My current stroker compression ratio is 11:1 with 0.030" deck height. I need to reduce the compression to about 8 to 8.5:1 for a TT setup. I'm currently using a 6" rod. If I change to 5.85" rods, the deck height will now be 0.180" dropping the compression ratio to just over 8:1.

I've never heard of an engine with this much deck height. Will this work? What are the potential problems if any? :cheers:

 

Changing the rod length would kill your quench, the distance between your pistons and the heads, you generally want between .030 and .040. Anyway it creates turbulence in the chamber which makes the air and fuel mixed better. You should change your pistons to dished ones, they retain the height in the quench area so that your quench is still good. It would be just as cheap to change pistons as it would be to change rods.

 

wouldn't it be cheaper to just change pistons?

 

Or heads. The rod swap idea is a horrible one. Although with .030 deck Clarence, you won't be giving up any worthwhile quench.

RACE ON!!!

[Afterthought.] Your present pistons on shorter rods will surely hit the crank.


[Modified by CFI-EFI, 4:54 PM 3/23/2004]

 

Thanx for the feedback...I only have 500miles on the new custom $$$ pistons for my 420. The rod option would cost about 1/2 as much but I guess I don't have choice.

:cheers:

 

i think if the pistons are that much money, I would likely look at getting larger chambered heads. this will keep your bore to stroke ratio where it needs to be, and give you the volume above the dome to run some boost. I agree that the .030" deck is excessive, not sure what your willing to do to get everything correct, or if the custom pistons were designed to work with that much deck height. can you give some specs on them? what is the compression height? you may have enough quench already if the height is designed to work at .030" deck height.

Chris


[Modified by lcvette, 8:38 AM 3/25/2004]

 

The problem isn't in having, "enough quench". His deck height is just fine the way it is, if he uses a head gasket with a compressed thickness of .005 - .015. GOOD luck.

RACE ON!!!

 

I was asking because different pistons especially custom pistons can be setup for the correct quesnch on various deck heighted blocks.. I was asking what the compression height was to aquire the information of where the piston@TDC is in relation to the deck he is currently running. then if we factor in the correct gasket thickness we can calc the current quench height. from here it is much easier to give him the most sound advise.. don't you agree?

also, he is planning on running a boosted application..has anyone really investigated the quench effect with cylinder pressures much higher then seen on N/A cars? might be interesting what is found out. I absolutely agree that dropping rod length is the wrong answer, and increasing gasket thickness is also another jury rigged way to go about it. hence the proper two methods wouldbe new pistons, or larger chambered heads!

Chris


[Modified by lcvette, 11:59 AM 3/25/2004]

 

He has already stated the piston is down the hole, by .030" That is why I made reference to the impossibly thin gaskets that would establish a respectable quench. Not really. There is no saving those pistons for what he wants to accomplish. By the time he decks his block far enough to get a decent quench, he'll have to put big block heads on it to get chambers large enough to drop the compression down to where he wants it. Detonation is caused by excessive cylinder pressures. It matters little, what causes the pressure. A good tight quench area will help mixture turbulence and inhibit detonation, regardless of the cause of the pressure. Don't you agree?

RACE ON!!!

 

lets see exactly what his combination is... we know 6" rods, whats the bore and stroke he's running? what are the present chamber volumes? what is the dome or dish volume of his pistons and last what is the compression height of the piston... whats the harm in divulging this information and then seeing what can be done? simply saying to swap pistons is not always the only answer..
I agree that he needs to have the quench area correct I was just asking if it made a difference on one of the two? after reading maximum boost, I found my answer as well as from your post! .. stating directly from "Maximum Boost" "quench area is sacred do not tamper with" "a 7:1 CR car with poor quench are may make less power be less boost tolerant and detonate more then a 9:1 CR setup correctly. so maybe getting his quench area to a .040" range, and then see what he's actually trying to accomplish may bring some light that he does not need to be so low on the CR. I dunno, just think that alot of people automatically think they need to drop compression of the face of the earth to go boosted! so give us some figures and your complete setup and lets crunch away and see if we can help find you a well laid out plan to consider.

Chris

 

Hey I'm learning something here!! :hurray:

The numbers are:

Bore: 4.155"
Stroke: 3.875"
Compression Height: 1.062"
Head Volume: 65cc
Piston Valve Relief Volume: 5cc
Gasket Thickness (compressed): 0.039" (Felpro #1010)

:cheers:

 

what are your power goals? what size turbos are you planning on running? what type of intercooler do you plan on using? what boost level do you have in mind? plug these in a post and lets see what exactly will be needed.. you may not need as much boost as you think with your motor to achieve some very stout numbers. also what camshaft will you be running what are the specs on it? all these things are going to play a key role in power output. also what type of fuel delivery system are you using? Carb or FI? this is really going to play a key role.. if fuel injection what management system are you currently running? give us as much information on your current setup and I can tell ya what your gonna need to give you an idea of what your going to be looking at and a scope of the project your considering.

Chris

Chris

 

I just looked up your signature, your currently running a 11.09 static compression ratio from my calcs, going up to a 76cc head would give you roughly 9.94 static compression ratio.. for your motors displacement, i think even at moderate boost levels of around 8-10 PSI with proper intercooling, you would see a very large gain in horsepower. everything is dependant on your top end package which looks like it is setup to flow very well. with the motor being very efficient already, you will see much more power per pound of boost then say a closer to stock setup trying to squeeze boosted air in to the motor. on a motor that size i would look at a pair of 60-1's from turbonetics as your displacement will need the flow on the exhaust side. if your manifolds/headers are well designed..equal lengthn style, and your downpipe sizing is properly matched i would think that on 8-10 PSI you would be very satisfied. if you are looking for an all out dyno buster, then you will need to swap in some pistons to properly setup for the turbos. also with the current deck and abundance of quench height, you may very well be looking at new pistons to properly build the TT setup. the rest of your package looks like it would be up to handling upwards of 1000RWHP on race gas and possibly some meth or alky injection. the best recommendation would be as stated above..bite the bullet and build it right..get the correct pistons for the job get the block machined to give you a proper quench height, i would aim for a minmum of 8.5 CR though and not be affraid to go as high as 9:1 as long as your intercooling, octane rating and fuel system are up to it and you dial in the DFI on the dyno, it should be a very wicked combo! again depending on power goals, anything upwards of 750 RWHP is going to get high dollar as your total fuel system will need to be upgraded with twin intank pumps or possible a sumped tank with -10 feed lines, depending on what the miniram fuel rails look like, you may be upgrading to custom rails. injector sizing will be in the 83+ LB range in Low z trim, but the DFI is equipped to handle this. you may want to post in the C4 forced induction /nitrous section to better understand what your looking at. also, the camaroz28 board has several cars built similarly to what your looking at doing.. you can get a good idea of what it takes and what kind of power to expect from what setups are proven and built on there.. my guess would be setup properly you could take that motor to the 1100-1200 RWHP mark but this will require soem serious planning, ALOT of money, and alot of custom fabrication. depending on who does your work, if you can weld thats a big plus if you have alot of time thats even better...lol the room issue might not be an issue if this is your racer..ie.. heatercore, AC removed.. mounting the turbos on a C4 takes some creative thinking and is not for the weak hearted. mouting them low will require a scavenging system for oil return and mounting two turbos of the size your motor will require high and in front will require alot of custom bracketry and plumbing. an intercooler large enough to support yout CFM requirement and flow cgharacteristic in an Air to Air is gonna be a monster.. perhaps a Air to water unit would better suit your needs..? meth or alky injection is a big plus and will help ensure lower intake temps and help fight detonation.. so many variable.. also is a TT somehting you have to build or possibly a SC application would be an easier way to attain a boosted motor. there are some nice numbers coming from cars running the F1 blower in your cube range. bracketing one of these and the rest of the plumbing would be infinately easier and cheaper in the long run.. just a thought! I know some people ghet set on turbos though and its a lost cause to sway them another direction..I'm one of those people! lol well if ya have any questions about turboing.. go purchase the book "Maximum Boost" it is the single most important purchase for the turbo build..it will become your new bible for perfformance! lol best of luck and keep us posted!

Chris

 

This started out as a question on the advisability of reducing the CR through deck height adjustment, ie shorter rods. I think I gave some pretty good answers. Even Icvette was forced to do some research, and learned something. Carry on.

RACE ON!!!

 

thanks for all the great info....changing the pistons will be much cheaper than changing the heads...those were relatively expensive.

I'll look into the SC option...I'm looking for 800-1000 RWHP.

:cheers:

 

It looks like shorter rods with the current pistons have already been ruled out. One area that I didn't see addressed was the piston shirt to crank counter weight clearance. Some if not all pistons wouldn't allow that much rod length change without opening another can of worms.

After posting and rereading the thread I see that this was already brought up. :thumbs:


[Modified by Lionsden, 9:44 PM 4/6/2004]

 

:thumbs:

RACE ON!!!