Head skimming compression question

jason · 11-07-06, 07:47 AM

I have a desire for a higher compressioon ratio to make my cam (edelbrock performer rpm 7122) work a bit better. i only have standard flat top's giving me something like 8.8. - 9.0 :1 compression and i know i need around 10.5 :1to one to make them work well. The engine has lost a lot of torque since fitting this cam.

i have standard World products windsor Jr heads (cast iron)that are 58 CC in chamber volume. Can people advise me :

1, Is the head skimming idea a good move in terms of improving driveability with this cam?

2, How much should i have removed, also bearing in mind the inlet manifold will need skimming to match it.

Any ideas ?

Alex Hirsbrunner · 11-07-06, 08:20 AM

Hi Jason,

One thing I would do first is verify how much piston to valve clearance you have with the current cam. This will effectively limit how much you can increase your CR through head milling without causing other more significant problems.

Al

speed220mph · 11-07-06, 09:25 AM

Jason: Skimming the head, or reducing clearance volume to increase compression is always good for power providing you have high enough octane fuel to handle it. A 10.5: is OK with a high-overlap cam and premium pump gas. A 10.3: works fine in my FE 427 with tunnel-port heads.

As Al says, you've got to make sure valve-to-piston clearance is sufficient. You'll also have to check combustion-chamber clearance volume and determine the cc's you'll need to arrive at the desired compression ratio. Now's my chance to hype my new book: I cover all of these checks in agonizing detail in my just published book, "The Racing Engine Builder's Handbook".

Paul Thompson 'Hooligan' · 11-07-06, 09:54 AM

Quote:

Originally Posted by speed220mph

Now's my chance to hype my new book: I cover all of these checks in agonizing detail in my just published book, "The Racing Engine Builder's Handbook".

Available here Florida Boating Accessories and Gifts - Books : Racing Engine Builder's HandbookHP1492: How to Build Winning Drag, Circle Track, Marine and Road RacingEngines

and mentioned to my wife for next month (hint hint

)

Alex Hirsbrunner · 11-07-06, 10:18 AM

I'm in. Just ordered it from Amazon - $13.57

Jac Mac · 11-07-06, 07:29 PM

Jason,

When raising compression you should first optimise your piston to head clearance @ TDC. In most case's this will give you a healthy boost in C.R. To do this you can use any of the following; longer rods, pistons with higher pin height, thinner head gasket, or machine block deck surface. By doing this first you will reduce the risk of detonation compared with what it would be if you simply machine the cylinder heads.

At the same time you must ensure that you maintain adequate valve to piston clearance.

You do not say which particular model/size engine you have but if it is a SBF 302 then you could fit 5.155" rods as per 289/ Boss 302 which are 0.065 longer than stock(5.090"), then machine the pistons to obtain correct deck clearance. This would probably save you from having to machine deck or cyl heads etc with any alignment issues that may arise.

Jac Mac

Lynn · 11-07-06, 08:42 PM

The small pic below is Tom's most recent book (the scamp didn't tell us that he's a multiple published author ;-) The bigger pic is a previous cover. Being a bit of a frustrated writer (minored in tech writing), color me impressed, Tom! - Lynn

Scott Calabro · 11-07-06, 09:45 PM

Jason,
Do you have forged pistons and easy availability of 93 octane gas ?
If you do, try advancing your initial & total ignition timing untill the engine just starts to "ping" under load. Then retard it just until it stops. You may want to try to enrich you F/A mixture slightly, as well as slightly colder plugs. Try this before you spend money on further engine architecture mods.
Regards,
Scott

Howard Jones · 11-07-06, 09:55 PM

I asked this question recently to the machine shop that assisted in my ballance job. The block prep guy said he would check the decks make sure they were square and equal dimensionally but didn't like to remove material unnecessarly from the newer Ford blocks because "they are a very thin casting now and don't have much to work with". Anyone else have any info on this? How much is too much?

speed220mph · 11-08-06, 12:03 PM

Lynn: I knew this would be a problem. There would be confusion between my two books, Engine Builder's Handbook and The Racing Engine Builder's Handbook. They are different. The first one is for doing a standard rebuild with some information for the novice on high-performance mods. The second is for the racer, what with a lot of machining, major modifications, info on racing technology such as materials, coatings and in-depth stuff only a race-engine builder needs to know.

jason · 11-09-06, 03:32 AM

Thanks for the feedback. I really want to avoid stripping the engine at this stage and was hoping to get some quick gains without internal engine surgery. I am using standard Cast pistons, and we can commonly use 97 octane gas here no problem. I know that decking the block would help optimise the CR, however as i said i really want to avoid stripping the engine , if i could gain 1 - 1.5 compression point by simple machining that would probabley make all the difference between the cam working for me or against me. I can probably do the head work without removing the engine from the Car.
Im not too worried by Valve clash as the cam only has a lift of .498". Im guessing that to get the desired C/R im looking at a reduction in cc from 58 to 54CC, question is, would that give me 1 more point compression? will, it make the cam more effective (Edelbrock state that the cam needs 9.5:1 to work properley). And also what alignement issues do i need to be aware of?

thanks guys!