compression ratio

[Rsmach1 71]

Maybe it's just me, I don't know, but some machine shops get me frustrated. They can operate the equipment but cannot do basic math to figure out what their doing. I think next time I would leek for a different machine shop (my 2 cents).

Nope not just you, you average machine shops don't know squat about HP engine building. I've been to a few that when I mentmentioned dynamic compression ratio I get the deer in the headlight look. Always better to find a performance based shop.

[det0326 179]

The local guy that did mine is one that the local round track drivers use so he is mostly chevy oriented but the basics should be the same tho. 

So what do you guys think my problem is? Do I have maybe earlier 351w pistons for the 9.480 deck height in a later year 351w with 9.5 deck height?

 

thanks Dave

[1969_Mach1 333]

The casting numbers on the block near the starter area will help with the year.  C9... is a 1969, D0... is a 1970, E series are 1980's and F series are 1990's. 1969 and 1970 blocks had a 9.480" deck height and 1971 and newer had a 9.503" deck height.  As far as your pistons, you'll have to look the application for them.  There may be a piston with a taller compression height.  That area gets a little confusing for 351W motors and most machine shops are not aware of the differences.  I learned the hard way on my first 351w.  Since then I don't select pistons until I know what the final deck height after its machined.

 

From the factory, every 351W had pistons below the deck.  But ideally you want the quench height that RSmach1 mentioned to minimize detonation.

 

I don't know why, but from my experience most shops that primarily build Chevrolet motors really struggle to get a Ford correct.  It makes no sense to me, but it seems to be that way.

[Rsmach1 71]

Based on the compression height of your pistons no, although the later 351w blocks are supposed to be 9.503 they do very somewhat, combine that along with minor shortening of rods when resized. I know this is a pain in arse, but I truly believe decking is going to be your best option. With the fixes I listed you could even go a little thinner on the head gasket and be fine compression wise, I just like to be on the conservative side.

[barnett468 418]

Yes barnett I have read that a lot of people recommend the cometic  I may need to check there as well. So at TDC I have .015 to top of piston (which are flat tops BTW) I would need a .020 head gasket thicknesst and not over a .030 head gasket thickness after compressing/torquing it down.Would this not make my compression ration too high.  My specs are as following

 

 

bore   4.040

stroke  3.5

piston vol.  12.00cc

piston down from deck  .015

 

I put the head on the block without a gasket and clay checked piston to valve clearance with a solid lifter and found that i had about .120. 

 

Thanks  Dave

 

 

ok, i just looked at you .025 below the deck and your 4 valve relief pistons . . not good at all.

 

what is your gearing?

 

call cometic and ask if they can make a .018 to .020 thick gasket for aluminum afr heads and let us know.

[barnett468 418]

i just talked to the engine builder a few minutes ago and i am quoting him here not that I believe him but he said I don't have a problem that .025 down the bore is typical for a street engine and that if I decked the block I would raise the compression ratio too high to run pump gas. Myself I don't understand where he gets this because what ever you take off the deck you simply add that amount to the gasket thickness. what you would be doing is improving the quench area if I understand this correctly, and I think I do.

 

Thanks Dave  

 

 

you are really trading one thig for another doing that and will have 9.76 compression which you could live with even with the huge quench providing you have at least 3.25 gears . . i would use rs machs calclations.

 

also, that cam needs at least 3.43 gears to not be a pig off the line.

 

depending on what your goal is, you might be better off with trick flow 11r 190 heads and mill them down or use the 58 cc afr 185 heads.

 

with your 58 cc heads and a .018 head gasket you get .043 quench and 10.3 compression which is still ok if you have at least 3.25 gears

 

with the 11r heads at 62 cc's you could use a .012 gasket with a 4.04 bore and hace .037 quench and 10.0 compression but i dont think you'll find a gasket that thin for aluminum heads.

 

if you keep your afr heads, you can enlarge the compustion chambers a bit if you need.

 

im pretty sure cometic can make an MLS gasket one that is around .018".

 

(800-752-9850 or 440-354-0777)

 

 

 

 

.

[det0326 179]

Thanks for the replies guys

 

the gear is 373 and I will check to see if I can get that .018 gasket.

 

This really sucks I am going to hate tearing down the rotating assembly to have it decked but If I have to I guess it could be worse, it could have being completely built.

 

 

Dave

[barnett468 418]

.

 

what trans do you have?

 

you do not have to tear it down to get it decked . . please read my post again.

[det0326 179]

hey barnett   I have an AOD that is going in it with the 4r70w wide gear ratio gear set

 

first gear  2.84-1       second gear  1.55-1      third gear   1-1      fourth gear   0.7-1

 

 

thanks Dave

[barnett468 418]

hey barnett   I have an AOD that is going in it with the 4r70w wide gear ratio gear set

 

first gear  2.84-1       second gear  1.55-1      third gear   1-1      fourth gear   0.7-1

 

 

thanks Dave

 

ok, that thing won't detonate if you just slap it together but i would still see abou a .018 gasket.

 

you nee a stall converter also.

[1969_Mach1 333]

Thanks for the replies guys

 

the gear is 373 and I will check to see if I can get that .018 gasket.

 

This really sucks I am going to hate tearing down the rotating assembly to have it decked but If I have to I guess it could be worse, it could have being completely built.

 

 

Dave

 

If the engine is still on a stand, tearing down a rotating assembly is simple, maybe an hour worth of work.  Just saying, if you cannot find a suitable head gasket I wouldn't settle for okay to avoid a little labor.  In the long run you'll be happier with it knowing it's done correctly.

 

If you do get it decked, find a shop that will square the block as well.  The cost should be the same or close to it.  A shop that discourages squaring the block means they simply don't have the fixture to set it up in their block resurfacer. 

 

If you talk to Cometic I would also as what type of surface finish is needed for their head gaskets.  They are good gaskets but not inexpensive, about $100 each.  So make certain they will work with your block and cyl head finish before purchasing.

[barnett468 418]

.

it also wouldn't hurt to check your block for flatness in the center from end to end and then kitty corner.

[det0326 179]

called Cometic and spoke with Sam a tech there and he said the thinnest MLS for aluminum heads is .027.  Thoughts? This will get closer but not perfect.

 

barnett you mentioned a stall converter, the cam specs states a 2500 stall would that be sufficient?

 

Dave 

[Rsmach1 71]

With the .027 gasket that will give you a 10.07:1 static compression, and a .052 squish. You will be pushing the upper limit of the squish (or quench) benefit. It will probably work if you limit how much timing throw at it and don't lug the engine. Note that I calculated this based on a 4.100 gasket bore. If Lunati recommend a 2500 stall that should be sufficient.

 

Randy

[1969_Mach1 333]

Did Cometic mention if the deck and head surfaces need a particular finish for their gaskets?  I know most MLS head gaskets require a different surface finish than composite head gaskets use.  And MLS gaskets don't tolerate surface pitting and imperfections as well either.  I'm not saying MLS gaskets are bad, because they are not.  I wanted to use MLS gaskets and the machine shop mentioned my block or cylinder heads were not surfaced with the correct finish for an MLS gasket.  Was your block deck surfaced at all to at least clean it up?

 

I guess what I am saying is make certain they will work, and not just the thickness alone, before buying them.

[Rsmach1 71]

+1 with 69_Mach1

[moodster 55]

Cometic head gaskets require a smoother surface then the old gaskets.  Newer cars use this style gasket and if my memory is correct I think you can progress to a wet sand at around 400 grit and that'll work fine.  However you can find the specs for it somewhere online... I've tracked them down before.

 

Also if you're going to through the trouble of squaring the block then you should balance the combustion chambers in the heads.  Otherwise your efforts can become mute if the chambers have different volumes.

 

david

[det0326 179]

Did Cometic mention if the deck and head surfaces need a particular finish for their gaskets?  I know most MLS head gaskets require a different surface finish than composite head gaskets use.  And MLS gaskets don't tolerate surface pitting and imperfections as well either.  I'm not saying MLS gaskets are bad, because they are not.  I wanted to use MLS gaskets and the machine shop mentioned my block or cylinder heads were not surfaced with the correct finish for an MLS gasket.  Was your block deck surfaced at all to at least clean it up?

 

I guess what I am saying is make certain they will work, and not just the thickness alone, before buying them.

No, deck hasn't been touched, I mentioned this to the builder when the rest of the machine work was done and he said that it was not necessary, I am a maintenance machine by trade and should have insisted that he do but I let him talk be out of it so this is my fault. However I did put a straight edge on it checked with feeler gauge and everything seems very flat with no pitting. I called cometic again and talked to Sam and he said that as long as it is flat and your finger nail didn't catch on anything you would be good. LOL I like the finger nail thing.  Wish I had the equipment to

 have done the machine work myself and wouldn't have all these problems.

 

Dave

[Rsmach1 71]

Also if you're going to through the trouble of squaring the block then you should balance the combustion chambers in the heads. Otherwise your efforts can become mute if the chambers have different volumes.

 

david

OP is running AFR heads, being they are all cnc machined, combustion chamber volumes should not be an issue.

[barnett468 418]

Yes, as mentioned that stall is fine.

 

+ 1 with Rsmach . . Yes you can run the .027" thick gasket and you will NOT have detonation so you can stop worrying now . . Just curve your distributor timing so it is optimal for your app as Rsmach suggested, run a 180 thermostat, use autolite racing 24 plugs and jet it just a hair on the rich side . .  also, a multi fire ignition box like an msd can reduce deto plus increase power.

 

Although your engine is not ideal, there is absolutely no reason to redo it . . also, the smaller bore of the cometic gasket wont increase compression much, besides you can get them in .080" and .100" over if you want, however, getting them the same size as your current bore is the best because it reduces the area outside of the bore that the gasses can get to.

 

Also, the higher your elevation, the lower your compression will be . . you loose around 4.7 - 5.0 psi for every 1000 foot increase.

 

Since you already checked the block surface, you are ready to go . . the cometics actually are "reasonably" forgiving for surface finish.

[Rsmach1 71]

Looks like the closest cometic is 4.060, with that you'll have 10.08 static and 8.32 dynamic, which is perfectly fine. As Barnett468 mentioned, run it a tad fat (rich) and go with a multi spark ignition. If you ever do another small block ford now you'll know what to look for and insist on. When you finish the car you need to post before and after pics at the same time, I can't wait to see it done and running :)

 

Randy

[det0326 179]

Thanks guys the gaskets are ordered. Should be here next week hopefully. 

 

I really appreciate all the help so nice to be a member of this forum.

 

Randy I can't wait either and that day is getting closer. It seems like every time I start on a new area of this car I think this is going to be a piece of cake then it turns into a nightmare.  Just like the gaskets I have to wait on now, if I could have used thicker gaskets I could have this engine put together tomorrow. I guess thats life tho its all about hurry up and wait.

 

Thanks again all

Dave

[barnett468 418]

.

You need p/n C5910-027 for the AFR heads

 

http://www.summitracing.com/parts/CGT-C5910-027

 

.027" thick . . 4.080 bore

 

Have you drilled your block for the coolant holes?

[det0326 179]

.

You need p/n C5910-027 for the AFR heads

 

http://www.summitracing.com/parts/CGT-C5910-027

 

.027" thick . . 4.080 bore

 

Have you drilled your block for the coolant holes?

Thanks barnett for the p/n

No have not drilled any holes in the block. where do I have to drill and what size. Does a 1991 block require drilling?

 

Dave

[barnett468 418]

Thanks barnett for the p/n

No have not drilled any holes in the block. where do I have to drill and what size. Does a 1991 block require drilling?

 

Dave

 

I would cal AFR and ask them.