Well, I'm starting to think about putting my engine back togther after its nitrous mishap this summer. Since I'm going turbo I need to drop the compression dramatically. What's the best way to do this?

I've got a 3.85" stroke with 6.0" rods and 24cc dished pistons running with stock ported heads(58cc) with larger valves. Current CR 9.9:1 Here's a few of my options, please feel free to add more.

1.) I could buy a set of AFR 210's with 76cc chambers which would get me down to 8.4:1 The only problem is that the heads cost $2500 plus the lt4 manifold $500 and the new fuel rails that I would need for it $200. So I'm looking at $3k+ which isn't good since I've still got to pay for the turbo setup and the engine rebuild, and the tranny too(again). But I would gain a bunch of hp with the better heads.


2.) My next option would be to buy a new crank. I could go down to a 3.75" stroke with the same rods and pistons which would increase the deck height to 0.050 And that should get me to 8.77:1 But I would loose torque with the shorter stroke. But it would only cost me $700 for the crank and $100(??) to get the rod journals ground down(I've got small holes on my rods) and $150 to get everything rebalanced. But thats still $1k


3.) My third option is to see how much we can grind out of the combustion chamber and add a thicker head gasket. Does anyone know how much you can grind the combustion chamber before the head weakens? If I can get it up to a 64 cc chamber and go from a .040 to a .074 head gasket that will get me down to 8.78:1 This will probably cost me $200 for the gaskets and ?? for the chamber work. But by far the cheapest.

I'd like to hear some opinions from you guys, pros and cons if you have any. I'd also like to hear what you guys think the "IDEAL" compression ratio for a FI motor is. Thanks.

 

I would get a new crank and save couple thousand $ for the turbo stuffs.

Bruce

 

UNshroud the valves until you get to about 62-64cc's it will both help flow at low lift and top end as well. then order a .080 thick cometic MLS gasket. AND/OR -If you send out the heads (and pistons to be coated it will have the same amount of detonation resistance) All for about 4-500 bucks Just shoot for 8-9.5:1 and you will be able to get good power out of pump gas. But Ideally you want to run as much compression as detonation will allow to give the best off boost performance.
That all being said I have a -37 dish with afr 54cc heads for a total of 8.8-1. I want all the dish in the piston, as it directs the force more down the hole. Dynamics is a cool class. I'll let you know how it works out.

 

a little diffrent to what some would recomend, and only another option, but you could get some spacer plates/thick copper gaskets made up. i only say coz i saw an advert in a mag over here (UK) advertising them. as long as they are propaly machined i can see then beeing a problem. can any of you guys? also will let you run whatever comp you want. just give them a gasket and tell them how thick you want it! there are a few tuning cumpaines over here using them on stock engines to reduce comp but thee are only power stuff. is there any reasons you couldnt use them (engine itegraty???)?

thanks Chris.

 

well, in two words, intake an pushrods. thickening the deck too much will start causing geometry issues and require more machining. htat works alright with ohc motors though

 

Brian,

You really have only 2 options here.. if you do anything else, you will have a horrible nightmare to deal with!

1- go with a larger -cc dish in a new pistons set and rebalance the rotating assembly..

2- larger chamber heads. this would be my recommendation, as you mentioned above you would reep some serious benefits witht hat large cube stroker over the stock heads.


going to a different stroke crank is wrong completely without going to a new piston.. the pin hole in the piston is setup for a 3.875" stroke and the edges of the pistons would stop to far away from the deck surface to still achieve a suitable quench area.. if you take away your quench area, you can essentially figure on having serious detonation issues even with a NA low compression motor. that quench area is designed to turbulate the mixture as it begins burning and eliminate combustion happening in one localized area which leads to detonation. (hot spots)

the most effective way to maintain performance and keep combustion as efficient as possible is to add displacement to both the combustion chamber and the piston. keeping a flat top piston and a huge chamber volume is really not a good idea especially in a performance vehicle as the pressure can be accentuated at the mating surface and add extra stress to the steel ring of the gasket. thats why when you see a dished piston, the edges of the pistons are still designed to reach the top of the block. try and achieve a .040"-.045" gap between the flat portion of the head and the edges of the piston to stave off detonation. this will aid more then dropping the compression ratio! and increase power making potential.

seeing as you already have a heafty dish in your pistons, i would consider going to a head with a chamber close to 65-68cc's you really don't need a 8.5:1 ratio. a high 8 or a 9 flat will be fine for pressure levels near 18-20 psi on proper fuel and tune. much more then 20psi and you are going to enter a realm that compression ratio isn't really your main issue. that is where you start lifting heads and have to have your block oringed, machined for 1/2" bolt/studs.

the AFR's will benefit you from a thicker head deck which will help reduce deflection under high pressures and maintain clamping uniformity across the block. they also add CC's to help bring the CR down, and will flow superior to the stock heads you have now.

another option is to go to the trick flow heads which offer a 64cc chamber if i am not mistaken.. this may not get you down as low as you're shooting for, but will help noticeably and will not require a change in your intake manifold. they are also 195cc intake runners which are notably bigger then the 170cc stock heads, and I am sure these can be ported out a good bit more. the head deck in them is sure to be a little thicker then the stockers, not as thick as the AFR's, but at half the price, it may be an option to consider.

anyways.. just throwing a few things out there at ya!

hope this helps man!

Chris

 

thanks Baldturbofreak. good point. im used to OHC units where sapcers are less of a problem (just loosen the timing belt a bit to get the extra length or fit a longer one).

this is a good thread though. very intersting.

thanks Chris.

 

Chris,

I did look into several of the other options. But I can't get a bigger dish on my piston due to the stroke size. They just can't make that big of a dish without hitting the rod pin. I'd have to buy a second crank and a new set of pistons which puts me up around $1k+ for machining and balancing and parts. I'm glad you mentioned something about the quench, its things like this that I don't always understand, but it's good that someone out there is keeping an eye on my ideas.


I did look into the trickflow heads with the 64cc chambers. They run about $900 for the bare set and $1350 for them assembled. So I'm looking at atleast $1k and from the very limited info that I could find on them they won't flow any better then my stock ported heads. I'm also not sure about the cc size, it's advertized as 64cc but listed several places as 62cc. And that would only get me to 9.53 or 9.35 for 62 and 64cc.


I think I'm still leaning towards grinding out the stock chambers and a thick cometic gasket. I'm hoping for the 9:1 which I should be able to get from a 0.074 gasket and a 60 cc chamber. I just wish there was a good accurate way to get a chamber size to get this calculated exactly.





I found this on the trickflow website

Lift Value
(in.) Intake Flow
(CFM) Exhaust Flow
(CFM)

--------------------------------------------------------------------------------


0.100 51 58
0.200 136 98
0.300 191 136
0.400 230 163
0.500 253 177
0.600 254 190

 

Brian,

the .074" cometic is a bandaid.. to large of a quench area. you are more likely todetonate at 9:1 witht hat gap then at 9.5 with the tighter clearance.. fill in these blank if ya can..

compression height of pistons:

dish value: -24cc

chamber volume: 58cc (stock)

depth piston sits in bore at TDC(looking for deck height here, what has been machined off):

Gasket thickness: I would go no more then a .045" compressed thickness in a cometic or SCE copper

then figure out what it will take to get to the correct compression ratio with the appropriate quench area..

THIS IS THE ONLY RIGHT WAY TO DO IT!!!!!!!!!!!!!!

any other way and you are gonna negate any benefit from what you are doing.. it is really that simple!

if it takes larger chamber heads then that is what you do.. don't take this lightly.. I cannot stress this fact enough!

spend the extra dough, get the better heads that will do everything you want, save the cubic inches you paid for, strengthen the sealing surface, flow better, achieve you correct compression ratio with proper quench area, and turn the boost up and be happy with a well layed out design. or... sell your motor and start from scratch building one designed specifically for boost maybe something with a little less throw in the crank and a little more dish in the piston.

If it were me, and I have had the opportunity to work and port AFR heads.. they are second to none! that is the route i would take. even a set of the street ported 210cc's or the larger 220 or even 227cc. the porting will help but not nearly as much as the volume of the runners. and not nearly as much as in a N/A setup. let that 76 do its job.. as long as you give it some vloume to push the air through and drop the intake restriction, you are going to make more power then you really know what to do with on the street!

Hope this helps and hope you do it right! you have been through the ringer already, and cutting another corner here to rush getting it together is only gonna set you back farther in time and money after you realize what you did won't get you where you wanna be!

Chris

 

Chris,

I do realize that using the headgasket to lower compression is a band aid. But I can't arguee with my finance department. I just don't have the extra $3k to spend on new heads. If I do use the headgasket I can upgrade the heads next season and get the benefits of the 210 AFR heads.

I do realize that increasing the quench area is a bad thing, but I've called cometic and they told me that they haven't heard of any problems with it. I've also talked to two people that run the .074 head gaskets, one is making 5-600hp on a turbo application and the other was making 1200hp on a supercharged ford. Perhpas they are running some high octane gas or something, but it seems to work.

I've got some engine dyno time lined up for this winter so once the engine is rebuilt we'll find out real quick if it'll work or not. I'm not planning on running more than 10psi on the engine the first year.