some real interesting articles on Teds website well worth a visit.

one question please  why ,with the aluminium heads, haven't you raised the target DCR to 9:1 from 8:1 with the iron heads?

many thanks

stuey 

In short, 8.0:1 DCR (dynamic compression ratio) is much easier to warranty than 8½:1 DCR.  8½:1 DCR is my own top limit for premium fuel and aluminum heads and this is only good until the pistons and combustion chambers start getting carbon deposits.  Hence, 8.0:1 DCR gives a safety cushion which allows for the random load of 89 octane fuel without having immediate engine damage.  I’ll add that my own DCR targets are based on U.S.A. octane ratings and should not be confused with other octane rating methods that are used around the world.

Lorena, Texas (South of Waco)

Ted,



I am back after a year off for medical stuff and have just read the article on your website regarding the two engine builds, and aluminum/iron head compression ratios. When I put my 312 back together using Mummert aluminum heads last year I used the original flat top standard bore pistons. The engine was sleeved in all eight cylinder during a rebuild before I bought the Tbird and all measured out fine so I lightly honed the cylinders and re-ringed the pistons. Rings seated quickly with no problems. I also used Mummerts 270 cam and the Best head gaskets provided by John. I "assumed" the compression ratio would be about 9.5/1 and using the 91 octane premium fuel here in California would be ok. I am running 3 94's with an Edelbrock 573, MSD/Petronix stuff with the 57 distributor, Sanderson headers with the Fordomatic. Probably have only 400 miles on the engine so far. Runs strong. Am I at risk for detonation damage? If so, can ignition timing be adjusted to help this or do I need to tear into it and replace the flat tops with dished pistons. Thanks.

57 Black Tbird 312/auto

Hi Mark

i'm mulling over a similar worry hence the question.  i have not fired her up yet due to financial restraints but soon.

i originally had a scr of 13:1 but after skimming the pistons/grinding the combustion chambers i'm down to 10.75:1.

Mr Mummerts aly heads are generally reported to measure at 60cc but mine came in between 56.0  - 57.3.

so i gouged them out to 57.3. i'm also using the 270 cam which according to Mr Mummerts "cam card" has IC at 64* ATDC

but even after retarding cam 2* my IC at 55* giving a DCR of 9.1:1 . i'm pretty well convinced our petrol is the same as yours just using diferent RON/PON numbers.  as i'm not a racer and will probably not use WOT (much) i'm hoping to get away with the pinking issue.

i stand to be shot down on all this but thats how i learn. i'm not an experienced petrol head i mostly ride a bicycle, to and from the pub!

thanks every body

stuey

UK

Stuey.  Retarding the camshaft should have the intake valve closing at a later timing event which is expected to lower the DCR.  Your 55° number leans towards the camshaft being advanced instead of retarded which will increase the DCR.  Look for the intake closing event being ABDC, not ATDC.

Lorena, Texas (South of Waco)

Mark.  DCR is just another tool to help in building an engine combination but it’s not absolutely definitive or the last word.  It does however do a better job than the SCR in predicting what a particular engine combination will do on pump gasoline.  Here are the calculations I get for your combination.  I used 110° for the camshaft lobe centerline angle and installed 2° advanced or at 108° intake lobe centerline.  Zero deck was also used in the calculation.

 

 

As long as good premium fuel is used, then you should be able to optimize the ignition timing assuming the carbs are jetted correctly.  It’s when you get that mislabeled or bad load of fuel that you may notice some clatter at which point backing up the timing should clear that up.  If you hear some clatter, then make that timing adjustment.  Just keep in mind that any backed up or retarded timing will affect overall performance and drivability.  Also play around with different brands of fuel if you get some clatter at different times.  A 160° thermostat versus a 180° one will also make a difference if you're on the edge. 

Lorena, Texas (South of Waco)

Ted,



Thanks for taking the time to sort this out for me. I buy premium fuel from a "name brand" busy gas station. I have a 160 T-stat buried somewhere in the shelves which I will swap. The temp gauge never goes over 190 most of the time unless it is in the 90's and I am stuck in stop and go traffic for an extended time. In those rare times I pull the hood release to let some air circulate in the Bird engine compartment. But I feel safer changing it. I am not using vacuum advance now and have at last check about 32 degrees total with something like 14 or 15 initial I think. That was a year ago before I had to take a break. I need to recheck it and see. My original tach drive dist is in need of a rebuild, breaker plate has some corrosion, etc. So I may try an MSD dist. for accuracy and shelve the OEM. The clatter you speak of is on heavy load, is that correct? I notice a slight "ping" occasionally under moderate load but not consistent or excessive. Since I don't use the Tbird for daily use, I may use some sort of additive (race fuel, toluene) to play it safe.

57 Black Tbird 312/auto

Hi Ted

thanks for reply it is much appreciated.   my mistake re ATDC should be ABDC

my big problem is in understanding cam specs.my cam is one of Johns 270 cams.

John sent the following cam card info

                                                  LIFT      DUR.        OPEN             CLOSE        AREA

IL-- lobe centerline 105.00 ATDC     0.018    261.51    28.50 BTDC     53.01 ABDC    22.74

after retarding 2*     107                                         26                 55  

the following was lifted from Johns website

Y-270-S   272°   224°@.050"     108° Lobe Separation     Valve lift  .440"        Valve Lash .018"

IO   28   BTDC                  IC   64   ABDC                  SPRING PRESSURE 95# SEAT   230# OPEN

EO  64   BBDC                  EC  28  ATDC . 

     note both sets  of info open at 28* but closure times are 9* adrift.  am i not understanding the effects of lash which is .018????

if the closure time is 64* i'm happy if it is 55* i must do further work.???

comments please

thanks

stuey

sorry about print size don't understand that either 

 

 

 

i think what i don't understand is in the above......the figure given in the cam spec is IL closes 53* ABDC at 18 thou lift.

that would be true at the valve given a 1:1 ratio rocker but i'm using Dove 1.6:1 ratio so that gives 28.8 thou at the valve minus 18 thou lash valve is open 10.8 thou. is that where my missing 11* degrees are hiding(the 64* quoted on Johns web site)   if so that then begs the question does the IL open at 39* BTDC???

please beer with me i'm more than a bit slow

stu

Stu, The .018" is measured at the valve. The numbers for lift and duration will be given for that clearance. The point at which the valve closes or opens relects the position of the cam in relation to the crank shaft. Don't get your self confused with the clearance issue. (Easy to do :w00t Also, and this is important, Make absolutely positively sure you have the degree wheel indexed correctly. If that's off the numbers you'll see are garbage. Clear as mud?

  by the way, how about a picture of the truck.........and maybe the PUB!!                               

Frank/Rebop

Bristol, In ( by Elkhart)