Is there a higher propensity for a big cube 434 SBC to detonate over a 355? Assuming the following being equal:

DCR- 9.0
Quench Distance - .045"
SCR - 11.5
Aluminum Heads with appropriate flow characteristics for Cubic Inches (probably about 275 for the small motor and around 330 for the big motor- standard cooled
93 pump gas
VE up around 100
Optimum Timing of course
EFI of course

or

Is it just way more complicated than that?

Steve

 

First thing you have to understand is that pump gas (unleaded) is junk. It will detonate just because it can.

Next any SCR above 9 to 1 is asking for trouble when you are dealing with today's gasoline!!!!!

Therefore as far as the propensity to detonate it has nothing whatsoever to do with the size of the engine - it's strickly C/R vs ign timing related. Personally I like the 9 to 1 and higher timing, instead of high C/R and low timing.

Denny Schmidt

 

Denny I expect 1racerdude to be on here shortly to argue that. I could be wrong but I think I heard him talking once about motor he had over 12:1 SCR running on pump gas. He might have been talking about race gas though.

Either way, dynamic compression ratio is the only thing the gas cares about, that and engine cooling

 

I'm running 11.8 to 1 Static and darned close to 9 DCR right now on my 355 on 93 pumpgas without any detonation problems. All quiet on the knock counter front. My total timing is at about 36 degrees. Good to hear that the larger bore shouldn't contribute to knock sensitivity.

Thanks for the reply

Steve

 

Funny, cause I've built street pump gas SBC, LT1 and LS1 motors with well over 11.5:1 and haven't had issues.

The issue with the 434 is the bore size, that's going to make the detonation more likely. With enough cam though a 11.5:1 434 should be a cake walk.

Bret

 

Hi Steve,
Bret's got you on the right track. One of your biggest concerns should be a properly designed quench region. As the bore gets larger, so must the charge front. Controlling it becomes the issue. You would be ahead of the game by increasing the VE of a small bore motor if you are going extreme, but I don't think your looking that direction.

Thinking about ditching the ltx, and going big bore ehh?

Dennis

 

Dennis,

No immediate plans to build the Big Bore SBC. The project is very interesting to me however. I'm thinking gobs of torque, plenty of power to 6500, extreemly reliable for bracket racing and occasional street use, OEM EFI, Still an M6 for the fun factor. I'm just gathering ideas right now. I am a believer that success is mostly in the heads so that's where a big chunk of money would go. Aftermarket blocks and rotating parts are reasonably priced so there's really not too many barriers. Ignition is a concern but not insurmountable with opti cam position sensor and LTCC setup to eliminate clearance problems associated with a rear mounted dizzy. I would think waterpump spacers and some machine work on a timing chain cover is all do-able. I'm currently constructing my garage/workshop so that where most of my time and money is going this winter so there probably won't be too many changes on the Pontiac in the short term, but when I get the itch to do something it sometimes takes over as my common sense fails me.

Steve

 

Ok you guys lets take all the computerized gadegts off the engine.

Disconnect the the computer from the timing advance and the fuel curve and then make it run at anything above 9 to 1!!!

The reality of your situation is that you really dont know what the IGN timing is because the computer modifyes it accordingly based on certain varibles, one of them being the knock-value. The computer also modifies the A/F ratio accordingly. When I say 9 to 1 its with a carb, with non-computer assisted ignition, and non-computer assisted anything.

Granted you can create a cam with timing events that will allow you to run a higher compression than 9 to 1 but you have to be smart enough to know when that can exceed the ability of the gas to function in its normal way.

Just my .02

Dennis (airflowdevelop)
Talked about the squish-quench area of the chamber/deck surface of the head. It is so true that when you can get this to function correctly you can achieve a better end-gas burn rate, thus more fuel molecules get used to produce power instead of waste gases out the tail-pipe.

Denny Schmidt

 

Yep, 9-1 DCR is max for pump gas(92 octane) NA.(with everything being designed for pump gas ya might get a little more). I have talked to a guy in 'Vegas that runs 9.65 on the street in an 'IMPY" with no problems. Static don't mean S***,it is just a number to be talked about and a guideline.
If ya got 13-1 and a IVC of 78/80*(real closing degrees) ya should be fine with GOOD/ultimate cooling. Ya won't get the REAL closing degrees off of a cam card,just close.

Large bore engines(over 4.030) won't go that high due to the area to be covered by the burn and the left overs that are unburned every stroke without a great deal of thought put into the design of the engine.

Now I will hear from everybody with a 4.060 bore and "they never had a problem".
A good deal of research has to be done on "the burn" to get it right with no detionation and walk the "MAXED out line"

 

I'll do you one better Denny....

3.50" stroke
6.25" rod

So LOTS OF DWELL

67cc chambers with a small dome, 11.7:1 SCR

25° initall 35° of lead all in at 3500rpm with a MSD pro billet

92octane gas

Holley 950HP Carb

IVC 65° ABDC

No detonion with good cooling.

It's an extreme example but i've done it, tweak that back some and I can make it work in 100° heat the same way.

Bret

 

Here's some great info on combustion, swirl, engine dynamics, and "The Soft Head" principles:

ENDYN www.theoldone.com

 

Yes, I know all of that I've talked to Larry before. He can get 13.2:1 on pump gas and 11:1+ with a blower on pump gas in pent roof Honda heads. This is the guy who started rasing the static compression in racing years ago in NASCAR and Pro Stock. Back when they ran 12:1 on race gas he got them to 16:1 and has done motors over 23:1 without detonation.

Bret

 

Bret

The question is can we duplicate Larry's results - with domestic push rod engines?

Do you think that - if we could design the chamber in a Chevy head any way we wanted too we could achieve the same results?

Actually the more I looked at the stuff on his site the more I think that the centerline chamber can be made to do the same things, given the budget to experiment.

Denny

 

That guy's the man.. LoL Love the way he put's things.. and he use of certain language.. GooD Read def...

 

Yeah.... given the budget. He's done some crazy things with compression for sure, I know to certain extents we can push the envelope but not with shelf parts. What Kaase did in the Engine Masters is pretty close to what can be done I would guess with chamber shapes and burn on 2 valve stuff.

Bret