mark kibort

Thats not even really a remote possiblity. everthing pulled off was put on in exactly the same way. timing was identical, both electrically and with the cams. the only possible change was the offset correction, where all other 2 valve and early 32 valve engines have one bank correct and the other bank not-correct. (fixed with the S4)

so, this is a great comparison. both bottom ends were brand new as well. (new rings and bearings)

so, its also makes sence as i did this before with my old 4.7 euro top end( sans no euro heads). when i added the 300ccs WITH the euro heads (bigger valves) it went from 240rwhp to 293hp. (both Ljet and euro top end)

However, Scots has a B1 cam, i had euro '82 cams, but i had a 3.5" exhaust system, scot's comes out at 3" so thats kind of a trade off.

where else could the HP come from, especially since both engines were built by me and incorporated the same things and dynoed within 3hp of each other?

mk

Rick Carter

It is unusual to increase displacement and all other things being equal to have a more efficient engine. It must be the venturi effect.

mark kibort

I see where you are coming from. However ,there are a lot of factors that change, even with there not being any other changes. as i said, we did an offset optimization on this shortblock. Greg Brown says thats good for 10hp, and that would bring things right in line with displacement increase percentages only if that was not done. However, that could only be proved imperically, if we found an Ljet with the pistons put on the rods with out the correct offset. oh yeah, i did this, but we had slightly different cams and a 3.5" exhaust vs 3". maybe scots motor has different piston to block clearances than the stock 85 shortblock i built. the facts are that my 4.7 went from 240rwhp to 292rwhp and scots went from 250-260ish rwhp to 290rwhp. (scot having the euro heads already and my old Us84 having less compression ratio and was pretty old and tired) so, are larger valves and larger compression worth 10-20hp?? maybe. thats why we see pure euro engines in the 260rwhp range with similar mods.

maybe our engines get more efficient with the proper air fuel sized charge in the combustion chamber. we have just increased the bore to stroke ratio, and maybe that has an effect. I dont know.

What i do know is that we carefully removed all components on the engine, not even changing alternator/powersteering pump belt tensions! nothing changed on the clutch, both new engines had the same new bearings and new piston rings. the cams and lifters were never even removed from the cam towers!

the only real difference was that we had scots block shaved .010". I dont think that would effect anything. i think on the compression ratio scale, it raises things .3:1 or something.

If you have any ideas of what could have changed, id be interested to hear it. but between the two engines, the only real changes were the displacement.
Interesting to see how HP is increased by increasing displacement by increasing diameter or increasing stroke. it maybe that increasing diameter may yeild more efficiency. Thats something for the scientists to confirm!

Mk

Rick Carter

Mark I'm just pushing your buttons, the fact remains it is what it is. You have objective empirical proof, good job!

mark kibort

I know . But hey, your right. 6% increase in displacement should only equal 6% or less in Power. but for some reason, ive got double that! (and we did this twice) im starting to think this is a diameter thing. certainly we dont see these kind of ratios with the stroker kits.

as a side note, my buddies stock S4 went from 5 liter to 6 liter, yet his hp gain was in the 300rwhp range (compared to 270rwhp of most S4s) so that was a 20% gain in displacment but only a 10% gain in HP. however, he gained 30% in torque. im wondering, with the diameter increase becomes the bigger factor here. I think most 6.5 liter strokers are always looking at 350rwhp and greater and thats an even 30% increase of both HP and displacement. ( 500cc of due to diameter and 1000cc due to stroke)

the only other thing we can look at, is what happens to the 4.5 liter engines when compared to 4.7 liters. i think the 1982 4.5s were 220hp and the 4.7s were 242. thats 4.4% displacement making 10% more power. Hmmm, diameter seems to help the most with HP!

mk

mark kibort

Here is the dyno sheet from scot's engine with Ljet (5 liter euro)

hacker-pschorr

Mark - post the before / after dyno charts here:
https://rennlist.com/forums/928-forum/346823-official-928-dyno-and-performance-bragging-thread.html

mark kibort

Here is a side by side comparison

mk

tommytomaso

it all makes sense , a larger dia, in my mind would give beter HP given you get a larger combustion in the same stroke length.
now would this same basics work why a longer stroke gets higher TQ? ie: longer leverage on the crank? and if so...could you be countering HP gains with TQ gains with the longer stroke?
lets just throw out our intake system and plop on an Eldebrock and a holly and see what happens...lol

928autobahndreamer

A little off topic, I know. But, when are you and Scott going to be able to put this new engine to the test at the track. I love hearing the track day stories and am anxiously awaiting the next race.

mark kibort

scot's car is almost ready for the race next weekend (May 5th at laguna seca)

he had some pretty significant body work from a couple of incidents in the last race weekends. we got the new front end on (hood and two fenders thanks to Dozer!) and the engine is dynoed and ready to go. 9 quarts of Amzoil for Scot and he will be there!

here is a pic of the damage scot had and the comparison dyno runs (before and after 4.7 liter to 5 liter , no other changes) Here is what his car is going to look like again

mk

mark kibort

actually, it sure seems that the % hp gains are greater for displacement for larger diameter same stroke vs just longer stroke. I think its due to your first point, but with the longer stroke, you still have a larger combustable mass, however, its over the same diameter, (surface area of the piston if that doesnt change) , so you do have gains of more torque, but also more hp in the end as well. i remember a 6 liter stroker curve on a stock S4 and the hp went up to near 310rwhp, but the torque went up from 275 to near 375 ft-lbs.
thats a gain of only 40 or so HP for a 1 liter change for a longer stroke. Scot's went from 4.7 to 5 liter and gained near 40hp with a piston diameter change only.

I would guess that if you could make a 6.5 liter block with 130mm piston diameters it would make a lot more hp than a 6.5 liter stroker. (but thats just a guess).

mk

Jim bailey - 928 International

Mark the stroker crank has more leverage but at the expense of piston side loading and the need for shorter rods shorter pistons. Short stroke engines have lower piston speeds so are potentially capable of higher RPM and RPM has the potential for more HP. In some cases larger bore opens up the area around the valves for better efficiency. And as you noted stroking a stock engine with no other changes makes a torque motor because the same cams valves and intake are NOT BIG enough to make big HPbut have high velocity at low RPM thus make torque. The same camshaft in a 911 2 liter race engine is pretty mild in a 3.6 liter !

mark kibort

Update:

scot ran a 1:42.6 qualified 10th in a pretty tough and even field.
I ran a 1:40.8 and am 2nd behind an S2000 who ran a 1:40.6 on a real clean lap and won the pole.

the 440rwhp camaro is right there, as well as several BMWs and the Vet. with 10 cars within 1 second of each other in ITE, it should be a good race.

race time: 1pm tomorrow

MK

IcemanG17

MK
Bummer is I'm gonna miss the race.......but I'm hoping for a 928 1-2.....hey a guy can dream okay!!