sbc HP per cubic inch

[digster]

Matt
  Many variables but in your years of dyno sessions have you seen any patterns develop as to best sbc bore stroke (sweet spot).
I've been making 700hp @ 6000 RPM. 4.125 bore X 4.00 stoke. My set up needs 750hp @ 7500 RPM. My new heads and cam will get me there.
Going to freshen up a bit in the off season. Just wondering if I need to re invent the wheel bore and stroke wise or if the 434 chev is still a good choice for this range? Thanks   

[masracingtd1167]

A lot of the bracket guy's that I know run that same bore and stroke and run very fast with it . A good pair of heads should really make good power . There are some really good 23 degree heads out there .  2http://www.speierracingheads.com/ 

[digster]

Thanks 1167.
 I run Brodix - 12 15 degree heads. My Old heads flowed 360@ .700. The new 12's are a solid casting with more porting. They flow 390@ .700.
I'll get my goals with a few updates to the old engine. Starting to second guess the set up. With comments like.  Not enough rod ratio (6 in rod). Not enough piston dwell time. Need a shorter stroke and spin it harder. Not sure if I agree with these guys. Some good things are hard to change.
 

[masracingtd1167]

I didn't know you were running a rolled over head . You should be able to make big power with a head like that . I use an 18 degree head on mine but I do run a shorter stroke 3.625 and a 4.155 bore . I agree you might have to spin it a little higher to get the results that you want . I think you should be able to make over 800 HP with it .

[Frontenginedragsters]

Digster:
 As we all know the engine is a air pump.
Dyno testing shows us that as long as torque continues to rise with RPM then the HP numbers rise also.
Its how the math works on a dyno. Torque x RPM divided by 5252.
It would help to know what heads, intake and camshaft you are running now.

Matt

[digster]

Thanks for the reply Matt
The current cam is smallish  I.630 E.630  268 270 @ 50 and ground on a spikey 106 Separation. Pulls very hard to 6000 rpm then its all done.
I've got a new spec. I think I'll try with the new heads. I.663 E.688  278 284 @ 50 on what should be a little broader power band 110 separation.
Matt new Heads are Brodix 12 with heavy port work 2.20 Intake valve 1.60 exhaust valves.
Stack injected, down nozzles and 16 to 1 comp.

[wideopen231]

I have cpl guys who dyno tell me that most of the time with all other things being right the motor will make between 2.2 and 2.4 times the airflow of the engine.I play with desktop dyno program a lot and most of the time its real close given correct info. It also comes up pretty close to that 2.2 to 2.4 x cfm deal. Not sure that verify's anything just food for thought. I think you said 390cfm,Bill said should be 800 plus and 2.2x390 would put you closer to 850.Again that with everything be a optimal.

[digster]

Thanks 231
 I assume smaller pump (engine) displacement could be made up by for by higher pump speed and still approximate the 2.2 HP per cfm rule. Using my assumption the pump would have no regard for bore vs stroke Just CFM. The pump configuration is what intrigues me.

[dusterdave173]

I dyno lots of engines--I have seen all kinds of stuff but you just can't ever go wrong with bigger bore--I see some very large bore with much shorter stroke than they could have for same effort or money do amazing things as long as they have enough head.
I think on smalller CI combos you have to get it just right with runner volume , cam etc based on what RPM you will really be turning--too often I see way too much runner on pretty small CI engines.
The combos you guys are discussing 434 with those awesome heads make it hard to go wrong--just add RPM until you get what you want out of it
An interesting benchmark of what will do what is Scott Shaffiroffs website--You see the engine combos he builds with what parts and then see HP numbers that are usually right--Scott is more than anything a perfect business man but talk to anyone that has an engine from there and they are usually very happy--of course we need to think of the alky and induction we use but it is just a decent yardstick of popular combos.
The thing is you can go 8.90 with Craigslist, stock block used small  junk--Going 8.50's is a decent little engine --going 8.0's is a serious and way more expensive deal as far as parts needed--I am thinking 8.50 as my long term goal is somthing I can do myself and afford- I have enjoyed making things as light as possible--it always rewards in a drag engine--if not all that much faster it makes for a great engine "personality" -after that, I read about you big dogs like you were astronauts

[masracingtd1167]

I agree with Dave A bigger bore will always make more HP . Most of the Comp Eliminator stuff is big bore like 4.125 up to 4.185 with a very short stroke like 2.800 and some are making close to 3.0 Hp per cube . 16 to 1 compression sounds kind of high to me . Maybe Matt or Dave could give some advice about that . From what I understand too much dynamic compression can cause severe detonation if the intake closes too early . I will look and see if I have some old cam cards from a couple of cams that worked for me .   

[wideopen231]

I think one reason bigger bore works so well is unshrouding the valves.Another maybe that with bigger bore higher compression requires less dome hight. even on my desktop dyno hp increase with .060 bore is more than with extra stroke.

[dusterdave173]

The bigger bores do unshroud the valves--it really helps
Most older "conventional wisdom " can be challenged these days but the old one about
Big Bore short stroke is still a real sweetheart
My best example is we had a truck puller--National Champ that wanted to stay that way--they had a simple CI rule
I can't remember exactly but it was not very big CI like 4-something
We built a 4.500 bore and made a custom crank to get that exact CI--it was pretty short stroke ( this being a big block Mopar) we then put a crazy big runner head( for that given CI) and cammed the heck out of it--filled with all the parts needed for happy RPM'ing we were stunned at the increase in HP over all the other things we had done in years past and he smashed the field until the rules changed.

These days most folks just put all the stroke and all the bore they can get no matter the math--some do good some are less than hoped for--now we have just opened the old rod ratio thing again-- and..I will leave that for another night.

[dreracecar]

Personaly I believe the long rod and piston dwell is more suited for blowers since one is atifically filling the cylinder rather than pulling air in with vacuum from the piston stroke. I would go with a shorter rod to get that piston moving as much air as possible quickly and not be lazy

[wideopen231]

To my understanding.The long rod  give more dwell time but also more piston speed which is also suppose to help the induction system.Faster piston speed adds to vaccume on the dwon stroke.I have read quite a few articles on subject and even tried  really long rod combo and cpl short rod combos on blower motors.My reasoning for short rod was if long rod help less than perfect intake runner and port then a blower could make use of shorter,hey what better induction sutem than one pushing 40 or 50 lbs of aire pressure into cylinder.

  Needelss to say ABA testing on this is next to impossible.I can not say there was any real on track gains.Then how often do you run blower combo in same air with same clutch,smae over drive and same jet on different weekends?

[MaggiFinn]

Here's a short video on rod ratio.