OK, I think I got the DCR program to work. I got the following info with with 4.040 bore:

cam straight up (107 ICL):
10.01 SCR
8.154 DCR

cam 4* adv (103 ICL):
10.01 SCR
8.383 DCR

 

I don't get it? ... where is KB response?

 

Top of the page highlighted in red Called Tech and Customer service and E-mailed both on Monday. This was their response 2 days later. Will post all corespondence.

 

I would be extremely grateful if somebody could confirm I have entered the correct numbers into this SCR/DCR calculator for my cam and engine.

I have a 355 sbc and it looks like these numbers are standard for this engine:
Stroke - 3.48
Rod Length - 5.7

I wasn't positive on the above but think I have the correct numbers in the calculator:

 

It looks OK, but on mine, the Intake Closing Angle in use (at the top of the calculater) is the same as the intake closing point on the cam timing tab.

 

If my calcs are correct, is 8.32 DCR too much for a street engine? If I retard the cam 4* it comes down to about 8.08 but I don't know how bad this will affect the engine performance.

The only other way I see to get it down would be to use a thicker gasket but then the quench goes up.

 

Yep....i have the same concern. As has been said many times, DCR is a black science and not the be-all. Sure is fun to try to dial into though! From my intense internet search it appears a DCR of 7.8-8.1:1 on street POM 92 is a good target. That said, even the GOOD PK calculator is at the mercy of a very sensitive and obscure SAE ICA value. PK computes a 56 ICA for a "929" cam when the cam lobe data indicates 64 deg at .004 tappet. Big difference! Who you gonna believe?

In addition quench targets are also a bit of a lightning-rod issue. With so many different opinions....it seems going with the consensus makes sense. Avg DCR from a few good DCR calculators. Shot for 7.9-8.1:1 DCR and keep quench .035-.050. Thats my plan until the next convincing argument comes along...!

 

Interestingly your post shows screens where you entered duration @ .050 yielding a ICA of 38 ABDC. But...the DCR screen you captured & posted is oddly correct based on the longer advertised (.004@tappet) which results in the much later and correct 60 ABDC.

I verified your DCR calcs at 8.32:1 with the ADV duration (268/280) and a DCR of 9.3:1 using the incorrect duration at .050@tappet. Bottom line is...I believe the 8.32:1 number...just the captured screens are misleading.

 

So I was supposed to use the 268/280 numbers instead of the 224/230? That is odd that the numbers came out right. I'll have to try it again tonight and fiddle with the different numbers.

I guess it would be nice to know, based on this comp cams sheet, what numbers I should have taken off the cam card and plugged into the CR calculator. If somebody could break that down for me I'd be grateful.

 

Thanks for your post. I was using the program wrong but coming up with the right numbers. Your post, in a round about way, helped me figure out how to use the program.

I was using the .050 timing numbers when I should have been using the .006 numbers. I was filling out the "Dynamic Stroke Length Calculator" tab first with the correct ADV Intake Closing Angle since the cam card had that data. Then I filled out the "Cam Timing...." tab, then the CR tab last. Since I had manually entered the 60 for the Intake Closing Angle first, the calculations came out right but the data on the screens was wrong.

It looks much better when I use it the way the FAQ says to and I use the correct .006 timing numbers instead of the incorrect .050 timing numbers.

 

BTW.....Did you get any other info/support on going with the 8.3:1 DCR?

 

Sure didn't; I'm still researching this pretty heavily. I sent a question to Comp Cams yesterday to see if they could offer any advice but haven't heard back yet. I was hoping since it dealt with me buying their cam for my engine they might offer some insight; we'll see.

 

I just talked to the guy over at Lunati; he was pretty helpful. Those guys have been hearing about DCR for the last few years but aren't really focused on it. It's still kind of an art to get these things right and nobody can or is willing to specifiy a specific DCR that will run without detonation. I understand why; there's just too many variables.

That said, even with the 8.3 DCR he recommended running their cam 4 degrees advanced at a 106 centerline and seeing how it goes. His gut feeling was that it would be fine.



Just called Comp Cams; similar response. Too many variables and can't really go on a specific DCR that will not have detonation. The popular thought of both these guys was that I'm running 10:1 static compression with aluminum heads and that they would give it a try. The comp guy threw in all the different factors such as timing, etc that could be adjusted that would make a difference. Guess I'm going to try the Comp XE268H with this combo.

 

Ok, this thread is dead but I wanted to post this information along with it for the archives. I think it is really good information for people in the same boat that I am.

I sent an email to Pat Kelley, they guy that created the CR Calculator everybody likes, asking him what his thoughts were on my 8.32:1 DCR for a steet engine on pump gas; keeing in mind that there are too many variables for anybody to know for sure without trying it. He gave me permission to post his response here. The disclaimer is that obviously he can't be responsible for any blown engines.

I gave him my specs as below and asked for his "personal opinion" on whether or not detonation would be a problem with this combination. Here are my specs and his reply:

355 SBC
64cc aluminum heads by Trick Flow
.026 Thick head gasket
4.100 gasket bore
4.030 bore
3.48 stroke
5.7 rod length
.022 deck clearance (a rough guess based on some basic numbers and knowing I have a stock non-decked block)
6.10 cc flat top pistons
268 Adv intake duration
280 Adv Exh Duration
110 lobe separation
106 Intake Lobe Centerline

Begin Reply

With aluminum heads you should be OK. 8.5 DCR is the value David Vizard (my primary source for this info) gives as the max for pump gas (and this requires some special considerations). He suggest 8.3 as max for a street driven engine. However, he never states if this is with aluminum or iron heads. I've take it to be iron since that is what he mostly talks about in his book. With iron, personally, I'd stay below 8.25 just for the extra safety. With aluminum you can go higher. Since most say you can/should go a half ratio higher in SCR with aluminum, I think 8.32 should be OK. One of the guys at Chevelles.com had/has a BBC with aluminum heads and 8.5 DCR. He has had problems with detonation.

The real key to running higher DCR's is cooling and fuel mixture. You
must keep the temps down. Make sure your cooling system is working at its best. Make sure it never, never goes lean.

One issue in your engine is the relatively large quench distance .048"
to .051". While this isn't overly excessive, tighter would be better. Of
course, if the engine is built, not much can be done. I ran my strip 355 at .032" and never had a problem, even shifting at 7000 rpm. This might be a bit on the snug side for a street motor, though, due to potential carbon buildup.

Just keep it cool and make sure the mixture is right and you should be good. Don't lug it either. Every year or so dribble some water through the carb with the engine idling This will remove any carbon in the chambers.

End Reply

To further the discussion, I asked if he felt the trade off of bringing my quench down and increasing the SCR and DCR was worth it.

His reply:

Yes, tightening the quench will increase the CR. It is worth it. A tighter quench increases turbulence in the chamber which greatly decreases the likelihood of detonation. A 9:1 engine is more likely to detonate with a wide, over .050", quench than a 10:1 engine with a tight, under .040", quench. Aluminum is more forgiving of this since the chambers run a lot cooler than iron (btw, this is the reason aluminum heads should be run with more compression than iron. This pulling of heat from the chamber decreases the power of the engine). At .060" virtually all quench effect is gone.

End reply

Hopefully this may help some fellow forum members when they run into this thread and have the same questions I did. This thread is by far the best discussion/explanation I've seen about SCR and DCR.

Thanks everybody!

 

I also talked to Crane who manufactured my cam. They told me to use the .050 timing instead of the .004 timing. Both the empire net and KB dcr calculator come up with the exact same number depending on what intake valve closing you input, either the .050 or seat timing but both give the exact same dcr if you input the same number.
According to Crane you are going to come out with a closer DCR if you use the .050 timing. They state their cams are more accurate to the .050 timing then the advertised duration. I guess you should just average the two and hope for the best since there seems to be no exact science here.

 

I found a drastic difference in the two calculators. Three phone calls, Three E-mails. Still no response from KB silvolite!!!

 

Your missing my point, do the empire net calc, for example my intake closing is 69, if I take that 69 and plug it into the empire & kb calculator instead of using there @.050 these are the results.
empire net DCR of 7.70631362932799
kb dcr of 7.708

The result with the empire & kb if using the .050 timing of 38 +15=53
empire net DCR of 8.61218098050469
kb dcr of 8.616


That is all I was saying if you input the actual seat cam intake valve closing into the kb calculator and not the .050 timing you will come up with close to the same number.
According to the Crane technician I spoke with he said to use the .050 timing and it will be more accurate. As to if he is right or not that remains to be seen.

 

So if I am understanding your post correctly, you are saying that the Crane Cam tech told you that the KB calculator is more accurate.

Am I following you correctly?

 

Just today I posed the SAE v. 0.050" for DCR to two leading PhD ME's at a leading NC motorsports engineering college ... neither are vizard (he ain't got a Master's or PhD anyway). If they reply w/anything worthwhile I'll be sure to post.

-edit- yes I find some bona fide engineers' opinions & work helpful ... just as I do those of an experienced long-time racer-builder. I have somewhat ecclectic interests ... Just yesterday I stood atop a platform at Darlington & watched alky USAC cars test ... 5 hours later the wife & I took in a live production of Mozart's Cosi fan tutte. Frankly, the opera was fun ... the USAC test was BORING.

 

Yes and no, niether calculator is different, they both use the same math
to figure dcr, so that much we know is correct.

1). The empire net uses seat timing (advertised duration IVC most cam cards show this as .006 or .004 timing) the kb uses @.050 duration IVC+15.

2). If you entered in the @.50 timing IVC +15 into the empire calculator as your intake closing point instead of the seat timing you will come up with what the KB calculator shows using the @.050 IVC+15 method as I showed above.

3). Now flip it, enter in the seat timing(.004 or .006) IVC into the KB calculator instead of the @.050 and it will show you what the empire calculator shows you with seat timing IVC.

Now, knowing that the math is the same:
KB uses @.050 timing IVC +15
Empire uses .004 or .006 IVC

I talked with the Crane tech and he claims that with there cams you will come up with a more accurate DCR using the .050+15 timing reference then the advertised duration.

So niether calculator is more right then the other it is up to you to input whichever IVC timing point you think is correct.

The real way to solve this is for someone still building there engine with a cam degree wheel to measure the ACTUAL intake valve closing point and see which is closer the adv duration or the @.050 +15.