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358 DIRT Modified Motor

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358 DIRT Modified Motor

Postby bloodnose » Sat May 11, 2013 9:14 am

G'day Guys
Very curious and have to ask a few questions about the building of a new engine.
We run slightly modified 358 Dirt Modified Spec motor rules on circle track. (Chevy)
We have to run a max 360ci displacement but can run any bore/stroke combo we want unlike the DIRT rules which require stock bore and stroke. We also have to run a max 11.5:1 compression ratio and 23 degree heads.
At the moment I run a 3.5" stroke, 4.00 inch bore, 6" rod, and the engine revs to about 7500rpm - we also run on methanol (carb only).
I run cast DART heads that have had a little work done.
The tracks we run on are approx 1/4" mile or smaller and do not usually go slick - I am looking for good pull off corners and starts.
Just want to know what you think is the best engine bore/stroke/rod combo.
The rules makers are also looking at going to the aluminium spec head next season but we are not allowed any porting or polishing - heads have to remain stock.
We are cracking approx 550hp at the moment with our current engine setup but some guys are cracking near 700hp with their SBF engines.
Hope to hear from you guys soon, and please give me some insight into your answers - I am trying to learn as I go along here.
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Re: 358 DIRT Modified Motor

Postby 900HP » Sat May 11, 2013 10:51 am

Here is a link to a 355 dirt modified motor I just finished. viewtopic.php?f=1&t=35525&hilit=dirt+dyno+results

We have no compression rule but unported world products sportsman II heads only (not much of a head)

I do not know what your cylinder head rules are other than 23* could you explain further. I'm a believer in big intake valve, big bore, and whatever stroke happens to fit for the C.I.D. rules. There are 23* chev heads that will blow right by 700HP if that's what you want.

If they are changing head rules next year is there any way to find out what the head will be? You could run that head this year and have a year of development time on the other teams.
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Re: 358 DIRT Modified Motor

Postby bloodnose » Sat May 11, 2013 11:41 am

Our current head rules are that the heads must be cast iron and 23 degree.
No angle milling.
Any port/polish allowed.
They are looking at allowing the brodix spec head http://www.brodix.com/heads/spec.php next season.

I am also currently only running 1.5 ratio roller rockers.

Am also running flat top pistons with a compression ratio so close to 11.5:1 you would not be able to lose anything off the head gasket....

As I said any advice would be great.

We are also allowed to run a dry sump system but currently I just run a good circle track pan and HV oil pump and have not had any surge etc.
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Re: 358 DIRT Modified Motor

Postby wyrmrider » Sat May 11, 2013 11:44 am

4.125+ bore 3.25 stroke 6.125 or 6 1/4 rods whichever are easiest to get pistons for
longer the rod the lighter the piston
not a good year to invest in new heads is it
small chamber but unshrouded, tight quench as little dome as possible
this year just unshroud the heads you have
I do not see any way to run next years heads without a letter from tech applying to everyone
8500 rpm so you'll need a lower gear and cam for off the corners
what are the block rules, bbc cam tunnel, oversized lifter bore rule, rocker ratio rule etc, roller cams legal?
with 11.5 and alcohol I'd think very high ratio rockers and a reasonably short on the seat cam to build cylinder pressure
is there a valve rule? If not Ti then hollow stem?
running a stock oil pump and distributor I'd at least get my timing off the front of the motor (if legal)
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Re: 358 DIRT Modified Motor

Postby bloodnose » Sat May 11, 2013 11:56 am

Block rules just state a cast iron small block. Pretty open in the way of lightening and machining.... All of what you said should be allowed.
We currently use roller cams etc - It is currently our off season so we are not doing to much till they finalise the head rules for next season ( plus I have a broken leg and can not get to far atm!)
Also as I said only use 1.5 ratio roller rockers..... Cam is pretty big - can not fit anything more inside the current cam tunnel.
Our cars are like NORTHEAST Modifieds so we can adjust any gear we want into the rear - for a guide on the tracks I run we currently use around 7.3
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Re: 358 DIRT Modified Motor

Postby 900HP » Sat May 11, 2013 12:09 pm

bloodnose wrote:Block rules just state a cast iron small block. Pretty open in the way of lightening and machining.... All of what you said should be allowed.
We currently use roller cams etc - It is currently our off season so we are not doing to much till they finalise the head rules for next season ( plus I have a broken leg and can not get to far atm!)
Also as I said only use 1.5 ratio roller rockers..... Cam is pretty big - can not fit anything more inside the current cam tunnel.
Our cars are like NORTHEAST Modifieds so we can adjust any gear we want into the rear - for a guide on the tracks I run we currently use around 7.3



Dart SHP pro-series block best bang for the buck. 174 lbs, big cam journal, big lifter bores, billet caps and ARP studs all from the factory. I would do 4.125" x 3.32" with 6.200" rod (which is what I did). Look at some of the cast iron 23* stuff Chad Speier has (Speier Racing Heads) he can also help you with the spec aluminum heads as well. There is a LOT in the right valve job and chamber work. Run as much piston as needed to get to 15:1 for alcohol or 14:1 for race gas. I would do 1.8/1.7 or 1.7/1.6 rocker set up with the lightest valves/valvetrain you can find. I don't think you can do better than the Holley Strip Dominator (keith dorton one) for this application. I don't know what your tire rule is but If you can hook it up I would look at something like a 263*/271* on a 108* if you want it to carry out past 8000. 255*/263* if you want more midrange but it will be all done by 7500. Also, dry sump or wet sump run the biggest ALUMINUM oil pan you can find, helps like an oil cooler.

This reflects my opinion on what I would do. Other opinions may differ.
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Re: 358 DIRT Modified Motor

Postby wyrmrider » Sat May 11, 2013 3:19 pm

you only use 1.5 or 1.5 is all that is legal?
raised cam big journals professional block allowed?
much better than breakable stock blocks IMHO
I'd be thinking of large diameter- large wheel lifters
Looks like the mo head is the one to use :)
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Re: 358 DIRT Modified Motor

Postby CNC BLOCKS » Sat May 11, 2013 4:16 pm

As mentioned the Pro block would be the way to go its lite compared to the Little-M or Bowtie BBC cam tunnel and .904 lifter bores.

There have been some guys that have used the 55MM babbit cam bearings for better valve control and it seems to be working.

I may have missed it can you run a dry sump?
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Re: 358 DIRT Modified Motor

Postby bloodnose » Sun May 12, 2013 2:59 am

G'Day guys - thanks for the replies.

Engine is SBC

So yes I can use the pro block - I am currently using 1.5 ratio rockers but am allowed to use whatever ratio we want.

As for dry sump - yes we can use it.

So now the main rules are 11.5:1 compression ratio, 361ci max displacement, cast iron block, 23 degree heads, alcohol engine, running 4150 carb. Almost everything else is open.

Lets see now what setup we can get :)

99% of tracks are 1/4" mile or smaller on dirt, we run a sprint car rear tyre setup but run a NORTHEAST DIRT Modified car. My gearing with my current engine package is around 7.3 most of the time pulling approx 7500rpm max.

Cheers
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Re: 358 DIRT Modified Motor

Postby wyrmrider » Sun May 12, 2013 9:40 am

one way would be to spec the common 406 compression height piston and use a 3.25 crank and a 6.250 rod
There are also pistons ready made for 4" stroke and 6" rod where you could us a 6.375 rod
11:1 is not much for alky and the longer the rod the longer the dwell at TDC and therefore the longer time to build cylinder pressure
you could use a calculator and see if a raised deck block and even longer rod would help
as cam king says in this thread you will need a custom cam
viewtopic.php?f=1&t=12839
especially if you go to a big wheel roller tappet and high ratio rockers -which I recommend- but ask Mike
here is another thread- there are many
viewtopic.php?f=1&t=26180
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Re: 358 DIRT Modified Motor

Postby SLPRACINGENGINES » Mon May 13, 2013 7:46 pm

im a big bore short stroke person in this case.good aftermarket block with roller cam bearings ..big bore will take you down the straight away ..and dont be afraid to turn it..i run the same thing in my son asphaly open wheel modified..we turn it 8500 rpm all night long..its a good little combination..
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Re: 358 DIRT Modified Motor

Postby SLPRACINGENGINES » Mon May 13, 2013 7:52 pm

i would talk with mike jones (camking)..i understand he likes to mix rocker arm ratio up.i believe he can fix you up..cam is very important on this engine..as an motor ..but esp this motor..because this thing will scare you how fast it will turn up and pull..
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Re: 358 DIRT Modified Motor

Postby wyrmrider » Mon May 13, 2013 9:39 pm

I agree that a consult with Mike is appropriate
he could help on the lifter size and rocker ratio targets and not have you spend on things you do not need
and If I'm out to lunch on the big bore un-shrouded big valve approach he will set me straight.
Find someone with experience with those heads, do big valves help, radius seats all those kinds of questions.
What bore size did Brocix flow them?, have they been flowed at different bore sizes?
If the raised cam makes a difference due to shorter pushrods or better angles he can tell you
The Bigger the cam tunnel the better IMHO

on the block
raised block question- would it help by at least a couple of percent
6.450-6.575 rod
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Re: 358 DIRT Modified Motor

Postby CNC BLOCKS » Tue May 14, 2013 5:06 am

wyrmrider wrote:I agree that a consult with Mike is appropriate
he could help on the lifter size and rocker ratio targets and not have you spend on things you do not need
and If I'm out to lunch on the big bore un-shrouded big valve approach he will set me straight.
Find someone with experience with those heads, do big valves help, radius seats all those kinds of questions.
What bore size did Brocix flow them?, have they been flowed at different bore sizes?
If the raised cam makes a difference due to shorter pushrods or better angles he can tell you
The Bigger the cam tunnel the better IMHO

on the block
raised block question- would it help by at least a couple of percent
6.450-6.575 rod



To much weight on a raised block
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Re: 358 DIRT Modified Motor

Postby wyrmrider » Tue May 14, 2013 8:42 am

OK CNC
is this a weight limit class or does everyone have to find parts of unobtanium?
If he has a minimum weight he could do raised block, if not he can't- how much weight difference is there< would be hard to find that much elsewhere
I'd still think of the raised cam tunnel- btw do they allow gear drive?
can you get a headgasket for a large bore block and lay it over one of those spec heads?
can you match the bores/ new chambers to the heads?
would such a move allow more flow?
cheers
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