I'm in the middle of a build and having some trouble committing to my SCR and DCR. This winter I've bought a TSP 228r cam and cnc'd 243's. The cam specs are 228/228 .588/.588 114 LSA. At .006 the values are 277/277 which is what I have been using in my DCR calculations. My problem is that my heads have already been milled to 60.5 cc chambers and with the stock gm mls gasket (0.51 compressed thickness) I calculate my SCR to be 11.27:1 and DCR to be 8.69:1. I've read the limits of 93 are about 8.8:1 and I don't like running that close to the upper limits. Also, by using the gm gasket and assuming the piston comes .008 out of the hole, my quench distance is .043.

Like I said, my heads are cnc'd 243's (gm world challenge heads ported by Lingenfelter) and so there really isn't much material I can take out of the chambers by sanding to regain some volume. There are some slight ridges from the different passes of the cutting bit in the chambers that I could smooth out, but I am guessing at most I could only take out .2-.3 ccs per chamber (which only drops SCR to 11.22:1 and DCR to 8.66:1, not much of a change).

What do you guys think? I'm in michigan where the summer might reach into the 90's once or twice a summer but other than that it is usually high 70's to low 80's. In the summer this car gets drove about every other day and never goes to the track so reliability is key. Probably 70% of the gas stations here have 93 with the rest have either 92 or 91.

Do you think I could get away with 91 or 92 if I was in a pinch. For the majority of my driving it would be 93 but I really like to have a certain amount of cushion. Or am I being way too concerned? Will I tune with a touch higher amount of timing all but eliminate any detonation that might come out of 91 or 92?

Sorry for the large post. I've attached pics of my DCR calculations and a pic of the chambers. Thanks guys.

Edit:
A few people have mentioned I might be calculating it wrong. Here are the numbers I have been using and I attached a pic of my cam card in post #11. I thought I was using the correct .006 values but can someone correct me if I am wrong by looking at the cam card?

TSP 228R @ 114 LSA:
Advertised Intake Duration: 277
Advertised Exhaust Duration: 277
Lobe Separation: 114
Intake Lobe Centerline: 110

#of cylinders: 8
Bore Diameter (in) 3.8976
Combustion chamber volume: 60.5
Head gasket thickness compressed: 0.051
Head gasket bore: 3.910
Piston to deck clearance: -0.008
Stock flattop pistons (no valve relief)
Crank Stroke: 3.622
Rod length: 6.098

Dynamic stroke length 2.712
Dynamic compression: 8.688

 

Slowhawk ran a similar cam and had 58cc chambers, ran great on pump gas. I think DCR is good to get you in the ballpark and then the tune determines whether pump gas will work or not. So make sure you get a good tune.

Nice setup btw.

 

You could retard your cam 1 or 2 degrees to drop the dynamic. Or maybe a thicker head gasket.

 

I thought about retarding the cam but I already have the cam in and everything sealed up and dont have an adjustable timing set. I don't want to go a thicker gasket route because that will increase my quench distance. I've read many times that the ideal quench is .030-.040, with tighter helping with possible detonation.

 

58 cc's DCR would be 9.0:1 and SCR 11.7:1 with this cam. If that's what he ran that is awesome. I know its all in the tune but will a good conservative tune allow me to run 91 or 92 if needed?

 

Not sure honestly, I think the tune can be set up for 91 octane. A good tuner won't leave you at the hairy edge of detonation.

I did a quick search over in the dyno section. Here is a PRC 2.5 with 59cc chambers, 224 cam (should be a tiny bit higher DCR than your 228):
https://ls1tech.com/forums/dynamomet...er-dyno-s.html

 

thats what knock sensors are for ,I thought.consider a EPS cam or a thicker head gasket.

 

I don't think that you calculated your static compression ratio correctly.
I calculated it using below calculators and it turned out to be in the range 10.7 to 10.8 maximum. So, your dynamic compression ratio should be in the safe range of 8.2 to 8.3.

http://www.kb-silvolite.com/calc.php?action=comp

http://www.wallaceracing.com/cr_test2.php

http://www.rbracing-rsr.com/compstaticcalc.html

http://www.gtsparkplugs.com/CompRatioCalc.html

 

I calculated my SCR at about 10.85 with the 228r and 5.3L PRC heads (59cc) and stock gasket. I got about 425 at the wheels with a 4.10 rear and a dynojet.

 

Where did I go wrong? I posted screenshots of my calculations in my first post.



Thats good to hear because that is pretty much identical to my setup minus a few cc's. How did you calculate that?

 

I edited post 1 to include the values I am using. Am I interpreting my cam card incorrectly? I thought I was supposed to use the .006 values. I attached it below. Thanks for all your help guys and Exidous, I'm hoping to be in that ballpark as well. Did you leave your fuel injectors stock?

 

I use the DCR excel spreadsheet made by Pianoprodigy (sp?), I get the same numbers as you:
http://i275.photobucket.com/albums/j...8_dcr_calc.jpg

I think you will be fine with your setup as long as you get a good tune.

 

Just tune it on 91 and run 93 as much as possible.

 

I see on that last sheet that you have the piston volume at "0". This is not correct for a flat top because of the area around the edge of the piston above the rings. I don't know the exact number but it is for an LS1. A little over a 1cc. Perhaps 1.2cc.

You can run the figures with a stock sized combustion chamber to see if you get 10.5:1. Perhaps you could work backward from that to figure your actual piston volume.

 

why are you using a negative value for piston to dick clearance?
I was using the same value (positive)

 

why are you using a negative value for piston to dick clearance?
I was using the same value (positive)

 

I think you calculated it right. The one's that are claiming lower probably calculated the gasket thickness to be thicker. Do you for sure know that .051 is the gasket thickness?

 

Samer: You use a negative value because the piston comes out of the hole .008. It's just how the calculator is set up.

Intenseblue: I was under the impression that the stock mls gaskets where thicker ( at .057) but Beau from SDPC told me they were in fact .051 compressed

 

I've heard .052, .054, and .058 all from vendors on here Two of those figures have been said by the same vendor, just different people at different times

 

I have never taken this measurement! My girlfriend in college said she wanted to tell her friends though.