......77 f250, 4x4, almost new engine, Complete rebuild with about 4k miles. My CPR is around 10.2 or so which means that I must use 93 octane gas and I'm only getting about 8 mpg. I would like to drop the CPR down to around 9 or so. Can I accomplish this by adding an exta head gasket or gaskets??? The pistons were 9 to 1 when installed. I realize I'm going to lose some HP but I can live with less HP. ALSO, I used ARP head bolts when the mechanic built the engine. Can I REUSE these bolts if I decide to install thicker head gaskets????? Any recommendations on how thick they should BE???...thanks, s.kuteman

 

how did the pistons go from 9:1 when installed to around 10.2:1? i am not understanding. the double head gasket thing has surely been done before but hardly recommended. SCE makes thick copper gaskets, if you think that alone will solve the problem. your ARP hardware should be reuseable. how sure are you of your current comp ratio? there are on-line calculators to find out what it really is, but you must have extremely accurate measurements to plug in to get an accurate answer. a little fudging on 10 or so inputs leads to a WAG. Wild *** Guess. not even a SWAG, Scientific Wild *** Guess

 

http://raceabilene.com/kelly/hotrod/calculators.html

 

...........Well, basically I'm just repeating what I was told by the mechanic who assembled the engine. If, I don't fillup at least every other tank with 93 octane i get the clatter sound which I definitely don't care for. I think a set of those thick copper gaskets would probably help enough to allow me to run a 50\50 ratio of 87 oct. and 89 oct. If, I could do it over I'd build a 460 with a 8.5 or so and just run 87 octane all the time. Gas is just too expensive and this truck is not a race car. Do you think I could just install a set of those copper gaskets and be happy with the way it turns out???....I checked that CPR program and I don't have all the info they want to allow it to make an acurate calculation..........thanks , s.kuteman

 

i found some numbers that give 10.19:1 they are, from the top, 8, 4.03, 4, 80, .041, 4.1, .01, 0. this is a flat top piston without any valve reliefs and a .010 deck clearance. the .041 is a Fel-Pro blue head gasket. changing this to .92 drops the CR 1 full point to 9.2:1. .92 is the thickest SCE gasket they make, i think. this is all based on the 80cc head volume, you might have to polish your chambers to get them to 80cc, you might even be able to get more? i'm surprised the CR came down a full point. i didn't think it would be that much, but this is just a HASWAG. that's a Half Assed Scientific Wild *** Guess. maybe someone else can help out?

 

BTW, is you EGR working? if not, it would prolly help a bunch. also, the timing and curve of the distributor plays a major role. what cam is in the engine? if the CR really is around 10.2:1, and you have a short duration or RV type cam, the cylinder pressure will be on the high side for pump gas.

 

I would try backing the timing down a few degrees that will usually cure the detonation of sub93 gas. I would back it down about 4 degrees and see if you can run midgrade gas. Then if no detonation then try regular. Also a cooler thermostat will reduce the motor temp and help with detonation. Or you could just use the thicker head gaskets but that would be my last resort.

 

You can also retard your cam timing by 4 degrees, if it is set to 0 degrees. That will reduce your Dynamic Compression Ratio? What cam do you have? What heads do you have to get 10.2:1 CR? are they Aussie heads?

I think that you can get a lot of help from the people here, but you need to find out what has been done to get you where you are now?

 

..........Ok guys, I finally found my cam info. Crane hydraulic energizer, #133042, 272HH10. Cam timing....
@.004....intake......opens..27 BTDC Closes 65 ABDC
exhaust......opens..67 BBDC Closes 25 ATDC
........There are some more numbers but Maybe these will be enough to tell you'll what you need to know. The mechanic and I have pretty much gone our separate ways so I'm kinda on my OWN from here on out. I am presently finishing up a power steering conversion on this truck and have got the whole front clip removed. The engine runs like no body's business but with gas being high and getting Higher I can't afford to keep it running on premium. thanks for your help , s.kuteman

 

That is a good cam for a motor with 10.2:1 CR. It has 67 degrees ABDC IVC. The Dynamic Compression Ratio is about 8.0:1. I used a Crane H278-2 cam with my 351C-4V closed chamber heads (62.8cc). With flatop pistons and these heads the CR was around 10.46:1 CR. The IVC on that cam was 72 degrees, which brought my DCR down to 7.86:1. I used 93 Octane gas and got 8 MPG also.

If you retard your cam by 4 degrees you will have a DCR of 7.73:1, but that will still require 91/92/93 Octane fuel. If you retard your cam by 8 degrees, you can get the DCR down to 7.48:1. I am not sure how to do that, but it should be doable. This should be 89 Octane territory.

If you use a 0.092" head gasket, your intake manifold will not reach the ports on both heads, since they now sit further apart. You will need spacer plates or multiple intake gaskets to bridge the gap. However your DCR will drop to 7.26:1. You should be able to run on Regular or 89 Octane.

I would try to find out what was done to get up to 10.2:1 CR, and undo that part. It may mean obtaining a set of stock 400 heads, if the change was to the heads and not the block. You might be able to rebuild them using the valves and springs from your current heads.

My next question is WHY? If you use 40 gallons/week(maybe you use a lot more?) and premium cost $0.20/gal more than 87 Octane, you will save $6.08/week. That is $316.16/year. It may take more than a year to break even on the conversion.

 

danlee, thanks for the analysis. I, will probably, just leave the engine as it is now . It runs really good and strong and I had stainless valves and double springs and the crane fulcrum kit all installed on the heads. It was configured as a flatbed when I purchased it in july of 01 and is still setup that way. I want to ultimately find an origional bed and put it back the way it came from the factory, i.e. I want it to look like a pickup. I would never beable to afford to drive this truck as a daily driver with the current price of gas. I guess this much adoo about nothing.........thanks for your help, s.kuteman

 

S. Kuteman,

I think that the motor is built about right. The Crane H272-2 is similar to the Comp Cams 275DEH that I recommend to some who want a highway cruiser.

I am still curious about the CR. To be specific, what was done to get 10.2:1? Are they 351C-4V heads?

 

..........The heads were stock 400 heads that had the old valve seats removed and hardened valve seats installed. Stainless valves, fulcrum kit, double valve springs,etc. The heads may have been shaved alittle but I don't remember the mechanic telling me that explicitly. The CPR was an educated guess between the mechanic and the machine shop. The pistons were purchased from Dynagear in Dallas, Tx. They were 9 : 1, flattops, same as the Badgers. The block was bored 40 over because i wanted as much metal as possible left in the block. The machinist had a hard time finding a block that hadn't been bored atleast once..........Thats about all I can tell you....Again, thanks for your help , s.kuteman

 

Next time you mess with the engine, put in the dished pistons. Sounds like the head was shaved considerably since stock 400 heads give 9-1 CR with flat tops.

 

................Here is another option I have considered.......Since this engine is operating with a 10.++ cpr, How would it perform IF.....I converted it over to Propane???It, would seem to be a perfect candidate for that type of situation.....I have been quoted a figure of 1500 to have the tank and all the plumbing installed...........They say that you loose about 10% of your HP when you run on propane but that really wouldn't affect me . I, don't know if it would be worth the expense......s.kuteman