Compression ratio

This is something that is often talked about here, but you don't hear much of people doing much with it, in my book this is some of the cheapest horsepower you will ever get in even your street car. Most of the aftermarket pistons we buy to rebuild our engines with will gve us a compression ratio boost, you get a .6 increase in the later 18Vs alone with just the piston, but we're still talking about a pretty lowly 8.7 to 1, and on street gas we can do so much better. In a recent post I made, about decking MGB heads on street engines to get the compression ratio to more repectable number, I could tell I was treading into new ground for alot of the folks here, they seem to be quite troubled about the amounts I was talking about cutting a street head .030-.040".
We had a test day for Fred McConnell anbd his MGB vintage race car, who post here, a month or so ago, as he gets his car ready for it's first ever competition drivers school. Fred bought this MGB as many of you know from a guy in SC, it was basicly a street racer if you will, gutted interior, roll cage and such and the guy had built a very normal cammed up street engine for it. While Fred has done alot with this car in the way of turning it into a legal race car with safety equipment, suspension enhancements and such, the slightly warmed over street engine has stayed part of his later plans as he gets started into racing, using the warmed over street engine to get his feet wet. At the tech session with a leak down test we found that while the rings were doing thier job and sealing well, the valve themselves were a different story, we were getting leakage across the board of 20-30% percent, also Fred HS4 SU carbs were a mess. After the tech session I advised Fred at a minumum to let me do a good valve job and get the valves sealing, and rebuild the carbs which badly needed it to even operate normally. After I got Fred's head and got a good look at it, I was very impressed with the core, all the components looked to be in pretty good shape. The head was the correct one for his 1967 engine, 12H 1326, and it CCed right at 42CCs it's supposed to be stock, which only gave Fred at the most about 9.0 to 1 compression, so it made perfectly good sense to deck the head and give him a compression ratio boost. In many talks that David Anton and myself have had, he often recommends on a cammed up performance street MGB to deck the head .040" from stock. Ok that seemed to shock a few folks here when I first posted so I thought I would go through the numbers a little more to give some you a idea of what that comes out to in static compression ratio.

Ok here is the data used to come up with these numbers:

Bore 3.20" .040" over stock MGB
Stroke 3.50"
Stock combustion chamber volume on a 12G 1326 MGB head 42ccs
Compressed head gasket thickness .035"
Head gasket bore diameter 3.245"
Piston deck height -.020"-.030" Now this one is just a good guess, but I believe I'm very much in the ballpark.
MGB AE replacment piston dish volume 6ccs

If you plug in all those number you get a static compression ratio of 9.1 to 1
Ok that's pretty close, but alittle better to the 8.7 to 1 AE pistons claim to be. The head gasket cylinder bore was measure across the bore not accounting for the valve relief area of the gasket, but if you throw another .020" at the head gasket the compression ratio only drops .1.

Ok using the number which could be slightly off, but never the less pretty close to what claimed compression ratio is, here's what happen when you start decking the head and decreasing the combustion chambers volumes.

If you decked a stock head .020", the CCs in the combustion chamber will drop from 42 to 40ccs, this would raise the CR to 9.4 to 1

Ok, now with the head decked .040", the combustion chamber volume dropped to 38ccs and the CR went to 9.7 to 1

In the case of Fred's head we decided on .060", I knew this would not be too much for future plans I have for this engine as we turn it into a real race engine, but still enough with his current piston and bottom end build to see a decent, yet still conservative power increase. .060" decked off the head dropped the combustion chambers down to 36ccs and the CR now went to 10.3 to 1

There a few loose ends in my calculations but nothing earth shattering, I fiquires Fred piston deck height at .020" and it probably no more than that. It will be next week or so in a MGB engine project before I can confirm these numbers a little better, but I'm real close. of course in a real race engine that piston deck height would be decreased, if for example you decreased the piston to deck height to -.005" the CR would raise .4 points to 10.7 to 1., If a flat top piston was used with this same engine and with the same -.005" piston deck height the CR would jump to 12.1 to 1 for example.

The main reason I posted this was to give guys out there building thier MGB engines a liitle bit of peace of mind knowing it's very much ok to deck your performance MGB engine head .020-.040" and still be very much suited for street gas. Now keep in mind you need to have your combustion chambers CCed to begin with, but the trend turned out to be that 020' decking dropped the combustion chamber's CCs 2 CCs, now mind you it would probably start turing out to be less than 2 CCS for every .020" if you deck further, but that would only come into play with a race engine, so .020" for a 2CC drop turns out to be a pretty good exmaple for a street engine. Anyway it was a interesting day yesterday playing with this stuff, and I thought I would share it with the crowd. The actual before and after CC readings on on Fred's head were recorded by me using a CC burret.

Back in the 70's I had the head decked 0.050" on my 18V engine. Ran good but was a bit picky on fuel quality and spark advance. Engine was otherwise stock except for a tube header and a Garrity (?) "Dyno-Tune" kit consisting on new dizzy springs and new needles for the HIFs

Are the head thicknesses from the factory machined accurately/ consistently enough that measuring the thickness would give a good indication as to whether the head had already been decked?

I'm not interested, usually, in ultimate performance, so how high a CR would you guess would still work with "Regular" (87 Octane) pump gas? 9.3:1? I think that's about what I'm running at the moment on 87 Octane. Gas prices around here are high enough (around $3.80/gallon) that I'd prefer to stay with the "cheap" stuff!

Hap, what kind of increase did you see in cylinder pressures generated by the engine by cutting the head .060"? I imagine the valve job raised numbers as well.

Derek up North Wrote:

Hap, could you PM me about the price of decking my head .020? Will I see you at Euro Auto Fest in Greer?

B-racer Wrote:

For reference, I'm running at 9.6:1 with a ported cylinder head and can run regular (87 octane) pump gas for everything except autocross - where the engine heats up enough that I can occasionally get pinging. In that situation I generally run a tank of premium.

(edit) I am also having a carb problem and going super-lean at 100% throttle, which is probably contributing to the ping at high temps with 87-octane gas. Gotta tear into those HIFs and see what's going on. :)

Hap, I understand what you mean about cylinder pressures, but in general they're a good measure of not only compression, but cam duration and valve timing. All else being the same, I was just wondering what pressure increase you will see with a .060" shave. I'd guess that'd put you in the 180s-190s as well, but I don't know what cam is in there.

Derek, there is a measurement in the Special Tuning manual for the head thickness. Apparently the factory thought the heads were consistent enough to use this measurement as a baseline

Hap et. al.

Do you think that re-shaping the early head chambers by removing the "point" between the valves allow even higher CRs for the non race gas crowd? It seems that this "point" in the shape of the chamber can become a hot spot and cause pinging. Is my armchair logic wrong?

Jeff, he got a Briteck 270 cam if I'm not mistaken.

Hap - I think you are absolutely correct that most people are too timid on compression ratio. Almost everyone puts a mild performance camshaft in their engines when they rebuild them - but they ignore the manufacturer's recommendation about compression ratio. If the duration is increased over stock then the GCR has got to go up to get even the same effective compression ratio.

As usual, Hap is right on.

When I did the engine in my '63 we pulled about .030" off the head, and IIRC. With flat top pistons and .040" overbore I think we figured about 9.2 to 1. It runs great on premium, and since I only use a tank or two/year it's a small price to pay. The engine has a lot more pep than the stock one in the GT!

My boss had a Mini for a while with a 1098 in it back in the late 70s. I don't remember the overbore, but he bought a set of Jahns flat top pistons and pulled some off the head. After I got it together and we ran it a while I did a compression test and it was over 200 psi. That little car was a blast to drive! He sold it and bought a really clean Volvo 444 of all things....but I'm drifting way off topic.

When I have the money:), I may do this as well, if I don't go with the v6 conversion. I just don't want to take away the Britishness of the car with some v6+
T5 from a camaro. I don't think I want THAT much torque/horsepower anyway.


peace, Kyle

fleshy1 Wrote:

I told someone in a PM, I would post a picture of the 8.0 to 1 supercharger piston we had made, so here it is, ain't it purty :)

Speedracer Wrote:

Removing that "point" does eliminate a hot spot, which will help prevent pinging, but your exhaust valve is still going to be the hot spot. Flowing your exhaust port will also help alleviate heat.

Oh God, I can't believe I'm writing this but here goes:

Incrasing compression is supposed to be about cramming the air/fuel mixture into a smaller space so that when ignition takes place, the release of energy takes less time and there's a bigger bang of shorter duration. Because you time where the bang takes place, you can make more of the energy transfer to the piston rather than being lost to the surrounding area or lost as heat. And that works to a certain point. After that, you start experiencing "pumping loss." That's where the energy required to compress the mixture is greater than the extra energy transferred to the piston. That happens quite quickly unless you change the profile of your cam. Also, the calculated compression ratio doesn't mean the same thing in every carr. The measured compression ratio and the actual compression can be very different depending on how much overlap there is between the intake and exhaust valve timing. Lastly, simply shaving the head (while cheap) can be a nightmare. Remember that you're doing it to increase momentary energy release in the combustion chamber and you're doing it by taking structural rigidity away from the surface of the head. For an increase from 8.0 to 8.7 it's probably not a big deal but if you're going much higher, it could be. Here's what to consider:

First, if you want to make power from compression and don't want to risk compromising the structural rigidity of the engine castings, change the pistons. You can even get pistons that help push the air/fuel mixture toward the spark plug to help propagate a good flame front and greater momentary energy transfer.

Second, Consider decking the block instead of the head. Getting the piston all the way up to deck height decreases the volume acheived at the moment of combustion but the block is a far stronger piece than the deck of the head. This may take custom pistons that move the ring lands and the wrist pin location. (Figute $500 instead of $250 for the readily available pistons)

Third, remember that there are diminishing returns and that more compression can cost you power unless you make additional changes to your engine - like cams and exhaust.

Lastly, remember the words Joe Huffaker told me when I was building my racecar. He said, "Fast, reliable, cheap. Pick two." Truer words were never spoken. Lopping a big hunk off your head is cheap but you trade off relaibility. You can buy more pistons but once you mill a head, there's no going back. Best of luck.

And for any of you motorheads that might also like higher mathematics, there's a great book on this topic titled "Internal Combustion Fundamentals" by John Heywood. John is the engineering department chair and runs the internal combustion lab at MIT. It belongs on your shelf. Basil

Basil,

I bought that book about a month ago and I'm about 2/3rds through reading it (skipping the diesel parts). Being an engineer, I like to see the equations!
I think it'll turn out to be a good reference book.

-Jim

Basil Adams Wrote:

Basil, you make a good point if you're building a really high horsepower engine from a clean page of paper, but it really only partly applies to our motors, outside of the 1275 with it's smallish combustion chamber, all the rest of the MG stuff, to achieve high compression, (now I'm not talkng the kind of compression anyone would run on the street but rather all out racing) you're doing both, getting as close to zero deck height as you can stand on your piston compression height whether, domed, dished or flat top, and still cutting the crap out of your head, you should see how much the Spitfire guys deck thier heads to get compression, I bet Dieter Griesenger's 1500 head has .200" cut off of it, and he still probably running dome pistons, we cut the LP 1500 heads.100" to get to 11.0 to 1, Rick Cline helped me out with that one. On a street MGB, .040" is nothing to cut the head, the head's deck is aprox. .225" thick, you've got guys out there racing MGBs with flat top pistons running 13.0+ to 1 compression ratios. Guys with street MGBs don't have access to new replacement pistons to bump thier compression a point without using some pretty high dollar custom pistons, yeah sure decking the block will acheive more of a compression bump than decking the head the same amount, but that involves a engine tear down, and that's not always in the cards for everyone, when I build a race engine both piston deck height and combustion chamber CCs are both taken into account and addressed. Decking the MGB head .040 is a quick and easy way for a street guy to buy him a little more CR and power very cheaply, and for sure isn't even bordering radical.

Cutting anything off the head deck makes a weaker structure and it's unrepairable. I know guys have done it for years because it's inexpensive but it still weakens the structure. With all the places that a 5 main B head cracks, the last thing I want is to induce more flexing and twisting by having a less-rigid head. To each their own.

eclecticalan Wrote: [quote]
Removing that "point" does eliminate a hot spot, which will help prevent pinging, but your exhaust valve is still going to be the hot spot.

it also reduces turbulance, and proper turbulance is necessary to reduce pinging at whatever compression.

a friend runs 1o.5:1 on 91 pump gas in a highly tricked out Alfa 2liter (in a Morris Minor!). (that sucker gets up and goes!)!

Remember that the Alfa is a twin cam, hemispheric head, cross flow. The effective compression ratio will be different than an MGB with the exact same calculated compression ratio. (And getting 160hp out of a 2.o litre Alfa on pump gas is pretty easy - I'll bet she flies :-)

static compression dont really mean much its cam compression that
really makes the difference. increasing static compression by cutting the head increases thermal efficency..installing a cam that increases volumetric
efficency increases dynamic compression . a standard street eng has about 80%
efficency in filling the combustion chamber,however a properly designed cam will fill the combustion chambers to 100% or more at the designed operating range.

BL-

That's the point - you can't get to 100%VE on our engines with siamese ports regardless of the cam. So even if the calculated (static)compression ratio is the same between an MGB and the Alfa, hemi, crossflow, 8 port head or a 16 valve miata, the effective compression is waaaaaaay different.

So.... if you skim the block or the head by X amount, what's that doing to your basic street engine valve rocker geometry? At what point do you run into problems?

The amounts we talking about for the street nothing you'll notice, larger amounts for a race engine, then you going to need to get shorter tubular pushrods, some engines are more sensitive to this than others. I ran some numbers to give you a comparision of if you took a earlier model MGB head (42ccs) and left the head alone and just decked the block say until you got .005 piston to deck clearence you would be at 9.6 to 1. compression ratio.
Ok, lets talk about that a little more, this is something I do all the time, even zero deck heights on race engines, zero deck height is a real pain and takes as minumum atleast two dummy builds to make sure you are right, all it takes is one connecting rod being resized a little more than the others and you've got to adjust (machine) the other pistons to match it. Now .005 deck clearence wouldn't be as much a PITA for the average street engine builder as zero deck height, which they wouldn't do anyway, but it would still require a dummy build, I know you know what a dummy build is from hanging out with me, but for everyone else that's installing the crankshaft, connecting rods and pistons to take measurements to determine how much you need to taken off the deck, then taking it all back down again. Before this became a engineering pissing battle, all this post was meant to be was a simple guideline of how much to cut your MGB head to get a given compression ratio based off a real experience rather than theory. If you read Basil comments it leads you to believe that taking .020-.040" would compromise your head casting, excuse me but that is ridiculous, sure the casting could be compromised if you cut it enough, but .040" or even .060" is nothing to the integrity of a .225" deck thickness of a MGB head. For the average DIY street engine builder this is still the easiest path to take to increase your compression ratio, which by all means regardless of anything you've read here will be a seat in the pants, smile on your face, noticable difference for very little money invested and if taking a .100" plus off many a LBC head casting hasn't ruined them under race conditions, then I know for a fact it will not compromise your MGB head, if it had I would have been in trouble many years ago, as well as many others. Engineering theories are great, real field experience, even better, nothing like really doing it to learn from it. Everything discussed by me here concerning a MGB street engine is still what I consider to be somewhat conservative, in otherwords I wouldn't want to build a customer's street engine to be anything over 10.0 to 1, in fact I rather keep it a little lower than that, just for the fact that some guy out there might not find his local pump to be good enough to support a higher compression ratio, but a 8.7 to 1, there are gains to be had without even coming close to pushing the limits, bumping your compression ratio a bit will be one of the best under $100 dollar improvements you could ever do, talking about the most bang for the buck, well here it is.

Taking .060" off a .225" deck is a lot more than "nothing" when it comes to structural rigidity and it's not a linear function. If it were "nothing" we could take all of the quarter inch plate on the Golden Gate Bridge or the hull of a submarine and shave it down and still expect the same rigidity. I'm not saying that your head can't take it but you do increase the amount of flex the head will have and therefore you increase the risk of cracks. As for rocker geometry, as you mill the head, shorter pushrods will help keep the rocker angle correct but you'll probably have to move the pedestals too to keep all of the geometry right. Best of luck - you guys choose how you want to make modifications - just be informed that the cheap way isn't necessarily the best or safest way. Basil

Basil, how much did Joe Huffaker, or Lim Lerch take off your head, when he built your race engine? This has been so tested for so long, under so much more severe conditions (racing), and with much greater cuts than we are talking about here, your concerns crack me up. heck I never enven saw a MGB head crack where the crack originated from the deck, they normally crack from the top downward.

Speedracer Wrote:

Hap, Huffaker built my chassis but has never built an engine for me. Every head I have has bigger chambers that stock to accept the pop-up pistons. But look at all the cracks in MGB heads right on the deck - usually between pushrod holes.

Tony, The trick with decking the block and using custom pistons is to move the wrist pin up so that you effectively move the piston crown down. Then when you deck the block, you increase compression and keep the piston below/at deck height. Don't go thinking that decking the block is without a price. There's a significant propensity for 5 main engines (noyt 3 mains) to weep water from around the front, left head stud if you take too much off the deck. There's a way to make sure that doesn't happen but it's not my trade secret so if you consider doing this, call me (707.762.0974) and I'll turn you on to the maestro who knows what toi do.

You're right about dished pistons. Since the flame front starts at a single point (the spark plug) the shape of that front (and of the energy pulse) is spherical around the point of ignition. The part of the pulse that's reflected by the combustion chamber and the cylinder walls is bounced around until it finds the point of least resistance - the moving piston (Remember that PV=K). The part that is aimed directly at the piston is different though. If you assume that the front is a shperic section, then having a piston with the inverse spheric section on top allows more instantaneous energy transfer - more of the energy contacts the piston crown simultaneously. But the dished piston also helps with squish and swirl. Remember that book I mentioned, "Internal Combustion Engine Fundamentals" by John Heywood? It does a great job of discussing how squish and swirl affect cylinder pressures. Give it a look - you'll be happy you did. Best of luck. Basil

Demi Elgin (Elgin Cams) published a summary of what he considered to be best engine building practice a while back, and in it he suggests that a quench distance (top of piston crown to surface of the head) of.040-.050 is pretty much optimum- any more than .050 you might as well open it up to .100, as there is greater pre-disposition to detonate between .050 & .100 than at higher compression ratios where the quench distance is is less than .050.

What does this mean? If you have a choice, get as close to zero deck as you can with the block and pistons and let the head gasket generate the quench height. A cut on the deck will have about twice the impact on compression ratio as a cut on the head of the same amount.

If I had a dished piston and was looking at getting best compression, I'd consider positive decking the block and milling the piston to zero deck, eliminating some of the piston dish.

Do not discount the effect of cam timing on dynamic compression ratio- late inlet valve closing lowers cranking pressure regardless of mechanical compression ratio



In a perfect world, I'd order pistons with about .040 higher compression height and minimize the amount of material decked to get a zero deck


Edited for spelling a couple times

These are issues I deal with on a regular basis, first off zero block/piston deck height is easier said than done. First thing that comes into play is sizing of the connecting rod's big end, this can vary each rod/piston assembly total length a few thousands, sometime more, anyone who has ever sized used connecting rods knows this, it's not uncommon to have onne rod or more out of a set that reguire a second go at the cap grinder and hone to achieve the roundness you're looking for. Uping the compression hieght on a custom made piston is a dangerous game, not so much with a flat top as it would be with a dome, because with a dome piston it can be real hard to lower that compression height if you miss it slightly. Most stock engine have a negative deck height, but nothing enormous, normally .020-.030" in the case of engines. One thing that comes into play when decking the block is rod stretch, most builders acount .030" for this, 9 times out of ten, the thickness of the head gasket is that or more, but don't totaly rely on .030" rod stretch to be enough, I saw many cases where zero deck height and .030" thick head gasket caused the piston to slightly contact the head outside of the combustion chamber, for this reason I would rather use deck height of just below zero, say .005".
Greg as far as machining dish pistons to adjust deck height you have to be careful doing that with off the shelf pistons unless you are talking off about very small amounts you greatly limited by the crown thickness of the piston, alot of your dish cast piston are cast in a way that you will see a reverse of the dish on the underside of piston to keep total weight down, so the.thickness across the piston crown can vary, then you got to take into the equation if you lessen one or two piston assemblies a single CC by machining for deck height, now you adjusted the compression ratio on those two cylinders as well, this becomes extremely tricky when you're mandated to certain compression ratio like with the SCCA limited prep rules. When determining deck hieght what most builder do is a 'dummy build" they install the crank, rods and pistons, then measure everything, normally you only find a few thousands varience from one cylinder to the next, to achieve a consistent deck height you would normally use the shortest assembly as your mark, then adjust the other ones (machine the piston's compression height) to match your mark. So in short you're better off to go with a compression height that will start you off at a negative deck and work your way down to it. Of course all of this is much easier to do if you're using a flat top piston, say you're using a flat top with valve reliefs like we do in the 1275 race engine, if you had to machine any decent amount off the top of the piston, you may have to remachine the depth of the valve reliefs as well. Most piston companies are trying to make the lightest piston assemblies thay can so bottom line is sure you can adjust a piston crown a few thousands, but you don't want to go hog wild. One of the problems alot of 948 racers had in the past was with the Venolia domed piston used in that engine, crown thickness in those piston was only about .100", it was pretty common for those guys to modify these piston even more, some who machined the crowns of these piston got themselves into trouble, as they compromised the crown thickness of a laredy thin crown, and if they for instance ran thier motor alittle lean, it made it much easier to burn a hole in a piston, I literally seen dozens of domed Venoila 948 race piston that had been burned through over the years, this is one reason folks like myself, Malvern and other went to JE to have 948 domed piston made with a thicker crown, since Venoila seemed unable to do it for us.

Greg a good example for us to look at, that I know is a good example for you and I is the SCCA G-production limited prep MGB, using a flat top piston, and a early head head with a starting 42cc combustion chamber, very little from the head will have to be removed from the head to get there. if you fiquire that the stock deck height is say -030" then you're already at 10.0 to 1 with a stock 42ccs in the early head and flat top pistons, with the later 38cc heads you already be to 10.8 to 1. So for me the best plan of attack would be to lessen the deck height but not all the way to zero, so you have a little bit of wiggle room to deck the head just a bit. For example if you reduced the deck height to -.010" you're at 10.5 to 1 with a 42cc head, and the amount you need to deck the head to flirt with the maxiumum allowed 11.0 to 1 is very little. However in the case of SCCA full prep race egnine where compression ratio is not mandated and you can run whatever you can keep to together, there are tons of different motors out there running 15.0+ to 1, so to get to that kind of number you have to do alot of things, dome pistons, deck heads, deck blocks. Then there are some other heads and block that you have the ablity or need to deck alot more, for example on a Triumph 1500 head with flat top piston, you must deck the head .100" witha zero deck height to get to the LP max of 11.0 to one, full prep 1500 racers deck thier head alot more than .100", that head has a rather thick deck to begin with so it can be done. Oh and there are people otu there running fairly high compression flat top MGB motors, that are deck well beyond anything I quoted here, everything that I mentioned here is pretty conservative.

Tony for sure you could make a piston that helps flame, the MGB has a rather large combustion chamber, so it's not near as much of a problem when compared to some other engines, on the 1275s (real small combustion chambers, like 26 ccs stock) we made a dish piston that the dish mirrors the combustion chamber to help with this. With that being said if you are using a flat top piston engine in your MGB, first off I would not mess with zero deck height, depending on your head gasket's crushed thickness you could have pistons that kiss the cylinder head at high rpm, that's where max rod stretch will come into play, I shoot for a deck height of -.005" the adjust my head to get where I wanted to be, but no bones about you can easily get to the compression ratio you want ( still streetable ) without the flatop pistons, and without going hog wild decking the head. A dish piston would actually probably be a better way to go for you, as laong as you are keeping this motor below 7000 rpms.

Absolutely easier said than done. True zero decking requires either selective fit of pistons and rods (and usually that still yields discrepancies since rods are rarely resized exactly the same.) Candidly, I'm not intent on getting a true zero deck, I'm just interested in reducing it with minimum material removal. if there's a couple thousandths of difference between cylinders, I'm not going to loose sleep over it, particularly on a low compression motor like an 11.0:1 LP engine

As far as changing compression height of the piston, using your example of the LP engine w/ a flat-top piston, a stock compression height is 1.66", rod length is 6.5", stroke length is 3.5/2 = stack height of 9.91 against a stock block deck height of 9.95, leaving .040 of deck clearance on the table, without considering any rod resize shrinkage. Ordering a pistion with a compression height of 1.68 or 1.69 will allow for a minimal squaring cut. I should note here I'm fundamentally opposed to removing iron from anywhere if I can achieve the end goal economically another way. If I'm ordering custom pistons, I'll order them with a greater compression height so I don't need to cut the block as much- first, there will be some structural compromise (I know, not huge, but I may have to cut again) and greater expense as multiple passes or lower translation speed for the flycutter is required.

On milling pistons, I've only really looked at Hepolite castings and JE forgings. The rim of the AE bowl appears to be incremental metal , there appears to be a large radius spherical section with a tight radius into the piston's ID wall. I'm guessing taking material off the rim of the bowl should not alter it's structure as this is 'baggage aluminum'.

Speedracer Wrote:

Tony,

Call me when you can and I'll turn you on to the cam guru. Your cam card is interesting but the 270 degree duration isn't fully described. Sometimes the duration is measured seat-to-seat and sometimes it's measured from .050" off the seat to .050" off the seat. The lift you're achieving is a little light too for your purposes. Remember that the best proxy for ability to fill the cylinder at higher rpms is the valve curtain area (Pi x diameter x total lift). On the race car with a very radical cam and valve pockets in the pistons we get total lift up over .550" Part of that is done to increase curtain area but part of it is also done to get nice, steep ramps to help the valve open quickly. You don't need that much but getting .450" is probably going to be necessary - the maestro can help you with that. Call or PM and I'll make an introduction. Basil 707.762.0974.

Basil Adams Wrote: