Zero decking the 250

[bookworm007]

I have read up on the other posts about this and know that normally you either use the 255 pistons, deck the block, or use jeep pistons and bore the block way out.
First let me fill in some background on my planned engine build. I'm building a 1970 250 that will have an aftermarket cam, DUI ignition, header, crossflow head ect. I already have the head and I am aware of the work needed to make this conversion and that it is not the cheapest easiest choice. My biggest problem is that my crossflow head has small combustion chambers like 42cc small and even if I opened it up to 52cc zero decking by the conventional means would result in way too high of compression.

I did some number crunching and found 3.72 dia piston (it comes in more dia to suit a .02, .04, and .06 over bore) with a 1.310 compression height. According to my math if you used one of these pistons on a 300 rod it would zero deck the block with no machining. The piston also has a pin size of .905 so it could be honed out and used with early 300 rods and pins or bushed and used with the newer rods.

Now the reason I am interested in them. On top of improving the rod ratio of the 250 these pistons have an ample 12.37 dish allowing more economical compression ratio. With my head it just allows me to stay on pump gas without boring out my block but with larger head you could keep the compression ratio down in the 9's with the right head gasket.

Here is the series of pistons i am looking at:
http://www.summitracing.com/search/Depa ... Rank%7cAsc

And here are the 3.72 dia ones I am specifically looking at for my build:
http://www.summitracing.com/parts/UEM-1743H-5MM/

I could only get a 10.7:1 with a 52cc head, .050 compressed head gasket and assuming the pistons sticks out .010 after you do minor machining to true the mating surfaces. If you had a 62cc head and made the same assumptions you could have a 9.5:1 compression ratio.

I posted this info on an aussie thread I was using to get specifics on using the crossflow head but I wanted a few more opinions who have messed around with trying to zero deck these blocks.

 

[CNC-Dude]

In the past few years, many engine builders have come to realize that an engines DCR is more critical when it comes to pump gas than the SCR we have used for years to define an engines compression ratio. While there are still boundaries you have to observe, the DCR is what the engine actually sees when you put all the pieces of the puzzle together and either causes the engine to knock, rattle or ping on the gas you are trying to run or run like a top. I have seen some some guidelines for the maximum DCR numbers you can get away with when trying to run different grades of fuels. For pump gas, it seems consistent that many builders have found the optimum DCR to be between 8:1 to 8.2:1, a DCR of 8.5:1 is doable but may require premium grade pump gas. The key to calculating the DCR is the IC(intake closing point) on your cam, basically you can run a higher static compression ratio(SCR) if you run a bigger camshaft(later IC point). Cam selection is a definate must when you are pushing the envelop and trying to make an engine build compatab;e with the gas we have today. In some cases you might find yourself having to settle for a camshaft you might not have normally chosen to obtain the DCR, but at this point, its all about compromise. Also an advantage to using aluminum heads is you can generally run a full point of compression(SCR) higher than a cast iron head because they bleed off heat better and dont hold heat as much in the chamber making them less likely to cause detonation with the same or higher compression. So, in a nut shell, build your engine around the DCR, not the SCR and you'll be fine. Yes, you can have a SCR of 10.5:1 while still having a DCR of 8-8.2:1 with the right cam selection.

 

[170-3tree]

I thought the hugest problem with cross flow was an extremely limited space to fit a distributor? Especially the DUI unit?

He is right about the dynamic compression. It is also very helpful that you can use offset pins to advance the cam which will raise the dcr a bit if needed. Also using a longer duration or wider lca will give more flexibility.

 

[CoupeBoy]

on a 250 the head is almost an inch higher than on a 200 so there is more wiggle room for the distributor.
-ron

 

[170-3tree]

Oh duh. Even with the massive DUI?

 

[bookworm007]

Yeah from what other members have told me the dui should fit, it might be snug but it will be up close to the intake manifold so i dont really have to worry about heat too much. Thank you for the tip on the dynamic compression ratio, with my 10.7:1 i should be able to run gas in the 90-92 octane range. Even better if i move up to a bigger cam.

Im planning on using crow cams 14892 cam the specs can be found on this page:
http://www.crowcams.com.au/media/catalo ... XF_all.pdf

I have also considered using the 14650 it would let me run regular most likely but it would need extra port work on the head which is tricky because if you do it wrong you actually hurt the performance on these heads.

Anymore input on the zero decking solution? Does it sounds doable? Would you use the earlier rods and open the small ends up a bit or use the later once and have them bushed?

 

[CNC-Dude]

I think the earlier rods with the small wrist pin has a spit hole in the rod beam, I would not use them for any sort of performance application. Also, I wouldn't do any machine work to the block until you get the mods done to allow the head to fit, there is always the possibilty you end up trashing the block and not be able to use it, so no sense in losing money in machine work until you can be assured that part of the operation is going to be a success.

 

[bookworm007]

True, that is good info on the rods. So bushing down the later ones sounds like the best plan, plus they will probably be easier to get a hold of. I am currently trying to get a hold of a set of gaskets for the head, but its been difficult so far. It seems that none of the auto parts stores in Australia have online stores, and there is only one ACL set on the Australian ebay that doesnt list the crush distance. But once i have those pieces I can start tearing into the block and getting it ready.

 

[gb500]

contact one of the posters on here - xrglen .
he is in wa (ie western australia) and can post you whatever you need.

email me on mhbs@people.net.au if you have any questions..

failing that i can get you some head gasket kits from thr local motor factors.

ill check out the prices today.: EDIt i see the set ACL GH100GTR is $138 inc post on ebay.com.au . if need be buy them and post to me and i can forward on .


i would assume that the acl crush dist is going to be in ordr of 50 thou. EDIT : but there is a tech email adress fro acl - email them and ask for crush distance

 

[bookworm007]

Alright i will try emailing ACL for more info. I figure i should get one full set of top end gaskets to start with, and then a few extra head gaskets as maintainence items. I tried that email and it says its not longer in service, I also noticed that rocklord put a post onto the hotsix forum trying to get in contact with xrglen with no response too. So if anyone has a current email for him that would be very helpful

I have never shipped anything from australia. What is the cheapest service for getting that type of item over here? Also there is a crossflow redline intake and headers that are very tempting but i know i would probably want to look into sea frieght for something like like or pay through the nose.

 

[chad]

What is the cheapest service for getting that type of item over here?

the mass buy approach. if there's some others who need the same stuff the group can share the costs lowering the individual's. On the other hand, if it small the fella who offered can ship cheeper than the co. will?

 

[MustangSix]

Don't forget to carefully check piston to valve clearance. With that piston/rod combo you are a lot closer than before and you may need reliefs.

 

[gb500]

as a guess - using this australia post calculator:

http://auspost.com.au/apps/international-parcel.html

approx $40 seafreight + some $9 securiy charge ( think this is applied by the usa only).. 6 weeks ?.
approx $57 airfreifgt + $9 ...2 weeks ?

based on a 2kg parcel (4.4lbs).
I measured a gasket set (once wrapped) i have at approx 85cm x 24 cm x 3cm thick ( you guys do the conversion to inches!) but it seems the calculator doesnt take/need the parcel dims into account

 

[bookworm007]

excelent that is very reasonable for shipping something half way around the world

If some one is willing to handle my shipping that would be excellent. And if you ever want a log head i will get it over there pronto If you are interested please PM me. Once again thanks for your help!

As for valve clearance hopefully it wont be an issue. If i really can get a .050 crush head gasket then there will be .040 clearance between the top of the piston and the mating surface of the head. And of course i will use the frozen tootsie roll trick to check clearance before i button the engine up.

 

[CNC-Dude]

Also keep in mind that you are now going to have to select a camshaft that will make your 10.7:1 SCR fall into the 8-8.2:1 DCR range, and neither of the 2 camshafts you have selected may do that. You are going to have to get more specific cam specs from the manufacturer before you can even begin to select a cam. The IC(intake closing)number is going to dictate what cam will have to be used to achieve the correct DCR ratio to use pump gas. While the SCR number is flexible, the 8.2:1 DCR number is an absolute, and you cannot exceed that number by any amount and expect to use pump gas, you will find that you are going to likely have to choose a cam you wouldn't have previously selected just to make the DCR compatible with the gas grade you will be running. As you can see from the target DCR numbers, you have a very narrow window to try and fall into to work at all with pump gas, and its going to be difficult since the SCR is so high to begin with and your cam choices might not be that great to start with to help you achieve that, and you may have to get a cam custom ground to get you there.

 

[bookworm007]

I thought i was using the proper info but maybe you can help me find my mistake.

I took the intake closing position from this form. The numbers i used are circled in red. I used the second ones because the dynamic ratio calculator needs the intake valves closing position.



Then i plugged the values into the compression ratio calculator. I used one that does both static and dynamic. I went with a more conservative 50cc head since i dont know how much I can take out of the one i have. Then i assumed the head gasket bore was 3.72 in. I figure if anything it would be bigger and help my compression ratio. Here are the values the calculator kick out for the 14892 cam and the 14650 cam.

 

[CNC-Dude]

Bookworm, I wasn't implying you had made a mistake, I was just meaning for you to be cautious and not assume either of the cams you were selecting would make your DCR what you needed. I never saw where you mentioned you had calculated the DCR using either one, so I didn't know you were as far along as you are with your findings. Also, make sure you post all your calculation findings as to the actual DCR with each cam, this can be very helpful to others that might find themselves in a similar situation with a little too much compression and a way to still complete their combo.

 

[bookworm007]

No worries i like to have my calculations checked out reguardless especially since i haven't played with dynamic compression ratios very much. And if anyone falls into a similar situation hopefully the thumbnails will help them since it seems finding dynamic compression takes a lot of variables.

 

[xctasy]

No worries i like to have my calculations checked out reguardless especially since i haven't played with dynamic compression ratios very much. And if anyone falls into a similar situation hopefully the thumbnails will help them since it seems finding dynamic compression takes a lot of variables.

Its a very easy calculation. John Dalton from a book in 1989 called Practical Gas Flow coverered it step wise.

Simple, easy, and a heck of a lot of fun. This dating back to 2003...

Enjoy it, brother

viewtopic.php?f=5&t=5518&p=38459&hilit=dynamic+compression#p38459

 

[xctasy]

No worries i like to have my calculations checked out reguardless especially since i haven't played with dynamic compression ratios very much. And if anyone falls into a similar situation hopefully the thumbnails will help them since it seems finding dynamic compression takes a lot of variables.

Its a very easy "plug and play" calculation, with many links to it on the internet.

http://cochise.uia.net/pkelley2/DynamicCR.html


-=Whittey=-

I found it way before the internet back in 1990 when doing Civil Engineering back at Otago Polytechnic. Two nice, simple sources were:-

John Dalton from a book in 1989 called Practical Gas Flow coverered it step wise
http://www.amazon.com/Practical-Gas-Flo ... b_title_bk

, but way before then, Vincent Black Shadow 196 mph LSR bike designer, the late Australian Phil Irving described it in Tuning For Speed.

See http://www.webbikeworld.com/books/tuning-for-speed.htm

Simple, easy, and a heck of a lot of fun. I used my Hewlit Packard Reverse Polish Notation 32C calculator to run through these numbers, and used Heatseakers Standard Ford Cross Flow and Non Cross flow 1990 cam shaft list of common cams to do a sensitivity check. You can verifiy that they are close to accurate

This summary post dating back to Hard Core Forum in 7 June 2003...

Effective compression is really just calculating the actual reduction in real, effective stroke of an engine. In the following case, its the 9:1 compression 170 HP gross M-code 250 2V engine made from 1971 to 1974 in XY,XA and XB Falcons using what I thought was standard 'at lash' 252 cam duration cam placed at recomended advance. In fact, the right cam for the AUssie 250 may have been Fords standard 256 degree cam as used on 2V 351 and stock 250 1-bbl engines on both sides of the Pacific, but here are the calcs to recheck...

This has an effective stroke of 3.264", and gives an effective compression of 7.654:1. This is a benchmark figure which shouldn't be gone under for longer duration cams. The static compression needs to be raised to ensure the on cam characteristics are maintained. If you go more than 20% below 7.654:1 for any subsequent cam combo on this engine, you risk unfavorable brake specific fuel consumption characteristcs, and performance and economy will go down hill..

Enjoy it, brother
viewtopic.php?f=5&t=5518#p38099

or high lighted viewtopic.php?f=5&t=5518&p=38459&hilit=dynamic+compression#p38459