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Compression Ratio and 44 IDF's

Folks,

All right. I have been having a slightly rich problem with my 2016cc. My exhaust pipe is getting quite blackened by carbon. I don't exactly smell gas, but suspect that I am running rich by the residue on my pipes and plugs (Bosch W8 ac). I am running a 78mm crank/90.5 P/C's, and FK7 cam with Scat

1.4:1 Ratio rockers. Ported and polished heads matched at the carbs, but not to the head. On the carbs, I am using F11 emulsion, 200 ac, and 135 main, with velocity stacks on Berg 580 polished intakes. The heads are ported and polished, and I think are 42 exhaust, 36 Intake. I'm running a merged 1 5/8's exhaust with a stinger and spark arrester.

The problem is that the carbs don't respond when trying to adjust the mixture screws. I used the settings from the Weber manual and the engine fired up immediately. Just idled a little fast. I've run the engine like this for a few seasons. (Strickly summer and fall) The engine never overheats, doesn't leak oil. Has never set off the Berg oil dipstick. I just can't help but think that I am robbing HP with this rich condition. The engine pulls hard and spins up like a motorcycle, but the front end of my dunebuggy would only lift in 2nd gear, and only maybe 1/2 inch off the ground, and it's a very light vehicle. The wheels would only break loose in rain. (Although I was running 295 65 R15's.) I've measure the deck height and cc'd the heads twice. I can't find the numbers just now, but remember the CR being at 6.9:1 each time I measured. I spoke with Jake Raby, and he correctly surmised that:

A) I read a lot of Gene Berg articles B) My compression ratio is too low C) The engine would get much snappier if I switched from 92 Octane to 89 Octane.

He got A and C correct. Is it possible that my compression ratio is too low and affecting the performance of the engine and carb mixture? I am also running electronic ignition. Pertronix I think. I have tried a Jacobs coil and a Bosch blue coil with the same result. Is there something I am missing here?

Your thoughts would be greatly appreciated. I would like to get a handle on this before I start the new season.

Thanks, Matt from NH

Reply to
Matthewd
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WAY TOO LOW compression. Even for a STOCK engine. In that area the Berg advice is downright BAD. Your engine cannot burn the fuel afficiently with that low CR, and since you have a hing performance Cam (not a race cam but a typical street cam anyway), you would need to RAISE the CR from ths stockl 7.4:1 to compensate from the compression pressure loss caused by valves being open longer. So you made the problem worse from BOTH ends, lowering the CR and creating need for MORE CR with the cam.

You need a CR of around 7.8-8:1 just for the engine to start working correctly. No matter what you do now, it won't until you get your CR up. High octane gasoline and 8.5:1 would be my suggestion. You won't believe the difference it makes.

Jan

Reply to
Jan Andersson

hello Matt....you say Jake nailed it on A and C, well i'm pretty sure he nailed it on B too....i have been a victim of low compression engines in the past, and they never really perform as they should...too inefficient....they always soot up the pipe, and typically suck in the gas mileage area(not that the average person cares about MPG when building a hi-po engine)....your compression is much lower than stock and your cam "bleeds" off quite a bit of compression due to the valve overlap it has...so your "dynamic" compression is less than you would think...were it mine(these days) i would have the heads flycut(unless you have cylinder shims to play with) and i would get the compression up to 8.5:1 or even 9.0:1......this is not advice, but what *I* would do if it were *mine*.....do your research(as you did before, but try more than one source) and decide what works best for you....good luck

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Reply to
Joey Tribiani

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