VETDRMS

I went ahead and checked the compression this evening while I was checking valve lash. Here are the numbers with corresponding cylinder:

1 - 185
3 - 185
5 - 184
7 - 184

2 - 186
4 - 185
6 - 190
8 - 185

They are all very consistent, but #6 is a little high. I think the lash may be a little loose on this cylinder, which would cause the intake valve to close sooner/open later, could this cause the higher number?

Also, do these cranking numbers sound right for my calculated 10.6:1 SCR with cam duration: 242/248 @ .050?

Thanks!

:cheers:

LAvetteman

Looks good. All numbers are plus or minus 10%.

My numbers on my Big Block were in the 145-150 range.

mbeeman350

Anytime the compression is within 5%, from ther highest reading to the lowest, the rings and valves are in great shape and doing their job!

:cheers:

VETDRMS

The largest % variance from the mean comes out to be 3.2%.

Excluding the erroneous high cylinder, the highest % variance from the mean is 1.08%.

Assuming there will be some variation between cylinder temperatures, ect., this sounds pretty darn close to me. I could have some gas sitting in the #6 cylinder as the couple times I opened the carb all the way dumped a LOT of gas, should #6 been open it could of got the lion's share of it. :lol:


:cheers:

JJMURFS

It is my understanding that the Dynamic Compression test has very little to do with Static Compression Ratio, and there are many variables, such as cam overlap, and the profile of the cam lobe (rollers has much quicker opening and closing than a flat tappet with the same duration). I have seen many very high compression engines (12:1 ++) have cranking compression readings much lower than your numbers, due to the cam having alot of degrees of overlap (when both the intake and exhaust open at the same time).

71coupe

The numbers you have are about the highest you want for a street driven car. Should make some serious power :yesnod:

ZD75blue

they look nice and even to me...

Might even be one of those "Needs more horsepower on the starter side!"

:cool:

VETDRMS

JJMURFS Yes, as expected my DCR is rather high: 8.5:1. This cam does have a lot of duration at .050, but seat-to-seat is kind of low: 280/286. With an intake closing at 66 BTDC.

ZD75blue I am running a 4.41:1 gear reduction starter good to 18:1 compression. It gets the job done and sounds :reddevil:.

Thanks for the input!

GrandSportC3

those numbers look good.. As others mentioned.. there can be a 5% variation...

Oldguard 7

:thumbs:

MotorHead

My compression was 230-235psi across the board with Sportsman II iron heads and it runs great on pump gas and I run 40 degrees total advance. I did it twice once with the old CC270H cam and then again with CC282 solid and I got pretty much the same numbers. I have been told that up to about 240psi is good for the street then it is track only.

This is the trickiest part of cam/head/piston-compression ratio selection. The higher duration cams require a higher compression ratio to keep the dynamic compression up. That is the reason I am going with 11:1 pistons in my 406ci with the same cam you have, it will be interesting to see what numbers I get. :D





[Modified by MotorHead, 10:07 PM 3/23/2004]

VETDRMS

I'm surprised you didn't have any issues with detonation. What was your DCR?

With this cam and my calculated compression of 10.65:1 your 11:1 will most likely not be that much higher. I assume your elevation will have some effect on the compression limit with pump gas.

I am very happy with this cam, I think you'll be pleased. :)

Travis

jimtreber

I also got caught up in the numbers with my combo, 406 with 12 to 1 and CC 280 HE cam. But with Rhoads lifters I approached 240 psi. The only problem I had was difficult hot starting. I had Comp Cams grind a 298 cam with additional overlap because of the Rhoads and the problem was solved.

WESCH

Hi

The compression test of my 427 CI .030 over solid lifter engine was from 161 to 177 PSI. Unfortunately, I used a push on type tester and they are known not to seal off so well .
The cam is a original L71 solid lifter cam with .520/.520 lift, 242/242 duration , 114 lobe ctr line angle.
Compression is 11.3 : 1 .
The engine had an overhaul about 10 K ago, .030 bore , new guts. The old heads only got a valve seat grind with creme only.

Gunther :seeya