I'm calculating the DCR for my cam with a 42.8 IVC, and I'm worried that I'm getting a very high number, around 9.7. These are the numbers I'm using, but in reality, I just dropped the block off at the machine shop today, and I'm having the heads ported soon. So, I'm not sure on all the numbers, but even when I get a SCR I like around 10.5 or so, the DCR is still way high. This cam felt great in my stock motor with stock heads though. What's wrong and can I fix it? I want to know what to tell my machinist and whoever does the headwork what I need. Thanks.

Rod: 5.7
Stroke: 3.48
Bore: 4.030
*Combustion Chamber volume: 58
Head Gasket Thickness: .046
*Head Gasket Bore: 4.125
*Piston to Deck Clearance: .025
Flat Top pistons with 4-valve relief

I'm assuming there won't be any dish on the piston different from stock since the stock pistons also had valve reliefs. The * quantities are what I need advice on. I'm guessing the heads will be close to LE1, just done locally. Do they usually increase the chamber size and how much? Do they have a lot of different gasket bore sizes? Am I just SOL if I stay with this cam?

 

You need to use the advertised IVC at say .006" not at the .050" mark. That is why you are getting a high DCR.

 

That would make sense. Any way to find that with the cam specs I have at .050?

 

What's the duration at .050" and the advertised duration? With a little math we should be able to come up with it.

 

I only have the specs at .050", but I have all the events and everything. Stupid Cam Motion doesn't have the intake lobe on their catalog anymore. Furthermore, the adv exhaust is 283 for this lobe, but the duration at .050 is almost a full 2* off what the cam doctor has!

EDIT: Looks like it might be 288/283 on a 114+4. Any suggestions?

 

What is the duration at .050" for the intake side? From what you have posted the advertised looks to be 288 on the intake side.

 

Duration at .050" is 226 for the intake. They didn't have the exact lobe, but I found one close to it. This gives me a IVC of 74 which is much better.

 

Yep. Doing the math I come up with 73.8 IVC. That should make a big difference on the DCR.

 

It does, now I'm getting 7.91 or so dcr and 10.73 scr.

60cc chambers
.039 gasket thickness (Fel-pro)
4.125 gasket bore
.02 PTDC
-2cc dish piston (valve reliefs)

This also gives me .059 quench distance, is that good or bad?

With a .046 head gasket and 0 dish piston, I get 7.95 dcr and 10.79 scr with a .066 QD. Any ideas?

 

yes put a thinner head gasket in it

 

Wouldn't 11:1 SCR be too much for a street engine on pump gas?

 

I agree with Bret. I see you have dropped the block off at the machine shop. Have him zero deck the block and then use the appropriate head gasket. This will enable you to get down to a .040" quench or a little less. That is way better than the .059" you were looking at. Also at a minimum I would run a 8.5:1 DCR.

Also you state a zero dish piston. What about valve reliefs in the pistons? They will effect the compression ratio. If you don't know what they are have them CC'd and plug that into your compression calculator. You are lucky in that you are just starting the build up. Now you have a chance not to leave easy horsepower on the table and have a more efficient motor.

By the way that is a very lazy cam you have. I don't know what your budget is but you might very well consider a custom cam. Are you planning a street car, street/strip or racing only?

 

Why is the cam very lazy? I bought it cause it was really cheap and around what I wanted, although it was a compromise. I'm going to call the shop tomorrow to ask about the pistons and tell them to zero deck the block. I've been looking and with the 4-valve relief, it's most a likely 6.1cc dish. I thought 8.5 dcr would be too much for a street/strip car on pump gas?

EDIT: Compared my cam to the CC305, and I have .38 more dcr all else equal. Compared to the CC503, I have about .20 less dcr. Compared to the CC306, I have .23 more. Doesn't seem too lazy to me.

 

Before I get too carried away is this a roller cam?

 

hydraulic roller. Thanks for your input too, btw.