information about stroking my gt
01-16-2009, 02:58 PM
#1
2nd Gear Member
Thread Starter
Join Date: Oct 2008
Location: massachussets
Posts: 467
information about stroking my gt
i am thinking about stroking my car this summer because its time to replace the bottom end. I don't really know that much about stroking and how it really works. obviously the piston has a longer stroke so could i just buy shorter rods? or do i have to buy a new crank to compliment the shorter rods? i'm just trying to gather information ahead of time so i can plan accordingly funds wise. I have looked up some stroker kits and they all seem pretty expensive. Is there a way to stroke a car on a budget for example buying a cobra crank with cobra pistons or do they have the same stroke as my gt? sry if some of these questions sound self explanitory i haven't really explored deep into the subject yet. any information would be helpful thanks
01-17-2009, 03:43 PM
#2
3rd Gear Member
Join Date: Apr 2007
Location: Georgia
Posts: 931
The modular stroker kits I've seen get the added stroke from the crank, and use stock length rods and pistons. I guess you could just buy a stoker crank and use your rods, and pistons, but adding stoke on an already jacked up bore to stroke ratio isnt gonna help. You'd be better off going with an oversized piston than stroking your car. Cobras have the same stroke crank as a gt crank.
01-17-2009, 05:20 PM
#4
2nd Gear Member
Thread Starter
Join Date: Oct 2008
Location: massachussets
Posts: 467
so just buying shorter rods won't do anything because its still going to have the same stroke. also i would like to get alittle higher compression. do flat top pistons perform well in strokers?? i wana be in the compression area if i were to put P.I. heads on my car. I could just do that, but i ported my npi's and liked the results. The only way to see what my true compression is now is to take the heads off and fill the combustion chamber with water right???
01-18-2009, 09:20 AM
#6
TECH SAVANT
Join Date: Dec 2006
Location: Saint Augustine, FL
Posts: 10,938
Compression ratio is the ratio of the cylinder's total volume at BDC to the total volume as TDC.
The total volume at TDC consists of the volume of the piston's "dishes" (if any); the volume of the cylindrical spaces caused by the piston's not rising all the way to the block's deck, and the thin slice created by the head gasket; and the volume of the head's combustion chamber.
Total volume at BDC = the TDC volume + ((bore² * pi) / 4 * stroke).
Increasing stroke, using proportionally shorter rods (required unless you want the pistons smashing into the spark plugs and cylinder head) and stock pistons will increase compression ratio, it has to as long as the cylinder volume at TDC doesn't change.
The total volume at TDC consists of the volume of the piston's "dishes" (if any); the volume of the cylindrical spaces caused by the piston's not rising all the way to the block's deck, and the thin slice created by the head gasket; and the volume of the head's combustion chamber.
Total volume at BDC = the TDC volume + ((bore² * pi) / 4 * stroke).
Increasing stroke, using proportionally shorter rods (required unless you want the pistons smashing into the spark plugs and cylinder head) and stock pistons will increase compression ratio, it has to as long as the cylinder volume at TDC doesn't change.
01-18-2009, 01:39 PM
#10
2nd Gear Member
Join Date: Dec 2007
Posts: 402
Man..some people should not be giving engine building advice...haha. 
You will need a stroker crank, stroker rods (shorter) and stroker pistons (shorter compression height)..all relative to deck height. Most combo's use a 3.750" stroke, a 5.850" rod and approximately a 1.20" compression height piston. You can't use a stock rod because 1), it's too long and 2) the crank journal is a different size on the stroker crank vs the stock crank rod journal. There are a few exceptions to these rules but that would be a pretty f'ed up combo if so. Yes, adding stroke or bore will increase compression ratio given a constant combustion chamber volume ...as mentioned earlier.
You will need a stroker crank, stroker rods (shorter) and stroker pistons (shorter compression height)..all relative to deck height. Most combo's use a 3.750" stroke, a 5.850" rod and approximately a 1.20" compression height piston. You can't use a stock rod because 1), it's too long and 2) the crank journal is a different size on the stroker crank vs the stock crank rod journal. There are a few exceptions to these rules but that would be a pretty f'ed up combo if so. Yes, adding stroke or bore will increase compression ratio given a constant combustion chamber volume ...as mentioned earlier.