225lbs of comp.

smitty · Post by **smitty** » Wed Jul 10, 2002 4:58 pm

a friend just bought a rail buggy we were tunning it did a comp. check all cylinders were at 225 to 228 lbs. thinking this was to high and that the deck height must be wrong we took off the heads and checked it,they were all at .051 it has 94 pistons 82stroke.the heads have 16th deeper reces than the cylinderbore matingsurface.we cant get it to stop dieseling when timeing is set at 7 before and 30 fulladvance, we put 93 octane in it that semmed to help but still had to retard the timeing to get it to shut off .neither of us have worked on any thing but mostly stock engines, dont want to tear it up because of this anyadvice of what we should do thanks

goatmaster187 · Post by **goatmaster187** » Wed Jul 10, 2002 5:02 pm

How do the plugs look? That compression seems a bit high. From my experience the highest I've seen is around 170lbs. Keep in mind this is in a stock engine with around 9:1. What is your static compression?

Lo Cash John · Post by **Lo Cash John** » Wed Jul 10, 2002 5:35 pm

Throw some 114 octane CAM2 in the tank and see how she does. BTW, where in GA are you?

Plastermaster · Post by **Plastermaster** » Wed Jul 10, 2002 5:59 pm

Good point on the plugs. I always thought dieseling was caused by running lean (providing the timing is right) but recently someone told me it is from running rich.
Which is it?

Ron

smitty · Post by **smitty** » Wed Jul 10, 2002 6:42 pm

the plugs were a lightish brown,but the tailpipe does seem to have sootybuildup.idels like a dream acelerates fine starts easyier than any new car,but were afraid that it is prededenating.

JohnConnolly · Post by **JohnConnolly** » Wed Jul 10, 2002 7:20 pm

what cam?

you'll help things a LOT by running the same initial advance, and about 10 degrees less total advance.

John
Aircooled.Net Inc.

<BLOCKQUOTE>quote:<HR>Originally posted by smitty:
the plugs were a lightish brown,but the tailpipe does seem to have sootybuildup.idels like a dream acelerates fine starts easyier than any new car,but were afraid that it is prededenating. <HR></BLOCKQUOTE>

smitty · Post by **smitty** » Wed Jul 10, 2002 8:19 pm

We dont have a clue what cam is in it. so should we just try seting the timeing at 20 before at full advance? What about using barrel shims to lower the comp. ? and still dont understand why the comp. would be that high,is this normal for these piston@stroke combo's /I havent ever seen but about 165lbs max on any engine I have ever worked on,still trying to learn thanks.

Sidewalk · Post by **Sidewalk** » Wed Jul 10, 2002 10:29 pm

<BLOCKQUOTE>quote:<HR>Originally posted by Plastermaster:
Good point on the plugs. I always thought dieseling was caused by running lean (providing the timing is right) but recently someone told me it is from running rich.
Which is it?

Ron<HR></BLOCKQUOTE>

Dieseling is caused by a condition inside the combustion chamber that is heating the fuel enough to auto-ignite. In this case, the high compression is heating the air enough to ignite the fuel. It can also be cause by metalic burrs in the head or carbon.

------------------
'88 Bronco II - Sonja
'59 Bug - Stella
'00 Buell X1 - Liz
"Yes, I am a smart ass."
"I'm a DUMBASS too"
But are you gutless?

Sarcasm is one service we offer.

Muffler Mike · Post by **Muffler Mike** » Thu Jul 11, 2002 5:22 am

If we went with the assumption (hate using that word) that the heads have say 55cc in them and you say .050 deck, your looking at 9.9:1 compression
(60cc chambers and it drops to 9.2)

------------------
Muffler Mike
www.MufflerMike.com

Post by **Marc** » Thu Jul 11, 2002 7:06 am

...the heads have 16th deeper reces than the cylinderbore matingsurface...[/B]

I don't understand what you're saying here; therefore the below comments assume that it doesn't affect your compression ratio.

Cranking compression figures are affected by several variables besides the C.R. (hence the need to know what cam you have - the bigger the cam, the lower the pressure will be at low speeds).
The timing doesn't have much to do with running-on (after all, the ignition is OFF, so what difference does it make?) other than the effect it can have on plug and cylinder temps at the moment you switch off.
The problem is the compression ratio, you need serious octane to stop detonation with that much cylinder pressure. Eliminating any sharp edges in the combustion chambers/valves that can "glow-plug" like Sidewalk says will help, as will running cold sparkplugs. I'd suggest NGK B8HS or Bosch W5AC.
At 10:1 you're looking at $4-$5 per gallon race fuel (technically illegal to use on the public highway) or methanol. If it's a playtoy sandrail or something that may be fine with you (provided you can keep it cool), but for street use you're probably going to want to drop the C.R. to a more conservative one - barrel spacers and/or machine work to open up the chambers are the usual remedies (you may need longer pushrods if you go with the former), it's also possible to have some dish volume milled into the pistons. A 2 3/4" dish would add almost 1 cc for every .010" depth, so if you're at 9.9 now, adding a .100" dish would drop you to 8.7. I don't know what the crown thickness is on your 94s so I'm just guessing that it should be safe to go that deep - anyone?

Pillow · Post by **Pillow** » Thu Jul 11, 2002 7:20 am

You must CC the heads as well as the deck to get a handle on what compression you are running.

Right now the CR is waaaaay to high.

------------------
Adrian Pillow
'66 VW Westfalia - "Biscuit"
'67 VW Deluxe Westfakia - "Dino"
'79 Porsche 911 SC - "Cochese"

Ryan72 · Post by **Ryan72** » Thu Jul 11, 2002 8:26 am

That amount of pressure per cylinder seems a tad high, its hard to say without knowing the cam in the beast, but lets just say with that much pressure per cylinder its quite a good amount of compression when compared to the amount of overlap/and or lift in the cam.

Ryan72 · Post by **Ryan72** » Thu Jul 11, 2002 8:29 am

CC'ing the heads would be a helpful start, getting and accurate calculation of compression ratio is important, My friend runs a bit over 9.6:1 using california 91 octane, and it runs great. It can only be accomplished however because of the cam he uses which is rather agressive for the size motor.

Sidewalk · Post by **Sidewalk** » Thu Jul 11, 2002 8:37 am

If you can, pull off a valve cover and rotate the engine, and note the timing (in crank degree's) that the valves open and close. You can get an idea of what cam you have based on these numbers. Odds are, it is a real short duration cam, and the engine was built for a larger cam.

------------------
'88 Bronco II - Sonja
'59 Bug - Stella
'00 Buell X1 - Liz
"Yes, I am a smart ass."
"I'm a DUMBASS too"
But are you gutless?

Sarcasm is one service we offer.