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Author Topic:   305 chevy compression
M1
Dirt Freak

Total posts: 257
posted March 19, 2004 05:06 PM  
I have a pro built 305 with stock 1978 or so dished pistons. After about 5 shows, I did a compression check on it, and it pumped about 130-140 psi per cylinder with 601 heads. Now I have a 1979 with the same pistons, and 434 heads, and has never been opened (as far as I can tell) and it pumped 170psi per hole.

Why the difference? would cam differences make the compression readins goofy? Or am I missing something here.

racer17j
Dirt Forum Champ
Total posts: 5390
posted March 19, 2004 05:16 PM  
A larger cam wil make a diff. on a crancking comp. test, due to the increase in dur. the intake valve will stil be slighly open when the piston is coming up on the compression stroke. The bigger the cam or dur the lower the cranking comp. will usually be, A cyl leakdown test is your bst bet to check for rings or valves not sealling properly. Hpe this helps.

Josh K.

fast_crew
Dirt Forum Racer

Total posts: 99
posted March 19, 2004 05:45 PM  
PS sorry for miss spelled words or missing letters i am gnna chuck this darn keboard lol

M1
Dirt Freak

Total posts: 257
posted March 19, 2004 06:07 PM  
The confusion starts with these three factors....

#1The "good engine" pumpes 130-140psi and was built by a very reputable engine builder.

The "other engine" is one I got for FREE and is 100% stock still.

#2They are both dished piston motors, and the 434's have larger chambers then the 601's so should automatically have less compression anyway.

#3And doesnt 170psi seem a little high for "astray pistons" and 434 heads?

Okie11m
Dirt Freak

Total posts: 287
posted March 19, 2004 06:55 PM  
What kind of gauge did you use? I had one with a bad check valve in it & it would give me low readings sometimes & high the next. Well not higher than it should be just higher than the last reading. A cam with more overlap will lower the comp. also.

M1
Dirt Freak

Total posts: 257
posted March 19, 2004 07:32 PM  
the gauge I am using is a cheepo, BUT I checked every cylinder more then once. I checked my banshee, snowmobile, and then the race motor. All of them were where they should be, so I am not doubting its concistancy, but maybe its accuracy.

Here are the specs for the cam......

Lift .410
duration 280
@50 230
lobe seperation 106

fast_crew
Dirt Forum Racer

Total posts: 99
posted March 20, 2004 02:51 PM  
The cam in your race motor isbigger then the cam in the stocker. Back your rockers off say 1/2-1 turn (from zero lash ) on a cyl and then check that cyl. it will be a good bit higher. The cam doesn't have the same effect on cyl. pressure at say 4500 rpms

Thanks josh k.

ruggs66
Dirt Maniac

Total posts: 193
posted March 26, 2004 09:13 PM  
the 106 lobe seperation is bleeding off some pressure when you check compression since the exhaust valve is opening when the intake is closing.
with a stock cam the intake completely shuts before the exhaust opens.

some engine builders want the 106 lobe sep. when using a big cam, they say it will give it some extra torque off of the corner & its supposed to rev up quicker.

I believe that you need better static compression to run a cam with 106 l.s. to use it to its full potential.

the cam in the 51 car is similar but with 111 lobe sep.

your car looked fast with the new moda.... have you tried the 222 cam from Howie? I have one of those cams from the 91 car. We took it out trying to make him faster, but we found out he had a poor carb all along.

ruggs66


M1
Dirt Freak

Total posts: 257
posted March 27, 2004 08:46 AM  
I always ran stock cams until last year. I called and asked them what cam they suggested for the application I had. I dont really like it, because the car is a slug on restarts, and out of the corners, down the striaghts, and into the corners....LOL

I want to try a better cam this year...the "222" is what you suggest?

ruggs66
Dirt Maniac

Total posts: 193
posted March 27, 2004 09:08 AM  
I don't have any imput Good or Bad on that cam since the 91 had a bad carb when he was running it.

but the smaller duration @ 50 will give you more power down low.

I have a Blue-Racer cam you could try. It should pull to 6000-6300 rpm & sound almost stock.
& I have the 222 cam also sitting on a shelf if you want to try that one....but I suggest the Blue Racer cam.

oh I believe its 414 lift so it will pass tech.


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