Raising compression

[zcarcrazy]

Has anyone tried (or knows someone who has) to raise the compression in a fuel injected engine. I have an F-54 block that I had thoughts of putting an N-47 head on. I'm going to completely rebuild the engine, so a new cam and that sort of equipment isn't out of the question. I also have a set of SU's, but everyone that I've talked to says to stay with injection if that's what I have. I think that the carbs are the flat top, but I can't remember right now. I don't want this engine for drag racing, but I may do some autocrosses or other track events as well as street driving. Any thoughts? Thanks.

[mike240]

i would assume that raising the compression on an f.i. engine would be very similar to a carb engine. there is a calculator(can't remember where i got it but i can send it to you if you like) that computes compression for any given head and such. it's very handy. i wish it had a calc to compute compression for shaving a certain amount off a given head because i'm trying to find out that myself, as i am trying to find out how much of an increase in hp you can get for raising compression a certain amount.

as far as carbs, if nothing is wrong with your f.i., i'd say leave it be but if you were gunna retrofit it with carbs, be sure to get the round top carbs vice the flat tops that you think you might have. hope that helps somewhat.

[zcarcrazy]

Thanks, Mike. That pretty much confirms what I was thinking. I've seen the address for that calculator in other threads. Maybe I can find it and check it out. I'm just guessing that the compression will be just under 10:1. At least I hope it will. That seems to be the number that most of the other posts are dealing with/around.:ninja:

[2ManyZs]

Here's the thread with the engine calculator

http://www.classiczcars.com/forums/showthread.php?s=&threadid=1292

[zcarcrazy]

Thanks so much for the link! It worked out to be almost exactly what I expected (9.8:1 +/-). I think that will be perfect for what I have planned. I'll definitely have more questions coming up. Thanks for all the help.

[mike240]

hey 2manyZs,

on that calculator, it shows the difference in CCs and compression between the HKS 1mm(FELPRO is the same) and 2mm head gaskets...now, would the difference in compression and volume apply the exact same if you were to shave the head 1mm? for example, compression with an E31(head i'm going to use) and a 2mm gasket(11.3 cc volume) on an L24 is 8.49:1, the same with a 1mm or felpro gasket(5.7cc volume) is 9.37:1. the difference in compression is .88. now wouldn't it stand to reason that given the E31 head and a felpro gasket, if you shaved 1mm(which i believe is about .050 in) off the head, it would respectively raise the compression .88(from the amount we calculated earlier) from 9.37:1 to 10.25. does this sound right? i'm trying to see what i need to do to get the highest compression i can whilst still using pump gas(and occasionally race gas for autocrossing). i want to get as much info as i can before i yank the E88 off and get the E31 and start tweaking it so i don't screw stuff up. let me know. thanks.

[2ManyZs]

Good question.... hmm, got me thinking on that one.:stupid:

It should be fairly close but remember, when using a different thickness gasket you are only changing the combustion chamber volume. If you mill the head, you will most likely change the quench area and shape of the combustion chamber along with the volume of the combustion chamber.

Of course, piston selection and overbore also enter into the the final numbers as well as piston deck height.

BTW, milling the head might be a cheaper way to go if you are going to have machine work done to the head, as those gaskets are over 100 bucks a piece last time I looked....

Also, don't forget to shim the cam towers when milling the head or you'll end up changing the cam timing.

Might be a good idea to ask these questions of a better engine person than I when you decide on exactly which parts you are going to use. I'm just a "shade-tree" engine man at best.....

[mike240]

i plan on getting the intake and head ported and polished along with a mild Crane cam(new chain and sprockets too) and rebuilding the SUs with the better flowing needles(they have a 'one letter and two digit' designation) but i can't remember it. and as far as the head gasket goes, i'm just going to get a felpro. i've heard nothing but good about felpro. they are less than $50 at advance auto. i'm not going to do any bottom end work. i just wanted to tweak it a little bit to get it going/running and then i'll start saving for the sbc conversion.

and funny you should mention the cam tower shims as i had a thick convo with a guy over on zcar. i was frustrated because i still don't see why someone can't just increase the tension in the chain vice shimming the towers. i mean, look at the way the rear derailleur works on a mountain bike. it keeps the tension in the chain and can be adjusted for more tension if you need it. why can't that work for head shaving? surely a measurement as small as .050 in could be compensated for by means other than shims. damn. but thanks for the info you did give me. i appreciate it.

[2ManyZs]

The bike reference brings up a good point, but you have to take into consideration that the centers of the drive gears doesn't change like it does when you shave the head. Now, the deraileur does keep the same tension on the chain, but the chain only goes from one sprocket to a different sized one, yet the distance between the center of the gear cluster on the rear wheel doesn't change in relation to the center of the drive gear.

When you mill the head, the distance between the centers of the drive gears will change, thus the gearing which is set for only one ratio (center to center)will not change no matter what you do to the chain via the tensioner. If you could change the diameter of one of the drive gears the point would be moot, as the chain effectively gets longer the more the tensioner has to move to restore the correct tension.

I doubt that explains it the way it should be in technical terms.. but if you think about it, it might make a little sense......or maybe not:ermm:

[Zedrally]

Aren't shims used only when the top of the head is milled?

When I had my last head job, the head job shop not only milled the bottom, but also the top; stop and think about this, if the head is warped it's not only the block to head surface but also the top of the head, ie cam cover to head.

In this case you have to add shims to the cam tower to maintain the geometry of the cam.

On the other hand if the head is straight and you're only shaving for compression then no shims would be needed.

[2ManyZs]

:stupid:

Uh, I was under the assumption that the top was being shaved as well, that is why he was asking about the shims being used.

The shims could be used either way if I'm thinking correctly, which I might not be.....I'll have to think about that idea some more, lest I stick my foot in my mouth....

When you mill the top of the head you have to shim the towers to maintain the correct cam to rocker geometry so that the rockers stay in contact with the center of the valve. The pivot of the rocker will move the tip around on the top of the valve relative to the length of the rocker adjuster. That would be a simple matter of geometry.

When the bottom of the head is milled for straightening or compression change, you change the distance between the centers of the driven gears, and the more the tensioner has to move to correct the slack in the chain, the more retarded the cam becomes as the chain has effectively gotten longer. Then it is a simple matter of advancing the cam sprocket to correct the cam timing. Of course, this also applies of the top of the block has been milled as well.

As a side note, no more than .008in or .2mm combined is the general rule of thumb for milling the head and/or block.

[Zedrally]

Originally posted by 2ManyZs

:stupid:

Uh, I was under the assumption that the top was being shaved as well, that is why he was asking about the shims being used.

I guess that would depend on whether the head is "straight", then again it could be standard shop practice to do both top & bottom whether you need it or not.

Originally posted by 2ManyZs

:stupid:

When you mill the top of the head you have to shim the towers to maintain the correct cam to rocker geometry so that the rockers stay in contact with the center of the valve. The pivot of the rocker will move the tip around on the top of the valve relative to the length of the rocker adjuster. That would be a simple matter of geometry.

The other aspect is that the distance between the cam and lash pad alters and this is the true reason for shimming, else valve events do not happen as they should, ie without shims the cam opens and closes earlier when factory valve clearences are used. Much easier to return the towers to the factory height than trying to work out new clearences