Jorlain

iTrader: (4)

That's good news. I'm a smidge farther down the hole than you, around .018-.019, and will be running the .026 Victor Reinz gasket, etc.

The bad news is, I'm really agonizing the cam selection. I just can't seem to make a decision.

86LG4Bird

iTrader: (1)

Agreed you should go for 4-6 E/I duration split.
No problem going 1.6 vs 1.5 on PTV clearance; your machinist isn't understanding that one.
Did you ever determine actual chamber volume / CR ?

Jorlain

iTrader: (4)

I don't have the chamber volume just yet, but I should have it sometime this week.

skinny z

THEN you can decide.

Jorlain

iTrader: (4)

Agreed.

For kicks, I stole the cam profile in your sig (274/282, .570/.570, 110LSA) and plugged it in.




Best numbers so far as far as computer modeling goes, with the exception that it falls off very rapidly after 5500 RPM.

I take it that you're not overly fond of this cam since you're switching, though.

Orr89RocZ

iTrader: (20)

Its a decent cam and will carry power above 5500 in a 355 with the right heads and intake lol but it all depends on the parts used

Jorlain

iTrader: (4)

Well to be honest, I was more or less planning on a 6000 RPM redline anyways, so that's really not an issue in my case.

skinny z

That cam was my attempt at specifying my own. I had a given compression ratio to work with and was RPM limited with my OEM cast crank and rods (although the rods were upgraded with better hardware). It's made up of Comp's XFI lobes. I looked for the most lift within the confines of 224 @ .050" and the least advertised duration. Those valve events maximized my DCR yet not so much that ignition timing would be an issue. The LSA and ICL carried over from my previous XR276HR as I found that cam to be a great street cam. I just wanted to take it further by making as much low speed torque as I could. It worked too as the car 60 footed the best it ever had (or close to it). I'm guessing peak power was around 5500 RPM. It spun to 6500 effortlessly. Problem was the short block went south and the new short block I acquired had too much SCR for the little 274. It worked well at 9.9:1. Not so well at nearly 10.5:1 (and iron heads). That forced my move to the XR288HR which I already had.

Jorlain

iTrader: (4)

Just to update, the combustion chambers measured up to the advertised 64cc.

Bore: 4.040
Stroke: 3.48
Head Chamber: 64cc
Valve Reliefs: 7cc
Gasket Thickness: .026
Gasket Bore: 4.1
Deck Height: .018
CR: 10.09

I get a DCR of 7.86:1 with the XR276HR-10, being that I'm at 1210 feet (which I guess I never realized until I heard it mentioned on the radio this weekend, being that NHRA is racing here this weekend).

skinny z

I'm getting 10.08 SCR but my calculator is saying 8.10 DCR which is a pretty good number. Not sure where the difference is coming from. Whose calculator do you use?
We've got a little discussion going on here:

https://www.thirdgen.org/forums/tech...-wanted-2.html

It might interest you.
I'd say from your numbers that while you might need premium fuel, others have run similar DCRs with iron Vortecs and mange with regular grade gasoline.
My 8.1 Vortec with the 276 always got 94 octane. The extra cost was worth it to me as a hedge against detonation. I always ran a full and somewhat aggressive timing curve. I also had plenty of vacuum advance, especially at cruise but I had to be cautious. It's easy to get 50 degrees of spark lead at highway cruising speeds. The mixture has to be spot on (not too lean) and you have to listen for pinging on light throttle tip in.

Jorlain

iTrader: (4)

The calculators I typically use are:

Static: http://www.wallaceracing.com/cr_test2.php
Dynamic: http://www.wallaceracing.com/dynamic-cr.php

If I put in 0 for altitude, I also get a DCR of 8.1:1. However, with my altitude of 1210, I get 7.86:1

My only real goal for octane requirements, is to keep it happy in 91 octane or less. Anything higher than 91 is just too difficult to find around here anymore. I think I know of one gas station within 100 miles that has 93 octane.

In the other thread you mentioned, I see you have a few builds there listing the Victor Reinz 5746 as a 4.120" bore. Did you measure it to be different than the advertised 4.1" bore?

skinny z

I just bought another Victor Reinz gasket and the specs this time said 4.125" for a bore size. That won't skew the results much. However, your adjustment for altitude definitely will. My calculator (Pat Kelly's) doesn't take into account the altitude.
For what it's worth, my calculated (and actual ) cranking pressures differ from my where I used to live at 600' to where I live now at 2200' by about 10 PSI. If the Pat Kelly calculator uses a sea level reference, then your numbers are spot on.
I'd say 91 octane with a well tuned engine should be no problem. Your Pro Toplines are aluminium aren't they? I haven't looked them up other than flow numbers.

Jorlain

iTrader: (4)

They're iron, actually.

skinny z

I still don't see a problem.
86LG4Bird (in the thread I posted a link to above) claims that with his 8.1:1 DCR Vortec 350, regular fuel isn't an issue. Now I can't say what he's running for a timing curve but still, based on that, and what I've managed to do with similar DCRs and iron heads, points towards success with 91 octane.
That said, DCRs approaching 8.5:1 and Vortecs (or similar like your Pro Toplines and my Pro Torkers) don't mix well.
Your altitude will help some too.