilty as charged.I trusted the Comp cams people. I may be too lean on the carb mixture. How do you adjust an Autolite 4100? The idle is set as low as I can adjust and it idles at 1100rpm.! I tried retarding the timing by ear and test drives. That darn damper! It is the same as far as marks and flat spot, which keys it into the crank. Just the weight is differant. However, this is something- the distibutoe gear was changed by a local balancing shop. The roller cam needs a steel gear on the distibutor, and the guy told me he had to drill the hole for the roll pin. Maybe he got the gear a little off.
I'm confused even more now. Are you timing the engine with the vacuum hose attached to the distributor? (remove and plug while timing). That should get your timing marks closer - assuming the plug wires are going to the right plugs. "Degreeing" a cam is done during the installation of the cam - not when purchasing it. If the cam had a dowel pin on it (that aligns with the top timing gear) then that's not the prob. With the new information regarding the hole for the pin, you might try pulling the distributor up and over a notch (both ways). If you know how to arrive at TDC, you could re-check that. Hmmm... maybe you should take it back to the guy who drilled the hole. :confused: I am certainly no cersamafied mechanic, and what I've told you is going on memory. Other guys on here know a lot more about this than I do. Just trying to help in the meantime.
cams from the usual reputable companies are rarely off enough to seriously affect the running of an engine. might not give optimum horsepower, haven't heard any horror stories lately about any manufacturers. maybe 2-4 degrees off, which costs HP, but doesn't do this to an engine. ford had 2 different sets of timing marks for the passenger side of the engine. one was at about 10:00 and the other at 11:00 when viewed from the front. very easy not to notice the discrepancy.
the cam's lack of being degreed is most likely not a problem unless the cam was installed one or 2 teeth off...in which case, it would run like crap, if it even ran at all. As many cam swaps as we've done, only one was more than 1 degree off...cam companies nowdays are making much more accurate camshafts than they used to. If you have the idle stop screw all the way out and the butterflies are 100% closed, I would be a vacuum leak would be the culprit. After all, a vacuum leak is nothing more than unmetered air entering the engine...and when that happens you can sometimes compensate by backing out the mixture screws to get it to idle. The dizzy gear would most likely not be much of a problem...even if the gear was installed a little off, you can make up for it by turning the distributor slightly. IMO, it sounds like you have a vacuum leak somewhere, which would allow extra air to enter the engine with no extra fuel....and be a partial cause of spark ping (detonation). At least, that's my thinking....for what it's worth. I
Tdc TDC of #1 piston is where I started out. I pulled the plug, put my finger in the hole ,felt compression till the rotor pointed to #1. Then I looked at the timing marks. They were right on. So far so good. Then i drove the car and it was detonating. So I retarded a little. Several times till it wouldn't idle. No luck at all. I tried advancing also. I tried jumping one dist. gear tooth both ways. So I tried my timing light(read the instructions). Plugged the vacumn hose.Timing marks were about 2 inches toward passenger side of the pointer. Well, heck. That just about does it for my pea sized brain. I'm stumped.:confused: I also checked all the spark plug wires for correct order. Maybe it is the carb. It is a 1.12" venturi, about 600cfm, so it should be able to deliver a good mixture. Maybe a vacumn leak? As vacumn increases, maybe its sucking air in from a leak and leaning out my mixture. I wonder how to check for that.
I think as I type and Mavman types faster. I did notice when I first started this engine that some air was leaking from the base of carb at the gasket. It was smoke after shutoff. I bought another (MR Gasket) gasket. Any ideas where the vac leaks can occur? I was wondering about the heated choke tube. I have none cuz the manifold end is rusted off. I plugged it with some rubber hose and also my finger. I noticed no differance. The vacumn there is tremendous. Compared to the dist. vac opening,which I couldn't even feel .
That carb needs to be tricked out to fit that cam. That is why you cant get a lower idle. I dont think you are running too much compression. Sounds like you are closer to correct timing, now you need to focus on the carb. I would get a holley or eldebrock carb and dont forget to consider a power valve that is suited for that cam. Dan
Dan , that is good advice. Your right. The guy at Ford's SVO help line told me that when he gave me the specs for that cam. I already had the carb and I thought it would be good enough for a while until I could break the motor in and afford the headers ,rear gears and new carb. (How do you keep your wife from finding out?) I'll check for vac leaks and such in the meantime. You guys are great. Thanks. And if anyone ele has any ideas ,I'll keep checking. Chris.
